path: root/doc/rfc/rfc9110.txt

Internet Engineering Task Force (IETF)                  R. Fielding, Ed.
Request for Comments: 9110                                         Adobe
STD: 97                                               M. Nottingham, Ed.
Obsoletes: 2818, 7230, 7231, 7232, 7233, 7235,                    Fastly
           7538, 7615, 7694                              J. Reschke, Ed.
Updates: 3864                                                 greenbytes
Category: Standards Track                                      June 2022
ISSN: 2070-1721


                             HTTP Semantics

Abstract

   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   level protocol for distributed, collaborative, hypertext information
   systems.  This document describes the overall architecture of HTTP,
   establishes common terminology, and defines aspects of the protocol
   that are shared by all versions.  In this definition are core
   protocol elements, extensibility mechanisms, and the "http" and
   "https" Uniform Resource Identifier (URI) schemes.

   This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232,
   7233, 7235, 7538, 7615, 7694, and portions of 7230.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9110.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction
     1.1.  Purpose
     1.2.  History and Evolution
     1.3.  Core Semantics
     1.4.  Specifications Obsoleted by This Document
   2.  Conformance
     2.1.  Syntax Notation
     2.2.  Requirements Notation
     2.3.  Length Requirements
     2.4.  Error Handling
     2.5.  Protocol Version
   3.  Terminology and Core Concepts
     3.1.  Resources
     3.2.  Representations
     3.3.  Connections, Clients, and Servers
     3.4.  Messages
     3.5.  User Agents
     3.6.  Origin Server
     3.7.  Intermediaries
     3.8.  Caches
     3.9.  Example Message Exchange
   4.  Identifiers in HTTP
     4.1.  URI References
     4.2.  HTTP-Related URI Schemes
       4.2.1.  http URI Scheme
       4.2.2.  https URI Scheme
       4.2.3.  http(s) Normalization and Comparison
       4.2.4.  Deprecation of userinfo in http(s) URIs
       4.2.5.  http(s) References with Fragment Identifiers
     4.3.  Authoritative Access
       4.3.1.  URI Origin
       4.3.2.  http Origins
       4.3.3.  https Origins
       4.3.4.  https Certificate Verification
       4.3.5.  IP-ID Reference Identity
   5.  Fields
     5.1.  Field Names
     5.2.  Field Lines and Combined Field Value
     5.3.  Field Order
     5.4.  Field Limits
     5.5.  Field Values
     5.6.  Common Rules for Defining Field Values
       5.6.1.  Lists (#rule ABNF Extension)
         5.6.1.1.  Sender Requirements
         5.6.1.2.  Recipient Requirements
       5.6.2.  Tokens
       5.6.3.  Whitespace
       5.6.4.  Quoted Strings
       5.6.5.  Comments
       5.6.6.  Parameters
       5.6.7.  Date/Time Formats
   6.  Message Abstraction
     6.1.  Framing and Completeness
     6.2.  Control Data
     6.3.  Header Fields
     6.4.  Content
       6.4.1.  Content Semantics
       6.4.2.  Identifying Content
     6.5.  Trailer Fields
       6.5.1.  Limitations on Use of Trailers
       6.5.2.  Processing Trailer Fields
     6.6.  Message Metadata
       6.6.1.  Date
       6.6.2.  Trailer
   7.  Routing HTTP Messages
     7.1.  Determining the Target Resource
     7.2.  Host and :authority
     7.3.  Routing Inbound Requests
       7.3.1.  To a Cache
       7.3.2.  To a Proxy
       7.3.3.  To the Origin
     7.4.  Rejecting Misdirected Requests
     7.5.  Response Correlation
     7.6.  Message Forwarding
       7.6.1.  Connection
       7.6.2.  Max-Forwards
       7.6.3.  Via
     7.7.  Message Transformations
     7.8.  Upgrade
   8.  Representation Data and Metadata
     8.1.  Representation Data
     8.2.  Representation Metadata
     8.3.  Content-Type
       8.3.1.  Media Type
       8.3.2.  Charset
       8.3.3.  Multipart Types
     8.4.  Content-Encoding
       8.4.1.  Content Codings
         8.4.1.1.  Compress Coding
         8.4.1.2.  Deflate Coding
         8.4.1.3.  Gzip Coding
     8.5.  Content-Language
       8.5.1.  Language Tags
     8.6.  Content-Length
     8.7.  Content-Location
     8.8.  Validator Fields
       8.8.1.  Weak versus Strong
       8.8.2.  Last-Modified
         8.8.2.1.  Generation
         8.8.2.2.  Comparison
       8.8.3.  ETag
         8.8.3.1.  Generation
         8.8.3.2.  Comparison
         8.8.3.3.  Example: Entity Tags Varying on Content-Negotiated
                 Resources
   9.  Methods
     9.1.  Overview
     9.2.  Common Method Properties
       9.2.1.  Safe Methods
       9.2.2.  Idempotent Methods
       9.2.3.  Methods and Caching
     9.3.  Method Definitions
       9.3.1.  GET
       9.3.2.  HEAD
       9.3.3.  POST
       9.3.4.  PUT
       9.3.5.  DELETE
       9.3.6.  CONNECT
       9.3.7.  OPTIONS
       9.3.8.  TRACE
   10. Message Context
     10.1.  Request Context Fields
       10.1.1.  Expect
       10.1.2.  From
       10.1.3.  Referer
       10.1.4.  TE
       10.1.5.  User-Agent
     10.2.  Response Context Fields
       10.2.1.  Allow
       10.2.2.  Location
       10.2.3.  Retry-After
       10.2.4.  Server
   11. HTTP Authentication
     11.1.  Authentication Scheme
     11.2.  Authentication Parameters
     11.3.  Challenge and Response
     11.4.  Credentials
     11.5.  Establishing a Protection Space (Realm)
     11.6.  Authenticating Users to Origin Servers
       11.6.1.  WWW-Authenticate
       11.6.2.  Authorization
       11.6.3.  Authentication-Info
     11.7.  Authenticating Clients to Proxies
       11.7.1.  Proxy-Authenticate
       11.7.2.  Proxy-Authorization
       11.7.3.  Proxy-Authentication-Info
   12. Content Negotiation
     12.1.  Proactive Negotiation
     12.2.  Reactive Negotiation
     12.3.  Request Content Negotiation
     12.4.  Content Negotiation Field Features
       12.4.1.  Absence
       12.4.2.  Quality Values
       12.4.3.  Wildcard Values
     12.5.  Content Negotiation Fields
       12.5.1.  Accept
       12.5.2.  Accept-Charset
       12.5.3.  Accept-Encoding
       12.5.4.  Accept-Language
       12.5.5.  Vary
   13. Conditional Requests
     13.1.  Preconditions
       13.1.1.  If-Match
       13.1.2.  If-None-Match
       13.1.3.  If-Modified-Since
       13.1.4.  If-Unmodified-Since
       13.1.5.  If-Range
     13.2.  Evaluation of Preconditions
       13.2.1.  When to Evaluate
       13.2.2.  Precedence of Preconditions
   14. Range Requests
     14.1.  Range Units
       14.1.1.  Range Specifiers
       14.1.2.  Byte Ranges
     14.2.  Range
     14.3.  Accept-Ranges
     14.4.  Content-Range
     14.5.  Partial PUT
     14.6.  Media Type multipart/byteranges
   15. Status Codes
     15.1.  Overview of Status Codes
     15.2.  Informational 1xx
       15.2.1.  100 Continue
       15.2.2.  101 Switching Protocols
     15.3.  Successful 2xx
       15.3.1.  200 OK
       15.3.2.  201 Created
       15.3.3.  202 Accepted
       15.3.4.  203 Non-Authoritative Information
       15.3.5.  204 No Content
       15.3.6.  205 Reset Content
       15.3.7.  206 Partial Content
         15.3.7.1.  Single Part
         15.3.7.2.  Multiple Parts
         15.3.7.3.  Combining Parts
     15.4.  Redirection 3xx
       15.4.1.  300 Multiple Choices
       15.4.2.  301 Moved Permanently
       15.4.3.  302 Found
       15.4.4.  303 See Other
       15.4.5.  304 Not Modified
       15.4.6.  305 Use Proxy
       15.4.7.  306 (Unused)
       15.4.8.  307 Temporary Redirect
       15.4.9.  308 Permanent Redirect
     15.5.  Client Error 4xx
       15.5.1.  400 Bad Request
       15.5.2.  401 Unauthorized
       15.5.3.  402 Payment Required
       15.5.4.  403 Forbidden
       15.5.5.  404 Not Found
       15.5.6.  405 Method Not Allowed
       15.5.7.  406 Not Acceptable
       15.5.8.  407 Proxy Authentication Required
       15.5.9.  408 Request Timeout
       15.5.10. 409 Conflict
       15.5.11. 410 Gone
       15.5.12. 411 Length Required
       15.5.13. 412 Precondition Failed
       15.5.14. 413 Content Too Large
       15.5.15. 414 URI Too Long
       15.5.16. 415 Unsupported Media Type
       15.5.17. 416 Range Not Satisfiable
       15.5.18. 417 Expectation Failed
       15.5.19. 418 (Unused)
       15.5.20. 421 Misdirected Request
       15.5.21. 422 Unprocessable Content
       15.5.22. 426 Upgrade Required
     15.6.  Server Error 5xx
       15.6.1.  500 Internal Server Error
       15.6.2.  501 Not Implemented
       15.6.3.  502 Bad Gateway
       15.6.4.  503 Service Unavailable
       15.6.5.  504 Gateway Timeout
       15.6.6.  505 HTTP Version Not Supported
   16. Extending HTTP
     16.1.  Method Extensibility
       16.1.1.  Method Registry
       16.1.2.  Considerations for New Methods
     16.2.  Status Code Extensibility
       16.2.1.  Status Code Registry
       16.2.2.  Considerations for New Status Codes
     16.3.  Field Extensibility
       16.3.1.  Field Name Registry
       16.3.2.  Considerations for New Fields
         16.3.2.1.  Considerations for New Field Names
         16.3.2.2.  Considerations for New Field Values
     16.4.  Authentication Scheme Extensibility
       16.4.1.  Authentication Scheme Registry
       16.4.2.  Considerations for New Authentication Schemes
     16.5.  Range Unit Extensibility
       16.5.1.  Range Unit Registry
       16.5.2.  Considerations for New Range Units
     16.6.  Content Coding Extensibility
       16.6.1.  Content Coding Registry
       16.6.2.  Considerations for New Content Codings
     16.7.  Upgrade Token Registry
   17. Security Considerations
     17.1.  Establishing Authority
     17.2.  Risks of Intermediaries
     17.3.  Attacks Based on File and Path Names
     17.4.  Attacks Based on Command, Code, or Query Injection
     17.5.  Attacks via Protocol Element Length
     17.6.  Attacks Using Shared-Dictionary Compression
     17.7.  Disclosure of Personal Information
     17.8.  Privacy of Server Log Information
     17.9.  Disclosure of Sensitive Information in URIs
     17.10. Application Handling of Field Names
     17.11. Disclosure of Fragment after Redirects
     17.12. Disclosure of Product Information
     17.13. Browser Fingerprinting
     17.14. Validator Retention
     17.15. Denial-of-Service Attacks Using Range
     17.16. Authentication Considerations
       17.16.1.  Confidentiality of Credentials
       17.16.2.  Credentials and Idle Clients
       17.16.3.  Protection Spaces
       17.16.4.  Additional Response Fields
   18. IANA Considerations
     18.1.  URI Scheme Registration
     18.2.  Method Registration
     18.3.  Status Code Registration
     18.4.  Field Name Registration
     18.5.  Authentication Scheme Registration
     18.6.  Content Coding Registration
     18.7.  Range Unit Registration
     18.8.  Media Type Registration
     18.9.  Port Registration
     18.10. Upgrade Token Registration
   19. References
     19.1.  Normative References
     19.2.  Informative References
   Appendix A.  Collected ABNF
   Appendix B.  Changes from Previous RFCs
     B.1.  Changes from RFC 2818
     B.2.  Changes from RFC 7230
     B.3.  Changes from RFC 7231
     B.4.  Changes from RFC 7232
     B.5.  Changes from RFC 7233
     B.6.  Changes from RFC 7235
     B.7.  Changes from RFC 7538
     B.8.  Changes from RFC 7615
     B.9.  Changes from RFC 7694
   Acknowledgements
   Index
   Authors' Addresses

1.  Introduction

1.1.  Purpose

   The Hypertext Transfer Protocol (HTTP) is a family of stateless,
   application-level, request/response protocols that share a generic
   interface, extensible semantics, and self-descriptive messages to
   enable flexible interaction with network-based hypertext information
   systems.

   HTTP hides the details of how a service is implemented by presenting
   a uniform interface to clients that is independent of the types of
   resources provided.  Likewise, servers do not need to be aware of
   each client's purpose: a request can be considered in isolation
   rather than being associated with a specific type of client or a
   predetermined sequence of application steps.  This allows general-
   purpose implementations to be used effectively in many different
   contexts, reduces interaction complexity, and enables independent
   evolution over time.

   HTTP is also designed for use as an intermediation protocol, wherein
   proxies and gateways can translate non-HTTP information systems into
   a more generic interface.

   One consequence of this flexibility is that the protocol cannot be
   defined in terms of what occurs behind the interface.  Instead, we
   are limited to defining the syntax of communication, the intent of
   received communication, and the expected behavior of recipients.  If
   the communication is considered in isolation, then successful actions
   ought to be reflected in corresponding changes to the observable
   interface provided by servers.  However, since multiple clients might
   act in parallel and perhaps at cross-purposes, we cannot require that
   such changes be observable beyond the scope of a single response.

1.2.  History and Evolution

   HTTP has been the primary information transfer protocol for the World
   Wide Web since its introduction in 1990.  It began as a trivial
   mechanism for low-latency requests, with a single method (GET) to
   request transfer of a presumed hypertext document identified by a
   given pathname.  As the Web grew, HTTP was extended to enclose
   requests and responses within messages, transfer arbitrary data
   formats using MIME-like media types, and route requests through
   intermediaries.  These protocols were eventually defined as HTTP/0.9
   and HTTP/1.0 (see [HTTP/1.0]).

   HTTP/1.1 was designed to refine the protocol's features while
   retaining compatibility with the existing text-based messaging
   syntax, improving its interoperability, scalability, and robustness
   across the Internet.  This included length-based data delimiters for
   both fixed and dynamic (chunked) content, a consistent framework for
   content negotiation, opaque validators for conditional requests,
   cache controls for better cache consistency, range requests for
   partial updates, and default persistent connections.  HTTP/1.1 was
   introduced in 1995 and published on the Standards Track in 1997
   [RFC2068], revised in 1999 [RFC2616], and revised again in 2014
   ([RFC7230] through [RFC7235]).

   HTTP/2 ([HTTP/2]) introduced a multiplexed session layer on top of
   the existing TLS and TCP protocols for exchanging concurrent HTTP
   messages with efficient field compression and server push.  HTTP/3
   ([HTTP/3]) provides greater independence for concurrent messages by
   using QUIC as a secure multiplexed transport over UDP instead of TCP.

   All three major versions of HTTP rely on the semantics defined by
   this document.  They have not obsoleted each other because each one
   has specific benefits and limitations depending on the context of
   use.  Implementations are expected to choose the most appropriate
   transport and messaging syntax for their particular context.

   This revision of HTTP separates the definition of semantics (this
   document) and caching ([CACHING]) from the current HTTP/1.1 messaging
   syntax ([HTTP/1.1]) to allow each major protocol version to progress
   independently while referring to the same core semantics.

1.3.  Core Semantics

   HTTP provides a uniform interface for interacting with a resource
   (Section 3.1) -- regardless of its type, nature, or implementation --
   by sending messages that manipulate or transfer representations
   (Section 3.2).

   Each message is either a request or a response.  A client constructs
   request messages that communicate its intentions and routes those
   messages toward an identified origin server.  A server listens for
   requests, parses each message received, interprets the message
   semantics in relation to the identified target resource, and responds
   to that request with one or more response messages.  The client
   examines received responses to see if its intentions were carried
   out, determining what to do next based on the status codes and
   content received.

   HTTP semantics include the intentions defined by each request method
   (Section 9), extensions to those semantics that might be described in
   request header fields, status codes that describe the response
   (Section 15), and other control data and resource metadata that might
   be given in response fields.

   Semantics also include representation metadata that describe how
   content is intended to be interpreted by a recipient, request header
   fields that might influence content selection, and the various
   selection algorithms that are collectively referred to as "content
   negotiation" (Section 12).

1.4.  Specifications Obsoleted by This Document

   +============================================+===========+=====+
   | Title                                      | Reference | See |
   +============================================+===========+=====+
   | HTTP Over TLS                              | [RFC2818] | B.1 |
   +--------------------------------------------+-----------+-----+
   | HTTP/1.1 Message Syntax and Routing [*]    | [RFC7230] | B.2 |
   +--------------------------------------------+-----------+-----+
   | HTTP/1.1 Semantics and Content             | [RFC7231] | B.3 |
   +--------------------------------------------+-----------+-----+
   | HTTP/1.1 Conditional Requests              | [RFC7232] | B.4 |
   +--------------------------------------------+-----------+-----+
   | HTTP/1.1 Range Requests                    | [RFC7233] | B.5 |
   +--------------------------------------------+-----------+-----+
   | HTTP/1.1 Authentication                    | [RFC7235] | B.6 |
   +--------------------------------------------+-----------+-----+
   | HTTP Status Code 308 (Permanent Redirect)  | [RFC7538] | B.7 |
   +--------------------------------------------+-----------+-----+
   | HTTP Authentication-Info and Proxy-        | [RFC7615] | B.8 |
   | Authentication-Info Response Header Fields |           |     |
   +--------------------------------------------+-----------+-----+
   | HTTP Client-Initiated Content-Encoding     | [RFC7694] | B.9 |
   +--------------------------------------------+-----------+-----+

                               Table 1

   [*] This document only obsoletes the portions of RFC 7230 that are
   independent of the HTTP/1.1 messaging syntax and connection
   management; the remaining bits of RFC 7230 are obsoleted by
   "HTTP/1.1" [HTTP/1.1].

2.  Conformance

2.1.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234], extended with the notation for case-
   sensitivity in strings defined in [RFC7405].

   It also uses a list extension, defined in Section 5.6.1, that allows
   for compact definition of comma-separated lists using a "#" operator
   (similar to how the "*" operator indicates repetition).  Appendix A
   shows the collected grammar with all list operators expanded to
   standard ABNF notation.

   As a convention, ABNF rule names prefixed with "obs-" denote obsolete
   grammar rules that appear for historical reasons.

   The following core rules are included by reference, as defined in
   Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
   CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
   (line feed), OCTET (any 8-bit sequence of data), SP (space), and
   VCHAR (any visible US-ASCII character).

   Section 5.6 defines some generic syntactic components for field
   values.

   This specification uses the terms "character", "character encoding
   scheme", "charset", and "protocol element" as they are defined in
   [RFC6365].

2.2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This specification targets conformance criteria according to the role
   of a participant in HTTP communication.  Hence, requirements are
   placed on senders, recipients, clients, servers, user agents,
   intermediaries, origin servers, proxies, gateways, or caches,
   depending on what behavior is being constrained by the requirement.
   Additional requirements are placed on implementations, resource
   owners, and protocol element registrations when they apply beyond the
   scope of a single communication.

   The verb "generate" is used instead of "send" where a requirement
   applies only to implementations that create the protocol element,
   rather than an implementation that forwards a received element
   downstream.

   An implementation is considered conformant if it complies with all of
   the requirements associated with the roles it partakes in HTTP.

   A sender MUST NOT generate protocol elements that do not match the
   grammar defined by the corresponding ABNF rules.  Within a given
   message, a sender MUST NOT generate protocol elements or syntax
   alternatives that are only allowed to be generated by participants in
   other roles (i.e., a role that the sender does not have for that
   message).

   Conformance to HTTP includes both conformance to the particular
   messaging syntax of the protocol version in use and conformance to
   the semantics of protocol elements sent.  For example, a client that
   claims conformance to HTTP/1.1 but fails to recognize the features
   required of HTTP/1.1 recipients will fail to interoperate with
   servers that adjust their responses in accordance with those claims.
   Features that reflect user choices, such as content negotiation and
   user-selected extensions, can impact application behavior beyond the
   protocol stream; sending protocol elements that inaccurately reflect
   a user's choices will confuse the user and inhibit choice.

   When an implementation fails semantic conformance, recipients of that
   implementation's messages will eventually develop workarounds to
   adjust their behavior accordingly.  A recipient MAY employ such
   workarounds while remaining conformant to this protocol if the
   workarounds are limited to the implementations at fault.  For
   example, servers often scan portions of the User-Agent field value,
   and user agents often scan the Server field value, to adjust their
   own behavior with respect to known bugs or poorly chosen defaults.

2.3.  Length Requirements

   A recipient SHOULD parse a received protocol element defensively,
   with only marginal expectations that the element will conform to its
   ABNF grammar and fit within a reasonable buffer size.

   HTTP does not have specific length limitations for many of its
   protocol elements because the lengths that might be appropriate will
   vary widely, depending on the deployment context and purpose of the
   implementation.  Hence, interoperability between senders and
   recipients depends on shared expectations regarding what is a
   reasonable length for each protocol element.  Furthermore, what is
   commonly understood to be a reasonable length for some protocol
   elements has changed over the course of the past three decades of
   HTTP use and is expected to continue changing in the future.

   At a minimum, a recipient MUST be able to parse and process protocol
   element lengths that are at least as long as the values that it
   generates for those same protocol elements in other messages.  For
   example, an origin server that publishes very long URI references to
   its own resources needs to be able to parse and process those same
   references when received as a target URI.

   Many received protocol elements are only parsed to the extent
   necessary to identify and forward that element downstream.  For
   example, an intermediary might parse a received field into its field
   name and field value components, but then forward the field without
   further parsing inside the field value.

2.4.  Error Handling

   A recipient MUST interpret a received protocol element according to
   the semantics defined for it by this specification, including
   extensions to this specification, unless the recipient has determined
   (through experience or configuration) that the sender incorrectly
   implements what is implied by those semantics.  For example, an
   origin server might disregard the contents of a received
   Accept-Encoding header field if inspection of the User-Agent header
   field indicates a specific implementation version that is known to
   fail on receipt of certain content codings.

   Unless noted otherwise, a recipient MAY attempt to recover a usable
   protocol element from an invalid construct.  HTTP does not define
   specific error handling mechanisms except when they have a direct
   impact on security, since different applications of the protocol
   require different error handling strategies.  For example, a Web
   browser might wish to transparently recover from a response where the
   Location header field doesn't parse according to the ABNF, whereas a
   systems control client might consider any form of error recovery to
   be dangerous.

   Some requests can be automatically retried by a client in the event
   of an underlying connection failure, as described in Section 9.2.2.

2.5.  Protocol Version

   HTTP's version number consists of two decimal digits separated by a
   "." (period or decimal point).  The first digit (major version)
   indicates the messaging syntax, whereas the second digit (minor
   version) indicates the highest minor version within that major
   version to which the sender is conformant (able to understand for
   future communication).

   While HTTP's core semantics don't change between protocol versions,
   their expression "on the wire" can change, and so the HTTP version
   number changes when incompatible changes are made to the wire format.
   Additionally, HTTP allows incremental, backwards-compatible changes
   to be made to the protocol without changing its version through the
   use of defined extension points (Section 16).

   The protocol version as a whole indicates the sender's conformance
   with the set of requirements laid out in that version's corresponding
   specification(s).  For example, the version "HTTP/1.1" is defined by
   the combined specifications of this document, "HTTP Caching"
   [CACHING], and "HTTP/1.1" [HTTP/1.1].

   HTTP's major version number is incremented when an incompatible
   message syntax is introduced.  The minor number is incremented when
   changes made to the protocol have the effect of adding to the message
   semantics or implying additional capabilities of the sender.

   The minor version advertises the sender's communication capabilities
   even when the sender is only using a backwards-compatible subset of
   the protocol, thereby letting the recipient know that more advanced
   features can be used in response (by servers) or in future requests
   (by clients).

   When a major version of HTTP does not define any minor versions, the
   minor version "0" is implied.  The "0" is used when referring to that
   protocol within elements that require a minor version identifier.

3.  Terminology and Core Concepts

   HTTP was created for the World Wide Web (WWW) architecture and has
   evolved over time to support the scalability needs of a worldwide
   hypertext system.  Much of that architecture is reflected in the
   terminology used to define HTTP.

3.1.  Resources

   The target of an HTTP request is called a "resource".  HTTP does not
   limit the nature of a resource; it merely defines an interface that
   might be used to interact with resources.  Most resources are
   identified by a Uniform Resource Identifier (URI), as described in
   Section 4.

   One design goal of HTTP is to separate resource identification from
   request semantics, which is made possible by vesting the request
   semantics in the request method (Section 9) and a few request-
   modifying header fields.  A resource cannot treat a request in a
   manner inconsistent with the semantics of the method of the request.
   For example, though the URI of a resource might imply semantics that
   are not safe, a client can expect the resource to avoid actions that
   are unsafe when processing a request with a safe method (see
   Section 9.2.1).

   HTTP relies upon the Uniform Resource Identifier (URI) standard [URI]
   to indicate the target resource (Section 7.1) and relationships
   between resources.

3.2.  Representations

   A "representation" is information that is intended to reflect a past,
   current, or desired state of a given resource, in a format that can
   be readily communicated via the protocol.  A representation consists
   of a set of representation metadata and a potentially unbounded
   stream of representation data (Section 8).

   HTTP allows "information hiding" behind its uniform interface by
   defining communication with respect to a transferable representation
   of the resource state, rather than transferring the resource itself.
   This allows the resource identified by a URI to be anything,
   including temporal functions like "the current weather in Laguna
   Beach", while potentially providing information that represents that
   resource at the time a message is generated [REST].

   The uniform interface is similar to a window through which one can
   observe and act upon a thing only through the communication of
   messages to an independent actor on the other side.  A shared
   abstraction is needed to represent ("take the place of") the current
   or desired state of that thing in our communications.  When a
   representation is hypertext, it can provide both a representation of
   the resource state and processing instructions that help guide the
   recipient's future interactions.

   A target resource might be provided with, or be capable of
   generating, multiple representations that are each intended to
   reflect the resource's current state.  An algorithm, usually based on
   content negotiation (Section 12), would be used to select one of
   those representations as being most applicable to a given request.
   This "selected representation" provides the data and metadata for
   evaluating conditional requests (Section 13) and constructing the
   content for 200 (OK), 206 (Partial Content), and 304 (Not Modified)
   responses to GET (Section 9.3.1).

3.3.  Connections, Clients, and Servers

   HTTP is a client/server protocol that operates over a reliable
   transport- or session-layer "connection".

   An HTTP "client" is a program that establishes a connection to a
   server for the purpose of sending one or more HTTP requests.  An HTTP
   "server" is a program that accepts connections in order to service
   HTTP requests by sending HTTP responses.

   The terms client and server refer only to the roles that these
   programs perform for a particular connection.  The same program might
   act as a client on some connections and a server on others.

   HTTP is defined as a stateless protocol, meaning that each request
   message's semantics can be understood in isolation, and that the
   relationship between connections and messages on them has no impact
   on the interpretation of those messages.  For example, a CONNECT
   request (Section 9.3.6) or a request with the Upgrade header field
   (Section 7.8) can occur at any time, not just in the first message on
   a connection.  Many implementations depend on HTTP's stateless design
   in order to reuse proxied connections or dynamically load balance
   requests across multiple servers.

   As a result, a server MUST NOT assume that two requests on the same
   connection are from the same user agent unless the connection is
   secured and specific to that agent.  Some non-standard HTTP
   extensions (e.g., [RFC4559]) have been known to violate this
   requirement, resulting in security and interoperability problems.

3.4.  Messages

   HTTP is a stateless request/response protocol for exchanging
   "messages" across a connection.  The terms "sender" and "recipient"
   refer to any implementation that sends or receives a given message,
   respectively.

   A client sends requests to a server in the form of a "request"
   message with a method (Section 9) and request target (Section 7.1).
   The request might also contain header fields (Section 6.3) for
   request modifiers, client information, and representation metadata,
   content (Section 6.4) intended for processing in accordance with the
   method, and trailer fields (Section 6.5) to communicate information
   collected while sending the content.

   A server responds to a client's request by sending one or more
   "response" messages, each including a status code (Section 15).  The
   response might also contain header fields for server information,
   resource metadata, and representation metadata, content to be
   interpreted in accordance with the status code, and trailer fields to
   communicate information collected while sending the content.

3.5.  User Agents

   The term "user agent" refers to any of the various client programs
   that initiate a request.

   The most familiar form of user agent is the general-purpose Web
   browser, but that's only a small percentage of implementations.
   Other common user agents include spiders (web-traversing robots),
   command-line tools, billboard screens, household appliances, scales,
   light bulbs, firmware update scripts, mobile apps, and communication
   devices in a multitude of shapes and sizes.

   Being a user agent does not imply that there is a human user directly
   interacting with the software agent at the time of a request.  In
   many cases, a user agent is installed or configured to run in the
   background and save its results for later inspection (or save only a
   subset of those results that might be interesting or erroneous).
   Spiders, for example, are typically given a start URI and configured
   to follow certain behavior while crawling the Web as a hypertext
   graph.

   Many user agents cannot, or choose not to, make interactive
   suggestions to their user or provide adequate warning for security or
   privacy concerns.  In the few cases where this specification requires
   reporting of errors to the user, it is acceptable for such reporting
   to only be observable in an error console or log file.  Likewise,
   requirements that an automated action be confirmed by the user before
   proceeding might be met via advance configuration choices, run-time
   options, or simple avoidance of the unsafe action; confirmation does
   not imply any specific user interface or interruption of normal
   processing if the user has already made that choice.

3.6.  Origin Server

   The term "origin server" refers to a program that can originate
   authoritative responses for a given target resource.

   The most familiar form of origin server are large public websites.
   However, like user agents being equated with browsers, it is easy to
   be misled into thinking that all origin servers are alike.  Common
   origin servers also include home automation units, configurable
   networking components, office machines, autonomous robots, news
   feeds, traffic cameras, real-time ad selectors, and video-on-demand
   platforms.

   Most HTTP communication consists of a retrieval request (GET) for a
   representation of some resource identified by a URI.  In the simplest
   case, this might be accomplished via a single bidirectional
   connection (===) between the user agent (UA) and the origin server
   (O).

            request   >
       UA ======================================= O
                                   <   response

                                  Figure 1

3.7.  Intermediaries

   HTTP enables the use of intermediaries to satisfy requests through a
   chain of connections.  There are three common forms of HTTP
   "intermediary": proxy, gateway, and tunnel.  In some cases, a single
   intermediary might act as an origin server, proxy, gateway, or
   tunnel, switching behavior based on the nature of each request.

            >             >             >             >
       UA =========== A =========== B =========== C =========== O
                  <             <             <             <

                                  Figure 2

   The figure above shows three intermediaries (A, B, and C) between the
   user agent and origin server.  A request or response message that
   travels the whole chain will pass through four separate connections.
   Some HTTP communication options might apply only to the connection
   with the nearest, non-tunnel neighbor, only to the endpoints of the
   chain, or to all connections along the chain.  Although the diagram
   is linear, each participant might be engaged in multiple,
   simultaneous communications.  For example, B might be receiving
   requests from many clients other than A, and/or forwarding requests
   to servers other than C, at the same time that it is handling A's
   request.  Likewise, later requests might be sent through a different
   path of connections, often based on dynamic configuration for load
   balancing.

   The terms "upstream" and "downstream" are used to describe
   directional requirements in relation to the message flow: all
   messages flow from upstream to downstream.  The terms "inbound" and
   "outbound" are used to describe directional requirements in relation
   to the request route: inbound means "toward the origin server",
   whereas outbound means "toward the user agent".

   A "proxy" is a message-forwarding agent that is chosen by the client,
   usually via local configuration rules, to receive requests for some
   type(s) of absolute URI and attempt to satisfy those requests via
   translation through the HTTP interface.  Some translations are
   minimal, such as for proxy requests for "http" URIs, whereas other
   requests might require translation to and from entirely different
   application-level protocols.  Proxies are often used to group an
   organization's HTTP requests through a common intermediary for the
   sake of security services, annotation services, or shared caching.
   Some proxies are designed to apply transformations to selected
   messages or content while they are being forwarded, as described in
   Section 7.7.

   A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as
   an origin server for the outbound connection but translates received
   requests and forwards them inbound to another server or servers.
   Gateways are often used to encapsulate legacy or untrusted
   information services, to improve server performance through
   "accelerator" caching, and to enable partitioning or load balancing
   of HTTP services across multiple machines.

   All HTTP requirements applicable to an origin server also apply to
   the outbound communication of a gateway.  A gateway communicates with
   inbound servers using any protocol that it desires, including private
   extensions to HTTP that are outside the scope of this specification.
   However, an HTTP-to-HTTP gateway that wishes to interoperate with
   third-party HTTP servers needs to conform to user agent requirements
   on the gateway's inbound connection.

   A "tunnel" acts as a blind relay between two connections without
   changing the messages.  Once active, a tunnel is not considered a
   party to the HTTP communication, though the tunnel might have been
   initiated by an HTTP request.  A tunnel ceases to exist when both
   ends of the relayed connection are closed.  Tunnels are used to
   extend a virtual connection through an intermediary, such as when
   Transport Layer Security (TLS, [TLS13]) is used to establish
   confidential communication through a shared firewall proxy.

   The above categories for intermediary only consider those acting as
   participants in the HTTP communication.  There are also
   intermediaries that can act on lower layers of the network protocol
   stack, filtering or redirecting HTTP traffic without the knowledge or
   permission of message senders.  Network intermediaries are
   indistinguishable (at a protocol level) from an on-path attacker,
   often introducing security flaws or interoperability problems due to
   mistakenly violating HTTP semantics.

   For example, an "interception proxy" [RFC3040] (also commonly known
   as a "transparent proxy" [RFC1919]) differs from an HTTP proxy
   because it is not chosen by the client.  Instead, an interception
   proxy filters or redirects outgoing TCP port 80 packets (and
   occasionally other common port traffic).  Interception proxies are
   commonly found on public network access points, as a means of
   enforcing account subscription prior to allowing use of non-local
   Internet services, and within corporate firewalls to enforce network
   usage policies.

3.8.  Caches

   A "cache" is a local store of previous response messages and the
   subsystem that controls its message storage, retrieval, and deletion.
   A cache stores cacheable responses in order to reduce the response
   time and network bandwidth consumption on future, equivalent
   requests.  Any client or server MAY employ a cache, though a cache
   cannot be used while acting as a tunnel.

   The effect of a cache is that the request/response chain is shortened
   if one of the participants along the chain has a cached response
   applicable to that request.  The following illustrates the resulting
   chain if B has a cached copy of an earlier response from O (via C)
   for a request that has not been cached by UA or A.

               >             >
          UA =========== A =========== B - - - - - - C - - - - - - O
                     <             <

                                  Figure 3

   A response is "cacheable" if a cache is allowed to store a copy of
   the response message for use in answering subsequent requests.  Even
   when a response is cacheable, there might be additional constraints
   placed by the client or by the origin server on when that cached
   response can be used for a particular request.  HTTP requirements for
   cache behavior and cacheable responses are defined in [CACHING].

   There is a wide variety of architectures and configurations of caches
   deployed across the World Wide Web and inside large organizations.
   These include national hierarchies of proxy caches to save bandwidth
   and reduce latency, content delivery networks that use gateway
   caching to optimize regional and global distribution of popular
   sites, collaborative systems that broadcast or multicast cache
   entries, archives of pre-fetched cache entries for use in off-line or
   high-latency environments, and so on.

3.9.  Example Message Exchange

   The following example illustrates a typical HTTP/1.1 message exchange
   for a GET request (Section 9.3.1) on the URI "http://www.example.com/
   hello.txt":

   Client request:

   GET /hello.txt HTTP/1.1
   User-Agent: curl/7.64.1
   Host: www.example.com
   Accept-Language: en, mi

   Server response:

   HTTP/1.1 200 OK
   Date: Mon, 27 Jul 2009 12:28:53 GMT
   Server: Apache
   Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
   ETag: "34aa387-d-1568eb00"
   Accept-Ranges: bytes
   Content-Length: 51
   Vary: Accept-Encoding
   Content-Type: text/plain

   Hello World! My content includes a trailing CRLF.

4.  Identifiers in HTTP

   Uniform Resource Identifiers (URIs) [URI] are used throughout HTTP as
   the means for identifying resources (Section 3.1).

4.1.  URI References

   URI references are used to target requests, indicate redirects, and
   define relationships.

   The definitions of "URI-reference", "absolute-URI", "relative-part",
   "authority", "port", "host", "path-abempty", "segment", and "query"
   are adopted from the URI generic syntax.  An "absolute-path" rule is
   defined for protocol elements that can contain a non-empty path
   component.  (This rule differs slightly from the path-abempty rule of
   RFC 3986, which allows for an empty path, and path-absolute rule,
   which does not allow paths that begin with "//".)  A "partial-URI"
   rule is defined for protocol elements that can contain a relative URI
   but not a fragment component.

     URI-reference = <URI-reference, see [URI], Section 4.1>
     absolute-URI  = <absolute-URI, see [URI], Section 4.3>
     relative-part = <relative-part, see [URI], Section 4.2>
     authority     = <authority, see [URI], Section 3.2>
     uri-host      = <host, see [URI], Section 3.2.2>
     port          = <port, see [URI], Section 3.2.3>
     path-abempty  = <path-abempty, see [URI], Section 3.3>
     segment       = <segment, see [URI], Section 3.3>
     query         = <query, see [URI], Section 3.4>

     absolute-path = 1*( "/" segment )
     partial-URI   = relative-part [ "?" query ]

   Each protocol element in HTTP that allows a URI reference will
   indicate in its ABNF production whether the element allows any form
   of reference (URI-reference), only a URI in absolute form (absolute-
   URI), only the path and optional query components (partial-URI), or
   some combination of the above.  Unless otherwise indicated, URI
   references are parsed relative to the target URI (Section 7.1).

   It is RECOMMENDED that all senders and recipients support, at a
   minimum, URIs with lengths of 8000 octets in protocol elements.  Note
   that this implies some structures and on-wire representations (for
   example, the request line in HTTP/1.1) will necessarily be larger in
   some cases.

4.2.  HTTP-Related URI Schemes

   IANA maintains the registry of URI Schemes [BCP35] at
   <https://www.iana.org/assignments/uri-schemes/>.  Although requests
   might target any URI scheme, the following schemes are inherent to
   HTTP servers:

   +============+====================================+=========+
   | URI Scheme | Description                        | Section |
   +============+====================================+=========+
   | http       | Hypertext Transfer Protocol        | 4.2.1   |
   +------------+------------------------------------+---------+
   | https      | Hypertext Transfer Protocol Secure | 4.2.2   |
   +------------+------------------------------------+---------+

                              Table 2

   Note that the presence of an "http" or "https" URI does not imply
   that there is always an HTTP server at the identified origin
   listening for connections.  Anyone can mint a URI, whether or not a
   server exists and whether or not that server currently maps that
   identifier to a resource.  The delegated nature of registered names
   and IP addresses creates a federated namespace whether or not an HTTP
   server is present.

4.2.1.  http URI Scheme

   The "http" URI scheme is hereby defined for minting identifiers
   within the hierarchical namespace governed by a potential HTTP origin
   server listening for TCP ([TCP]) connections on a given port.

     http-URI = "http" "://" authority path-abempty [ "?" query ]

   The origin server for an "http" URI is identified by the authority
   component, which includes a host identifier ([URI], Section 3.2.2)
   and optional port number ([URI], Section 3.2.3).  If the port
   subcomponent is empty or not given, TCP port 80 (the reserved port
   for WWW services) is the default.  The origin determines who has the
   right to respond authoritatively to requests that target the
   identified resource, as defined in Section 4.3.2.

   A sender MUST NOT generate an "http" URI with an empty host
   identifier.  A recipient that processes such a URI reference MUST
   reject it as invalid.

   The hierarchical path component and optional query component identify
   the target resource within that origin server's namespace.

4.2.2.  https URI Scheme

   The "https" URI scheme is hereby defined for minting identifiers
   within the hierarchical namespace governed by a potential origin
   server listening for TCP connections on a given port and capable of
   establishing a TLS ([TLS13]) connection that has been secured for
   HTTP communication.  In this context, "secured" specifically means
   that the server has been authenticated as acting on behalf of the
   identified authority and all HTTP communication with that server has
   confidentiality and integrity protection that is acceptable to both
   client and server.

     https-URI = "https" "://" authority path-abempty [ "?" query ]

   The origin server for an "https" URI is identified by the authority
   component, which includes a host identifier ([URI], Section 3.2.2)
   and optional port number ([URI], Section 3.2.3).  If the port
   subcomponent is empty or not given, TCP port 443 (the reserved port
   for HTTP over TLS) is the default.  The origin determines who has the
   right to respond authoritatively to requests that target the
   identified resource, as defined in Section 4.3.3.

   A sender MUST NOT generate an "https" URI with an empty host
   identifier.  A recipient that processes such a URI reference MUST
   reject it as invalid.

   The hierarchical path component and optional query component identify
   the target resource within that origin server's namespace.

   A client MUST ensure that its HTTP requests for an "https" resource
   are secured, prior to being communicated, and that it only accepts
   secured responses to those requests.  Note that the definition of
   what cryptographic mechanisms are acceptable to client and server are
   usually negotiated and can change over time.

   Resources made available via the "https" scheme have no shared
   identity with the "http" scheme.  They are distinct origins with
   separate namespaces.  However, extensions to HTTP that are defined as
   applying to all origins with the same host, such as the Cookie
   protocol [COOKIE], allow information set by one service to impact
   communication with other services within a matching group of host
   domains.  Such extensions ought to be designed with great care to
   prevent information obtained from a secured connection being
   inadvertently exchanged within an unsecured context.

4.2.3.  http(s) Normalization and Comparison

   URIs with an "http" or "https" scheme are normalized and compared
   according to the methods defined in Section 6 of [URI], using the
   defaults described above for each scheme.

   HTTP does not require the use of a specific method for determining
   equivalence.  For example, a cache key might be compared as a simple
   string, after syntax-based normalization, or after scheme-based
   normalization.

   Scheme-based normalization (Section 6.2.3 of [URI]) of "http" and
   "https" URIs involves the following additional rules:

   *  If the port is equal to the default port for a scheme, the normal
      form is to omit the port subcomponent.

   *  When not being used as the target of an OPTIONS request, an empty
      path component is equivalent to an absolute path of "/", so the
      normal form is to provide a path of "/" instead.

   *  The scheme and host are case-insensitive and normally provided in
      lowercase; all other components are compared in a case-sensitive
      manner.

   *  Characters other than those in the "reserved" set are equivalent
      to their percent-encoded octets: the normal form is to not encode
      them (see Sections 2.1 and 2.2 of [URI]).

   For example, the following three URIs are equivalent:

      http://example.com:80/~smith/home.html
      http://EXAMPLE.com/%7Esmith/home.html
      http://EXAMPLE.com:/%7esmith/home.html

   Two HTTP URIs that are equivalent after normalization (using any
   method) can be assumed to identify the same resource, and any HTTP
   component MAY perform normalization.  As a result, distinct resources
   SHOULD NOT be identified by HTTP URIs that are equivalent after
   normalization (using any method defined in Section 6.2 of [URI]).

4.2.4.  Deprecation of userinfo in http(s) URIs

   The URI generic syntax for authority also includes a userinfo
   subcomponent ([URI], Section 3.2.1) for including user authentication
   information in the URI.  In that subcomponent, the use of the format
   "user:password" is deprecated.

   Some implementations make use of the userinfo component for internal
   configuration of authentication information, such as within command
   invocation options, configuration files, or bookmark lists, even
   though such usage might expose a user identifier or password.

   A sender MUST NOT generate the userinfo subcomponent (and its "@"
   delimiter) when an "http" or "https" URI reference is generated
   within a message as a target URI or field value.

   Before making use of an "http" or "https" URI reference received from
   an untrusted source, a recipient SHOULD parse for userinfo and treat
   its presence as an error; it is likely being used to obscure the
   authority for the sake of phishing attacks.

4.2.5.  http(s) References with Fragment Identifiers

   Fragment identifiers allow for indirect identification of a secondary
   resource, independent of the URI scheme, as defined in Section 3.5 of
   [URI].  Some protocol elements that refer to a URI allow inclusion of
   a fragment, while others do not.  They are distinguished by use of
   the ABNF rule for elements where fragment is allowed; otherwise, a
   specific rule that excludes fragments is used.

      |  *Note:* The fragment identifier component is not part of the
      |  scheme definition for a URI scheme (see Section 4.3 of [URI]),
      |  thus does not appear in the ABNF definitions for the "http" and
      |  "https" URI schemes above.

4.3.  Authoritative Access

   Authoritative access refers to dereferencing a given identifier, for
   the sake of access to the identified resource, in a way that the
   client believes is authoritative (controlled by the resource owner).
   The process for determining whether access is granted is defined by
   the URI scheme and often uses data within the URI components, such as
   the authority component when the generic syntax is used.  However,
   authoritative access is not limited to the identified mechanism.

   Section 4.3.1 defines the concept of an origin as an aid to such
   uses, and the subsequent subsections explain how to establish that a
   peer has the authority to represent an origin.

   See Section 17.1 for security considerations related to establishing
   authority.

4.3.1.  URI Origin

   The "origin" for a given URI is the triple of scheme, host, and port
   after normalizing the scheme and host to lowercase and normalizing
   the port to remove any leading zeros.  If port is elided from the
   URI, the default port for that scheme is used.  For example, the URI

      https://Example.Com/happy.js

   would have the origin

      { "https", "example.com", "443" }

   which can also be described as the normalized URI prefix with port
   always present:

      https://example.com:443

   Each origin defines its own namespace and controls how identifiers
   within that namespace are mapped to resources.  In turn, how the
   origin responds to valid requests, consistently over time, determines
   the semantics that users will associate with a URI, and the
   usefulness of those semantics is what ultimately transforms these
   mechanisms into a resource for users to reference and access in the
   future.

   Two origins are distinct if they differ in scheme, host, or port.
   Even when it can be verified that the same entity controls two
   distinct origins, the two namespaces under those origins are distinct
   unless explicitly aliased by a server authoritative for that origin.

   Origin is also used within HTML and related Web protocols, beyond the
   scope of this document, as described in [RFC6454].

4.3.2.  http Origins

   Although HTTP is independent of the transport protocol, the "http"
   scheme (Section 4.2.1) is specific to associating authority with
   whomever controls the origin server listening for TCP connections on
   the indicated port of whatever host is identified within the
   authority component.  This is a very weak sense of authority because
   it depends on both client-specific name resolution mechanisms and
   communication that might not be secured from an on-path attacker.
   Nevertheless, it is a sufficient minimum for binding "http"
   identifiers to an origin server for consistent resolution within a
   trusted environment.

   If the host identifier is provided as an IP address, the origin
   server is the listener (if any) on the indicated TCP port at that IP
   address.  If host is a registered name, the registered name is an
   indirect identifier for use with a name resolution service, such as
   DNS, to find an address for an appropriate origin server.

   When an "http" URI is used within a context that calls for access to
   the indicated resource, a client MAY attempt access by resolving the
   host identifier to an IP address, establishing a TCP connection to
   that address on the indicated port, and sending over that connection
   an HTTP request message containing a request target that matches the
   client's target URI (Section 7.1).

   If the server responds to such a request with a non-interim HTTP
   response message, as described in Section 15, then that response is
   considered an authoritative answer to the client's request.

   Note, however, that the above is not the only means for obtaining an
   authoritative response, nor does it imply that an authoritative
   response is always necessary (see [CACHING]).  For example, the Alt-
   Svc header field [ALTSVC] allows an origin server to identify other
   services that are also authoritative for that origin.  Access to
   "http" identified resources might also be provided by protocols
   outside the scope of this document.

4.3.3.  https Origins

   The "https" scheme (Section 4.2.2) associates authority based on the
   ability of a server to use the private key corresponding to a
   certificate that the client considers to be trustworthy for the
   identified origin server.  The client usually relies upon a chain of
   trust, conveyed from some prearranged or configured trust anchor, to
   deem a certificate trustworthy (Section 4.3.4).

   In HTTP/1.1 and earlier, a client will only attribute authority to a
   server when they are communicating over a successfully established
   and secured connection specifically to that URI origin's host.  The
   connection establishment and certificate verification are used as
   proof of authority.

   In HTTP/2 and HTTP/3, a client will attribute authority to a server
   when they are communicating over a successfully established and
   secured connection if the URI origin's host matches any of the hosts
   present in the server's certificate and the client believes that it
   could open a connection to that host for that URI.  In practice, a
   client will make a DNS query to check that the origin's host contains
   the same server IP address as the established connection.  This
   restriction can be removed by the origin server sending an equivalent
   ORIGIN frame [RFC8336].

   The request target's host and port value are passed within each HTTP
   request, identifying the origin and distinguishing it from other
   namespaces that might be controlled by the same server (Section 7.2).
   It is the origin's responsibility to ensure that any services
   provided with control over its certificate's private key are equally
   responsible for managing the corresponding "https" namespaces or at
   least prepared to reject requests that appear to have been
   misdirected (Section 7.4).

   An origin server might be unwilling to process requests for certain
   target URIs even when they have the authority to do so.  For example,
   when a host operates distinct services on different ports (e.g., 443
   and 8000), checking the target URI at the origin server is necessary
   (even after the connection has been secured) because a network
   attacker might cause connections for one port to be received at some
   other port.  Failing to check the target URI might allow such an
   attacker to replace a response to one target URI (e.g.,
   "https://example.com/foo") with a seemingly authoritative response
   from the other port (e.g., "https://example.com:8000/foo").

   Note that the "https" scheme does not rely on TCP and the connected
   port number for associating authority, since both are outside the
   secured communication and thus cannot be trusted as definitive.
   Hence, the HTTP communication might take place over any channel that
   has been secured, as defined in Section 4.2.2, including protocols
   that don't use TCP.

   When an "https" URI is used within a context that calls for access to
   the indicated resource, a client MAY attempt access by resolving the
   host identifier to an IP address, establishing a TCP connection to
   that address on the indicated port, securing the connection end-to-
   end by successfully initiating TLS over TCP with confidentiality and
   integrity protection, and sending over that connection an HTTP
   request message containing a request target that matches the client's
   target URI (Section 7.1).

   If the server responds to such a request with a non-interim HTTP
   response message, as described in Section 15, then that response is
   considered an authoritative answer to the client's request.

   Note, however, that the above is not the only means for obtaining an
   authoritative response, nor does it imply that an authoritative
   response is always necessary (see [CACHING]).

4.3.4.  https Certificate Verification

   To establish a secured connection to dereference a URI, a client MUST
   verify that the service's identity is an acceptable match for the
   URI's origin server.  Certificate verification is used to prevent
   server impersonation by an on-path attacker or by an attacker that
   controls name resolution.  This process requires that a client be
   configured with a set of trust anchors.

   In general, a client MUST verify the service identity using the
   verification process defined in Section 6 of [RFC6125].  The client
   MUST construct a reference identity from the service's host: if the
   host is a literal IP address (Section 4.3.5), the reference identity
   is an IP-ID, otherwise the host is a name and the reference identity
   is a DNS-ID.

   A reference identity of type CN-ID MUST NOT be used by clients.  As
   noted in Section 6.2.1 of [RFC6125], a reference identity of type CN-
   ID might be used by older clients.

   A client might be specially configured to accept an alternative form
   of server identity verification.  For example, a client might be
   connecting to a server whose address and hostname are dynamic, with
   an expectation that the service will present a specific certificate
   (or a certificate matching some externally defined reference
   identity) rather than one matching the target URI's origin.

   In special cases, it might be appropriate for a client to simply
   ignore the server's identity, but it must be understood that this
   leaves a connection open to active attack.

   If the certificate is not valid for the target URI's origin, a user
   agent MUST either obtain confirmation from the user before proceeding
   (see Section 3.5) or terminate the connection with a bad certificate
   error.  Automated clients MUST log the error to an appropriate audit
   log (if available) and SHOULD terminate the connection (with a bad
   certificate error).  Automated clients MAY provide a configuration
   setting that disables this check, but MUST provide a setting which
   enables it.

4.3.5.  IP-ID Reference Identity

   A server that is identified using an IP address literal in the "host"
   field of an "https" URI has a reference identity of type IP-ID.  An
   IP version 4 address uses the "IPv4address" ABNF rule, and an IP
   version 6 address uses the "IP-literal" production with the
   "IPv6address" option; see Section 3.2.2 of [URI].  A reference
   identity of IP-ID contains the decoded bytes of the IP address.

   An IP version 4 address is 4 octets, and an IP version 6 address is
   16 octets.  Use of IP-ID is not defined for any other IP version.
   The iPAddress choice in the certificate subjectAltName extension does
   not explicitly include the IP version and so relies on the length of
   the address to distinguish versions; see Section 4.2.1.6 of
   [RFC5280].

   A reference identity of type IP-ID matches if the address is
   identical to an iPAddress value of the subjectAltName extension of
   the certificate.

5.  Fields

   HTTP uses "fields" to provide data in the form of extensible name/
   value pairs with a registered key namespace.  Fields are sent and
   received within the header and trailer sections of messages
   (Section 6).

5.1.  Field Names

   A field name labels the corresponding field value as having the
   semantics defined by that name.  For example, the Date header field
   is defined in Section 6.6.1 as containing the origination timestamp
   for the message in which it appears.

     field-name     = token

   Field names are case-insensitive and ought to be registered within
   the "Hypertext Transfer Protocol (HTTP) Field Name Registry"; see
   Section 16.3.1.

   The interpretation of a field does not change between minor versions
   of the same major HTTP version, though the default behavior of a
   recipient in the absence of such a field can change.  Unless
   specified otherwise, fields are defined for all versions of HTTP.  In
   particular, the Host and Connection fields ought to be recognized by
   all HTTP implementations whether or not they advertise conformance
   with HTTP/1.1.

   New fields can be introduced without changing the protocol version if
   their defined semantics allow them to be safely ignored by recipients
   that do not recognize them; see Section 16.3.

   A proxy MUST forward unrecognized header fields unless the field name
   is listed in the Connection header field (Section 7.6.1) or the proxy
   is specifically configured to block, or otherwise transform, such
   fields.  Other recipients SHOULD ignore unrecognized header and
   trailer fields.  Adhering to these requirements allows HTTP's
   functionality to be extended without updating or removing deployed
   intermediaries.

5.2.  Field Lines and Combined Field Value

   Field sections are composed of any number of "field lines", each with
   a "field name" (see Section 5.1) identifying the field, and a "field
   line value" that conveys data for that instance of the field.

   When a field name is only present once in a section, the combined
   "field value" for that field consists of the corresponding field line
   value.  When a field name is repeated within a section, its combined
   field value consists of the list of corresponding field line values
   within that section, concatenated in order, with each field line
   value separated by a comma.

   For example, this section:

   Example-Field: Foo, Bar
   Example-Field: Baz

   contains two field lines, both with the field name "Example-Field".
   The first field line has a field line value of "Foo, Bar", while the
   second field line value is "Baz".  The field value for "Example-
   Field" is the list "Foo, Bar, Baz".

5.3.  Field Order

   A recipient MAY combine multiple field lines within a field section
   that have the same field name into one field line, without changing
   the semantics of the message, by appending each subsequent field line
   value to the initial field line value in order, separated by a comma
   (",") and optional whitespace (OWS, defined in Section 5.6.3).  For
   consistency, use comma SP.

   The order in which field lines with the same name are received is
   therefore significant to the interpretation of the field value; a
   proxy MUST NOT change the order of these field line values when
   forwarding a message.

   This means that, aside from the well-known exception noted below, a
   sender MUST NOT generate multiple field lines with the same name in a
   message (whether in the headers or trailers) or append a field line
   when a field line of the same name already exists in the message,
   unless that field's definition allows multiple field line values to
   be recombined as a comma-separated list (i.e., at least one
   alternative of the field's definition allows a comma-separated list,
   such as an ABNF rule of #(values) defined in Section 5.6.1).

      |  *Note:* In practice, the "Set-Cookie" header field ([COOKIE])
      |  often appears in a response message across multiple field lines
      |  and does not use the list syntax, violating the above
      |  requirements on multiple field lines with the same field name.
      |  Since it cannot be combined into a single field value,
      |  recipients ought to handle "Set-Cookie" as a special case while
      |  processing fields.  (See Appendix A.2.3 of [Kri2001] for
      |  details.)

   The order in which field lines with differing field names are
   received in a section is not significant.  However, it is good
   practice to send header fields that contain additional control data
   first, such as Host on requests and Date on responses, so that
   implementations can decide when not to handle a message as early as
   possible.

   A server MUST NOT apply a request to the target resource until it
   receives the entire request header section, since later header field
   lines might include conditionals, authentication credentials, or
   deliberately misleading duplicate header fields that could impact
   request processing.

5.4.  Field Limits

   HTTP does not place a predefined limit on the length of each field
   line, field value, or on the length of a header or trailer section as
   a whole, as described in Section 2.  Various ad hoc limitations on
   individual lengths are found in practice, often depending on the
   specific field's semantics.

   A server that receives a request header field line, field value, or
   set of fields larger than it wishes to process MUST respond with an
   appropriate 4xx (Client Error) status code.  Ignoring such header
   fields would increase the server's vulnerability to request smuggling
   attacks (Section 11.2 of [HTTP/1.1]).

   A client MAY discard or truncate received field lines that are larger
   than the client wishes to process if the field semantics are such
   that the dropped value(s) can be safely ignored without changing the
   message framing or response semantics.

5.5.  Field Values

   HTTP field values consist of a sequence of characters in a format
   defined by the field's grammar.  Each field's grammar is usually
   defined using ABNF ([RFC5234]).

     field-value    = *field-content
     field-content  = field-vchar
                      [ 1*( SP / HTAB / field-vchar ) field-vchar ]
     field-vchar    = VCHAR / obs-text
     obs-text       = %x80-FF

   A field value does not include leading or trailing whitespace.  When
   a specific version of HTTP allows such whitespace to appear in a
   message, a field parsing implementation MUST exclude such whitespace
   prior to evaluating the field value.

   Field values are usually constrained to the range of US-ASCII
   characters [USASCII].  Fields needing a greater range of characters
   can use an encoding, such as the one defined in [RFC8187].
   Historically, HTTP allowed field content with text in the ISO-8859-1
   charset [ISO-8859-1], supporting other charsets only through use of
   [RFC2047] encoding.  Specifications for newly defined fields SHOULD
   limit their values to visible US-ASCII octets (VCHAR), SP, and HTAB.
   A recipient SHOULD treat other allowed octets in field content (i.e.,
   obs-text) as opaque data.

   Field values containing CR, LF, or NUL characters are invalid and
   dangerous, due to the varying ways that implementations might parse
   and interpret those characters; a recipient of CR, LF, or NUL within
   a field value MUST either reject the message or replace each of those
   characters with SP before further processing or forwarding of that
   message.  Field values containing other CTL characters are also
   invalid; however, recipients MAY retain such characters for the sake
   of robustness when they appear within a safe context (e.g., an
   application-specific quoted string that will not be processed by any
   downstream HTTP parser).

   Fields that only anticipate a single member as the field value are
   referred to as "singleton fields".

   Fields that allow multiple members as the field value are referred to
   as "list-based fields".  The list operator extension of Section 5.6.1
   is used as a common notation for defining field values that can
   contain multiple members.

   Because commas (",") are used as the delimiter between members, they
   need to be treated with care if they are allowed as data within a
   member.  This is true for both list-based and singleton fields, since
   a singleton field might be erroneously sent with multiple members and
   detecting such errors improves interoperability.  Fields that expect
   to contain a comma within a member, such as within an HTTP-date or
   URI-reference element, ought to be defined with delimiters around
   that element to distinguish any comma within that data from potential
   list separators.

   For example, a textual date and a URI (either of which might contain
   a comma) could be safely carried in list-based field values like
   these:

   Example-URIs: "http://example.com/a.html,foo",
                 "http://without-a-comma.example.com/"
   Example-Dates: "Sat, 04 May 1996", "Wed, 14 Sep 2005"

   Note that double-quote delimiters are almost always used with the
   quoted-string production (Section 5.6.4); using a different syntax
   inside double-quotes will likely cause unnecessary confusion.

   Many fields (such as Content-Type, defined in Section 8.3) use a
   common syntax for parameters that allows both unquoted (token) and
   quoted (quoted-string) syntax for a parameter value (Section 5.6.6).
   Use of common syntax allows recipients to reuse existing parser
   components.  When allowing both forms, the meaning of a parameter
   value ought to be the same whether it was received as a token or a
   quoted string.

      |  *Note:* For defining field value syntax, this specification
      |  uses an ABNF rule named after the field name to define the
      |  allowed grammar for that field's value (after said value has
      |  been extracted from the underlying messaging syntax and
      |  multiple instances combined into a list).

5.6.  Common Rules for Defining Field Values

5.6.1.  Lists (#rule ABNF Extension)

   A #rule extension to the ABNF rules of [RFC5234] is used to improve
   readability in the definitions of some list-based field values.

   A construct "#" is defined, similar to "*", for defining comma-
   delimited lists of elements.  The full form is "<n>#<m>element"
   indicating at least <n> and at most <m> elements, each separated by a
   single comma (",") and optional whitespace (OWS, defined in
   Section 5.6.3).

5.6.1.1.  Sender Requirements

   In any production that uses the list construct, a sender MUST NOT
   generate empty list elements.  In other words, a sender has to
   generate lists that satisfy the following syntax:

     1#element => element *( OWS "," OWS element )

   and:

     #element => [ 1#element ]

   and for n >= 1 and m > 1:

     <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )

   Appendix A shows the collected ABNF for senders after the list
   constructs have been expanded.

5.6.1.2.  Recipient Requirements

   Empty elements do not contribute to the count of elements present.  A
   recipient MUST parse and ignore a reasonable number of empty list
   elements: enough to handle common mistakes by senders that merge
   values, but not so much that they could be used as a denial-of-
   service mechanism.  In other words, a recipient MUST accept lists
   that satisfy the following syntax:

     #element => [ element ] *( OWS "," OWS [ element ] )

   Note that because of the potential presence of empty list elements,
   the RFC 5234 ABNF cannot enforce the cardinality of list elements,
   and consequently all cases are mapped as if there was no cardinality
   specified.

   For example, given these ABNF productions:

     example-list      = 1#example-list-elmt
     example-list-elmt = token ; see Section 5.6.2

   Then the following are valid values for example-list (not including
   the double quotes, which are present for delimitation only):

     "foo,bar"
     "foo ,bar,"
     "foo , ,bar,charlie"

   In contrast, the following values would be invalid, since at least
   one non-empty element is required by the example-list production:

     ""
     ","
     ",   ,"

5.6.2.  Tokens

   Tokens are short textual identifiers that do not include whitespace
   or delimiters.

     token          = 1*tchar

     tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                    / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
                    / DIGIT / ALPHA
                    ; any VCHAR, except delimiters

   Many HTTP field values are defined using common syntax components,
   separated by whitespace or specific delimiting characters.
   Delimiters are chosen from the set of US-ASCII visual characters not
   allowed in a token (DQUOTE and "(),/:;<=>?@[\]{}").

5.6.3.  Whitespace

   This specification uses three rules to denote the use of linear
   whitespace: OWS (optional whitespace), RWS (required whitespace), and
   BWS ("bad" whitespace).

   The OWS rule is used where zero or more linear whitespace octets
   might appear.  For protocol elements where optional whitespace is
   preferred to improve readability, a sender SHOULD generate the
   optional whitespace as a single SP; otherwise, a sender SHOULD NOT
   generate optional whitespace except as needed to overwrite invalid or
   unwanted protocol elements during in-place message filtering.

   The RWS rule is used when at least one linear whitespace octet is
   required to separate field tokens.  A sender SHOULD generate RWS as a
   single SP.

   OWS and RWS have the same semantics as a single SP.  Any content
   known to be defined as OWS or RWS MAY be replaced with a single SP
   before interpreting it or forwarding the message downstream.

   The BWS rule is used where the grammar allows optional whitespace
   only for historical reasons.  A sender MUST NOT generate BWS in
   messages.  A recipient MUST parse for such bad whitespace and remove
   it before interpreting the protocol element.

   BWS has no semantics.  Any content known to be defined as BWS MAY be
   removed before interpreting it or forwarding the message downstream.

     OWS            = *( SP / HTAB )
                    ; optional whitespace
     RWS            = 1*( SP / HTAB )
                    ; required whitespace
     BWS            = OWS
                    ; "bad" whitespace

5.6.4.  Quoted Strings

   A string of text is parsed as a single value if it is quoted using
   double-quote marks.

     quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE
     qdtext         = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text

   The backslash octet ("\") can be used as a single-octet quoting
   mechanism within quoted-string and comment constructs.  Recipients
   that process the value of a quoted-string MUST handle a quoted-pair
   as if it were replaced by the octet following the backslash.

     quoted-pair    = "\" ( HTAB / SP / VCHAR / obs-text )

   A sender SHOULD NOT generate a quoted-pair in a quoted-string except
   where necessary to quote DQUOTE and backslash octets occurring within
   that string.  A sender SHOULD NOT generate a quoted-pair in a comment
   except where necessary to quote parentheses ["(" and ")"] and
   backslash octets occurring within that comment.

5.6.5.  Comments

   Comments can be included in some HTTP fields by surrounding the
   comment text with parentheses.  Comments are only allowed in fields
   containing "comment" as part of their field value definition.

     comment        = "(" *( ctext / quoted-pair / comment ) ")"
     ctext          = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text

5.6.6.  Parameters

   Parameters are instances of name/value pairs; they are often used in
   field values as a common syntax for appending auxiliary information
   to an item.  Each parameter is usually delimited by an immediately
   preceding semicolon.

     parameters      = *( OWS ";" OWS [ parameter ] )
     parameter       = parameter-name "=" parameter-value
     parameter-name  = token
     parameter-value = ( token / quoted-string )

   Parameter names are case-insensitive.  Parameter values might or
   might not be case-sensitive, depending on the semantics of the
   parameter name.  Examples of parameters and some equivalent forms can
   be seen in media types (Section 8.3.1) and the Accept header field
   (Section 12.5.1).

   A parameter value that matches the token production can be
   transmitted either as a token or within a quoted-string.  The quoted
   and unquoted values are equivalent.

      |  *Note:* Parameters do not allow whitespace (not even "bad"
      |  whitespace) around the "=" character.

5.6.7.  Date/Time Formats

   Prior to 1995, there were three different formats commonly used by
   servers to communicate timestamps.  For compatibility with old
   implementations, all three are defined here.  The preferred format is
   a fixed-length and single-zone subset of the date and time
   specification used by the Internet Message Format [RFC5322].

     HTTP-date    = IMF-fixdate / obs-date

   An example of the preferred format is

     Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate

   Examples of the two obsolete formats are

     Sunday, 06-Nov-94 08:49:37 GMT   ; obsolete RFC 850 format
     Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format

   A recipient that parses a timestamp value in an HTTP field MUST
   accept all three HTTP-date formats.  When a sender generates a field
   that contains one or more timestamps defined as HTTP-date, the sender
   MUST generate those timestamps in the IMF-fixdate format.

   An HTTP-date value represents time as an instance of Coordinated
   Universal Time (UTC).  The first two formats indicate UTC by the
   three-letter abbreviation for Greenwich Mean Time, "GMT", a
   predecessor of the UTC name; values in the asctime format are assumed
   to be in UTC.

   A "clock" is an implementation capable of providing a reasonable
   approximation of the current instant in UTC.  A clock implementation
   ought to use NTP ([RFC5905]), or some similar protocol, to
   synchronize with UTC.

   Preferred format:

     IMF-fixdate  = day-name "," SP date1 SP time-of-day SP GMT
     ; fixed length/zone/capitalization subset of the format
     ; see Section 3.3 of [RFC5322]

     day-name     = %s"Mon" / %s"Tue" / %s"Wed"
                  / %s"Thu" / %s"Fri" / %s"Sat" / %s"Sun"

     date1        = day SP month SP year
                  ; e.g., 02 Jun 1982

     day          = 2DIGIT
     month        = %s"Jan" / %s"Feb" / %s"Mar" / %s"Apr"
                  / %s"May" / %s"Jun" / %s"Jul" / %s"Aug"
                  / %s"Sep" / %s"Oct" / %s"Nov" / %s"Dec"
     year         = 4DIGIT

     GMT          = %s"GMT"

     time-of-day  = hour ":" minute ":" second
                  ; 00:00:00 - 23:59:60 (leap second)

     hour         = 2DIGIT
     minute       = 2DIGIT
     second       = 2DIGIT

   Obsolete formats:

     obs-date     = rfc850-date / asctime-date

     rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT
     date2        = day "-" month "-" 2DIGIT
                  ; e.g., 02-Jun-82

     day-name-l   = %s"Monday" / %s"Tuesday" / %s"Wednesday"
                  / %s"Thursday" / %s"Friday" / %s"Saturday"
                  / %s"Sunday"

     asctime-date = day-name SP date3 SP time-of-day SP year
     date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))
                  ; e.g., Jun  2

   HTTP-date is case sensitive.  Note that Section 4.2 of [CACHING]
   relaxes this for cache recipients.

   A sender MUST NOT generate additional whitespace in an HTTP-date
   beyond that specifically included as SP in the grammar.  The
   semantics of day-name, day, month, year, and time-of-day are the same
   as those defined for the Internet Message Format constructs with the
   corresponding name ([RFC5322], Section 3.3).

   Recipients of a timestamp value in rfc850-date format, which uses a
   two-digit year, MUST interpret a timestamp that appears to be more
   than 50 years in the future as representing the most recent year in
   the past that had the same last two digits.

   Recipients of timestamp values are encouraged to be robust in parsing
   timestamps unless otherwise restricted by the field definition.  For
   example, messages are occasionally forwarded over HTTP from a non-
   HTTP source that might generate any of the date and time
   specifications defined by the Internet Message Format.

      |  *Note:* HTTP requirements for timestamp formats apply only to
      |  their usage within the protocol stream.  Implementations are
      |  not required to use these formats for user presentation,
      |  request logging, etc.

6.  Message Abstraction

   Each major version of HTTP defines its own syntax for communicating
   messages.  This section defines an abstract data type for HTTP
   messages based on a generalization of those message characteristics,
   common structure, and capacity for conveying semantics.  This
   abstraction is used to define requirements on senders and recipients
   that are independent of the HTTP version, such that a message in one
   version can be relayed through other versions without changing its
   meaning.

   A "message" consists of the following:

   *  control data to describe and route the message,

   *  a headers lookup table of name/value pairs for extending that
      control data and conveying additional information about the
      sender, message, content, or context,

   *  a potentially unbounded stream of content, and

   *  a trailers lookup table of name/value pairs for communicating
      information obtained while sending the content.

   Framing and control data is sent first, followed by a header section
   containing fields for the headers table.  When a message includes
   content, the content is sent after the header section, potentially
   followed by a trailer section that might contain fields for the
   trailers table.

   Messages are expected to be processed as a stream, wherein the
   purpose of that stream and its continued processing is revealed while
   being read.  Hence, control data describes what the recipient needs
   to know immediately, header fields describe what needs to be known
   before receiving content, the content (when present) presumably
   contains what the recipient wants or needs to fulfill the message
   semantics, and trailer fields provide optional metadata that was
   unknown prior to sending the content.

   Messages are intended to be "self-descriptive": everything a
   recipient needs to know about the message can be determined by
   looking at the message itself, after decoding or reconstituting parts
   that have been compressed or elided in transit, without requiring an
   understanding of the sender's current application state (established
   via prior messages).  However, a client MUST retain knowledge of the
   request when parsing, interpreting, or caching a corresponding
   response.  For example, responses to the HEAD method look just like
   the beginning of a response to GET but cannot be parsed in the same
   manner.

   Note that this message abstraction is a generalization across many
   versions of HTTP, including features that might not be found in some
   versions.  For example, trailers were introduced within the HTTP/1.1
   chunked transfer coding as a trailer section after the content.  An
   equivalent feature is present in HTTP/2 and HTTP/3 within the header
   block that terminates each stream.

6.1.  Framing and Completeness

   Message framing indicates how each message begins and ends, such that
   each message can be distinguished from other messages or noise on the
   same connection.  Each major version of HTTP defines its own framing
   mechanism.

   HTTP/0.9 and early deployments of HTTP/1.0 used closure of the
   underlying connection to end a response.  For backwards
   compatibility, this implicit framing is also allowed in HTTP/1.1.
   However, implicit framing can fail to distinguish an incomplete
   response if the connection closes early.  For that reason, almost all
   modern implementations use explicit framing in the form of length-
   delimited sequences of message data.

   A message is considered "complete" when all of the octets indicated
   by its framing are available.  Note that, when no explicit framing is
   used, a response message that is ended by the underlying connection's
   close is considered complete even though it might be
   indistinguishable from an incomplete response, unless a transport-
   level error indicates that it is not complete.

6.2.  Control Data

   Messages start with control data that describe its primary purpose.
   Request message control data includes a request method (Section 9),
   request target (Section 7.1), and protocol version (Section 2.5).
   Response message control data includes a status code (Section 15),
   optional reason phrase, and protocol version.

   In HTTP/1.1 ([HTTP/1.1]) and earlier, control data is sent as the
   first line of a message.  In HTTP/2 ([HTTP/2]) and HTTP/3 ([HTTP/3]),
   control data is sent as pseudo-header fields with a reserved name
   prefix (e.g., ":authority").

   Every HTTP message has a protocol version.  Depending on the version
   in use, it might be identified within the message explicitly or
   inferred by the connection over which the message is received.
   Recipients use that version information to determine limitations or
   potential for later communication with that sender.

   When a message is forwarded by an intermediary, the protocol version
   is updated to reflect the version used by that intermediary.  The Via
   header field (Section 7.6.3) is used to communicate upstream protocol
   information within a forwarded message.

   A client SHOULD send a request version equal to the highest version
   to which the client is conformant and whose major version is no
   higher than the highest version supported by the server, if this is
   known.  A client MUST NOT send a version to which it is not
   conformant.

   A client MAY send a lower request version if it is known that the
   server incorrectly implements the HTTP specification, but only after
   the client has attempted at least one normal request and determined
   from the response status code or header fields (e.g., Server) that
   the server improperly handles higher request versions.

   A server SHOULD send a response version equal to the highest version
   to which the server is conformant that has a major version less than
   or equal to the one received in the request.  A server MUST NOT send
   a version to which it is not conformant.  A server can send a 505
   (HTTP Version Not Supported) response if it wishes, for any reason,
   to refuse service of the client's major protocol version.

   A recipient that receives a message with a major version number that
   it implements and a minor version number higher than what it
   implements SHOULD process the message as if it were in the highest
   minor version within that major version to which the recipient is
   conformant.  A recipient can assume that a message with a higher
   minor version, when sent to a recipient that has not yet indicated
   support for that higher version, is sufficiently backwards-compatible
   to be safely processed by any implementation of the same major
   version.

6.3.  Header Fields

   Fields (Section 5) that are sent or received before the content are
   referred to as "header fields" (or just "headers", colloquially).

   The "header section" of a message consists of a sequence of header
   field lines.  Each header field might modify or extend message
   semantics, describe the sender, define the content, or provide
   additional context.

      |  *Note:* We refer to named fields specifically as a "header
      |  field" when they are only allowed to be sent in the header
      |  section.

6.4.  Content

   HTTP messages often transfer a complete or partial representation as
   the message "content": a stream of octets sent after the header
   section, as delineated by the message framing.

   This abstract definition of content reflects the data after it has
   been extracted from the message framing.  For example, an HTTP/1.1
   message body (Section 6 of [HTTP/1.1]) might consist of a stream of
   data encoded with the chunked transfer coding -- a sequence of data
   chunks, one zero-length chunk, and a trailer section -- whereas the
   content of that same message includes only the data stream after the
   transfer coding has been decoded; it does not include the chunk
   lengths, chunked framing syntax, nor the trailer fields
   (Section 6.5).

      |  *Note:* Some field names have a "Content-" prefix.  This is an
      |  informal convention; while some of these fields refer to the
      |  content of the message, as defined above, others are scoped to
      |  the selected representation (Section 3.2).  See the individual
      |  field's definition to disambiguate.

6.4.1.  Content Semantics

   The purpose of content in a request is defined by the method
   semantics (Section 9).

   For example, a representation in the content of a PUT request
   (Section 9.3.4) represents the desired state of the target resource
   after the request is successfully applied, whereas a representation
   in the content of a POST request (Section 9.3.3) represents
   information to be processed by the target resource.

   In a response, the content's purpose is defined by the request
   method, response status code (Section 15), and response fields
   describing that content.  For example, the content of a 200 (OK)
   response to GET (Section 9.3.1) represents the current state of the
   target resource, as observed at the time of the message origination
   date (Section 6.6.1), whereas the content of the same status code in
   a response to POST might represent either the processing result or
   the new state of the target resource after applying the processing.

   The content of a 206 (Partial Content) response to GET contains
   either a single part of the selected representation or a multipart
   message body containing multiple parts of that representation, as
   described in Section 15.3.7.

   Response messages with an error status code usually contain content
   that represents the error condition, such that the content describes
   the error state and what steps are suggested for resolving it.

   Responses to the HEAD request method (Section 9.3.2) never include
   content; the associated response header fields indicate only what
   their values would have been if the request method had been GET
   (Section 9.3.1).

   2xx (Successful) responses to a CONNECT request method
   (Section 9.3.6) switch the connection to tunnel mode instead of
   having content.

   All 1xx (Informational), 204 (No Content), and 304 (Not Modified)
   responses do not include content.

   All other responses do include content, although that content might
   be of zero length.

6.4.2.  Identifying Content

   When a complete or partial representation is transferred as message
   content, it is often desirable for the sender to supply, or the
   recipient to determine, an identifier for a resource corresponding to
   that specific representation.  For example, a client making a GET
   request on a resource for "the current weather report" might want an
   identifier specific to the content returned (e.g., "weather report
   for Laguna Beach at 20210720T1711").  This can be useful for sharing
   or bookmarking content from resources that are expected to have
   changing representations over time.

   For a request message:

   *  If the request has a Content-Location header field, then the
      sender asserts that the content is a representation of the
      resource identified by the Content-Location field value.  However,
      such an assertion cannot be trusted unless it can be verified by
      other means (not defined by this specification).  The information
      might still be useful for revision history links.

   *  Otherwise, the content is unidentified by HTTP, but a more
      specific identifier might be supplied within the content itself.

   For a response message, the following rules are applied in order
   until a match is found:

   1.  If the request method is HEAD or the response status code is 204
       (No Content) or 304 (Not Modified), there is no content in the
       response.

   2.  If the request method is GET and the response status code is 200
       (OK), the content is a representation of the target resource
       (Section 7.1).

   3.  If the request method is GET and the response status code is 203
       (Non-Authoritative Information), the content is a potentially
       modified or enhanced representation of the target resource as
       provided by an intermediary.

   4.  If the request method is GET and the response status code is 206
       (Partial Content), the content is one or more parts of a
       representation of the target resource.

   5.  If the response has a Content-Location header field and its field
       value is a reference to the same URI as the target URI, the
       content is a representation of the target resource.

   6.  If the response has a Content-Location header field and its field
       value is a reference to a URI different from the target URI, then
       the sender asserts that the content is a representation of the
       resource identified by the Content-Location field value.
       However, such an assertion cannot be trusted unless it can be
       verified by other means (not defined by this specification).

   7.  Otherwise, the content is unidentified by HTTP, but a more
       specific identifier might be supplied within the content itself.

6.5.  Trailer Fields

   Fields (Section 5) that are located within a "trailer section" are
   referred to as "trailer fields" (or just "trailers", colloquially).
   Trailer fields can be useful for supplying message integrity checks,
   digital signatures, delivery metrics, or post-processing status
   information.

   Trailer fields ought to be processed and stored separately from the
   fields in the header section to avoid contradicting message semantics
   known at the time the header section was complete.  The presence or
   absence of certain header fields might impact choices made for the
   routing or processing of the message as a whole before the trailers
   are received; those choices cannot be unmade by the later discovery
   of trailer fields.

6.5.1.  Limitations on Use of Trailers

   A trailer section is only possible when supported by the version of
   HTTP in use and enabled by an explicit framing mechanism.  For
   example, the chunked transfer coding in HTTP/1.1 allows a trailer
   section to be sent after the content (Section 7.1.2 of [HTTP/1.1]).

   Many fields cannot be processed outside the header section because
   their evaluation is necessary prior to receiving the content, such as
   those that describe message framing, routing, authentication, request
   modifiers, response controls, or content format.  A sender MUST NOT
   generate a trailer field unless the sender knows the corresponding
   header field name's definition permits the field to be sent in
   trailers.

   Trailer fields can be difficult to process by intermediaries that
   forward messages from one protocol version to another.  If the entire
   message can be buffered in transit, some intermediaries could merge
   trailer fields into the header section (as appropriate) before it is
   forwarded.  However, in most cases, the trailers are simply
   discarded.  A recipient MUST NOT merge a trailer field into a header
   section unless the recipient understands the corresponding header
   field definition and that definition explicitly permits and defines
   how trailer field values can be safely merged.

   The presence of the keyword "trailers" in the TE header field
   (Section 10.1.4) of a request indicates that the client is willing to
   accept trailer fields, on behalf of itself and any downstream
   clients.  For requests from an intermediary, this implies that all
   downstream clients are willing to accept trailer fields in the
   forwarded response.  Note that the presence of "trailers" does not
   mean that the client(s) will process any particular trailer field in
   the response; only that the trailer section(s) will not be dropped by
   any of the clients.

   Because of the potential for trailer fields to be discarded in
   transit, a server SHOULD NOT generate trailer fields that it believes
   are necessary for the user agent to receive.

6.5.2.  Processing Trailer Fields

   The "Trailer" header field (Section 6.6.2) can be sent to indicate
   fields likely to be sent in the trailer section, which allows
   recipients to prepare for their receipt before processing the
   content.  For example, this could be useful if a field name indicates
   that a dynamic checksum should be calculated as the content is
   received and then immediately checked upon receipt of the trailer
   field value.

   Like header fields, trailer fields with the same name are processed
   in the order received; multiple trailer field lines with the same
   name have the equivalent semantics as appending the multiple values
   as a list of members.  Trailer fields that might be generated more
   than once during a message MUST be defined as a list-based field even
   if each member value is only processed once per field line received.

   At the end of a message, a recipient MAY treat the set of received
   trailer fields as a data structure of name/value pairs, similar to
   (but separate from) the header fields.  Additional processing
   expectations, if any, can be defined within the field specification
   for a field intended for use in trailers.

6.6.  Message Metadata

   Fields that describe the message itself, such as when and how the
   message has been generated, can appear in both requests and
   responses.

6.6.1.  Date

   The "Date" header field represents the date and time at which the
   message was originated, having the same semantics as the Origination
   Date Field (orig-date) defined in Section 3.6.1 of [RFC5322].  The
   field value is an HTTP-date, as defined in Section 5.6.7.

     Date = HTTP-date

   An example is

   Date: Tue, 15 Nov 1994 08:12:31 GMT

   A sender that generates a Date header field SHOULD generate its field
   value as the best available approximation of the date and time of
   message generation.  In theory, the date ought to represent the
   moment just before generating the message content.  In practice, a
   sender can generate the date value at any time during message
   origination.

   An origin server with a clock (as defined in Section 5.6.7) MUST
   generate a Date header field in all 2xx (Successful), 3xx
   (Redirection), and 4xx (Client Error) responses, and MAY generate a
   Date header field in 1xx (Informational) and 5xx (Server Error)
   responses.

   An origin server without a clock MUST NOT generate a Date header
   field.

   A recipient with a clock that receives a response message without a
   Date header field MUST record the time it was received and append a
   corresponding Date header field to the message's header section if it
   is cached or forwarded downstream.

   A recipient with a clock that receives a response with an invalid
   Date header field value MAY replace that value with the time that
   response was received.

   A user agent MAY send a Date header field in a request, though
   generally will not do so unless it is believed to convey useful
   information to the server.  For example, custom applications of HTTP
   might convey a Date if the server is expected to adjust its
   interpretation of the user's request based on differences between the
   user agent and server clocks.

6.6.2.  Trailer

   The "Trailer" header field provides a list of field names that the
   sender anticipates sending as trailer fields within that message.
   This allows a recipient to prepare for receipt of the indicated
   metadata before it starts processing the content.

     Trailer = #field-name

   For example, a sender might indicate that a signature will be
   computed as the content is being streamed and provide the final
   signature as a trailer field.  This allows a recipient to perform the
   same check on the fly as it receives the content.

   A sender that intends to generate one or more trailer fields in a
   message SHOULD generate a Trailer header field in the header section
   of that message to indicate which fields might be present in the
   trailers.

   If an intermediary discards the trailer section in transit, the
   Trailer field could provide a hint of what metadata was lost, though
   there is no guarantee that a sender of Trailer will always follow
   through by sending the named fields.

7.  Routing HTTP Messages

   HTTP request message routing is determined by each client based on
   the target resource, the client's proxy configuration, and
   establishment or reuse of an inbound connection.  The corresponding
   response routing follows the same connection chain back to the
   client.

7.1.  Determining the Target Resource

   Although HTTP is used in a wide variety of applications, most clients
   rely on the same resource identification mechanism and configuration
   techniques as general-purpose Web browsers.  Even when communication
   options are hard-coded in a client's configuration, we can think of
   their combined effect as a URI reference (Section 4.1).

   A URI reference is resolved to its absolute form in order to obtain
   the "target URI".  The target URI excludes the reference's fragment
   component, if any, since fragment identifiers are reserved for
   client-side processing ([URI], Section 3.5).

   To perform an action on a "target resource", the client sends a
   request message containing enough components of its parsed target URI
   to enable recipients to identify that same resource.  For historical
   reasons, the parsed target URI components, collectively referred to
   as the "request target", are sent within the message control data and
   the Host header field (Section 7.2).

   There are two unusual cases for which the request target components
   are in a method-specific form:

   *  For CONNECT (Section 9.3.6), the request target is the host name
      and port number of the tunnel destination, separated by a colon.

   *  For OPTIONS (Section 9.3.7), the request target can be a single
      asterisk ("*").

   See the respective method definitions for details.  These forms MUST
   NOT be used with other methods.

   Upon receipt of a client's request, a server reconstructs the target
   URI from the received components in accordance with their local
   configuration and incoming connection context.  This reconstruction
   is specific to each major protocol version.  For example, Section 3.3
   of [HTTP/1.1] defines how a server determines the target URI of an
   HTTP/1.1 request.

      |  *Note:* Previous specifications defined the recomposed target
      |  URI as a distinct concept, the "effective request URI".

7.2.  Host and :authority

   The "Host" header field in a request provides the host and port
   information from the target URI, enabling the origin server to
   distinguish among resources while servicing requests for multiple
   host names.

   In HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3], the Host header field is, in
   some cases, supplanted by the ":authority" pseudo-header field of a
   request's control data.

     Host = uri-host [ ":" port ] ; Section 4

   The target URI's authority information is critical for handling a
   request.  A user agent MUST generate a Host header field in a request
   unless it sends that information as an ":authority" pseudo-header
   field.  A user agent that sends Host SHOULD send it as the first
   field in the header section of a request.

   For example, a GET request to the origin server for
   <http://www.example.org/pub/WWW/> would begin with:

   GET /pub/WWW/ HTTP/1.1
   Host: www.example.org

   Since the host and port information acts as an application-level
   routing mechanism, it is a frequent target for malware seeking to
   poison a shared cache or redirect a request to an unintended server.
   An interception proxy is particularly vulnerable if it relies on the
   host and port information for redirecting requests to internal
   servers, or for use as a cache key in a shared cache, without first
   verifying that the intercepted connection is targeting a valid IP
   address for that host.

7.3.  Routing Inbound Requests

   Once the target URI and its origin are determined, a client decides
   whether a network request is necessary to accomplish the desired
   semantics and, if so, where that request is to be directed.

7.3.1.  To a Cache

   If the client has a cache [CACHING] and the request can be satisfied
   by it, then the request is usually directed there first.

7.3.2.  To a Proxy

   If the request is not satisfied by a cache, then a typical client
   will check its configuration to determine whether a proxy is to be
   used to satisfy the request.  Proxy configuration is implementation-
   dependent, but is often based on URI prefix matching, selective
   authority matching, or both, and the proxy itself is usually
   identified by an "http" or "https" URI.

   If an "http" or "https" proxy is applicable, the client connects
   inbound by establishing (or reusing) a connection to that proxy and
   then sending it an HTTP request message containing a request target
   that matches the client's target URI.

7.3.3.  To the Origin

   If no proxy is applicable, a typical client will invoke a handler
   routine (specific to the target URI's scheme) to obtain access to the
   identified resource.  How that is accomplished is dependent on the
   target URI scheme and defined by its associated specification.

   Section 4.3.2 defines how to obtain access to an "http" resource by
   establishing (or reusing) an inbound connection to the identified
   origin server and then sending it an HTTP request message containing
   a request target that matches the client's target URI.

   Section 4.3.3 defines how to obtain access to an "https" resource by
   establishing (or reusing) an inbound secured connection to an origin
   server that is authoritative for the identified origin and then
   sending it an HTTP request message containing a request target that
   matches the client's target URI.

7.4.  Rejecting Misdirected Requests

   Once a request is received by a server and parsed sufficiently to
   determine its target URI, the server decides whether to process the
   request itself, forward the request to another server, redirect the
   client to a different resource, respond with an error, or drop the
   connection.  This decision can be influenced by anything about the
   request or connection context, but is specifically directed at
   whether the server has been configured to process requests for that
   target URI and whether the connection context is appropriate for that
   request.

   For example, a request might have been misdirected, deliberately or
   accidentally, such that the information within a received Host header
   field differs from the connection's host or port.  If the connection
   is from a trusted gateway, such inconsistency might be expected;
   otherwise, it might indicate an attempt to bypass security filters,
   trick the server into delivering non-public content, or poison a
   cache.  See Section 17 for security considerations regarding message
   routing.

   Unless the connection is from a trusted gateway, an origin server
   MUST reject a request if any scheme-specific requirements for the
   target URI are not met.  In particular, a request for an "https"
   resource MUST be rejected unless it has been received over a
   connection that has been secured via a certificate valid for that
   target URI's origin, as defined by Section 4.2.2.

   The 421 (Misdirected Request) status code in a response indicates
   that the origin server has rejected the request because it appears to
   have been misdirected (Section 15.5.20).

7.5.  Response Correlation

   A connection might be used for multiple request/response exchanges.
   The mechanism used to correlate between request and response messages
   is version dependent; some versions of HTTP use implicit ordering of
   messages, while others use an explicit identifier.

   All responses, regardless of the status code (including interim
   responses) can be sent at any time after a request is received, even
   if the request is not yet complete.  A response can complete before
   its corresponding request is complete (Section 6.1).  Likewise,
   clients are not expected to wait any specific amount of time for a
   response.  Clients (including intermediaries) might abandon a request
   if the response is not received within a reasonable period of time.

   A client that receives a response while it is still sending the
   associated request SHOULD continue sending that request unless it
   receives an explicit indication to the contrary (see, e.g.,
   Section 9.5 of [HTTP/1.1] and Section 6.4 of [HTTP/2]).

7.6.  Message Forwarding

   As described in Section 3.7, intermediaries can serve a variety of
   roles in the processing of HTTP requests and responses.  Some
   intermediaries are used to improve performance or availability.
   Others are used for access control or to filter content.  Since an
   HTTP stream has characteristics similar to a pipe-and-filter
   architecture, there are no inherent limits to the extent an
   intermediary can enhance (or interfere) with either direction of the
   stream.

   Intermediaries are expected to forward messages even when protocol
   elements are not recognized (e.g., new methods, status codes, or
   field names) since that preserves extensibility for downstream
   recipients.

   An intermediary not acting as a tunnel MUST implement the Connection
   header field, as specified in Section 7.6.1, and exclude fields from
   being forwarded that are only intended for the incoming connection.

   An intermediary MUST NOT forward a message to itself unless it is
   protected from an infinite request loop.  In general, an intermediary
   ought to recognize its own server names, including any aliases, local
   variations, or literal IP addresses, and respond to such requests
   directly.

   An HTTP message can be parsed as a stream for incremental processing
   or forwarding downstream.  However, senders and recipients cannot
   rely on incremental delivery of partial messages, since some
   implementations will buffer or delay message forwarding for the sake
   of network efficiency, security checks, or content transformations.

7.6.1.  Connection

   The "Connection" header field allows the sender to list desired
   control options for the current connection.

     Connection        = #connection-option
     connection-option = token

   Connection options are case-insensitive.

   When a field aside from Connection is used to supply control
   information for or about the current connection, the sender MUST list
   the corresponding field name within the Connection header field.
   Note that some versions of HTTP prohibit the use of fields for such
   information, and therefore do not allow the Connection field.

   Intermediaries MUST parse a received Connection header field before a
   message is forwarded and, for each connection-option in this field,
   remove any header or trailer field(s) from the message with the same
   name as the connection-option, and then remove the Connection header
   field itself (or replace it with the intermediary's own control
   options for the forwarded message).

   Hence, the Connection header field provides a declarative way of
   distinguishing fields that are only intended for the immediate
   recipient ("hop-by-hop") from those fields that are intended for all
   recipients on the chain ("end-to-end"), enabling the message to be
   self-descriptive and allowing future connection-specific extensions
   to be deployed without fear that they will be blindly forwarded by
   older intermediaries.

   Furthermore, intermediaries SHOULD remove or replace fields that are
   known to require removal before forwarding, whether or not they
   appear as a connection-option, after applying those fields'
   semantics.  This includes but is not limited to:

   *  Proxy-Connection (Appendix C.2.2 of [HTTP/1.1])

   *  Keep-Alive (Section 19.7.1 of [RFC2068])

   *  TE (Section 10.1.4)

   *  Transfer-Encoding (Section 6.1 of [HTTP/1.1])

   *  Upgrade (Section 7.8)

   A sender MUST NOT send a connection option corresponding to a field
   that is intended for all recipients of the content.  For example,
   Cache-Control is never appropriate as a connection option
   (Section 5.2 of [CACHING]).

   Connection options do not always correspond to a field present in the
   message, since a connection-specific field might not be needed if
   there are no parameters associated with a connection option.  In
   contrast, a connection-specific field received without a
   corresponding connection option usually indicates that the field has
   been improperly forwarded by an intermediary and ought to be ignored
   by the recipient.

   When defining a new connection option that does not correspond to a
   field, specification authors ought to reserve the corresponding field
   name anyway in order to avoid later collisions.  Such reserved field
   names are registered in the "Hypertext Transfer Protocol (HTTP) Field
   Name Registry" (Section 16.3.1).

7.6.2.  Max-Forwards

   The "Max-Forwards" header field provides a mechanism with the TRACE
   (Section 9.3.8) and OPTIONS (Section 9.3.7) request methods to limit
   the number of times that the request is forwarded by proxies.  This
   can be useful when the client is attempting to trace a request that
   appears to be failing or looping mid-chain.

     Max-Forwards = 1*DIGIT

   The Max-Forwards value is a decimal integer indicating the remaining
   number of times this request message can be forwarded.

   Each intermediary that receives a TRACE or OPTIONS request containing
   a Max-Forwards header field MUST check and update its value prior to
   forwarding the request.  If the received value is zero (0), the
   intermediary MUST NOT forward the request; instead, the intermediary
   MUST respond as the final recipient.  If the received Max-Forwards
   value is greater than zero, the intermediary MUST generate an updated
   Max-Forwards field in the forwarded message with a field value that
   is the lesser of a) the received value decremented by one (1) or b)
   the recipient's maximum supported value for Max-Forwards.

   A recipient MAY ignore a Max-Forwards header field received with any
   other request methods.

7.6.3.  Via

   The "Via" header field indicates the presence of intermediate
   protocols and recipients between the user agent and the server (on
   requests) or between the origin server and the client (on responses),
   similar to the "Received" header field in email (Section 3.6.7 of
   [RFC5322]).  Via can be used for tracking message forwards, avoiding
   request loops, and identifying the protocol capabilities of senders
   along the request/response chain.

     Via = #( received-protocol RWS received-by [ RWS comment ] )

     received-protocol = [ protocol-name "/" ] protocol-version
                       ; see Section 7.8
     received-by       = pseudonym [ ":" port ]
     pseudonym         = token

   Each member of the Via field value represents a proxy or gateway that
   has forwarded the message.  Each intermediary appends its own
   information about how the message was received, such that the end
   result is ordered according to the sequence of forwarding recipients.

   A proxy MUST send an appropriate Via header field, as described
   below, in each message that it forwards.  An HTTP-to-HTTP gateway
   MUST send an appropriate Via header field in each inbound request
   message and MAY send a Via header field in forwarded response
   messages.

   For each intermediary, the received-protocol indicates the protocol
   and protocol version used by the upstream sender of the message.
   Hence, the Via field value records the advertised protocol
   capabilities of the request/response chain such that they remain
   visible to downstream recipients; this can be useful for determining
   what backwards-incompatible features might be safe to use in
   response, or within a later request, as described in Section 2.5.
   For brevity, the protocol-name is omitted when the received protocol
   is HTTP.

   The received-by portion is normally the host and optional port number
   of a recipient server or client that subsequently forwarded the
   message.  However, if the real host is considered to be sensitive
   information, a sender MAY replace it with a pseudonym.  If a port is
   not provided, a recipient MAY interpret that as meaning it was
   received on the default port, if any, for the received-protocol.

   A sender MAY generate comments to identify the software of each
   recipient, analogous to the User-Agent and Server header fields.
   However, comments in Via are optional, and a recipient MAY remove
   them prior to forwarding the message.

   For example, a request message could be sent from an HTTP/1.0 user
   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
   forward the request to a public proxy at p.example.net, which
   completes the request by forwarding it to the origin server at
   www.example.com.  The request received by www.example.com would then
   have the following Via header field:

   Via: 1.0 fred, 1.1 p.example.net

   An intermediary used as a portal through a network firewall SHOULD
   NOT forward the names and ports of hosts within the firewall region
   unless it is explicitly enabled to do so.  If not enabled, such an
   intermediary SHOULD replace each received-by host of any host behind
   the firewall by an appropriate pseudonym for that host.

   An intermediary MAY combine an ordered subsequence of Via header
   field list members into a single member if the entries have identical
   received-protocol values.  For example,

   Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

   could be collapsed to

   Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

   A sender SHOULD NOT combine multiple list members unless they are all
   under the same organizational control and the hosts have already been
   replaced by pseudonyms.  A sender MUST NOT combine members that have
   different received-protocol values.

7.7.  Message Transformations

   Some intermediaries include features for transforming messages and
   their content.  A proxy might, for example, convert between image
   formats in order to save cache space or to reduce the amount of
   traffic on a slow link.  However, operational problems might occur
   when these transformations are applied to content intended for
   critical applications, such as medical imaging or scientific data
   analysis, particularly when integrity checks or digital signatures
   are used to ensure that the content received is identical to the
   original.

   An HTTP-to-HTTP proxy is called a "transforming proxy" if it is
   designed or configured to modify messages in a semantically
   meaningful way (i.e., modifications, beyond those required by normal
   HTTP processing, that change the message in a way that would be
   significant to the original sender or potentially significant to
   downstream recipients).  For example, a transforming proxy might be
   acting as a shared annotation server (modifying responses to include
   references to a local annotation database), a malware filter, a
   format transcoder, or a privacy filter.  Such transformations are
   presumed to be desired by whichever client (or client organization)
   chose the proxy.

   If a proxy receives a target URI with a host name that is not a fully
   qualified domain name, it MAY add its own domain to the host name it
   received when forwarding the request.  A proxy MUST NOT change the
   host name if the target URI contains a fully qualified domain name.

   A proxy MUST NOT modify the "absolute-path" and "query" parts of the
   received target URI when forwarding it to the next inbound server
   except as required by that forwarding protocol.  For example, a proxy
   forwarding a request to an origin server via HTTP/1.1 will replace an
   empty path with "/" (Section 3.2.1 of [HTTP/1.1]) or "*"
   (Section 3.2.4 of [HTTP/1.1]), depending on the request method.

   A proxy MUST NOT transform the content (Section 6.4) of a response
   message that contains a no-transform cache directive (Section 5.2.2.6
   of [CACHING]).  Note that this does not apply to message
   transformations that do not change the content, such as the addition
   or removal of transfer codings (Section 7 of [HTTP/1.1]).

   A proxy MAY transform the content of a message that does not contain
   a no-transform cache directive.  A proxy that transforms the content
   of a 200 (OK) response can inform downstream recipients that a
   transformation has been applied by changing the response status code
   to 203 (Non-Authoritative Information) (Section 15.3.4).

   A proxy SHOULD NOT modify header fields that provide information
   about the endpoints of the communication chain, the resource state,
   or the selected representation (other than the content) unless the
   field's definition specifically allows such modification or the
   modification is deemed necessary for privacy or security.

7.8.  Upgrade

   The "Upgrade" header field is intended to provide a simple mechanism
   for transitioning from HTTP/1.1 to some other protocol on the same
   connection.

   A client MAY send a list of protocol names in the Upgrade header
   field of a request to invite the server to switch to one or more of
   the named protocols, in order of descending preference, before
   sending the final response.  A server MAY ignore a received Upgrade
   header field if it wishes to continue using the current protocol on
   that connection.  Upgrade cannot be used to insist on a protocol
   change.

     Upgrade          = #protocol

     protocol         = protocol-name ["/" protocol-version]
     protocol-name    = token
     protocol-version = token

   Although protocol names are registered with a preferred case,
   recipients SHOULD use case-insensitive comparison when matching each
   protocol-name to supported protocols.

   A server that sends a 101 (Switching Protocols) response MUST send an
   Upgrade header field to indicate the new protocol(s) to which the
   connection is being switched; if multiple protocol layers are being
   switched, the sender MUST list the protocols in layer-ascending
   order.  A server MUST NOT switch to a protocol that was not indicated
   by the client in the corresponding request's Upgrade header field.  A
   server MAY choose to ignore the order of preference indicated by the
   client and select the new protocol(s) based on other factors, such as
   the nature of the request or the current load on the server.

   A server that sends a 426 (Upgrade Required) response MUST send an
   Upgrade header field to indicate the acceptable protocols, in order
   of descending preference.

   A server MAY send an Upgrade header field in any other response to
   advertise that it implements support for upgrading to the listed
   protocols, in order of descending preference, when appropriate for a
   future request.

   The following is a hypothetical example sent by a client:

   GET /hello HTTP/1.1
   Host: www.example.com
   Connection: upgrade
   Upgrade: websocket, IRC/6.9, RTA/x11

   The capabilities and nature of the application-level communication
   after the protocol change is entirely dependent upon the new
   protocol(s) chosen.  However, immediately after sending the 101
   (Switching Protocols) response, the server is expected to continue
   responding to the original request as if it had received its
   equivalent within the new protocol (i.e., the server still has an
   outstanding request to satisfy after the protocol has been changed,
   and is expected to do so without requiring the request to be
   repeated).

   For example, if the Upgrade header field is received in a GET request
   and the server decides to switch protocols, it first responds with a
   101 (Switching Protocols) message in HTTP/1.1 and then immediately
   follows that with the new protocol's equivalent of a response to a
   GET on the target resource.  This allows a connection to be upgraded
   to protocols with the same semantics as HTTP without the latency cost
   of an additional round trip.  A server MUST NOT switch protocols
   unless the received message semantics can be honored by the new
   protocol; an OPTIONS request can be honored by any protocol.

   The following is an example response to the above hypothetical
   request:

   HTTP/1.1 101 Switching Protocols
   Connection: upgrade
   Upgrade: websocket

   [... data stream switches to websocket with an appropriate response
   (as defined by new protocol) to the "GET /hello" request ...]

   A sender of Upgrade MUST also send an "Upgrade" connection option in
   the Connection header field (Section 7.6.1) to inform intermediaries
   not to forward this field.  A server that receives an Upgrade header
   field in an HTTP/1.0 request MUST ignore that Upgrade field.

   A client cannot begin using an upgraded protocol on the connection
   until it has completely sent the request message (i.e., the client
   can't change the protocol it is sending in the middle of a message).
   If a server receives both an Upgrade and an Expect header field with
   the "100-continue" expectation (Section 10.1.1), the server MUST send
   a 100 (Continue) response before sending a 101 (Switching Protocols)
   response.

   The Upgrade header field only applies to switching protocols on top
   of the existing connection; it cannot be used to switch the
   underlying connection (transport) protocol, nor to switch the
   existing communication to a different connection.  For those
   purposes, it is more appropriate to use a 3xx (Redirection) response
   (Section 15.4).

   This specification only defines the protocol name "HTTP" for use by
   the family of Hypertext Transfer Protocols, as defined by the HTTP
   version rules of Section 2.5 and future updates to this
   specification.  Additional protocol names ought to be registered
   using the registration procedure defined in Section 16.7.

8.  Representation Data and Metadata

8.1.  Representation Data

   The representation data associated with an HTTP message is either
   provided as the content of the message or referred to by the message
   semantics and the target URI.  The representation data is in a format
   and encoding defined by the representation metadata header fields.

   The data type of the representation data is determined via the header
   fields Content-Type and Content-Encoding.  These define a two-layer,
   ordered encoding model:

     representation-data := Content-Encoding( Content-Type( data ) )

8.2.  Representation Metadata

   Representation header fields provide metadata about the
   representation.  When a message includes content, the representation
   header fields describe how to interpret that data.  In a response to
   a HEAD request, the representation header fields describe the
   representation data that would have been enclosed in the content if
   the same request had been a GET.

8.3.  Content-Type

   The "Content-Type" header field indicates the media type of the
   associated representation: either the representation enclosed in the
   message content or the selected representation, as determined by the
   message semantics.  The indicated media type defines both the data
   format and how that data is intended to be processed by a recipient,
   within the scope of the received message semantics, after any content
   codings indicated by Content-Encoding are decoded.

     Content-Type = media-type

   Media types are defined in Section 8.3.1.  An example of the field is

   Content-Type: text/html; charset=ISO-8859-4

   A sender that generates a message containing content SHOULD generate
   a Content-Type header field in that message unless the intended media
   type of the enclosed representation is unknown to the sender.  If a
   Content-Type header field is not present, the recipient MAY either
   assume a media type of "application/octet-stream" ([RFC2046],
   Section 4.5.1) or examine the data to determine its type.

   In practice, resource owners do not always properly configure their
   origin server to provide the correct Content-Type for a given
   representation.  Some user agents examine the content and, in certain
   cases, override the received type (for example, see [Sniffing]).
   This "MIME sniffing" risks drawing incorrect conclusions about the
   data, which might expose the user to additional security risks (e.g.,
   "privilege escalation").  Furthermore, distinct media types often
   share a common data format, differing only in how the data is
   intended to be processed, which is impossible to distinguish by
   inspecting the data alone.  When sniffing is implemented,
   implementers are encouraged to provide a means for the user to
   disable it.

   Although Content-Type is defined as a singleton field, it is
   sometimes incorrectly generated multiple times, resulting in a
   combined field value that appears to be a list.  Recipients often
   attempt to handle this error by using the last syntactically valid
   member of the list, leading to potential interoperability and
   security issues if different implementations have different error
   handling behaviors.

8.3.1.  Media Type

   HTTP uses media types [RFC2046] in the Content-Type (Section 8.3) and
   Accept (Section 12.5.1) header fields in order to provide open and
   extensible data typing and type negotiation.  Media types define both
   a data format and various processing models: how to process that data
   in accordance with the message context.

     media-type = type "/" subtype parameters
     type       = token
     subtype    = token

   The type and subtype tokens are case-insensitive.

   The type/subtype MAY be followed by semicolon-delimited parameters
   (Section 5.6.6) in the form of name/value pairs.  The presence or
   absence of a parameter might be significant to the processing of a
   media type, depending on its definition within the media type
   registry.  Parameter values might or might not be case-sensitive,
   depending on the semantics of the parameter name.

   For example, the following media types are equivalent in describing
   HTML text data encoded in the UTF-8 character encoding scheme, but
   the first is preferred for consistency (the "charset" parameter value
   is defined as being case-insensitive in [RFC2046], Section 4.1.2):

     text/html;charset=utf-8
     Text/HTML;Charset="utf-8"
     text/html; charset="utf-8"
     text/html;charset=UTF-8

   Media types ought to be registered with IANA according to the
   procedures defined in [BCP13].

8.3.2.  Charset

   HTTP uses "charset" names to indicate or negotiate the character
   encoding scheme ([RFC6365], Section 2) of a textual representation.
   In the fields defined by this document, charset names appear either
   in parameters (Content-Type), or, for Accept-Encoding, in the form of
   a plain token.  In both cases, charset names are matched case-
   insensitively.

   Charset names ought to be registered in the IANA "Character Sets"
   registry (<https://www.iana.org/assignments/character-sets>)
   according to the procedures defined in Section 2 of [RFC2978].

      |  *Note:* In theory, charset names are defined by the "mime-
      |  charset" ABNF rule defined in Section 2.3 of [RFC2978] (as
      |  corrected in [Err1912]).  That rule allows two characters that
      |  are not included in "token" ("{" and "}"), but no charset name
      |  registered at the time of this writing includes braces (see
      |  [Err5433]).

8.3.3.  Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more representations within a single message body.  All
   multipart types share a common syntax, as defined in Section 5.1.1 of
   [RFC2046], and include a boundary parameter as part of the media type
   value.  The message body is itself a protocol element; a sender MUST
   generate only CRLF to represent line breaks between body parts.

   HTTP message framing does not use the multipart boundary as an
   indicator of message body length, though it might be used by
   implementations that generate or process the content.  For example,
   the "multipart/form-data" type is often used for carrying form data
   in a request, as described in [RFC7578], and the "multipart/
   byteranges" type is defined by this specification for use in some 206
   (Partial Content) responses (see Section 15.3.7).

8.4.  Content-Encoding

   The "Content-Encoding" header field indicates what content codings
   have been applied to the representation, beyond those inherent in the
   media type, and thus what decoding mechanisms have to be applied in
   order to obtain data in the media type referenced by the Content-Type
   header field.  Content-Encoding is primarily used to allow a
   representation's data to be compressed without losing the identity of
   its underlying media type.

     Content-Encoding = #content-coding

   An example of its use is

   Content-Encoding: gzip

   If one or more encodings have been applied to a representation, the
   sender that applied the encodings MUST generate a Content-Encoding
   header field that lists the content codings in the order in which
   they were applied.  Note that the coding named "identity" is reserved
   for its special role in Accept-Encoding and thus SHOULD NOT be
   included.

   Additional information about the encoding parameters can be provided
   by other header fields not defined by this specification.

   Unlike Transfer-Encoding (Section 6.1 of [HTTP/1.1]), the codings
   listed in Content-Encoding are a characteristic of the
   representation; the representation is defined in terms of the coded
   form, and all other metadata about the representation is about the
   coded form unless otherwise noted in the metadata definition.
   Typically, the representation is only decoded just prior to rendering
   or analogous usage.

   If the media type includes an inherent encoding, such as a data
   format that is always compressed, then that encoding would not be
   restated in Content-Encoding even if it happens to be the same
   algorithm as one of the content codings.  Such a content coding would
   only be listed if, for some bizarre reason, it is applied a second
   time to form the representation.  Likewise, an origin server might
   choose to publish the same data as multiple representations that
   differ only in whether the coding is defined as part of Content-Type
   or Content-Encoding, since some user agents will behave differently
   in their handling of each response (e.g., open a "Save as ..." dialog
   instead of automatic decompression and rendering of content).

   An origin server MAY respond with a status code of 415 (Unsupported
   Media Type) if a representation in the request message has a content
   coding that is not acceptable.

8.4.1.  Content Codings

   Content coding values indicate an encoding transformation that has
   been or can be applied to a representation.  Content codings are
   primarily used to allow a representation to be compressed or
   otherwise usefully transformed without losing the identity of its
   underlying media type and without loss of information.  Frequently,
   the representation is stored in coded form, transmitted directly, and
   only decoded by the final recipient.

     content-coding   = token

   All content codings are case-insensitive and ought to be registered
   within the "HTTP Content Coding Registry", as described in
   Section 16.6

   Content-coding values are used in the Accept-Encoding
   (Section 12.5.3) and Content-Encoding (Section 8.4) header fields.

8.4.1.1.  Compress Coding

   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
   [Welch] that is commonly produced by the UNIX file compression
   program "compress".  A recipient SHOULD consider "x-compress" to be
   equivalent to "compress".

8.4.1.2.  Deflate Coding

   The "deflate" coding is a "zlib" data format [RFC1950] containing a
   "deflate" compressed data stream [RFC1951] that uses a combination of
   the Lempel-Ziv (LZ77) compression algorithm and Huffman coding.

      |  *Note:* Some non-conformant implementations send the "deflate"
      |  compressed data without the zlib wrapper.

8.4.1.3.  Gzip Coding

   The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy
   Check (CRC) that is commonly produced by the gzip file compression
   program [RFC1952].  A recipient SHOULD consider "x-gzip" to be
   equivalent to "gzip".

8.5.  Content-Language

   The "Content-Language" header field describes the natural language(s)
   of the intended audience for the representation.  Note that this
   might not be equivalent to all the languages used within the
   representation.

     Content-Language = #language-tag

   Language tags are defined in Section 8.5.1.  The primary purpose of
   Content-Language is to allow a user to identify and differentiate
   representations according to the users' own preferred language.
   Thus, if the content is intended only for a Danish-literate audience,
   the appropriate field is

   Content-Language: da

   If no Content-Language is specified, the default is that the content
   is intended for all language audiences.  This might mean that the
   sender does not consider it to be specific to any natural language,
   or that the sender does not know for which language it is intended.

   Multiple languages MAY be listed for content that is intended for
   multiple audiences.  For example, a rendition of the "Treaty of
   Waitangi", presented simultaneously in the original Maori and English
   versions, would call for

   Content-Language: mi, en

   However, just because multiple languages are present within a
   representation does not mean that it is intended for multiple
   linguistic audiences.  An example would be a beginner's language
   primer, such as "A First Lesson in Latin", which is clearly intended
   to be used by an English-literate audience.  In this case, the
   Content-Language would properly only include "en".

   Content-Language MAY be applied to any media type -- it is not
   limited to textual documents.

8.5.1.  Language Tags

   A language tag, as defined in [RFC5646], identifies a natural
   language spoken, written, or otherwise conveyed by human beings for
   communication of information to other human beings.  Computer
   languages are explicitly excluded.

   HTTP uses language tags within the Accept-Language and
   Content-Language header fields.  Accept-Language uses the broader
   language-range production defined in Section 12.5.4, whereas
   Content-Language uses the language-tag production defined below.

     language-tag = <Language-Tag, see [RFC5646], Section 2.1>

   A language tag is a sequence of one or more case-insensitive subtags,
   each separated by a hyphen character ("-", %x2D).  In most cases, a
   language tag consists of a primary language subtag that identifies a
   broad family of related languages (e.g., "en" = English), which is
   optionally followed by a series of subtags that refine or narrow that
   language's range (e.g., "en-CA" = the variety of English as
   communicated in Canada).  Whitespace is not allowed within a language
   tag.  Example tags include:

     fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN

   See [RFC5646] for further information.

8.6.  Content-Length

   The "Content-Length" header field indicates the associated
   representation's data length as a decimal non-negative integer number
   of octets.  When transferring a representation as content, Content-
   Length refers specifically to the amount of data enclosed so that it
   can be used to delimit framing (e.g., Section 6.2 of [HTTP/1.1]).  In
   other cases, Content-Length indicates the selected representation's
   current length, which can be used by recipients to estimate transfer
   time or to compare with previously stored representations.

     Content-Length = 1*DIGIT

   An example is

   Content-Length: 3495

   A user agent SHOULD send Content-Length in a request when the method
   defines a meaning for enclosed content and it is not sending
   Transfer-Encoding.  For example, a user agent normally sends Content-
   Length in a POST request even when the value is 0 (indicating empty
   content).  A user agent SHOULD NOT send a Content-Length header field
   when the request message does not contain content and the method
   semantics do not anticipate such data.

   A server MAY send a Content-Length header field in a response to a
   HEAD request (Section 9.3.2); a server MUST NOT send Content-Length
   in such a response unless its field value equals the decimal number
   of octets that would have been sent in the content of a response if
   the same request had used the GET method.

   A server MAY send a Content-Length header field in a 304 (Not
   Modified) response to a conditional GET request (Section 15.4.5); a
   server MUST NOT send Content-Length in such a response unless its
   field value equals the decimal number of octets that would have been
   sent in the content of a 200 (OK) response to the same request.

   A server MUST NOT send a Content-Length header field in any response
   with a status code of 1xx (Informational) or 204 (No Content).  A
   server MUST NOT send a Content-Length header field in any 2xx
   (Successful) response to a CONNECT request (Section 9.3.6).

   Aside from the cases defined above, in the absence of Transfer-
   Encoding, an origin server SHOULD send a Content-Length header field
   when the content size is known prior to sending the complete header
   section.  This will allow downstream recipients to measure transfer
   progress, know when a received message is complete, and potentially
   reuse the connection for additional requests.

   Any Content-Length field value greater than or equal to zero is
   valid.  Since there is no predefined limit to the length of content,
   a recipient MUST anticipate potentially large decimal numerals and
   prevent parsing errors due to integer conversion overflows or
   precision loss due to integer conversion (Section 17.5).

   Because Content-Length is used for message delimitation in HTTP/1.1,
   its field value can impact how the message is parsed by downstream
   recipients even when the immediate connection is not using HTTP/1.1.
   If the message is forwarded by a downstream intermediary, a Content-
   Length field value that is inconsistent with the received message
   framing might cause a security failure due to request smuggling or
   response splitting.

   As a result, a sender MUST NOT forward a message with a Content-
   Length header field value that is known to be incorrect.

   Likewise, a sender MUST NOT forward a message with a Content-Length
   header field value that does not match the ABNF above, with one
   exception: a recipient of a Content-Length header field value
   consisting of the same decimal value repeated as a comma-separated
   list (e.g, "Content-Length: 42, 42") MAY either reject the message as
   invalid or replace that invalid field value with a single instance of
   the decimal value, since this likely indicates that a duplicate was
   generated or combined by an upstream message processor.

8.7.  Content-Location

   The "Content-Location" header field references a URI that can be used
   as an identifier for a specific resource corresponding to the
   representation in this message's content.  In other words, if one
   were to perform a GET request on this URI at the time of this
   message's generation, then a 200 (OK) response would contain the same
   representation that is enclosed as content in this message.

     Content-Location = absolute-URI / partial-URI

   The field value is either an absolute-URI or a partial-URI.  In the
   latter case (Section 4), the referenced URI is relative to the target
   URI ([URI], Section 5).

   The Content-Location value is not a replacement for the target URI
   (Section 7.1).  It is representation metadata.  It has the same
   syntax and semantics as the header field of the same name defined for
   MIME body parts in Section 4 of [RFC2557].  However, its appearance
   in an HTTP message has some special implications for HTTP recipients.

   If Content-Location is included in a 2xx (Successful) response
   message and its value refers (after conversion to absolute form) to a
   URI that is the same as the target URI, then the recipient MAY
   consider the content to be a current representation of that resource
   at the time indicated by the message origination date.  For a GET
   (Section 9.3.1) or HEAD (Section 9.3.2) request, this is the same as
   the default semantics when no Content-Location is provided by the
   server.  For a state-changing request like PUT (Section 9.3.4) or
   POST (Section 9.3.3), it implies that the server's response contains
   the new representation of that resource, thereby distinguishing it
   from representations that might only report about the action (e.g.,
   "It worked!").  This allows authoring applications to update their
   local copies without the need for a subsequent GET request.

   If Content-Location is included in a 2xx (Successful) response
   message and its field value refers to a URI that differs from the
   target URI, then the origin server claims that the URI is an
   identifier for a different resource corresponding to the enclosed
   representation.  Such a claim can only be trusted if both identifiers
   share the same resource owner, which cannot be programmatically
   determined via HTTP.

   *  For a response to a GET or HEAD request, this is an indication
      that the target URI refers to a resource that is subject to
      content negotiation and the Content-Location field value is a more
      specific identifier for the selected representation.

   *  For a 201 (Created) response to a state-changing method, a
      Content-Location field value that is identical to the Location
      field value indicates that this content is a current
      representation of the newly created resource.

   *  Otherwise, such a Content-Location indicates that this content is
      a representation reporting on the requested action's status and
      that the same report is available (for future access with GET) at
      the given URI.  For example, a purchase transaction made via a
      POST request might include a receipt document as the content of
      the 200 (OK) response; the Content-Location field value provides
      an identifier for retrieving a copy of that same receipt in the
      future.

   A user agent that sends Content-Location in a request message is
   stating that its value refers to where the user agent originally
   obtained the content of the enclosed representation (prior to any
   modifications made by that user agent).  In other words, the user
   agent is providing a back link to the source of the original
   representation.

   An origin server that receives a Content-Location field in a request
   message MUST treat the information as transitory request context
   rather than as metadata to be saved verbatim as part of the
   representation.  An origin server MAY use that context to guide in
   processing the request or to save it for other uses, such as within
   source links or versioning metadata.  However, an origin server MUST
   NOT use such context information to alter the request semantics.

   For example, if a client makes a PUT request on a negotiated resource
   and the origin server accepts that PUT (without redirection), then
   the new state of that resource is expected to be consistent with the
   one representation supplied in that PUT; the Content-Location cannot
   be used as a form of reverse content selection identifier to update
   only one of the negotiated representations.  If the user agent had
   wanted the latter semantics, it would have applied the PUT directly
   to the Content-Location URI.

8.8.  Validator Fields

   Resource metadata is referred to as a "validator" if it can be used
   within a precondition (Section 13.1) to make a conditional request
   (Section 13).  Validator fields convey a current validator for the
   selected representation (Section 3.2).

   In responses to safe requests, validator fields describe the selected
   representation chosen by the origin server while handling the
   response.  Note that, depending on the method and status code
   semantics, the selected representation for a given response is not
   necessarily the same as the representation enclosed as response
   content.

   In a successful response to a state-changing request, validator
   fields describe the new representation that has replaced the prior
   selected representation as a result of processing the request.

   For example, an ETag field in a 201 (Created) response communicates
   the entity tag of the newly created resource's representation, so
   that the entity tag can be used as a validator in later conditional
   requests to prevent the "lost update" problem.

   This specification defines two forms of metadata that are commonly
   used to observe resource state and test for preconditions:
   modification dates (Section 8.8.2) and opaque entity tags
   (Section 8.8.3).  Additional metadata that reflects resource state
   has been defined by various extensions of HTTP, such as Web
   Distributed Authoring and Versioning [WEBDAV], that are beyond the
   scope of this specification.

8.8.1.  Weak versus Strong

   Validators come in two flavors: strong or weak.  Weak validators are
   easy to generate but are far less useful for comparisons.  Strong
   validators are ideal for comparisons but can be very difficult (and
   occasionally impossible) to generate efficiently.  Rather than impose
   that all forms of resource adhere to the same strength of validator,
   HTTP exposes the type of validator in use and imposes restrictions on
   when weak validators can be used as preconditions.

   A "strong validator" is representation metadata that changes value
   whenever a change occurs to the representation data that would be
   observable in the content of a 200 (OK) response to GET.

   A strong validator might change for reasons other than a change to
   the representation data, such as when a semantically significant part
   of the representation metadata is changed (e.g., Content-Type), but
   it is in the best interests of the origin server to only change the
   value when it is necessary to invalidate the stored responses held by
   remote caches and authoring tools.

   Cache entries might persist for arbitrarily long periods, regardless
   of expiration times.  Thus, a cache might attempt to validate an
   entry using a validator that it obtained in the distant past.  A
   strong validator is unique across all versions of all representations
   associated with a particular resource over time.  However, there is
   no implication of uniqueness across representations of different
   resources (i.e., the same strong validator might be in use for
   representations of multiple resources at the same time and does not
   imply that those representations are equivalent).

   There are a variety of strong validators used in practice.  The best
   are based on strict revision control, wherein each change to a
   representation always results in a unique node name and revision
   identifier being assigned before the representation is made
   accessible to GET.  A collision-resistant hash function applied to
   the representation data is also sufficient if the data is available
   prior to the response header fields being sent and the digest does
   not need to be recalculated every time a validation request is
   received.  However, if a resource has distinct representations that
   differ only in their metadata, such as might occur with content
   negotiation over media types that happen to share the same data
   format, then the origin server needs to incorporate additional
   information in the validator to distinguish those representations.

   In contrast, a "weak validator" is representation metadata that might
   not change for every change to the representation data.  This
   weakness might be due to limitations in how the value is calculated
   (e.g., clock resolution), an inability to ensure uniqueness for all
   possible representations of the resource, or a desire of the resource
   owner to group representations by some self-determined set of
   equivalency rather than unique sequences of data.

   An origin server SHOULD change a weak entity tag whenever it
   considers prior representations to be unacceptable as a substitute
   for the current representation.  In other words, a weak entity tag
   ought to change whenever the origin server wants caches to invalidate
   old responses.

   For example, the representation of a weather report that changes in
   content every second, based on dynamic measurements, might be grouped
   into sets of equivalent representations (from the origin server's
   perspective) with the same weak validator in order to allow cached
   representations to be valid for a reasonable period of time (perhaps
   adjusted dynamically based on server load or weather quality).
   Likewise, a representation's modification time, if defined with only
   one-second resolution, might be a weak validator if it is possible
   for the representation to be modified twice during a single second
   and retrieved between those modifications.

   Likewise, a validator is weak if it is shared by two or more
   representations of a given resource at the same time, unless those
   representations have identical representation data.  For example, if
   the origin server sends the same validator for a representation with
   a gzip content coding applied as it does for a representation with no
   content coding, then that validator is weak.  However, two
   simultaneous representations might share the same strong validator if
   they differ only in the representation metadata, such as when two
   different media types are available for the same representation data.

   Strong validators are usable for all conditional requests, including
   cache validation, partial content ranges, and "lost update"
   avoidance.  Weak validators are only usable when the client does not
   require exact equality with previously obtained representation data,
   such as when validating a cache entry or limiting a web traversal to
   recent changes.

8.8.2.  Last-Modified

   The "Last-Modified" header field in a response provides a timestamp
   indicating the date and time at which the origin server believes the
   selected representation was last modified, as determined at the
   conclusion of handling the request.

     Last-Modified = HTTP-date

   An example of its use is

   Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

8.8.2.1.  Generation

   An origin server SHOULD send Last-Modified for any selected
   representation for which a last modification date can be reasonably
   and consistently determined, since its use in conditional requests
   and evaluating cache freshness ([CACHING]) can substantially reduce
   unnecessary transfers and significantly improve service availability
   and scalability.

   A representation is typically the sum of many parts behind the
   resource interface.  The last-modified time would usually be the most
   recent time that any of those parts were changed.  How that value is
   determined for any given resource is an implementation detail beyond
   the scope of this specification.

   An origin server SHOULD obtain the Last-Modified value of the
   representation as close as possible to the time that it generates the
   Date field value for its response.  This allows a recipient to make
   an accurate assessment of the representation's modification time,
   especially if the representation changes near the time that the
   response is generated.

   An origin server with a clock (as defined in Section 5.6.7) MUST NOT
   generate a Last-Modified date that is later than the server's time of
   message origination (Date, Section 6.6.1).  If the last modification
   time is derived from implementation-specific metadata that evaluates
   to some time in the future, according to the origin server's clock,
   then the origin server MUST replace that value with the message
   origination date.  This prevents a future modification date from
   having an adverse impact on cache validation.

   An origin server without a clock MUST NOT generate a Last-Modified
   date for a response unless that date value was assigned to the
   resource by some other system (presumably one with a clock).

8.8.2.2.  Comparison

   A Last-Modified time, when used as a validator in a request, is
   implicitly weak unless it is possible to deduce that it is strong,
   using the following rules:

   *  The validator is being compared by an origin server to the actual
      current validator for the representation and,

   *  That origin server reliably knows that the associated
      representation did not change twice during the second covered by
      the presented validator;

   or

   *  The validator is about to be used by a client in an
      If-Modified-Since, If-Unmodified-Since, or If-Range header field,
      because the client has a cache entry for the associated
      representation, and

   *  That cache entry includes a Date value which is at least one
      second after the Last-Modified value and the client has reason to
      believe that they were generated by the same clock or that there
      is enough difference between the Last-Modified and Date values to
      make clock synchronization issues unlikely;

   or

   *  The validator is being compared by an intermediate cache to the
      validator stored in its cache entry for the representation, and

   *  That cache entry includes a Date value which is at least one
      second after the Last-Modified value and the cache has reason to
      believe that they were generated by the same clock or that there
      is enough difference between the Last-Modified and Date values to
      make clock synchronization issues unlikely.

   This method relies on the fact that if two different responses were
   sent by the origin server during the same second, but both had the
   same Last-Modified time, then at least one of those responses would
   have a Date value equal to its Last-Modified time.

8.8.3.  ETag

   The "ETag" field in a response provides the current entity tag for
   the selected representation, as determined at the conclusion of
   handling the request.  An entity tag is an opaque validator for
   differentiating between multiple representations of the same
   resource, regardless of whether those multiple representations are
   due to resource state changes over time, content negotiation
   resulting in multiple representations being valid at the same time,
   or both.  An entity tag consists of an opaque quoted string, possibly
   prefixed by a weakness indicator.

     ETag       = entity-tag

     entity-tag = [ weak ] opaque-tag
     weak       = %s"W/"
     opaque-tag = DQUOTE *etagc DQUOTE
     etagc      = %x21 / %x23-7E / obs-text
                ; VCHAR except double quotes, plus obs-text

      |  *Note:* Previously, opaque-tag was defined to be a quoted-
      |  string ([RFC2616], Section 3.11); thus, some recipients might
      |  perform backslash unescaping.  Servers therefore ought to avoid
      |  backslash characters in entity tags.

   An entity tag can be more reliable for validation than a modification
   date in situations where it is inconvenient to store modification
   dates, where the one-second resolution of HTTP-date values is not
   sufficient, or where modification dates are not consistently
   maintained.

   Examples:

   ETag: "xyzzy"
   ETag: W/"xyzzy"
   ETag: ""

   An entity tag can be either a weak or strong validator, with strong
   being the default.  If an origin server provides an entity tag for a
   representation and the generation of that entity tag does not satisfy
   all of the characteristics of a strong validator (Section 8.8.1),
   then the origin server MUST mark the entity tag as weak by prefixing
   its opaque value with "W/" (case-sensitive).

   A sender MAY send the ETag field in a trailer section (see
   Section 6.5).  However, since trailers are often ignored, it is
   preferable to send ETag as a header field unless the entity tag is
   generated while sending the content.

8.8.3.1.  Generation

   The principle behind entity tags is that only the service author
   knows the implementation of a resource well enough to select the most
   accurate and efficient validation mechanism for that resource, and
   that any such mechanism can be mapped to a simple sequence of octets
   for easy comparison.  Since the value is opaque, there is no need for
   the client to be aware of how each entity tag is constructed.

   For example, a resource that has implementation-specific versioning
   applied to all changes might use an internal revision number, perhaps
   combined with a variance identifier for content negotiation, to
   accurately differentiate between representations.  Other
   implementations might use a collision-resistant hash of
   representation content, a combination of various file attributes, or
   a modification timestamp that has sub-second resolution.

   An origin server SHOULD send an ETag for any selected representation
   for which detection of changes can be reasonably and consistently
   determined, since the entity tag's use in conditional requests and
   evaluating cache freshness ([CACHING]) can substantially reduce
   unnecessary transfers and significantly improve service availability,
   scalability, and reliability.

8.8.3.2.  Comparison

   There are two entity tag comparison functions, depending on whether
   or not the comparison context allows the use of weak validators:

   "Strong comparison":  two entity tags are equivalent if both are not
      weak and their opaque-tags match character-by-character.

   "Weak comparison":  two entity tags are equivalent if their opaque-
      tags match character-by-character, regardless of either or both
      being tagged as "weak".

   The example below shows the results for a set of entity tag pairs and
   both the weak and strong comparison function results:

   +========+========+===================+=================+
   | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
   +========+========+===================+=================+
   | W/"1"  | W/"1"  | no match          | match           |
   +--------+--------+-------------------+-----------------+
   | W/"1"  | W/"2"  | no match          | no match        |
   +--------+--------+-------------------+-----------------+
   | W/"1"  | "1"    | no match          | match           |
   +--------+--------+-------------------+-----------------+
   | "1"    | "1"    | match             | match           |
   +--------+--------+-------------------+-----------------+

                            Table 3

8.8.3.3.  Example: Entity Tags Varying on Content-Negotiated Resources

   Consider a resource that is subject to content negotiation
   (Section 12), and where the representations sent in response to a GET
   request vary based on the Accept-Encoding request header field
   (Section 12.5.3):

   >> Request:

   GET /index HTTP/1.1
   Host: www.example.com
   Accept-Encoding: gzip

   In this case, the response might or might not use the gzip content
   coding.  If it does not, the response might look like:

   >> Response:

   HTTP/1.1 200 OK
   Date: Fri, 26 Mar 2010 00:05:00 GMT
   ETag: "123-a"
   Content-Length: 70
   Vary: Accept-Encoding
   Content-Type: text/plain

   Hello World!
   Hello World!
   Hello World!
   Hello World!
   Hello World!

   An alternative representation that does use gzip content coding would
   be:

   >> Response:

   HTTP/1.1 200 OK
   Date: Fri, 26 Mar 2010 00:05:00 GMT
   ETag: "123-b"
   Content-Length: 43
   Vary: Accept-Encoding
   Content-Type: text/plain
   Content-Encoding: gzip

   ...binary data...

      |  *Note:* Content codings are a property of the representation
      |  data, so a strong entity tag for a content-encoded
      |  representation has to be distinct from the entity tag of an
      |  unencoded representation to prevent potential conflicts during
      |  cache updates and range requests.  In contrast, transfer
      |  codings (Section 7 of [HTTP/1.1]) apply only during message
      |  transfer and do not result in distinct entity tags.

9.  Methods

9.1.  Overview

   The request method token is the primary source of request semantics;
   it indicates the purpose for which the client has made this request
   and what is expected by the client as a successful result.

   The request method's semantics might be further specialized by the
   semantics of some header fields when present in a request if those
   additional semantics do not conflict with the method.  For example, a
   client can send conditional request header fields (Section 13.1) to
   make the requested action conditional on the current state of the
   target resource.

   HTTP is designed to be usable as an interface to distributed object
   systems.  The request method invokes an action to be applied to a
   target resource in much the same way that a remote method invocation
   can be sent to an identified object.

     method = token

   The method token is case-sensitive because it might be used as a
   gateway to object-based systems with case-sensitive method names.  By
   convention, standardized methods are defined in all-uppercase US-
   ASCII letters.

   Unlike distributed objects, the standardized request methods in HTTP
   are not resource-specific, since uniform interfaces provide for
   better visibility and reuse in network-based systems [REST].  Once
   defined, a standardized method ought to have the same semantics when
   applied to any resource, though each resource determines for itself
   whether those semantics are implemented or allowed.

   This specification defines a number of standardized methods that are
   commonly used in HTTP, as outlined by the following table.

   +=========+============================================+=========+
   | Method  | Description                                | Section |
   | Name    |                                            |         |
   +=========+============================================+=========+
   | GET     | Transfer a current representation of the   | 9.3.1   |
   |         | target resource.                           |         |
   +---------+--------------------------------------------+---------+
   | HEAD    | Same as GET, but do not transfer the       | 9.3.2   |
   |         | response content.                          |         |
   +---------+--------------------------------------------+---------+
   | POST    | Perform resource-specific processing on    | 9.3.3   |
   |         | the request content.                       |         |
   +---------+--------------------------------------------+---------+
   | PUT     | Replace all current representations of the | 9.3.4   |
   |         | target resource with the request content.  |         |
   +---------+--------------------------------------------+---------+
   | DELETE  | Remove all current representations of the  | 9.3.5   |
   |         | target resource.                           |         |
   +---------+--------------------------------------------+---------+
   | CONNECT | Establish a tunnel to the server           | 9.3.6   |
   |         | identified by the target resource.         |         |
   +---------+--------------------------------------------+---------+
   | OPTIONS | Describe the communication options for the | 9.3.7   |
   |         | target resource.                           |         |
   +---------+--------------------------------------------+---------+
   | TRACE   | Perform a message loop-back test along the | 9.3.8   |
   |         | path to the target resource.               |         |
   +---------+--------------------------------------------+---------+

                                Table 4

   All general-purpose servers MUST support the methods GET and HEAD.
   All other methods are OPTIONAL.

   The set of methods allowed by a target resource can be listed in an
   Allow header field (Section 10.2.1).  However, the set of allowed
   methods can change dynamically.  An origin server that receives a
   request method that is unrecognized or not implemented SHOULD respond
   with the 501 (Not Implemented) status code.  An origin server that
   receives a request method that is recognized and implemented, but not
   allowed for the target resource, SHOULD respond with the 405 (Method
   Not Allowed) status code.

   Additional methods, outside the scope of this specification, have
   been specified for use in HTTP.  All such methods ought to be
   registered within the "Hypertext Transfer Protocol (HTTP) Method
   Registry", as described in Section 16.1.

9.2.  Common Method Properties

9.2.1.  Safe Methods

   Request methods are considered "safe" if their defined semantics are
   essentially read-only; i.e., the client does not request, and does
   not expect, any state change on the origin server as a result of
   applying a safe method to a target resource.  Likewise, reasonable
   use of a safe method is not expected to cause any harm, loss of
   property, or unusual burden on the origin server.

   This definition of safe methods does not prevent an implementation
   from including behavior that is potentially harmful, that is not
   entirely read-only, or that causes side effects while invoking a safe
   method.  What is important, however, is that the client did not
   request that additional behavior and cannot be held accountable for
   it.  For example, most servers append request information to access
   log files at the completion of every response, regardless of the
   method, and that is considered safe even though the log storage might
   become full and cause the server to fail.  Likewise, a safe request
   initiated by selecting an advertisement on the Web will often have
   the side effect of charging an advertising account.

   Of the request methods defined by this specification, the GET, HEAD,
   OPTIONS, and TRACE methods are defined to be safe.

   The purpose of distinguishing between safe and unsafe methods is to
   allow automated retrieval processes (spiders) and cache performance
   optimization (pre-fetching) to work without fear of causing harm.  In
   addition, it allows a user agent to apply appropriate constraints on
   the automated use of unsafe methods when processing potentially
   untrusted content.

   A user agent SHOULD distinguish between safe and unsafe methods when
   presenting potential actions to a user, such that the user can be
   made aware of an unsafe action before it is requested.

   When a resource is constructed such that parameters within the target
   URI have the effect of selecting an action, it is the resource
   owner's responsibility to ensure that the action is consistent with
   the request method semantics.  For example, it is common for Web-
   based content editing software to use actions within query
   parameters, such as "page?do=delete".  If the purpose of such a
   resource is to perform an unsafe action, then the resource owner MUST
   disable or disallow that action when it is accessed using a safe
   request method.  Failure to do so will result in unfortunate side
   effects when automated processes perform a GET on every URI reference
   for the sake of link maintenance, pre-fetching, building a search
   index, etc.

9.2.2.  Idempotent Methods

   A request method is considered "idempotent" if the intended effect on
   the server of multiple identical requests with that method is the
   same as the effect for a single such request.  Of the request methods
   defined by this specification, PUT, DELETE, and safe request methods
   are idempotent.

   Like the definition of safe, the idempotent property only applies to
   what has been requested by the user; a server is free to log each
   request separately, retain a revision control history, or implement
   other non-idempotent side effects for each idempotent request.

   Idempotent methods are distinguished because the request can be
   repeated automatically if a communication failure occurs before the
   client is able to read the server's response.  For example, if a
   client sends a PUT request and the underlying connection is closed
   before any response is received, then the client can establish a new
   connection and retry the idempotent request.  It knows that repeating
   the request will have the same intended effect, even if the original
   request succeeded, though the response might differ.

   A client SHOULD NOT automatically retry a request with a non-
   idempotent method unless it has some means to know that the request
   semantics are actually idempotent, regardless of the method, or some
   means to detect that the original request was never applied.

   For example, a user agent can repeat a POST request automatically if
   it knows (through design or configuration) that the request is safe
   for that resource.  Likewise, a user agent designed specifically to
   operate on a version control repository might be able to recover from
   partial failure conditions by checking the target resource
   revision(s) after a failed connection, reverting or fixing any
   changes that were partially applied, and then automatically retrying
   the requests that failed.

   Some clients take a riskier approach and attempt to guess when an
   automatic retry is possible.  For example, a client might
   automatically retry a POST request if the underlying transport
   connection closed before any part of a response is received,
   particularly if an idle persistent connection was used.

   A proxy MUST NOT automatically retry non-idempotent requests.  A
   client SHOULD NOT automatically retry a failed automatic retry.

9.2.3.  Methods and Caching

   For a cache to store and use a response, the associated method needs
   to explicitly allow caching and to detail under what conditions a
   response can be used to satisfy subsequent requests; a method
   definition that does not do so cannot be cached.  For additional
   requirements see [CACHING].

   This specification defines caching semantics for GET, HEAD, and POST,
   although the overwhelming majority of cache implementations only
   support GET and HEAD.

9.3.  Method Definitions

9.3.1.  GET

   The GET method requests transfer of a current selected representation
   for the target resource.  A successful response reflects the quality
   of "sameness" identified by the target URI (Section 1.2.2 of [URI]).
   Hence, retrieving identifiable information via HTTP is usually
   performed by making a GET request on an identifier associated with
   the potential for providing that information in a 200 (OK) response.

   GET is the primary mechanism of information retrieval and the focus
   of almost all performance optimizations.  Applications that produce a
   URI for each important resource can benefit from those optimizations
   while enabling their reuse by other applications, creating a network
   effect that promotes further expansion of the Web.

   It is tempting to think of resource identifiers as remote file system
   pathnames and of representations as being a copy of the contents of
   such files.  In fact, that is how many resources are implemented (see
   Section 17.3 for related security considerations).  However, there
   are no such limitations in practice.

   The HTTP interface for a resource is just as likely to be implemented
   as a tree of content objects, a programmatic view on various database
   records, or a gateway to other information systems.  Even when the
   URI mapping mechanism is tied to a file system, an origin server
   might be configured to execute the files with the request as input
   and send the output as the representation rather than transfer the
   files directly.  Regardless, only the origin server needs to know how
   each resource identifier corresponds to an implementation and how
   that implementation manages to select and send a current
   representation of the target resource.

   A client can alter the semantics of GET to be a "range request",
   requesting transfer of only some part(s) of the selected
   representation, by sending a Range header field in the request
   (Section 14.2).

   Although request message framing is independent of the method used,
   content received in a GET request has no generally defined semantics,
   cannot alter the meaning or target of the request, and might lead
   some implementations to reject the request and close the connection
   because of its potential as a request smuggling attack (Section 11.2
   of [HTTP/1.1]).  A client SHOULD NOT generate content in a GET
   request unless it is made directly to an origin server that has
   previously indicated, in or out of band, that such a request has a
   purpose and will be adequately supported.  An origin server SHOULD
   NOT rely on private agreements to receive content, since participants
   in HTTP communication are often unaware of intermediaries along the
   request chain.

   The response to a GET request is cacheable; a cache MAY use it to
   satisfy subsequent GET and HEAD requests unless otherwise indicated
   by the Cache-Control header field (Section 5.2 of [CACHING]).

   When information retrieval is performed with a mechanism that
   constructs a target URI from user-provided information, such as the
   query fields of a form using GET, potentially sensitive data might be
   provided that would not be appropriate for disclosure within a URI
   (see Section 17.9).  In some cases, the data can be filtered or
   transformed such that it would not reveal such information.  In
   others, particularly when there is no benefit from caching a
   response, using the POST method (Section 9.3.3) instead of GET can
   transmit such information in the request content rather than within
   the target URI.

9.3.2.  HEAD

   The HEAD method is identical to GET except that the server MUST NOT
   send content in the response.  HEAD is used to obtain metadata about
   the selected representation without transferring its representation
   data, often for the sake of testing hypertext links or finding recent
   modifications.

   The server SHOULD send the same header fields in response to a HEAD
   request as it would have sent if the request method had been GET.
   However, a server MAY omit header fields for which a value is
   determined only while generating the content.  For example, some
   servers buffer a dynamic response to GET until a minimum amount of
   data is generated so that they can more efficiently delimit small
   responses or make late decisions with regard to content selection.
   Such a response to GET might contain Content-Length and Vary fields,
   for example, that are not generated within a HEAD response.  These
   minor inconsistencies are considered preferable to generating and
   discarding the content for a HEAD request, since HEAD is usually
   requested for the sake of efficiency.

   Although request message framing is independent of the method used,
   content received in a HEAD request has no generally defined
   semantics, cannot alter the meaning or target of the request, and
   might lead some implementations to reject the request and close the
   connection because of its potential as a request smuggling attack
   (Section 11.2 of [HTTP/1.1]).  A client SHOULD NOT generate content
   in a HEAD request unless it is made directly to an origin server that
   has previously indicated, in or out of band, that such a request has
   a purpose and will be adequately supported.  An origin server SHOULD
   NOT rely on private agreements to receive content, since participants
   in HTTP communication are often unaware of intermediaries along the
   request chain.

   The response to a HEAD request is cacheable; a cache MAY use it to
   satisfy subsequent HEAD requests unless otherwise indicated by the
   Cache-Control header field (Section 5.2 of [CACHING]).  A HEAD
   response might also affect previously cached responses to GET; see
   Section 4.3.5 of [CACHING].

9.3.3.  POST

   The POST method requests that the target resource process the
   representation enclosed in the request according to the resource's
   own specific semantics.  For example, POST is used for the following
   functions (among others):

   *  Providing a block of data, such as the fields entered into an HTML
      form, to a data-handling process;

   *  Posting a message to a bulletin board, newsgroup, mailing list,
      blog, or similar group of articles;

   *  Creating a new resource that has yet to be identified by the
      origin server; and

   *  Appending data to a resource's existing representation(s).

   An origin server indicates response semantics by choosing an
   appropriate status code depending on the result of processing the
   POST request; almost all of the status codes defined by this
   specification could be received in a response to POST (the exceptions
   being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not
   Satisfiable)).

   If one or more resources has been created on the origin server as a
   result of successfully processing a POST request, the origin server
   SHOULD send a 201 (Created) response containing a Location header
   field that provides an identifier for the primary resource created
   (Section 10.2.2) and a representation that describes the status of
   the request while referring to the new resource(s).

   Responses to POST requests are only cacheable when they include
   explicit freshness information (see Section 4.2.1 of [CACHING]) and a
   Content-Location header field that has the same value as the POST's
   target URI (Section 8.7).  A cached POST response can be reused to
   satisfy a later GET or HEAD request.  In contrast, a POST request
   cannot be satisfied by a cached POST response because POST is
   potentially unsafe; see Section 4 of [CACHING].

   If the result of processing a POST would be equivalent to a
   representation of an existing resource, an origin server MAY redirect
   the user agent to that resource by sending a 303 (See Other) response
   with the existing resource's identifier in the Location field.  This
   has the benefits of providing the user agent a resource identifier
   and transferring the representation via a method more amenable to
   shared caching, though at the cost of an extra request if the user
   agent does not already have the representation cached.

9.3.4.  PUT

   The PUT method requests that the state of the target resource be
   created or replaced with the state defined by the representation
   enclosed in the request message content.  A successful PUT of a given
   representation would suggest that a subsequent GET on that same
   target resource will result in an equivalent representation being
   sent in a 200 (OK) response.  However, there is no guarantee that
   such a state change will be observable, since the target resource
   might be acted upon by other user agents in parallel, or might be
   subject to dynamic processing by the origin server, before any
   subsequent GET is received.  A successful response only implies that
   the user agent's intent was achieved at the time of its processing by
   the origin server.

   If the target resource does not have a current representation and the
   PUT successfully creates one, then the origin server MUST inform the
   user agent by sending a 201 (Created) response.  If the target
   resource does have a current representation and that representation
   is successfully modified in accordance with the state of the enclosed
   representation, then the origin server MUST send either a 200 (OK) or
   a 204 (No Content) response to indicate successful completion of the
   request.

   An origin server SHOULD verify that the PUT representation is
   consistent with its configured constraints for the target resource.
   For example, if an origin server determines a resource's
   representation metadata based on the URI, then the origin server
   needs to ensure that the content received in a successful PUT request
   is consistent with that metadata.  When a PUT representation is
   inconsistent with the target resource, the origin server SHOULD
   either make them consistent, by transforming the representation or
   changing the resource configuration, or respond with an appropriate
   error message containing sufficient information to explain why the
   representation is unsuitable.  The 409 (Conflict) or 415 (Unsupported
   Media Type) status codes are suggested, with the latter being
   specific to constraints on Content-Type values.

   For example, if the target resource is configured to always have a
   Content-Type of "text/html" and the representation being PUT has a
   Content-Type of "image/jpeg", the origin server ought to do one of:

   a.  reconfigure the target resource to reflect the new media type;

   b.  transform the PUT representation to a format consistent with that
       of the resource before saving it as the new resource state; or,

   c.  reject the request with a 415 (Unsupported Media Type) response
       indicating that the target resource is limited to "text/html",
       perhaps including a link to a different resource that would be a
       suitable target for the new representation.

   HTTP does not define exactly how a PUT method affects the state of an
   origin server beyond what can be expressed by the intent of the user
   agent request and the semantics of the origin server response.  It
   does not define what a resource might be, in any sense of that word,
   beyond the interface provided via HTTP.  It does not define how
   resource state is "stored", nor how such storage might change as a
   result of a change in resource state, nor how the origin server
   translates resource state into representations.  Generally speaking,
   all implementation details behind the resource interface are
   intentionally hidden by the server.

   This extends to how header and trailer fields are stored; while
   common header fields like Content-Type will typically be stored and
   returned upon subsequent GET requests, header and trailer field
   handling is specific to the resource that received the request.  As a
   result, an origin server SHOULD ignore unrecognized header and
   trailer fields received in a PUT request (i.e., not save them as part
   of the resource state).

   An origin server MUST NOT send a validator field (Section 8.8), such
   as an ETag or Last-Modified field, in a successful response to PUT
   unless the request's representation data was saved without any
   transformation applied to the content (i.e., the resource's new
   representation data is identical to the content received in the PUT
   request) and the validator field value reflects the new
   representation.  This requirement allows a user agent to know when
   the representation it sent (and retains in memory) is the result of
   the PUT, and thus it doesn't need to be retrieved again from the
   origin server.  The new validator(s) received in the response can be
   used for future conditional requests in order to prevent accidental
   overwrites (Section 13.1).

   The fundamental difference between the POST and PUT methods is
   highlighted by the different intent for the enclosed representation.
   The target resource in a POST request is intended to handle the
   enclosed representation according to the resource's own semantics,
   whereas the enclosed representation in a PUT request is defined as
   replacing the state of the target resource.  Hence, the intent of PUT
   is idempotent and visible to intermediaries, even though the exact
   effect is only known by the origin server.

   Proper interpretation of a PUT request presumes that the user agent
   knows which target resource is desired.  A service that selects a
   proper URI on behalf of the client, after receiving a state-changing
   request, SHOULD be implemented using the POST method rather than PUT.
   If the origin server will not make the requested PUT state change to
   the target resource and instead wishes to have it applied to a
   different resource, such as when the resource has been moved to a
   different URI, then the origin server MUST send an appropriate 3xx
   (Redirection) response; the user agent MAY then make its own decision
   regarding whether or not to redirect the request.

   A PUT request applied to the target resource can have side effects on
   other resources.  For example, an article might have a URI for
   identifying "the current version" (a resource) that is separate from
   the URIs identifying each particular version (different resources
   that at one point shared the same state as the current version
   resource).  A successful PUT request on "the current version" URI
   might therefore create a new version resource in addition to changing
   the state of the target resource, and might also cause links to be
   added between the related resources.

   Some origin servers support use of the Content-Range header field
   (Section 14.4) as a request modifier to perform a partial PUT, as
   described in Section 14.5.

   Responses to the PUT method are not cacheable.  If a successful PUT
   request passes through a cache that has one or more stored responses
   for the target URI, those stored responses will be invalidated (see
   Section 4.4 of [CACHING]).

9.3.5.  DELETE

   The DELETE method requests that the origin server remove the
   association between the target resource and its current
   functionality.  In effect, this method is similar to the "rm" command
   in UNIX: it expresses a deletion operation on the URI mapping of the
   origin server rather than an expectation that the previously
   associated information be deleted.

   If the target resource has one or more current representations, they
   might or might not be destroyed by the origin server, and the
   associated storage might or might not be reclaimed, depending
   entirely on the nature of the resource and its implementation by the
   origin server (which are beyond the scope of this specification).
   Likewise, other implementation aspects of a resource might need to be
   deactivated or archived as a result of a DELETE, such as database or
   gateway connections.  In general, it is assumed that the origin
   server will only allow DELETE on resources for which it has a
   prescribed mechanism for accomplishing the deletion.

   Relatively few resources allow the DELETE method -- its primary use
   is for remote authoring environments, where the user has some
   direction regarding its effect.  For example, a resource that was
   previously created using a PUT request, or identified via the
   Location header field after a 201 (Created) response to a POST
   request, might allow a corresponding DELETE request to undo those
   actions.  Similarly, custom user agent implementations that implement
   an authoring function, such as revision control clients using HTTP
   for remote operations, might use DELETE based on an assumption that
   the server's URI space has been crafted to correspond to a version
   repository.

   If a DELETE method is successfully applied, the origin server SHOULD
   send

   *  a 202 (Accepted) status code if the action will likely succeed but
      has not yet been enacted,

   *  a 204 (No Content) status code if the action has been enacted and
      no further information is to be supplied, or

   *  a 200 (OK) status code if the action has been enacted and the
      response message includes a representation describing the status.

   Although request message framing is independent of the method used,
   content received in a DELETE request has no generally defined
   semantics, cannot alter the meaning or target of the request, and
   might lead some implementations to reject the request and close the
   connection because of its potential as a request smuggling attack
   (Section 11.2 of [HTTP/1.1]).  A client SHOULD NOT generate content
   in a DELETE request unless it is made directly to an origin server
   that has previously indicated, in or out of band, that such a request
   has a purpose and will be adequately supported.  An origin server
   SHOULD NOT rely on private agreements to receive content, since
   participants in HTTP communication are often unaware of
   intermediaries along the request chain.

   Responses to the DELETE method are not cacheable.  If a successful
   DELETE request passes through a cache that has one or more stored
   responses for the target URI, those stored responses will be
   invalidated (see Section 4.4 of [CACHING]).

9.3.6.  CONNECT

   The CONNECT method requests that the recipient establish a tunnel to
   the destination origin server identified by the request target and,
   if successful, thereafter restrict its behavior to blind forwarding
   of data, in both directions, until the tunnel is closed.  Tunnels are
   commonly used to create an end-to-end virtual connection, through one
   or more proxies, which can then be secured using TLS (Transport Layer
   Security, [TLS13]).

   CONNECT uses a special form of request target, unique to this method,
   consisting of only the host and port number of the tunnel
   destination, separated by a colon.  There is no default port; a
   client MUST send the port number even if the CONNECT request is based
   on a URI reference that contains an authority component with an
   elided port (Section 4.1).  For example,

   CONNECT server.example.com:80 HTTP/1.1
   Host: server.example.com

   A server MUST reject a CONNECT request that targets an empty or
   invalid port number, typically by responding with a 400 (Bad Request)
   status code.

   Because CONNECT changes the request/response nature of an HTTP
   connection, specific HTTP versions might have different ways of
   mapping its semantics into the protocol's wire format.

   CONNECT is intended for use in requests to a proxy.  The recipient
   can establish a tunnel either by directly connecting to the server
   identified by the request target or, if configured to use another
   proxy, by forwarding the CONNECT request to the next inbound proxy.
   An origin server MAY accept a CONNECT request, but most origin
   servers do not implement CONNECT.

   Any 2xx (Successful) response indicates that the sender (and all
   inbound proxies) will switch to tunnel mode immediately after the
   response header section; data received after that header section is
   from the server identified by the request target.  Any response other
   than a successful response indicates that the tunnel has not yet been
   formed.

   A tunnel is closed when a tunnel intermediary detects that either
   side has closed its connection: the intermediary MUST attempt to send
   any outstanding data that came from the closed side to the other
   side, close both connections, and then discard any remaining data
   left undelivered.

   Proxy authentication might be used to establish the authority to
   create a tunnel.  For example,

   CONNECT server.example.com:443 HTTP/1.1
   Host: server.example.com:443
   Proxy-Authorization: basic aGVsbG86d29ybGQ=

   There are significant risks in establishing a tunnel to arbitrary
   servers, particularly when the destination is a well-known or
   reserved TCP port that is not intended for Web traffic.  For example,
   a CONNECT to "example.com:25" would suggest that the proxy connect to
   the reserved port for SMTP traffic; if allowed, that could trick the
   proxy into relaying spam email.  Proxies that support CONNECT SHOULD
   restrict its use to a limited set of known ports or a configurable
   list of safe request targets.

   A server MUST NOT send any Transfer-Encoding or Content-Length header
   fields in a 2xx (Successful) response to CONNECT.  A client MUST
   ignore any Content-Length or Transfer-Encoding header fields received
   in a successful response to CONNECT.

   A CONNECT request message does not have content.  The interpretation
   of data sent after the header section of the CONNECT request message
   is specific to the version of HTTP in use.

   Responses to the CONNECT method are not cacheable.

9.3.7.  OPTIONS

   The OPTIONS method requests information about the communication
   options available for the target resource, at either the origin
   server or an intervening intermediary.  This method allows a client
   to determine the options and/or requirements associated with a
   resource, or the capabilities of a server, without implying a
   resource action.

   An OPTIONS request with an asterisk ("*") as the request target
   (Section 7.1) applies to the server in general rather than to a
   specific resource.  Since a server's communication options typically
   depend on the resource, the "*" request is only useful as a "ping" or
   "no-op" type of method; it does nothing beyond allowing the client to
   test the capabilities of the server.  For example, this can be used
   to test a proxy for HTTP/1.1 conformance (or lack thereof).

   If the request target is not an asterisk, the OPTIONS request applies
   to the options that are available when communicating with the target
   resource.

   A server generating a successful response to OPTIONS SHOULD send any
   header that might indicate optional features implemented by the
   server and applicable to the target resource (e.g., Allow), including
   potential extensions not defined by this specification.  The response
   content, if any, might also describe the communication options in a
   machine or human-readable representation.  A standard format for such
   a representation is not defined by this specification, but might be
   defined by future extensions to HTTP.

   A client MAY send a Max-Forwards header field in an OPTIONS request
   to target a specific recipient in the request chain (see
   Section 7.6.2).  A proxy MUST NOT generate a Max-Forwards header
   field while forwarding a request unless that request was received
   with a Max-Forwards field.

   A client that generates an OPTIONS request containing content MUST
   send a valid Content-Type header field describing the representation
   media type.  Note that this specification does not define any use for
   such content.

   Responses to the OPTIONS method are not cacheable.

9.3.8.  TRACE

   The TRACE method requests a remote, application-level loop-back of
   the request message.  The final recipient of the request SHOULD
   reflect the message received, excluding some fields described below,
   back to the client as the content of a 200 (OK) response.  The
   "message/http" format (Section 10.1 of [HTTP/1.1]) is one way to do
   so.  The final recipient is either the origin server or the first
   server to receive a Max-Forwards value of zero (0) in the request
   (Section 7.6.2).

   A client MUST NOT generate fields in a TRACE request containing
   sensitive data that might be disclosed by the response.  For example,
   it would be foolish for a user agent to send stored user credentials
   (Section 11) or cookies [COOKIE] in a TRACE request.  The final
   recipient of the request SHOULD exclude any request fields that are
   likely to contain sensitive data when that recipient generates the
   response content.

   TRACE allows the client to see what is being received at the other
   end of the request chain and use that data for testing or diagnostic
   information.  The value of the Via header field (Section 7.6.3) is of
   particular interest, since it acts as a trace of the request chain.
   Use of the Max-Forwards header field allows the client to limit the
   length of the request chain, which is useful for testing a chain of
   proxies forwarding messages in an infinite loop.

   A client MUST NOT send content in a TRACE request.

   Responses to the TRACE method are not cacheable.

10.  Message Context

10.1.  Request Context Fields

   The request header fields below provide additional information about
   the request context, including information about the user, user
   agent, and resource behind the request.

10.1.1.  Expect

   The "Expect" header field in a request indicates a certain set of
   behaviors (expectations) that need to be supported by the server in
   order to properly handle this request.

     Expect =      #expectation
     expectation = token [ "=" ( token / quoted-string ) parameters ]

   The Expect field value is case-insensitive.

   The only expectation defined by this specification is "100-continue"
   (with no defined parameters).

   A server that receives an Expect field value containing a member
   other than 100-continue MAY respond with a 417 (Expectation Failed)
   status code to indicate that the unexpected expectation cannot be
   met.

   A "100-continue" expectation informs recipients that the client is
   about to send (presumably large) content in this request and wishes
   to receive a 100 (Continue) interim response if the method, target
   URI, and header fields are not sufficient to cause an immediate
   success, redirect, or error response.  This allows the client to wait
   for an indication that it is worthwhile to send the content before
   actually doing so, which can improve efficiency when the data is huge
   or when the client anticipates that an error is likely (e.g., when
   sending a state-changing method, for the first time, without
   previously verified authentication credentials).

   For example, a request that begins with

   PUT /somewhere/fun HTTP/1.1
   Host: origin.example.com
   Content-Type: video/h264
   Content-Length: 1234567890987
   Expect: 100-continue

   allows the origin server to immediately respond with an error
   message, such as 401 (Unauthorized) or 405 (Method Not Allowed),
   before the client starts filling the pipes with an unnecessary data
   transfer.

   Requirements for clients:

   *  A client MUST NOT generate a 100-continue expectation in a request
      that does not include content.

   *  A client that will wait for a 100 (Continue) response before
      sending the request content MUST send an Expect header field
      containing a 100-continue expectation.

   *  A client that sends a 100-continue expectation is not required to
      wait for any specific length of time; such a client MAY proceed to
      send the content even if it has not yet received a response.
      Furthermore, since 100 (Continue) responses cannot be sent through
      an HTTP/1.0 intermediary, such a client SHOULD NOT wait for an
      indefinite period before sending the content.

   *  A client that receives a 417 (Expectation Failed) status code in
      response to a request containing a 100-continue expectation SHOULD
      repeat that request without a 100-continue expectation, since the
      417 response merely indicates that the response chain does not
      support expectations (e.g., it passes through an HTTP/1.0 server).

   Requirements for servers:

   *  A server that receives a 100-continue expectation in an HTTP/1.0
      request MUST ignore that expectation.

   *  A server MAY omit sending a 100 (Continue) response if it has
      already received some or all of the content for the corresponding
      request, or if the framing indicates that there is no content.

   *  A server that sends a 100 (Continue) response MUST ultimately send
      a final status code, once it receives and processes the request
      content, unless the connection is closed prematurely.

   *  A server that responds with a final status code before reading the
      entire request content SHOULD indicate whether it intends to close
      the connection (e.g., see Section 9.6 of [HTTP/1.1]) or continue
      reading the request content.

   Upon receiving an HTTP/1.1 (or later) request that has a method,
   target URI, and complete header section that contains a 100-continue
   expectation and an indication that request content will follow, an
   origin server MUST send either:

   *  an immediate response with a final status code, if that status can
      be determined by examining just the method, target URI, and header
      fields, or

   *  an immediate 100 (Continue) response to encourage the client to
      send the request content.

   The origin server MUST NOT wait for the content before sending the
   100 (Continue) response.

   Upon receiving an HTTP/1.1 (or later) request that has a method,
   target URI, and complete header section that contains a 100-continue
   expectation and indicates a request content will follow, a proxy MUST
   either:

   *  send an immediate response with a final status code, if that
      status can be determined by examining just the method, target URI,
      and header fields, or

   *  forward the request toward the origin server by sending a
      corresponding request-line and header section to the next inbound
      server.

   If the proxy believes (from configuration or past interaction) that
   the next inbound server only supports HTTP/1.0, the proxy MAY
   generate an immediate 100 (Continue) response to encourage the client
   to begin sending the content.

10.1.2.  From

   The "From" header field contains an Internet email address for a
   human user who controls the requesting user agent.  The address ought
   to be machine-usable, as defined by "mailbox" in Section 3.4 of
   [RFC5322]:

     From    = mailbox

     mailbox = <mailbox, see [RFC5322], Section 3.4>

   An example is:

   From: spider-admin@example.org

   The From header field is rarely sent by non-robotic user agents.  A
   user agent SHOULD NOT send a From header field without explicit
   configuration by the user, since that might conflict with the user's
   privacy interests or their site's security policy.

   A robotic user agent SHOULD send a valid From header field so that
   the person responsible for running the robot can be contacted if
   problems occur on servers, such as if the robot is sending excessive,
   unwanted, or invalid requests.

   A server SHOULD NOT use the From header field for access control or
   authentication, since its value is expected to be visible to anyone
   receiving or observing the request and is often recorded within
   logfiles and error reports without any expectation of privacy.

10.1.3.  Referer

   The "Referer" [sic] header field allows the user agent to specify a
   URI reference for the resource from which the target URI was obtained
   (i.e., the "referrer", though the field name is misspelled).  A user
   agent MUST NOT include the fragment and userinfo components of the
   URI reference [URI], if any, when generating the Referer field value.

     Referer = absolute-URI / partial-URI

   The field value is either an absolute-URI or a partial-URI.  In the
   latter case (Section 4), the referenced URI is relative to the target
   URI ([URI], Section 5).

   The Referer header field allows servers to generate back-links to
   other resources for simple analytics, logging, optimized caching,
   etc.  It also allows obsolete or mistyped links to be found for
   maintenance.  Some servers use the Referer header field as a means of
   denying links from other sites (so-called "deep linking") or
   restricting cross-site request forgery (CSRF), but not all requests
   contain it.

   Example:

   Referer: http://www.example.org/hypertext/Overview.html

   If the target URI was obtained from a source that does not have its
   own URI (e.g., input from the user keyboard, or an entry within the
   user's bookmarks/favorites), the user agent MUST either exclude the
   Referer header field or send it with a value of "about:blank".

   The Referer header field value need not convey the full URI of the
   referring resource; a user agent MAY truncate parts other than the
   referring origin.

   The Referer header field has the potential to reveal information
   about the request context or browsing history of the user, which is a
   privacy concern if the referring resource's identifier reveals
   personal information (such as an account name) or a resource that is
   supposed to be confidential (such as behind a firewall or internal to
   a secured service).  Most general-purpose user agents do not send the
   Referer header field when the referring resource is a local "file" or
   "data" URI.  A user agent SHOULD NOT send a Referer header field if
   the referring resource was accessed with a secure protocol and the
   request target has an origin differing from that of the referring
   resource, unless the referring resource explicitly allows Referer to
   be sent.  A user agent MUST NOT send a Referer header field in an
   unsecured HTTP request if the referring resource was accessed with a
   secure protocol.  See Section 17.9 for additional security
   considerations.

   Some intermediaries have been known to indiscriminately remove
   Referer header fields from outgoing requests.  This has the
   unfortunate side effect of interfering with protection against CSRF
   attacks, which can be far more harmful to their users.
   Intermediaries and user agent extensions that wish to limit
   information disclosure in Referer ought to restrict their changes to
   specific edits, such as replacing internal domain names with
   pseudonyms or truncating the query and/or path components.  An
   intermediary SHOULD NOT modify or delete the Referer header field
   when the field value shares the same scheme and host as the target
   URI.

10.1.4.  TE

   The "TE" header field describes capabilities of the client with
   regard to transfer codings and trailer sections.

   As described in Section 6.5, a TE field with a "trailers" member sent
   in a request indicates that the client will not discard trailer
   fields.

   TE is also used within HTTP/1.1 to advise servers about which
   transfer codings the client is able to accept in a response.  As of
   publication, only HTTP/1.1 uses transfer codings (see Section 7 of
   [HTTP/1.1]).

   The TE field value is a list of members, with each member (aside from
   "trailers") consisting of a transfer coding name token with an
   optional weight indicating the client's relative preference for that
   transfer coding (Section 12.4.2) and optional parameters for that
   transfer coding.

     TE                 = #t-codings
     t-codings          = "trailers" / ( transfer-coding [ weight ] )
     transfer-coding    = token *( OWS ";" OWS transfer-parameter )
     transfer-parameter = token BWS "=" BWS ( token / quoted-string )

   A sender of TE MUST also send a "TE" connection option within the
   Connection header field (Section 7.6.1) to inform intermediaries not
   to forward this field.

10.1.5.  User-Agent

   The "User-Agent" header field contains information about the user
   agent originating the request, which is often used by servers to help
   identify the scope of reported interoperability problems, to work
   around or tailor responses to avoid particular user agent
   limitations, and for analytics regarding browser or operating system
   use.  A user agent SHOULD send a User-Agent header field in each
   request unless specifically configured not to do so.

     User-Agent = product *( RWS ( product / comment ) )

   The User-Agent field value consists of one or more product
   identifiers, each followed by zero or more comments (Section 5.6.5),
   which together identify the user agent software and its significant
   subproducts.  By convention, the product identifiers are listed in
   decreasing order of their significance for identifying the user agent
   software.  Each product identifier consists of a name and optional
   version.

     product         = token ["/" product-version]
     product-version = token

   A sender SHOULD limit generated product identifiers to what is
   necessary to identify the product; a sender MUST NOT generate
   advertising or other nonessential information within the product
   identifier.  A sender SHOULD NOT generate information in
   product-version that is not a version identifier (i.e., successive
   versions of the same product name ought to differ only in the
   product-version portion of the product identifier).

   Example:

   User-Agent: CERN-LineMode/2.15 libwww/2.17b3

   A user agent SHOULD NOT generate a User-Agent header field containing
   needlessly fine-grained detail and SHOULD limit the addition of
   subproducts by third parties.  Overly long and detailed User-Agent
   field values increase request latency and the risk of a user being
   identified against their wishes ("fingerprinting").

   Likewise, implementations are encouraged not to use the product
   tokens of other implementations in order to declare compatibility
   with them, as this circumvents the purpose of the field.  If a user
   agent masquerades as a different user agent, recipients can assume
   that the user intentionally desires to see responses tailored for
   that identified user agent, even if they might not work as well for
   the actual user agent being used.

10.2.  Response Context Fields

   The response header fields below provide additional information about
   the response, beyond what is implied by the status code, including
   information about the server, about the target resource, or about
   related resources.

10.2.1.  Allow

   The "Allow" header field lists the set of methods advertised as
   supported by the target resource.  The purpose of this field is
   strictly to inform the recipient of valid request methods associated
   with the resource.

     Allow = #method

   Example of use:

   Allow: GET, HEAD, PUT

   The actual set of allowed methods is defined by the origin server at
   the time of each request.  An origin server MUST generate an Allow
   header field in a 405 (Method Not Allowed) response and MAY do so in
   any other response.  An empty Allow field value indicates that the
   resource allows no methods, which might occur in a 405 response if
   the resource has been temporarily disabled by configuration.

   A proxy MUST NOT modify the Allow header field -- it does not need to
   understand all of the indicated methods in order to handle them
   according to the generic message handling rules.

10.2.2.  Location

   The "Location" header field is used in some responses to refer to a
   specific resource in relation to the response.  The type of
   relationship is defined by the combination of request method and
   status code semantics.

     Location = URI-reference

   The field value consists of a single URI-reference.  When it has the
   form of a relative reference ([URI], Section 4.2), the final value is
   computed by resolving it against the target URI ([URI], Section 5).

   For 201 (Created) responses, the Location value refers to the primary
   resource created by the request.  For 3xx (Redirection) responses,
   the Location value refers to the preferred target resource for
   automatically redirecting the request.

   If the Location value provided in a 3xx (Redirection) response does
   not have a fragment component, a user agent MUST process the
   redirection as if the value inherits the fragment component of the
   URI reference used to generate the target URI (i.e., the redirection
   inherits the original reference's fragment, if any).

   For example, a GET request generated for the URI reference
   "http://www.example.org/~tim" might result in a 303 (See Other)
   response containing the header field:

   Location: /People.html#tim

   which suggests that the user agent redirect to
   "http://www.example.org/People.html#tim"

   Likewise, a GET request generated for the URI reference
   "http://www.example.org/index.html#larry" might result in a 301
   (Moved Permanently) response containing the header field:

   Location: http://www.example.net/index.html

   which suggests that the user agent redirect to
   "http://www.example.net/index.html#larry", preserving the original
   fragment identifier.

   There are circumstances in which a fragment identifier in a Location
   value would not be appropriate.  For example, the Location header
   field in a 201 (Created) response is supposed to provide a URI that
   is specific to the created resource.

      |  *Note:* Some recipients attempt to recover from Location header
      |  fields that are not valid URI references.  This specification
      |  does not mandate or define such processing, but does allow it
      |  for the sake of robustness.  A Location field value cannot
      |  allow a list of members because the comma list separator is a
      |  valid data character within a URI-reference.  If an invalid
      |  message is sent with multiple Location field lines, a recipient
      |  along the path might combine those field lines into one value.
      |  Recovery of a valid Location field value from that situation is
      |  difficult and not interoperable across implementations.

      |  *Note:* The Content-Location header field (Section 8.7) differs
      |  from Location in that the Content-Location refers to the most
      |  specific resource corresponding to the enclosed representation.
      |  It is therefore possible for a response to contain both the
      |  Location and Content-Location header fields.

10.2.3.  Retry-After

   Servers send the "Retry-After" header field to indicate how long the
   user agent ought to wait before making a follow-up request.  When
   sent with a 503 (Service Unavailable) response, Retry-After indicates
   how long the service is expected to be unavailable to the client.
   When sent with any 3xx (Redirection) response, Retry-After indicates
   the minimum time that the user agent is asked to wait before issuing
   the redirected request.

   The Retry-After field value can be either an HTTP-date or a number of
   seconds to delay after receiving the response.

     Retry-After = HTTP-date / delay-seconds

   A delay-seconds value is a non-negative decimal integer, representing
   time in seconds.

     delay-seconds  = 1*DIGIT

   Two examples of its use are

   Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
   Retry-After: 120

   In the latter example, the delay is 2 minutes.

10.2.4.  Server

   The "Server" header field contains information about the software
   used by the origin server to handle the request, which is often used
   by clients to help identify the scope of reported interoperability
   problems, to work around or tailor requests to avoid particular
   server limitations, and for analytics regarding server or operating
   system use.  An origin server MAY generate a Server header field in
   its responses.

     Server = product *( RWS ( product / comment ) )

   The Server header field value consists of one or more product
   identifiers, each followed by zero or more comments (Section 5.6.5),
   which together identify the origin server software and its
   significant subproducts.  By convention, the product identifiers are
   listed in decreasing order of their significance for identifying the
   origin server software.  Each product identifier consists of a name
   and optional version, as defined in Section 10.1.5.

   Example:

   Server: CERN/3.0 libwww/2.17

   An origin server SHOULD NOT generate a Server header field containing
   needlessly fine-grained detail and SHOULD limit the addition of
   subproducts by third parties.  Overly long and detailed Server field
   values increase response latency and potentially reveal internal
   implementation details that might make it (slightly) easier for
   attackers to find and exploit known security holes.

11.  HTTP Authentication

11.1.  Authentication Scheme

   HTTP provides a general framework for access control and
   authentication, via an extensible set of challenge-response
   authentication schemes, which can be used by a server to challenge a
   client request and by a client to provide authentication information.
   It uses a case-insensitive token to identify the authentication
   scheme:

     auth-scheme    = token

   Aside from the general framework, this document does not specify any
   authentication schemes.  New and existing authentication schemes are
   specified independently and ought to be registered within the
   "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry".
   For example, the "basic" and "digest" authentication schemes are
   defined by [RFC7617] and [RFC7616], respectively.

11.2.  Authentication Parameters

   The authentication scheme is followed by additional information
   necessary for achieving authentication via that scheme as either a
   comma-separated list of parameters or a single sequence of characters
   capable of holding base64-encoded information.

     token68        = 1*( ALPHA / DIGIT /
                          "-" / "." / "_" / "~" / "+" / "/" ) *"="

   The token68 syntax allows the 66 unreserved URI characters ([URI]),
   plus a few others, so that it can hold a base64, base64url (URL and
   filename safe alphabet), base32, or base16 (hex) encoding, with or
   without padding, but excluding whitespace ([RFC4648]).

   Authentication parameters are name/value pairs, where the name token
   is matched case-insensitively and each parameter name MUST only occur
   once per challenge.

     auth-param     = token BWS "=" BWS ( token / quoted-string )

   Parameter values can be expressed either as "token" or as "quoted-
   string" (Section 5.6).  Authentication scheme definitions need to
   accept both notations, both for senders and recipients, to allow
   recipients to use generic parsing components regardless of the
   authentication scheme.

   For backwards compatibility, authentication scheme definitions can
   restrict the format for senders to one of the two variants.  This can
   be important when it is known that deployed implementations will fail
   when encountering one of the two formats.

11.3.  Challenge and Response

   A 401 (Unauthorized) response message is used by an origin server to
   challenge the authorization of a user agent, including a
   WWW-Authenticate header field containing at least one challenge
   applicable to the requested resource.

   A 407 (Proxy Authentication Required) response message is used by a
   proxy to challenge the authorization of a client, including a
   Proxy-Authenticate header field containing at least one challenge
   applicable to the proxy for the requested resource.

     challenge   = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

      |  *Note:* Many clients fail to parse a challenge that contains an
      |  unknown scheme.  A workaround for this problem is to list well-
      |  supported schemes (such as "basic") first.

   A user agent that wishes to authenticate itself with an origin server
   -- usually, but not necessarily, after receiving a 401 (Unauthorized)
   -- can do so by including an Authorization header field with the
   request.

   A client that wishes to authenticate itself with a proxy -- usually,
   but not necessarily, after receiving a 407 (Proxy Authentication
   Required) -- can do so by including a Proxy-Authorization header
   field with the request.

11.4.  Credentials

   Both the Authorization field value and the Proxy-Authorization field
   value contain the client's credentials for the realm of the resource
   being requested, based upon a challenge received in a response
   (possibly at some point in the past).  When creating their values,
   the user agent ought to do so by selecting the challenge with what it
   considers to be the most secure auth-scheme that it understands,
   obtaining credentials from the user as appropriate.  Transmission of
   credentials within header field values implies significant security
   considerations regarding the confidentiality of the underlying
   connection, as described in Section 17.16.1.

     credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

   Upon receipt of a request for a protected resource that omits
   credentials, contains invalid credentials (e.g., a bad password) or
   partial credentials (e.g., when the authentication scheme requires
   more than one round trip), an origin server SHOULD send a 401
   (Unauthorized) response that contains a WWW-Authenticate header field
   with at least one (possibly new) challenge applicable to the
   requested resource.

   Likewise, upon receipt of a request that omits proxy credentials or
   contains invalid or partial proxy credentials, a proxy that requires
   authentication SHOULD generate a 407 (Proxy Authentication Required)
   response that contains a Proxy-Authenticate header field with at
   least one (possibly new) challenge applicable to the proxy.

   A server that receives valid credentials that are not adequate to
   gain access ought to respond with the 403 (Forbidden) status code
   (Section 15.5.4).

   HTTP does not restrict applications to this simple challenge-response
   framework for access authentication.  Additional mechanisms can be
   used, such as authentication at the transport level or via message
   encapsulation, and with additional header fields specifying
   authentication information.  However, such additional mechanisms are
   not defined by this specification.

   Note that various custom mechanisms for user authentication use the
   Set-Cookie and Cookie header fields, defined in [COOKIE], for passing
   tokens related to authentication.

11.5.  Establishing a Protection Space (Realm)

   The "realm" authentication parameter is reserved for use by
   authentication schemes that wish to indicate a scope of protection.

   A "protection space" is defined by the origin (see Section 4.3.1) of
   the server being accessed, in combination with the realm value if
   present.  These realms allow the protected resources on a server to
   be partitioned into a set of protection spaces, each with its own
   authentication scheme and/or authorization database.  The realm value
   is a string, generally assigned by the origin server, that can have
   additional semantics specific to the authentication scheme.  Note
   that a response can have multiple challenges with the same auth-
   scheme but with different realms.

   The protection space determines the domain over which credentials can
   be automatically applied.  If a prior request has been authorized,
   the user agent MAY reuse the same credentials for all other requests
   within that protection space for a period of time determined by the
   authentication scheme, parameters, and/or user preferences (such as a
   configurable inactivity timeout).

   The extent of a protection space, and therefore the requests to which
   credentials might be automatically applied, is not necessarily known
   to clients without additional information.  An authentication scheme
   might define parameters that describe the extent of a protection
   space.  Unless specifically allowed by the authentication scheme, a
   single protection space cannot extend outside the scope of its
   server.

   For historical reasons, a sender MUST only generate the quoted-string
   syntax.  Recipients might have to support both token and quoted-
   string syntax for maximum interoperability with existing clients that
   have been accepting both notations for a long time.

11.6.  Authenticating Users to Origin Servers

11.6.1.  WWW-Authenticate

   The "WWW-Authenticate" response header field indicates the
   authentication scheme(s) and parameters applicable to the target
   resource.

     WWW-Authenticate = #challenge

   A server generating a 401 (Unauthorized) response MUST send a WWW-
   Authenticate header field containing at least one challenge.  A
   server MAY generate a WWW-Authenticate header field in other response
   messages to indicate that supplying credentials (or different
   credentials) might affect the response.

   A proxy forwarding a response MUST NOT modify any WWW-Authenticate
   header fields in that response.

   User agents are advised to take special care in parsing the field
   value, as it might contain more than one challenge, and each
   challenge can contain a comma-separated list of authentication
   parameters.  Furthermore, the header field itself can occur multiple
   times.

   For instance:

   WWW-Authenticate: Basic realm="simple", Newauth realm="apps",
                    type=1, title="Login to \"apps\""

   This header field contains two challenges, one for the "Basic" scheme
   with a realm value of "simple" and another for the "Newauth" scheme
   with a realm value of "apps".  It also contains two additional
   parameters, "type" and "title".

   Some user agents do not recognize this form, however.  As a result,
   sending a WWW-Authenticate field value with more than one member on
   the same field line might not be interoperable.

      |  *Note:* The challenge grammar production uses the list syntax
      |  as well.  Therefore, a sequence of comma, whitespace, and comma
      |  can be considered either as applying to the preceding
      |  challenge, or to be an empty entry in the list of challenges.
      |  In practice, this ambiguity does not affect the semantics of
      |  the header field value and thus is harmless.

11.6.2.  Authorization

   The "Authorization" header field allows a user agent to authenticate
   itself with an origin server -- usually, but not necessarily, after
   receiving a 401 (Unauthorized) response.  Its value consists of
   credentials containing the authentication information of the user
   agent for the realm of the resource being requested.

     Authorization = credentials

   If a request is authenticated and a realm specified, the same
   credentials are presumed to be valid for all other requests within
   this realm (assuming that the authentication scheme itself does not
   require otherwise, such as credentials that vary according to a
   challenge value or using synchronized clocks).

   A proxy forwarding a request MUST NOT modify any Authorization header
   fields in that request.  See Section 3.5 of [CACHING] for details of
   and requirements pertaining to handling of the Authorization header
   field by HTTP caches.

11.6.3.  Authentication-Info

   HTTP authentication schemes can use the "Authentication-Info"
   response field to communicate information after the client's
   authentication credentials have been accepted.  This information can
   include a finalization message from the server (e.g., it can contain
   the server authentication).

   The field value is a list of parameters (name/value pairs), using the
   "auth-param" syntax defined in Section 11.3.  This specification only
   describes the generic format; authentication schemes using
   Authentication-Info will define the individual parameters.  The
   "Digest" Authentication Scheme, for instance, defines multiple
   parameters in Section 3.5 of [RFC7616].

     Authentication-Info = #auth-param

   The Authentication-Info field can be used in any HTTP response,
   independently of request method and status code.  Its semantics are
   defined by the authentication scheme indicated by the Authorization
   header field (Section 11.6.2) of the corresponding request.

   A proxy forwarding a response is not allowed to modify the field
   value in any way.

   Authentication-Info can be sent as a trailer field (Section 6.5) when
   the authentication scheme explicitly allows this.

11.7.  Authenticating Clients to Proxies

11.7.1.  Proxy-Authenticate

   The "Proxy-Authenticate" header field consists of at least one
   challenge that indicates the authentication scheme(s) and parameters
   applicable to the proxy for this request.  A proxy MUST send at least
   one Proxy-Authenticate header field in each 407 (Proxy Authentication
   Required) response that it generates.

     Proxy-Authenticate = #challenge

   Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
   only to the next outbound client on the response chain.  This is
   because only the client that chose a given proxy is likely to have
   the credentials necessary for authentication.  However, when multiple
   proxies are used within the same administrative domain, such as
   office and regional caching proxies within a large corporate network,
   it is common for credentials to be generated by the user agent and
   passed through the hierarchy until consumed.  Hence, in such a
   configuration, it will appear as if Proxy-Authenticate is being
   forwarded because each proxy will send the same challenge set.

   Note that the parsing considerations for WWW-Authenticate apply to
   this header field as well; see Section 11.6.1 for details.

11.7.2.  Proxy-Authorization

   The "Proxy-Authorization" header field allows the client to identify
   itself (or its user) to a proxy that requires authentication.  Its
   value consists of credentials containing the authentication
   information of the client for the proxy and/or realm of the resource
   being requested.

     Proxy-Authorization = credentials

   Unlike Authorization, the Proxy-Authorization header field applies
   only to the next inbound proxy that demanded authentication using the
   Proxy-Authenticate header field.  When multiple proxies are used in a
   chain, the Proxy-Authorization header field is consumed by the first
   inbound proxy that was expecting to receive credentials.  A proxy MAY
   relay the credentials from the client request to the next proxy if
   that is the mechanism by which the proxies cooperatively authenticate
   a given request.

11.7.3.  Proxy-Authentication-Info

   The "Proxy-Authentication-Info" response header field is equivalent
   to Authentication-Info, except that it applies to proxy
   authentication (Section 11.3) and its semantics are defined by the
   authentication scheme indicated by the Proxy-Authorization header
   field (Section 11.7.2) of the corresponding request:

     Proxy-Authentication-Info = #auth-param

   However, unlike Authentication-Info, the Proxy-Authentication-Info
   header field applies only to the next outbound client on the response
   chain.  This is because only the client that chose a given proxy is
   likely to have the credentials necessary for authentication.
   However, when multiple proxies are used within the same
   administrative domain, such as office and regional caching proxies
   within a large corporate network, it is common for credentials to be
   generated by the user agent and passed through the hierarchy until
   consumed.  Hence, in such a configuration, it will appear as if
   Proxy-Authentication-Info is being forwarded because each proxy will
   send the same field value.

   Proxy-Authentication-Info can be sent as a trailer field
   (Section 6.5) when the authentication scheme explicitly allows this.

12.  Content Negotiation

   When responses convey content, whether indicating a success or an
   error, the origin server often has different ways of representing
   that information; for example, in different formats, languages, or
   encodings.  Likewise, different users or user agents might have
   differing capabilities, characteristics, or preferences that could
   influence which representation, among those available, would be best
   to deliver.  For this reason, HTTP provides mechanisms for content
   negotiation.

   This specification defines three patterns of content negotiation that
   can be made visible within the protocol: "proactive" negotiation,
   where the server selects the representation based upon the user
   agent's stated preferences; "reactive" negotiation, where the server
   provides a list of representations for the user agent to choose from;
   and "request content" negotiation, where the user agent selects the
   representation for a future request based upon the server's stated
   preferences in past responses.

   Other patterns of content negotiation include "conditional content",
   where the representation consists of multiple parts that are
   selectively rendered based on user agent parameters, "active
   content", where the representation contains a script that makes
   additional (more specific) requests based on the user agent
   characteristics, and "Transparent Content Negotiation" ([RFC2295]),
   where content selection is performed by an intermediary.  These
   patterns are not mutually exclusive, and each has trade-offs in
   applicability and practicality.

   Note that, in all cases, HTTP is not aware of the resource semantics.
   The consistency with which an origin server responds to requests,
   over time and over the varying dimensions of content negotiation, and
   thus the "sameness" of a resource's observed representations over
   time, is determined entirely by whatever entity or algorithm selects
   or generates those responses.

12.1.  Proactive Negotiation

   When content negotiation preferences are sent by the user agent in a
   request to encourage an algorithm located at the server to select the
   preferred representation, it is called "proactive negotiation"
   (a.k.a., "server-driven negotiation").  Selection is based on the
   available representations for a response (the dimensions over which
   it might vary, such as language, content coding, etc.) compared to
   various information supplied in the request, including both the
   explicit negotiation header fields below and implicit
   characteristics, such as the client's network address or parts of the
   User-Agent field.

   Proactive negotiation is advantageous when the algorithm for
   selecting from among the available representations is difficult to
   describe to a user agent, or when the server desires to send its
   "best guess" to the user agent along with the first response (when
   that "best guess" is good enough for the user, this avoids the round-
   trip delay of a subsequent request).  In order to improve the
   server's guess, a user agent MAY send request header fields that
   describe its preferences.

   Proactive negotiation has serious disadvantages:

   *  It is impossible for the server to accurately determine what might
      be "best" for any given user, since that would require complete
      knowledge of both the capabilities of the user agent and the
      intended use for the response (e.g., does the user want to view it
      on screen or print it on paper?);

   *  Having the user agent describe its capabilities in every request
      can be both very inefficient (given that only a small percentage
      of responses have multiple representations) and a potential risk
      to the user's privacy;

   *  It complicates the implementation of an origin server and the
      algorithms for generating responses to a request; and,

   *  It limits the reusability of responses for shared caching.

   A user agent cannot rely on proactive negotiation preferences being
   consistently honored, since the origin server might not implement
   proactive negotiation for the requested resource or might decide that
   sending a response that doesn't conform to the user agent's
   preferences is better than sending a 406 (Not Acceptable) response.

   A Vary header field (Section 12.5.5) is often sent in a response
   subject to proactive negotiation to indicate what parts of the
   request information were used in the selection algorithm.

   The request header fields Accept, Accept-Charset, Accept-Encoding,
   and Accept-Language are defined below for a user agent to engage in
   proactive negotiation of the response content.  The preferences sent
   in these fields apply to any content in the response, including
   representations of the target resource, representations of error or
   processing status, and potentially even the miscellaneous text
   strings that might appear within the protocol.

12.2.  Reactive Negotiation

   With "reactive negotiation" (a.k.a., "agent-driven negotiation"),
   selection of content (regardless of the status code) is performed by
   the user agent after receiving an initial response.  The mechanism
   for reactive negotiation might be as simple as a list of references
   to alternative representations.

   If the user agent is not satisfied by the initial response content,
   it can perform a GET request on one or more of the alternative
   resources to obtain a different representation.  Selection of such
   alternatives might be performed automatically (by the user agent) or
   manually (e.g., by the user selecting from a hypertext menu).

   A server might choose not to send an initial representation, other
   than the list of alternatives, and thereby indicate that reactive
   negotiation by the user agent is preferred.  For example, the
   alternatives listed in responses with the 300 (Multiple Choices) and
   406 (Not Acceptable) status codes include information about available
   representations so that the user or user agent can react by making a
   selection.

   Reactive negotiation is advantageous when the response would vary
   over commonly used dimensions (such as type, language, or encoding),
   when the origin server is unable to determine a user agent's
   capabilities from examining the request, and generally when public
   caches are used to distribute server load and reduce network usage.

   Reactive negotiation suffers from the disadvantages of transmitting a
   list of alternatives to the user agent, which degrades user-perceived
   latency if transmitted in the header section, and needing a second
   request to obtain an alternate representation.  Furthermore, this
   specification does not define a mechanism for supporting automatic
   selection, though it does not prevent such a mechanism from being
   developed.

12.3.  Request Content Negotiation

   When content negotiation preferences are sent in a server's response,
   the listed preferences are called "request content negotiation"
   because they intend to influence selection of an appropriate content
   for subsequent requests to that resource.  For example, the Accept
   (Section 12.5.1) and Accept-Encoding (Section 12.5.3) header fields
   can be sent in a response to indicate preferred media types and
   content codings for subsequent requests to that resource.

   Similarly, Section 3.1 of [RFC5789] defines the "Accept-Patch"
   response header field, which allows discovery of which content types
   are accepted in PATCH requests.

12.4.  Content Negotiation Field Features

12.4.1.  Absence

   For each of the content negotiation fields, a request that does not
   contain the field implies that the sender has no preference on that
   dimension of negotiation.

   If a content negotiation header field is present in a request and
   none of the available representations for the response can be
   considered acceptable according to it, the origin server can either
   honor the header field by sending a 406 (Not Acceptable) response or
   disregard the header field by treating the response as if it is not
   subject to content negotiation for that request header field.  This
   does not imply, however, that the client will be able to use the
   representation.

      |  *Note:* A user agent sending these header fields makes it
      |  easier for a server to identify an individual by virtue of the
      |  user agent's request characteristics (Section 17.13).

12.4.2.  Quality Values

   The content negotiation fields defined by this specification use a
   common parameter, named "q" (case-insensitive), to assign a relative
   "weight" to the preference for that associated kind of content.  This
   weight is referred to as a "quality value" (or "qvalue") because the
   same parameter name is often used within server configurations to
   assign a weight to the relative quality of the various
   representations that can be selected for a resource.

   The weight is normalized to a real number in the range 0 through 1,
   where 0.001 is the least preferred and 1 is the most preferred; a
   value of 0 means "not acceptable".  If no "q" parameter is present,
   the default weight is 1.

     weight = OWS ";" OWS "q=" qvalue
     qvalue = ( "0" [ "." 0*3DIGIT ] )
            / ( "1" [ "." 0*3("0") ] )

   A sender of qvalue MUST NOT generate more than three digits after the
   decimal point.  User configuration of these values ought to be
   limited in the same fashion.

12.4.3.  Wildcard Values

   Most of these header fields, where indicated, define a wildcard value
   ("*") to select unspecified values.  If no wildcard is present,
   values that are not explicitly mentioned in the field are considered
   unacceptable.  Within Vary, the wildcard value means that the
   variance is unlimited.

      |  *Note:* In practice, using wildcards in content negotiation has
      |  limited practical value because it is seldom useful to say, for
      |  example, "I prefer image/* more or less than (some other
      |  specific value)".  By sending Accept: */*;q=0, clients can
      |  explicitly request a 406 (Not Acceptable) response if a more
      |  preferred format is not available, but they still need to be
      |  able to handle a different response since the server is allowed
      |  to ignore their preference.

12.5.  Content Negotiation Fields

12.5.1.  Accept

   The "Accept" header field can be used by user agents to specify their
   preferences regarding response media types.  For example, Accept
   header fields can be used to indicate that the request is
   specifically limited to a small set of desired types, as in the case
   of a request for an in-line image.

   When sent by a server in a response, Accept provides information
   about which content types are preferred in the content of a
   subsequent request to the same resource.

     Accept = #( media-range [ weight ] )

     media-range    = ( "*/*"
                        / ( type "/" "*" )
                        / ( type "/" subtype )
                      ) parameters

   The asterisk "*" character is used to group media types into ranges,
   with "*/*" indicating all media types and "type/*" indicating all
   subtypes of that type.  The media-range can include media type
   parameters that are applicable to that range.

   Each media-range might be followed by optional applicable media type
   parameters (e.g., charset), followed by an optional "q" parameter for
   indicating a relative weight (Section 12.4.2).

   Previous specifications allowed additional extension parameters to
   appear after the weight parameter.  The accept extension grammar
   (accept-params, accept-ext) has been removed because it had a
   complicated definition, was not being used in practice, and is more
   easily deployed through new header fields.  Senders using weights
   SHOULD send "q" last (after all media-range parameters).  Recipients
   SHOULD process any parameter named "q" as weight, regardless of
   parameter ordering.

      |  *Note:* Use of the "q" parameter name to control content
      |  negotiation would interfere with any media type parameter
      |  having the same name.  Hence, the media type registry disallows
      |  parameters named "q".

   The example

   Accept: audio/*; q=0.2, audio/basic

   is interpreted as "I prefer audio/basic, but send me any audio type
   if it is the best available after an 80% markdown in quality".

   A more elaborate example is

   Accept: text/plain; q=0.5, text/html,
          text/x-dvi; q=0.8, text/x-c

   Verbally, this would be interpreted as "text/html and text/x-c are
   the equally preferred media types, but if they do not exist, then
   send the text/x-dvi representation, and if that does not exist, send
   the text/plain representation".

   Media ranges can be overridden by more specific media ranges or
   specific media types.  If more than one media range applies to a
   given type, the most specific reference has precedence.  For example,

   Accept: text/*, text/plain, text/plain;format=flowed, */*

   have the following precedence:

   1.  text/plain;format=flowed

   2.  text/plain

   3.  text/*

   4.  */*

   The media type quality factor associated with a given type is
   determined by finding the media range with the highest precedence
   that matches the type.  For example,

   Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed,
          text/plain;format=fixed;q=0.4, */*;q=0.5

   would cause the following values to be associated:

   +==========================+===============+
   | Media Type               | Quality Value |
   +==========================+===============+
   | text/plain;format=flowed | 1             |
   +--------------------------+---------------+
   | text/plain               | 0.7           |
   +--------------------------+---------------+
   | text/html                | 0.3           |
   +--------------------------+---------------+
   | image/jpeg               | 0.5           |
   +--------------------------+---------------+
   | text/plain;format=fixed  | 0.4           |
   +--------------------------+---------------+
   | text/html;level=3        | 0.7           |
   +--------------------------+---------------+

                     Table 5

      |  *Note:* A user agent might be provided with a default set of
      |  quality values for certain media ranges.  However, unless the
      |  user agent is a closed system that cannot interact with other
      |  rendering agents, this default set ought to be configurable by
      |  the user.

12.5.2.  Accept-Charset

   The "Accept-Charset" header field can be sent by a user agent to
   indicate its preferences for charsets in textual response content.
   For example, this field allows user agents capable of understanding
   more comprehensive or special-purpose charsets to signal that
   capability to an origin server that is capable of representing
   information in those charsets.

     Accept-Charset = #( ( token / "*" ) [ weight ] )

   Charset names are defined in Section 8.3.2.  A user agent MAY
   associate a quality value with each charset to indicate the user's
   relative preference for that charset, as defined in Section 12.4.2.
   An example is

   Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

   The special value "*", if present in the Accept-Charset header field,
   matches every charset that is not mentioned elsewhere in the field.

      |  *Note:* Accept-Charset is deprecated because UTF-8 has become
      |  nearly ubiquitous and sending a detailed list of user-preferred
      |  charsets wastes bandwidth, increases latency, and makes passive
      |  fingerprinting far too easy (Section 17.13).  Most general-
      |  purpose user agents do not send Accept-Charset unless
      |  specifically configured to do so.

12.5.3.  Accept-Encoding

   The "Accept-Encoding" header field can be used to indicate
   preferences regarding the use of content codings (Section 8.4.1).

   When sent by a user agent in a request, Accept-Encoding indicates the
   content codings acceptable in a response.

   When sent by a server in a response, Accept-Encoding provides
   information about which content codings are preferred in the content
   of a subsequent request to the same resource.

   An "identity" token is used as a synonym for "no encoding" in order
   to communicate when no encoding is preferred.

     Accept-Encoding  = #( codings [ weight ] )
     codings          = content-coding / "identity" / "*"

   Each codings value MAY be given an associated quality value (weight)
   representing the preference for that encoding, as defined in
   Section 12.4.2.  The asterisk "*" symbol in an Accept-Encoding field
   matches any available content coding not explicitly listed in the
   field.

   Examples:

   Accept-Encoding: compress, gzip
   Accept-Encoding:
   Accept-Encoding: *
   Accept-Encoding: compress;q=0.5, gzip;q=1.0
   Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0

   A server tests whether a content coding for a given representation is
   acceptable using these rules:

   1.  If no Accept-Encoding header field is in the request, any content
       coding is considered acceptable by the user agent.

   2.  If the representation has no content coding, then it is
       acceptable by default unless specifically excluded by the Accept-
       Encoding header field stating either "identity;q=0" or "*;q=0"
       without a more specific entry for "identity".

   3.  If the representation's content coding is one of the content
       codings listed in the Accept-Encoding field value, then it is
       acceptable unless it is accompanied by a qvalue of 0.  (As
       defined in Section 12.4.2, a qvalue of 0 means "not acceptable".)

   A representation could be encoded with multiple content codings.
   However, most content codings are alternative ways to accomplish the
   same purpose (e.g., data compression).  When selecting between
   multiple content codings that have the same purpose, the acceptable
   content coding with the highest non-zero qvalue is preferred.

   An Accept-Encoding header field with a field value that is empty
   implies that the user agent does not want any content coding in
   response.  If a non-empty Accept-Encoding header field is present in
   a request and none of the available representations for the response
   have a content coding that is listed as acceptable, the origin server
   SHOULD send a response without any content coding unless the identity
   coding is indicated as unacceptable.

   When the Accept-Encoding header field is present in a response, it
   indicates what content codings the resource was willing to accept in
   the associated request.  The field value is evaluated the same way as
   in a request.

   Note that this information is specific to the associated request; the
   set of supported encodings might be different for other resources on
   the same server and could change over time or depend on other aspects
   of the request (such as the request method).

   Servers that fail a request due to an unsupported content coding
   ought to respond with a 415 (Unsupported Media Type) status and
   include an Accept-Encoding header field in that response, allowing
   clients to distinguish between issues related to content codings and
   media types.  In order to avoid confusion with issues related to
   media types, servers that fail a request with a 415 status for
   reasons unrelated to content codings MUST NOT include the Accept-
   Encoding header field.

   The most common use of Accept-Encoding is in responses with a 415
   (Unsupported Media Type) status code, in response to optimistic use
   of a content coding by clients.  However, the header field can also
   be used to indicate to clients that content codings are supported in
   order to optimize future interactions.  For example, a resource might
   include it in a 2xx (Successful) response when the request content
   was big enough to justify use of a compression coding but the client
   failed do so.

12.5.4.  Accept-Language

   The "Accept-Language" header field can be used by user agents to
   indicate the set of natural languages that are preferred in the
   response.  Language tags are defined in Section 8.5.1.

     Accept-Language = #( language-range [ weight ] )
     language-range  =
               <language-range, see [RFC4647], Section 2.1>

   Each language-range can be given an associated quality value
   representing an estimate of the user's preference for the languages
   specified by that range, as defined in Section 12.4.2.  For example,

   Accept-Language: da, en-gb;q=0.8, en;q=0.7

   would mean: "I prefer Danish, but will accept British English and
   other types of English".

   Note that some recipients treat the order in which language tags are
   listed as an indication of descending priority, particularly for tags
   that are assigned equal quality values (no value is the same as q=1).
   However, this behavior cannot be relied upon.  For consistency and to
   maximize interoperability, many user agents assign each language tag
   a unique quality value while also listing them in order of decreasing
   quality.  Additional discussion of language priority lists can be
   found in Section 2.3 of [RFC4647].

   For matching, Section 3 of [RFC4647] defines several matching
   schemes.  Implementations can offer the most appropriate matching
   scheme for their requirements.  The "Basic Filtering" scheme
   ([RFC4647], Section 3.3.1) is identical to the matching scheme that
   was previously defined for HTTP in Section 14.4 of [RFC2616].

   It might be contrary to the privacy expectations of the user to send
   an Accept-Language header field with the complete linguistic
   preferences of the user in every request (Section 17.13).

   Since intelligibility is highly dependent on the individual user,
   user agents need to allow user control over the linguistic preference
   (either through configuration of the user agent itself or by
   defaulting to a user controllable system setting).  A user agent that
   does not provide such control to the user MUST NOT send an Accept-
   Language header field.

      |  *Note:* User agents ought to provide guidance to users when
      |  setting a preference, since users are rarely familiar with the
      |  details of language matching as described above.  For example,
      |  users might assume that on selecting "en-gb", they will be
      |  served any kind of English document if British English is not
      |  available.  A user agent might suggest, in such a case, to add
      |  "en" to the list for better matching behavior.

12.5.5.  Vary

   The "Vary" header field in a response describes what parts of a
   request message, aside from the method and target URI, might have
   influenced the origin server's process for selecting the content of
   this response.

     Vary = #( "*" / field-name )

   A Vary field value is either the wildcard member "*" or a list of
   request field names, known as the selecting header fields, that might
   have had a role in selecting the representation for this response.
   Potential selecting header fields are not limited to fields defined
   by this specification.

   A list containing the member "*" signals that other aspects of the
   request might have played a role in selecting the response
   representation, possibly including aspects outside the message syntax
   (e.g., the client's network address).  A recipient will not be able
   to determine whether this response is appropriate for a later request
   without forwarding the request to the origin server.  A proxy MUST
   NOT generate "*" in a Vary field value.

   For example, a response that contains

   Vary: accept-encoding, accept-language

   indicates that the origin server might have used the request's
   Accept-Encoding and Accept-Language header fields (or lack thereof)
   as determining factors while choosing the content for this response.

   A Vary field containing a list of field names has two purposes:

   1.  To inform cache recipients that they MUST NOT use this response
       to satisfy a later request unless the later request has the same
       values for the listed header fields as the original request
       (Section 4.1 of [CACHING]) or reuse of the response has been
       validated by the origin server.  In other words, Vary expands the
       cache key required to match a new request to the stored cache
       entry.

   2.  To inform user agent recipients that this response was subject to
       content negotiation (Section 12) and a different representation
       might be sent in a subsequent request if other values are
       provided in the listed header fields (proactive negotiation).

   An origin server SHOULD generate a Vary header field on a cacheable
   response when it wishes that response to be selectively reused for
   subsequent requests.  Generally, that is the case when the response
   content has been tailored to better fit the preferences expressed by
   those selecting header fields, such as when an origin server has
   selected the response's language based on the request's
   Accept-Language header field.

   Vary might be elided when an origin server considers variance in
   content selection to be less significant than Vary's performance
   impact on caching, particularly when reuse is already limited by
   cache response directives (Section 5.2 of [CACHING]).

   There is no need to send the Authorization field name in Vary because
   reuse of that response for a different user is prohibited by the
   field definition (Section 11.6.2).  Likewise, if the response content
   has been selected or influenced by network region, but the origin
   server wants the cached response to be reused even if recipients move
   from one region to another, then there is no need for the origin
   server to indicate such variance in Vary.

13.  Conditional Requests

   A conditional request is an HTTP request with one or more request
   header fields that indicate a precondition to be tested before
   applying the request method to the target resource.  Section 13.2
   defines when to evaluate preconditions and their order of precedence
   when more than one precondition is present.

   Conditional GET requests are the most efficient mechanism for HTTP
   cache updates [CACHING].  Conditionals can also be applied to state-
   changing methods, such as PUT and DELETE, to prevent the "lost
   update" problem: one client accidentally overwriting the work of
   another client that has been acting in parallel.

13.1.  Preconditions

   Preconditions are usually defined with respect to a state of the
   target resource as a whole (its current value set) or the state as
   observed in a previously obtained representation (one value in that
   set).  If a resource has multiple current representations, each with
   its own observable state, a precondition will assume that the mapping
   of each request to a selected representation (Section 3.2) is
   consistent over time.  Regardless, if the mapping is inconsistent or
   the server is unable to select an appropriate representation, then no
   harm will result when the precondition evaluates to false.

   Each precondition defined below consists of a comparison between a
   set of validators obtained from prior representations of the target
   resource to the current state of validators for the selected
   representation (Section 8.8).  Hence, these preconditions evaluate
   whether the state of the target resource has changed since a given
   state known by the client.  The effect of such an evaluation depends
   on the method semantics and choice of conditional, as defined in
   Section 13.2.

   Other preconditions, defined by other specifications as extension
   fields, might place conditions on all recipients, on the state of the
   target resource in general, or on a group of resources.  For
   instance, the "If" header field in WebDAV can make a request
   conditional on various aspects of multiple resources, such as locks,
   if the recipient understands and implements that field ([WEBDAV],
   Section 10.4).

   Extensibility of preconditions is only possible when the precondition
   can be safely ignored if unknown (like If-Modified-Since), when
   deployment can be assumed for a given use case, or when
   implementation is signaled by some other property of the target
   resource.  This encourages a focus on mutually agreed deployment of
   common standards.

13.1.1.  If-Match

   The "If-Match" header field makes the request method conditional on
   the recipient origin server either having at least one current
   representation of the target resource, when the field value is "*",
   or having a current representation of the target resource that has an
   entity tag matching a member of the list of entity tags provided in
   the field value.

   An origin server MUST use the strong comparison function when
   comparing entity tags for If-Match (Section 8.8.3.2), since the
   client intends this precondition to prevent the method from being
   applied if there have been any changes to the representation data.

     If-Match = "*" / #entity-tag

   Examples:

   If-Match: "xyzzy"
   If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
   If-Match: *

   If-Match is most often used with state-changing methods (e.g., POST,
   PUT, DELETE) to prevent accidental overwrites when multiple user
   agents might be acting in parallel on the same resource (i.e., to
   prevent the "lost update" problem).  In general, it can be used with
   any method that involves the selection or modification of a
   representation to abort the request if the selected representation's
   current entity tag is not a member within the If-Match field value.

   When an origin server receives a request that selects a
   representation and that request includes an If-Match header field,
   the origin server MUST evaluate the If-Match condition per
   Section 13.2 prior to performing the method.

   To evaluate a received If-Match header field:

   1.  If the field value is "*", the condition is true if the origin
       server has a current representation for the target resource.

   2.  If the field value is a list of entity tags, the condition is
       true if any of the listed tags match the entity tag of the
       selected representation.

   3.  Otherwise, the condition is false.

   An origin server that evaluates an If-Match condition MUST NOT
   perform the requested method if the condition evaluates to false.
   Instead, the origin server MAY indicate that the conditional request
   failed by responding with a 412 (Precondition Failed) status code.
   Alternatively, if the request is a state-changing operation that
   appears to have already been applied to the selected representation,
   the origin server MAY respond with a 2xx (Successful) status code
   (i.e., the change requested by the user agent has already succeeded,
   but the user agent might not be aware of it, perhaps because the
   prior response was lost or an equivalent change was made by some
   other user agent).

   Allowing an origin server to send a success response when a change
   request appears to have already been applied is more efficient for
   many authoring use cases, but comes with some risk if multiple user
   agents are making change requests that are very similar but not
   cooperative.  For example, multiple user agents writing to a common
   resource as a semaphore (e.g., a nonatomic increment) are likely to
   collide and potentially lose important state transitions.  For those
   kinds of resources, an origin server is better off being stringent in
   sending 412 for every failed precondition on an unsafe method.  In
   other cases, excluding the ETag field from a success response might
   encourage the user agent to perform a GET as its next request to
   eliminate confusion about the resource's current state.

   A client MAY send an If-Match header field in a GET request to
   indicate that it would prefer a 412 (Precondition Failed) response if
   the selected representation does not match.  However, this is only
   useful in range requests (Section 14) for completing a previously
   received partial representation when there is no desire for a new
   representation.  If-Range (Section 13.1.5) is better suited for range
   requests when the client prefers to receive a new representation.

   A cache or intermediary MAY ignore If-Match because its
   interoperability features are only necessary for an origin server.

   Note that an If-Match header field with a list value containing "*"
   and other values (including other instances of "*") is syntactically
   invalid (therefore not allowed to be generated) and furthermore is
   unlikely to be interoperable.

13.1.2.  If-None-Match

   The "If-None-Match" header field makes the request method conditional
   on a recipient cache or origin server either not having any current
   representation of the target resource, when the field value is "*",
   or having a selected representation with an entity tag that does not
   match any of those listed in the field value.

   A recipient MUST use the weak comparison function when comparing
   entity tags for If-None-Match (Section 8.8.3.2), since weak entity
   tags can be used for cache validation even if there have been changes
   to the representation data.

     If-None-Match = "*" / #entity-tag

   Examples:

   If-None-Match: "xyzzy"
   If-None-Match: W/"xyzzy"
   If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
   If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
   If-None-Match: *

   If-None-Match is primarily used in conditional GET requests to enable
   efficient updates of cached information with a minimum amount of
   transaction overhead.  When a client desires to update one or more
   stored responses that have entity tags, the client SHOULD generate an
   If-None-Match header field containing a list of those entity tags
   when making a GET request; this allows recipient servers to send a
   304 (Not Modified) response to indicate when one of those stored
   responses matches the selected representation.

   If-None-Match can also be used with a value of "*" to prevent an
   unsafe request method (e.g., PUT) from inadvertently modifying an
   existing representation of the target resource when the client
   believes that the resource does not have a current representation
   (Section 9.2.1).  This is a variation on the "lost update" problem
   that might arise if more than one client attempts to create an
   initial representation for the target resource.

   When an origin server receives a request that selects a
   representation and that request includes an If-None-Match header
   field, the origin server MUST evaluate the If-None-Match condition
   per Section 13.2 prior to performing the method.

   To evaluate a received If-None-Match header field:

   1.  If the field value is "*", the condition is false if the origin
       server has a current representation for the target resource.

   2.  If the field value is a list of entity tags, the condition is
       false if one of the listed tags matches the entity tag of the
       selected representation.

   3.  Otherwise, the condition is true.

   An origin server that evaluates an If-None-Match condition MUST NOT
   perform the requested method if the condition evaluates to false;
   instead, the origin server MUST respond with either a) the 304 (Not
   Modified) status code if the request method is GET or HEAD or b) the
   412 (Precondition Failed) status code for all other request methods.

   Requirements on cache handling of a received If-None-Match header
   field are defined in Section 4.3.2 of [CACHING].

   Note that an If-None-Match header field with a list value containing
   "*" and other values (including other instances of "*") is
   syntactically invalid (therefore not allowed to be generated) and
   furthermore is unlikely to be interoperable.

13.1.3.  If-Modified-Since

   The "If-Modified-Since" header field makes a GET or HEAD request
   method conditional on the selected representation's modification date
   being more recent than the date provided in the field value.
   Transfer of the selected representation's data is avoided if that
   data has not changed.

     If-Modified-Since = HTTP-date

   An example of the field is:

   If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

   A recipient MUST ignore If-Modified-Since if the request contains an
   If-None-Match header field; the condition in If-None-Match is
   considered to be a more accurate replacement for the condition in If-
   Modified-Since, and the two are only combined for the sake of
   interoperating with older intermediaries that might not implement
   If-None-Match.

   A recipient MUST ignore the If-Modified-Since header field if the
   received field value is not a valid HTTP-date, the field value has
   more than one member, or if the request method is neither GET nor
   HEAD.

   A recipient MUST ignore the If-Modified-Since header field if the
   resource does not have a modification date available.

   A recipient MUST interpret an If-Modified-Since field value's
   timestamp in terms of the origin server's clock.

   If-Modified-Since is typically used for two distinct purposes: 1) to
   allow efficient updates of a cached representation that does not have
   an entity tag and 2) to limit the scope of a web traversal to
   resources that have recently changed.

   When used for cache updates, a cache will typically use the value of
   the cached message's Last-Modified header field to generate the field
   value of If-Modified-Since.  This behavior is most interoperable for
   cases where clocks are poorly synchronized or when the server has
   chosen to only honor exact timestamp matches (due to a problem with
   Last-Modified dates that appear to go "back in time" when the origin
   server's clock is corrected or a representation is restored from an
   archived backup).  However, caches occasionally generate the field
   value based on other data, such as the Date header field of the
   cached message or the clock time at which the message was received,
   particularly when the cached message does not contain a Last-Modified
   header field.

   When used for limiting the scope of retrieval to a recent time
   window, a user agent will generate an If-Modified-Since field value
   based on either its own clock or a Date header field received from
   the server in a prior response.  Origin servers that choose an exact
   timestamp match based on the selected representation's Last-Modified
   header field will not be able to help the user agent limit its data
   transfers to only those changed during the specified window.

   When an origin server receives a request that selects a
   representation and that request includes an If-Modified-Since header
   field without an If-None-Match header field, the origin server SHOULD
   evaluate the If-Modified-Since condition per Section 13.2 prior to
   performing the method.

   To evaluate a received If-Modified-Since header field:

   1.  If the selected representation's last modification date is
       earlier or equal to the date provided in the field value, the
       condition is false.

   2.  Otherwise, the condition is true.

   An origin server that evaluates an If-Modified-Since condition SHOULD
   NOT perform the requested method if the condition evaluates to false;
   instead, the origin server SHOULD generate a 304 (Not Modified)
   response, including only those metadata that are useful for
   identifying or updating a previously cached response.

   Requirements on cache handling of a received If-Modified-Since header
   field are defined in Section 4.3.2 of [CACHING].

13.1.4.  If-Unmodified-Since

   The "If-Unmodified-Since" header field makes the request method
   conditional on the selected representation's last modification date
   being earlier than or equal to the date provided in the field value.
   This field accomplishes the same purpose as If-Match for cases where
   the user agent does not have an entity tag for the representation.

     If-Unmodified-Since = HTTP-date

   An example of the field is:

   If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

   A recipient MUST ignore If-Unmodified-Since if the request contains
   an If-Match header field; the condition in If-Match is considered to
   be a more accurate replacement for the condition in If-Unmodified-
   Since, and the two are only combined for the sake of interoperating
   with older intermediaries that might not implement If-Match.

   A recipient MUST ignore the If-Unmodified-Since header field if the
   received field value is not a valid HTTP-date (including when the
   field value appears to be a list of dates).

   A recipient MUST ignore the If-Unmodified-Since header field if the
   resource does not have a modification date available.

   A recipient MUST interpret an If-Unmodified-Since field value's
   timestamp in terms of the origin server's clock.

   If-Unmodified-Since is most often used with state-changing methods
   (e.g., POST, PUT, DELETE) to prevent accidental overwrites when
   multiple user agents might be acting in parallel on a resource that
   does not supply entity tags with its representations (i.e., to
   prevent the "lost update" problem).  In general, it can be used with
   any method that involves the selection or modification of a
   representation to abort the request if the selected representation's
   last modification date has changed since the date provided in the If-
   Unmodified-Since field value.

   When an origin server receives a request that selects a
   representation and that request includes an If-Unmodified-Since
   header field without an If-Match header field, the origin server MUST
   evaluate the If-Unmodified-Since condition per Section 13.2 prior to
   performing the method.

   To evaluate a received If-Unmodified-Since header field:

   1.  If the selected representation's last modification date is
       earlier than or equal to the date provided in the field value,
       the condition is true.

   2.  Otherwise, the condition is false.

   An origin server that evaluates an If-Unmodified-Since condition MUST
   NOT perform the requested method if the condition evaluates to false.
   Instead, the origin server MAY indicate that the conditional request
   failed by responding with a 412 (Precondition Failed) status code.
   Alternatively, if the request is a state-changing operation that
   appears to have already been applied to the selected representation,
   the origin server MAY respond with a 2xx (Successful) status code
   (i.e., the change requested by the user agent has already succeeded,
   but the user agent might not be aware of it, perhaps because the
   prior response was lost or an equivalent change was made by some
   other user agent).

   Allowing an origin server to send a success response when a change
   request appears to have already been applied is more efficient for
   many authoring use cases, but comes with some risk if multiple user
   agents are making change requests that are very similar but not
   cooperative.  In those cases, an origin server is better off being
   stringent in sending 412 for every failed precondition on an unsafe
   method.

   A client MAY send an If-Unmodified-Since header field in a GET
   request to indicate that it would prefer a 412 (Precondition Failed)
   response if the selected representation has been modified.  However,
   this is only useful in range requests (Section 14) for completing a
   previously received partial representation when there is no desire
   for a new representation.  If-Range (Section 13.1.5) is better suited
   for range requests when the client prefers to receive a new
   representation.

   A cache or intermediary MAY ignore If-Unmodified-Since because its
   interoperability features are only necessary for an origin server.

13.1.5.  If-Range

   The "If-Range" header field provides a special conditional request
   mechanism that is similar to the If-Match and If-Unmodified-Since
   header fields but that instructs the recipient to ignore the Range
   header field if the validator doesn't match, resulting in transfer of
   the new selected representation instead of a 412 (Precondition
   Failed) response.

   If a client has a partial copy of a representation and wishes to have
   an up-to-date copy of the entire representation, it could use the
   Range header field with a conditional GET (using either or both of
   If-Unmodified-Since and If-Match.)  However, if the precondition
   fails because the representation has been modified, the client would
   then have to make a second request to obtain the entire current
   representation.

   The "If-Range" header field allows a client to "short-circuit" the
   second request.  Informally, its meaning is as follows: if the
   representation is unchanged, send me the part(s) that I am requesting
   in Range; otherwise, send me the entire representation.

     If-Range = entity-tag / HTTP-date

   A valid entity-tag can be distinguished from a valid HTTP-date by
   examining the first three characters for a DQUOTE.

   A client MUST NOT generate an If-Range header field in a request that
   does not contain a Range header field.  A server MUST ignore an If-
   Range header field received in a request that does not contain a
   Range header field.  An origin server MUST ignore an If-Range header
   field received in a request for a target resource that does not
   support Range requests.

   A client MUST NOT generate an If-Range header field containing an
   entity tag that is marked as weak.  A client MUST NOT generate an If-
   Range header field containing an HTTP-date unless the client has no
   entity tag for the corresponding representation and the date is a
   strong validator in the sense defined by Section 8.8.2.2.

   A server that receives an If-Range header field on a Range request
   MUST evaluate the condition per Section 13.2 prior to performing the
   method.

   To evaluate a received If-Range header field containing an HTTP-date:

   1.  If the HTTP-date validator provided is not a strong validator in
       the sense defined by Section 8.8.2.2, the condition is false.

   2.  If the HTTP-date validator provided exactly matches the
       Last-Modified field value for the selected representation, the
       condition is true.

   3.  Otherwise, the condition is false.

   To evaluate a received If-Range header field containing an
   entity-tag:

   1.  If the entity-tag validator provided exactly matches the ETag
       field value for the selected representation using the strong
       comparison function (Section 8.8.3.2), the condition is true.

   2.  Otherwise, the condition is false.

   A recipient of an If-Range header field MUST ignore the Range header
   field if the If-Range condition evaluates to false.  Otherwise, the
   recipient SHOULD process the Range header field as requested.

   Note that the If-Range comparison is by exact match, including when
   the validator is an HTTP-date, and so it differs from the "earlier
   than or equal to" comparison used when evaluating an
   If-Unmodified-Since conditional.

13.2.  Evaluation of Preconditions

13.2.1.  When to Evaluate

   Except when excluded below, a recipient cache or origin server MUST
   evaluate received request preconditions after it has successfully
   performed its normal request checks and just before it would process
   the request content (if any) or perform the action associated with
   the request method.  A server MUST ignore all received preconditions
   if its response to the same request without those conditions, prior
   to processing the request content, would have been a status code
   other than a 2xx (Successful) or 412 (Precondition Failed).  In other
   words, redirects and failures that can be detected before significant
   processing occurs take precedence over the evaluation of
   preconditions.

   A server that is not the origin server for the target resource and
   cannot act as a cache for requests on the target resource MUST NOT
   evaluate the conditional request header fields defined by this
   specification, and it MUST forward them if the request is forwarded,
   since the generating client intends that they be evaluated by a
   server that can provide a current representation.  Likewise, a server
   MUST ignore the conditional request header fields defined by this
   specification when received with a request method that does not
   involve the selection or modification of a selected representation,
   such as CONNECT, OPTIONS, or TRACE.

   Note that protocol extensions can modify the conditions under which
   preconditions are evaluated or the consequences of their evaluation.
   For example, the immutable cache directive (defined by [RFC8246])
   instructs caches to forgo forwarding conditional requests when they
   hold a fresh response.

   Although conditional request header fields are defined as being
   usable with the HEAD method (to keep HEAD's semantics consistent with
   those of GET), there is no point in sending a conditional HEAD
   because a successful response is around the same size as a 304 (Not
   Modified) response and more useful than a 412 (Precondition Failed)
   response.

13.2.2.  Precedence of Preconditions

   When more than one conditional request header field is present in a
   request, the order in which the fields are evaluated becomes
   important.  In practice, the fields defined in this document are
   consistently implemented in a single, logical order, since "lost
   update" preconditions have more strict requirements than cache
   validation, a validated cache is more efficient than a partial
   response, and entity tags are presumed to be more accurate than date
   validators.

   A recipient cache or origin server MUST evaluate the request
   preconditions defined by this specification in the following order:

   1.  When recipient is the origin server and If-Match is present,
       evaluate the If-Match precondition:

       *  if true, continue to step 3

       *  if false, respond 412 (Precondition Failed) unless it can be
          determined that the state-changing request has already
          succeeded (see Section 13.1.1)

   2.  When recipient is the origin server, If-Match is not present, and
       If-Unmodified-Since is present, evaluate the If-Unmodified-Since
       precondition:

       *  if true, continue to step 3

       *  if false, respond 412 (Precondition Failed) unless it can be
          determined that the state-changing request has already
          succeeded (see Section 13.1.4)

   3.  When If-None-Match is present, evaluate the If-None-Match
       precondition:

       *  if true, continue to step 5

       *  if false for GET/HEAD, respond 304 (Not Modified)

       *  if false for other methods, respond 412 (Precondition Failed)

   4.  When the method is GET or HEAD, If-None-Match is not present, and
       If-Modified-Since is present, evaluate the If-Modified-Since
       precondition:

       *  if true, continue to step 5

       *  if false, respond 304 (Not Modified)

   5.  When the method is GET and both Range and If-Range are present,
       evaluate the If-Range precondition:

       *  if true and the Range is applicable to the selected
          representation, respond 206 (Partial Content)

       *  otherwise, ignore the Range header field and respond 200 (OK)

   6.  Otherwise,

       *  perform the requested method and respond according to its
          success or failure.

   Any extension to HTTP that defines additional conditional request
   header fields ought to define the order for evaluating such fields in
   relation to those defined in this document and other conditionals
   that might be found in practice.

14.  Range Requests

   Clients often encounter interrupted data transfers as a result of
   canceled requests or dropped connections.  When a client has stored a
   partial representation, it is desirable to request the remainder of
   that representation in a subsequent request rather than transfer the
   entire representation.  Likewise, devices with limited local storage
   might benefit from being able to request only a subset of a larger
   representation, such as a single page of a very large document, or
   the dimensions of an embedded image.

   Range requests are an OPTIONAL feature of HTTP, designed so that
   recipients not implementing this feature (or not supporting it for
   the target resource) can respond as if it is a normal GET request
   without impacting interoperability.  Partial responses are indicated
   by a distinct status code to not be mistaken for full responses by
   caches that might not implement the feature.

14.1.  Range Units

   Representation data can be partitioned into subranges when there are
   addressable structural units inherent to that data's content coding
   or media type.  For example, octet (a.k.a. byte) boundaries are a
   structural unit common to all representation data, allowing
   partitions of the data to be identified as a range of bytes at some
   offset from the start or end of that data.

   This general notion of a "range unit" is used in the Accept-Ranges
   (Section 14.3) response header field to advertise support for range
   requests, the Range (Section 14.2) request header field to delineate
   the parts of a representation that are requested, and the
   Content-Range (Section 14.4) header field to describe which part of a
   representation is being transferred.

     range-unit       = token

   All range unit names are case-insensitive and ought to be registered
   within the "HTTP Range Unit Registry", as defined in Section 16.5.1.

   Range units are intended to be extensible, as described in
   Section 16.5.

14.1.1.  Range Specifiers

   Ranges are expressed in terms of a range unit paired with a set of
   range specifiers.  The range unit name determines what kinds of
   range-spec are applicable to its own specifiers.  Hence, the
   following grammar is generic: each range unit is expected to specify
   requirements on when int-range, suffix-range, and other-range are
   allowed.

   A range request can specify a single range or a set of ranges within
   a single representation.

     ranges-specifier = range-unit "=" range-set
     range-set        = 1#range-spec
     range-spec       = int-range
                      / suffix-range
                      / other-range

   An int-range is a range expressed as two non-negative integers or as
   one non-negative integer through to the end of the representation
   data.  The range unit specifies what the integers mean (e.g., they
   might indicate unit offsets from the beginning, inclusive numbered
   parts, etc.).

     int-range     = first-pos "-" [ last-pos ]
     first-pos     = 1*DIGIT
     last-pos      = 1*DIGIT

   An int-range is invalid if the last-pos value is present and less
   than the first-pos.

   A suffix-range is a range expressed as a suffix of the representation
   data with the provided non-negative integer maximum length (in range
   units).  In other words, the last N units of the representation data.

     suffix-range  = "-" suffix-length
     suffix-length = 1*DIGIT

   To provide for extensibility, the other-range rule is a mostly
   unconstrained grammar that allows application-specific or future
   range units to define additional range specifiers.

     other-range   = 1*( %x21-2B / %x2D-7E )
                   ; 1*(VCHAR excluding comma)

   A ranges-specifier is invalid if it contains any range-spec that is
   invalid or undefined for the indicated range-unit.

   A valid ranges-specifier is "satisfiable" if it contains at least one
   range-spec that is satisfiable, as defined by the indicated
   range-unit.  Otherwise, the ranges-specifier is "unsatisfiable".

14.1.2.  Byte Ranges

   The "bytes" range unit is used to express subranges of a
   representation data's octet sequence.  Each byte range is expressed
   as an integer range at some offset, relative to either the beginning
   (int-range) or end (suffix-range) of the representation data.  Byte
   ranges do not use the other-range specifier.

   The first-pos value in a bytes int-range gives the offset of the
   first byte in a range.  The last-pos value gives the offset of the
   last byte in the range; that is, the byte positions specified are
   inclusive.  Byte offsets start at zero.

   If the representation data has a content coding applied, each byte
   range is calculated with respect to the encoded sequence of bytes,
   not the sequence of underlying bytes that would be obtained after
   decoding.

   Examples of bytes range specifiers:

   *  The first 500 bytes (byte offsets 0-499, inclusive):

           bytes=0-499

   *  The second 500 bytes (byte offsets 500-999, inclusive):

           bytes=500-999

   A client can limit the number of bytes requested without knowing the
   size of the selected representation.  If the last-pos value is
   absent, or if the value is greater than or equal to the current
   length of the representation data, the byte range is interpreted as
   the remainder of the representation (i.e., the server replaces the
   value of last-pos with a value that is one less than the current
   length of the selected representation).

   A client can refer to the last N bytes (N > 0) of the selected
   representation using a suffix-range.  If the selected representation
   is shorter than the specified suffix-length, the entire
   representation is used.

   Additional examples, assuming a representation of length 10000:

   *  The final 500 bytes (byte offsets 9500-9999, inclusive):

           bytes=-500

      Or:

           bytes=9500-

   *  The first and last bytes only (bytes 0 and 9999):

           bytes=0-0,-1

   *  The first, middle, and last 1000 bytes:

           bytes= 0-999, 4500-5499, -1000

   *  Other valid (but not canonical) specifications of the second 500
      bytes (byte offsets 500-999, inclusive):

           bytes=500-600,601-999
           bytes=500-700,601-999

   For a GET request, a valid bytes range-spec is satisfiable if it is
   either:

   *  an int-range with a first-pos that is less than the current length
      of the selected representation or

   *  a suffix-range with a non-zero suffix-length.

   When a selected representation has zero length, the only satisfiable
   form of range-spec in a GET request is a suffix-range with a non-zero
   suffix-length.

   In the byte-range syntax, first-pos, last-pos, and suffix-length are
   expressed as decimal number of octets.  Since there is no predefined
   limit to the length of content, recipients MUST anticipate
   potentially large decimal numerals and prevent parsing errors due to
   integer conversion overflows.

14.2.  Range

   The "Range" header field on a GET request modifies the method
   semantics to request transfer of only one or more subranges of the
   selected representation data (Section 8.1), rather than the entire
   selected representation.

     Range = ranges-specifier

   A server MAY ignore the Range header field.  However, origin servers
   and intermediate caches ought to support byte ranges when possible,
   since they support efficient recovery from partially failed transfers
   and partial retrieval of large representations.

   A server MUST ignore a Range header field received with a request
   method that is unrecognized or for which range handling is not
   defined.  For this specification, GET is the only method for which
   range handling is defined.

   An origin server MUST ignore a Range header field that contains a
   range unit it does not understand.  A proxy MAY discard a Range
   header field that contains a range unit it does not understand.

   A server that supports range requests MAY ignore or reject a Range
   header field that contains an invalid ranges-specifier
   (Section 14.1.1), a ranges-specifier with more than two overlapping
   ranges, or a set of many small ranges that are not listed in
   ascending order, since these are indications of either a broken
   client or a deliberate denial-of-service attack (Section 17.15).  A
   client SHOULD NOT request multiple ranges that are inherently less
   efficient to process and transfer than a single range that
   encompasses the same data.

   A server that supports range requests MAY ignore a Range header field
   when the selected representation has no content (i.e., the selected
   representation's data is of zero length).

   A client that is requesting multiple ranges SHOULD list those ranges
   in ascending order (the order in which they would typically be
   received in a complete representation) unless there is a specific
   need to request a later part earlier.  For example, a user agent
   processing a large representation with an internal catalog of parts
   might need to request later parts first, particularly if the
   representation consists of pages stored in reverse order and the user
   agent wishes to transfer one page at a time.

   The Range header field is evaluated after evaluating the precondition
   header fields defined in Section 13.1, and only if the result in
   absence of the Range header field would be a 200 (OK) response.  In
   other words, Range is ignored when a conditional GET would result in
   a 304 (Not Modified) response.

   The If-Range header field (Section 13.1.5) can be used as a
   precondition to applying the Range header field.

   If all of the preconditions are true, the server supports the Range
   header field for the target resource, the received Range field-value
   contains a valid ranges-specifier with a range-unit supported for
   that target resource, and that ranges-specifier is satisfiable with
   respect to the selected representation, the server SHOULD send a 206
   (Partial Content) response with content containing one or more
   partial representations that correspond to the satisfiable
   range-spec(s) requested.

   The above does not imply that a server will send all requested
   ranges.  In some cases, it may only be possible (or efficient) to
   send a portion of the requested ranges first, while expecting the
   client to re-request the remaining portions later if they are still
   desired (see Section 15.3.7).

   If all of the preconditions are true, the server supports the Range
   header field for the target resource, the received Range field-value
   contains a valid ranges-specifier, and either the range-unit is not
   supported for that target resource or the ranges-specifier is
   unsatisfiable with respect to the selected representation, the server
   SHOULD send a 416 (Range Not Satisfiable) response.

14.3.  Accept-Ranges

   The "Accept-Ranges" field in a response indicates whether an upstream
   server supports range requests for the target resource.

     Accept-Ranges     = acceptable-ranges
     acceptable-ranges = 1#range-unit

   For example, a server that supports byte-range requests
   (Section 14.1.2) can send the field

   Accept-Ranges: bytes

   to indicate that it supports byte range requests for that target
   resource, thereby encouraging its use by the client for future
   partial requests on the same request path.  Range units are defined
   in Section 14.1.

   A client MAY generate range requests regardless of having received an
   Accept-Ranges field.  The information only provides advice for the
   sake of improving performance and reducing unnecessary network
   transfers.

   Conversely, a client MUST NOT assume that receiving an Accept-Ranges
   field means that future range requests will return partial responses.
   The content might change, the server might only support range
   requests at certain times or under certain conditions, or a different
   intermediary might process the next request.

   A server that does not support any kind of range request for the
   target resource MAY send

   Accept-Ranges: none

   to advise the client not to attempt a range request on the same
   request path.  The range unit "none" is reserved for this purpose.

   The Accept-Ranges field MAY be sent in a trailer section, but is
   preferred to be sent as a header field because the information is
   particularly useful for restarting large information transfers that
   have failed in mid-content (before the trailer section is received).

14.4.  Content-Range

   The "Content-Range" header field is sent in a single part 206
   (Partial Content) response to indicate the partial range of the
   selected representation enclosed as the message content, sent in each
   part of a multipart 206 response to indicate the range enclosed
   within each body part (Section 14.6), and sent in 416 (Range Not
   Satisfiable) responses to provide information about the selected
   representation.

     Content-Range       = range-unit SP
                           ( range-resp / unsatisfied-range )

     range-resp          = incl-range "/" ( complete-length / "*" )
     incl-range          = first-pos "-" last-pos
     unsatisfied-range   = "*/" complete-length

     complete-length     = 1*DIGIT

   If a 206 (Partial Content) response contains a Content-Range header
   field with a range unit (Section 14.1) that the recipient does not
   understand, the recipient MUST NOT attempt to recombine it with a
   stored representation.  A proxy that receives such a message SHOULD
   forward it downstream.

   Content-Range might also be sent as a request modifier to request a
   partial PUT, as described in Section 14.5, based on private
   agreements between client and origin server.  A server MUST ignore a
   Content-Range header field received in a request with a method for
   which Content-Range support is not defined.

   For byte ranges, a sender SHOULD indicate the complete length of the
   representation from which the range has been extracted, unless the
   complete length is unknown or difficult to determine.  An asterisk
   character ("*") in place of the complete-length indicates that the
   representation length was unknown when the header field was
   generated.

   The following example illustrates when the complete length of the
   selected representation is known by the sender to be 1234 bytes:

   Content-Range: bytes 42-1233/1234

   and this second example illustrates when the complete length is
   unknown:

   Content-Range: bytes 42-1233/*

   A Content-Range field value is invalid if it contains a range-resp
   that has a last-pos value less than its first-pos value, or a
   complete-length value less than or equal to its last-pos value.  The
   recipient of an invalid Content-Range MUST NOT attempt to recombine
   the received content with a stored representation.

   A server generating a 416 (Range Not Satisfiable) response to a byte-
   range request SHOULD send a Content-Range header field with an
   unsatisfied-range value, as in the following example:

   Content-Range: bytes */1234

   The complete-length in a 416 response indicates the current length of
   the selected representation.

   The Content-Range header field has no meaning for status codes that
   do not explicitly describe its semantic.  For this specification,
   only the 206 (Partial Content) and 416 (Range Not Satisfiable) status
   codes describe a meaning for Content-Range.

   The following are examples of Content-Range values in which the
   selected representation contains a total of 1234 bytes:

   *  The first 500 bytes:

      Content-Range: bytes 0-499/1234

   *  The second 500 bytes:

      Content-Range: bytes 500-999/1234

   *  All except for the first 500 bytes:

      Content-Range: bytes 500-1233/1234

   *  The last 500 bytes:

      Content-Range: bytes 734-1233/1234

14.5.  Partial PUT

   Some origin servers support PUT of a partial representation when the
   user agent sends a Content-Range header field (Section 14.4) in the
   request, though such support is inconsistent and depends on private
   agreements with user agents.  In general, it requests that the state
   of the target resource be partly replaced with the enclosed content
   at an offset and length indicated by the Content-Range value, where
   the offset is relative to the current selected representation.

   An origin server SHOULD respond with a 400 (Bad Request) status code
   if it receives Content-Range on a PUT for a target resource that does
   not support partial PUT requests.

   Partial PUT is not backwards compatible with the original definition
   of PUT.  It may result in the content being written as a complete
   replacement for the current representation.

   Partial resource updates are also possible by targeting a separately
   identified resource with state that overlaps or extends a portion of
   the larger resource, or by using a different method that has been
   specifically defined for partial updates (for example, the PATCH
   method defined in [RFC5789]).

14.6.  Media Type multipart/byteranges

   When a 206 (Partial Content) response message includes the content of
   multiple ranges, they are transmitted as body parts in a multipart
   message body ([RFC2046], Section 5.1) with the media type of
   "multipart/byteranges".

   The "multipart/byteranges" media type includes one or more body
   parts, each with its own Content-Type and Content-Range fields.  The
   required boundary parameter specifies the boundary string used to
   separate each body part.

   Implementation Notes:

   1.  Additional CRLFs might precede the first boundary string in the
       body.

   2.  Although [RFC2046] permits the boundary string to be quoted, some
       existing implementations handle a quoted boundary string
       incorrectly.

   3.  A number of clients and servers were coded to an early draft of
       the byteranges specification that used a media type of
       "multipart/x-byteranges", which is almost (but not quite)
       compatible with this type.

   Despite the name, the "multipart/byteranges" media type is not
   limited to byte ranges.  The following example uses an "exampleunit"
   range unit:

   HTTP/1.1 206 Partial Content
   Date: Tue, 14 Nov 1995 06:25:24 GMT
   Last-Modified: Tue, 14 July 04:58:08 GMT
   Content-Length: 2331785
   Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

   --THIS_STRING_SEPARATES
   Content-Type: video/example
   Content-Range: exampleunit 1.2-4.3/25

   ...the first range...
   --THIS_STRING_SEPARATES
   Content-Type: video/example
   Content-Range: exampleunit 11.2-14.3/25

   ...the second range
   --THIS_STRING_SEPARATES--

   The following information serves as the registration form for the
   "multipart/byteranges" media type.

   Type name:  multipart

   Subtype name:  byteranges

   Required parameters:  boundary

   Optional parameters:  N/A

   Encoding considerations:  only "7bit", "8bit", or "binary" are
      permitted

   Security considerations:  see Section 17

   Interoperability considerations:  N/A

   Published specification:  RFC 9110 (see Section 14.6)

   Applications that use this media type:  HTTP components supporting
      multiple ranges in a single request

   Fragment identifier considerations:  N/A

   Additional information:  Deprecated alias names for this type:  N/A

                            Magic number(s):  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  See Aut
      hors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See Authors' Addresses section.

   Change controller:  IESG

15.  Status Codes

   The status code of a response is a three-digit integer code that
   describes the result of the request and the semantics of the
   response, including whether the request was successful and what
   content is enclosed (if any).  All valid status codes are within the
   range of 100 to 599, inclusive.

   The first digit of the status code defines the class of response.
   The last two digits do not have any categorization role.  There are
   five values for the first digit:

   *  1xx (Informational): The request was received, continuing process

   *  2xx (Successful): The request was successfully received,
      understood, and accepted

   *  3xx (Redirection): Further action needs to be taken in order to
      complete the request

   *  4xx (Client Error): The request contains bad syntax or cannot be
      fulfilled

   *  5xx (Server Error): The server failed to fulfill an apparently
      valid request

   HTTP status codes are extensible.  A client is not required to
   understand the meaning of all registered status codes, though such
   understanding is obviously desirable.  However, a client MUST
   understand the class of any status code, as indicated by the first
   digit, and treat an unrecognized status code as being equivalent to
   the x00 status code of that class.

   For example, if a client receives an unrecognized status code of 471,
   it can see from the first digit that there was something wrong with
   its request and treat the response as if it had received a 400 (Bad
   Request) status code.  The response message will usually contain a
   representation that explains the status.

   Values outside the range 100..599 are invalid.  Implementations often
   use three-digit integer values outside of that range (i.e., 600..999)
   for internal communication of non-HTTP status (e.g., library errors).
   A client that receives a response with an invalid status code SHOULD
   process the response as if it had a 5xx (Server Error) status code.

   A single request can have multiple associated responses: zero or more
   "interim" (non-final) responses with status codes in the
   "informational" (1xx) range, followed by exactly one "final" response
   with a status code in one of the other ranges.

15.1.  Overview of Status Codes

   The status codes listed below are defined in this specification.  The
   reason phrases listed here are only recommendations -- they can be
   replaced by local equivalents or left out altogether without
   affecting the protocol.

   Responses with status codes that are defined as heuristically
   cacheable (e.g., 200, 203, 204, 206, 300, 301, 308, 404, 405, 410,
   414, and 501 in this specification) can be reused by a cache with
   heuristic expiration unless otherwise indicated by the method
   definition or explicit cache controls [CACHING]; all other status
   codes are not heuristically cacheable.

   Additional status codes, outside the scope of this specification,
   have been specified for use in HTTP.  All such status codes ought to
   be registered within the "Hypertext Transfer Protocol (HTTP) Status
   Code Registry", as described in Section 16.2.

15.2.  Informational 1xx

   The 1xx (Informational) class of status code indicates an interim
   response for communicating connection status or request progress
   prior to completing the requested action and sending a final
   response.  Since HTTP/1.0 did not define any 1xx status codes, a
   server MUST NOT send a 1xx response to an HTTP/1.0 client.

   A 1xx response is terminated by the end of the header section; it
   cannot contain content or trailers.

   A client MUST be able to parse one or more 1xx responses received
   prior to a final response, even if the client does not expect one.  A
   user agent MAY ignore unexpected 1xx responses.

   A proxy MUST forward 1xx responses unless the proxy itself requested
   the generation of the 1xx response.  For example, if a proxy adds an
   "Expect: 100-continue" header field when it forwards a request, then
   it need not forward the corresponding 100 (Continue) response(s).

15.2.1.  100 Continue

   The 100 (Continue) status code indicates that the initial part of a
   request has been received and has not yet been rejected by the
   server.  The server intends to send a final response after the
   request has been fully received and acted upon.

   When the request contains an Expect header field that includes a
   100-continue expectation, the 100 response indicates that the server
   wishes to receive the request content, as described in
   Section 10.1.1.  The client ought to continue sending the request and
   discard the 100 response.

   If the request did not contain an Expect header field containing the
   100-continue expectation, the client can simply discard this interim
   response.

15.2.2.  101 Switching Protocols

   The 101 (Switching Protocols) status code indicates that the server
   understands and is willing to comply with the client's request, via
   the Upgrade header field (Section 7.8), for a change in the
   application protocol being used on this connection.  The server MUST
   generate an Upgrade header field in the response that indicates which
   protocol(s) will be in effect after this response.

   It is assumed that the server will only agree to switch protocols
   when it is advantageous to do so.  For example, switching to a newer
   version of HTTP might be advantageous over older versions, and
   switching to a real-time, synchronous protocol might be advantageous
   when delivering resources that use such features.

15.3.  Successful 2xx

   The 2xx (Successful) class of status code indicates that the client's
   request was successfully received, understood, and accepted.

15.3.1.  200 OK

   The 200 (OK) status code indicates that the request has succeeded.
   The content sent in a 200 response depends on the request method.
   For the methods defined by this specification, the intended meaning
   of the content can be summarized as:

   +================+============================================+
   | Request Method | Response content is a representation of:   |
   +================+============================================+
   | GET            | the target resource                        |
   +----------------+--------------------------------------------+
   | HEAD           | the target resource, like GET, but without |
   |                | transferring the representation data       |
   +----------------+--------------------------------------------+
   | POST           | the status of, or results obtained from,   |
   |                | the action                                 |
   +----------------+--------------------------------------------+
   | PUT, DELETE    | the status of the action                   |
   +----------------+--------------------------------------------+
   | OPTIONS        | communication options for the target       |
   |                | resource                                   |
   +----------------+--------------------------------------------+
   | TRACE          | the request message as received by the     |
   |                | server returning the trace                 |
   +----------------+--------------------------------------------+

                               Table 6

   Aside from responses to CONNECT, a 200 response is expected to
   contain message content unless the message framing explicitly
   indicates that the content has zero length.  If some aspect of the
   request indicates a preference for no content upon success, the
   origin server ought to send a 204 (No Content) response instead.  For
   CONNECT, there is no content because the successful result is a
   tunnel, which begins immediately after the 200 response header
   section.

   A 200 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

   In 200 responses to GET or HEAD, an origin server SHOULD send any
   available validator fields (Section 8.8) for the selected
   representation, with both a strong entity tag and a Last-Modified
   date being preferred.

   In 200 responses to state-changing methods, any validator fields
   (Section 8.8) sent in the response convey the current validators for
   the new representation formed as a result of successfully applying
   the request semantics.  Note that the PUT method (Section 9.3.4) has
   additional requirements that might preclude sending such validators.

15.3.2.  201 Created

   The 201 (Created) status code indicates that the request has been
   fulfilled and has resulted in one or more new resources being
   created.  The primary resource created by the request is identified
   by either a Location header field in the response or, if no Location
   header field is received, by the target URI.

   The 201 response content typically describes and links to the
   resource(s) created.  Any validator fields (Section 8.8) sent in the
   response convey the current validators for a new representation
   created by the request.  Note that the PUT method (Section 9.3.4) has
   additional requirements that might preclude sending such validators.

15.3.3.  202 Accepted

   The 202 (Accepted) status code indicates that the request has been
   accepted for processing, but the processing has not been completed.
   The request might or might not eventually be acted upon, as it might
   be disallowed when processing actually takes place.  There is no
   facility in HTTP for re-sending a status code from an asynchronous
   operation.

   The 202 response is intentionally noncommittal.  Its purpose is to
   allow a server to accept a request for some other process (perhaps a
   batch-oriented process that is only run once per day) without
   requiring that the user agent's connection to the server persist
   until the process is completed.  The representation sent with this
   response ought to describe the request's current status and point to
   (or embed) a status monitor that can provide the user with an
   estimate of when the request will be fulfilled.

15.3.4.  203 Non-Authoritative Information

   The 203 (Non-Authoritative Information) status code indicates that
   the request was successful but the enclosed content has been modified
   from that of the origin server's 200 (OK) response by a transforming
   proxy (Section 7.7).  This status code allows the proxy to notify
   recipients when a transformation has been applied, since that
   knowledge might impact later decisions regarding the content.  For
   example, future cache validation requests for the content might only
   be applicable along the same request path (through the same proxies).

   A 203 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

15.3.5.  204 No Content

   The 204 (No Content) status code indicates that the server has
   successfully fulfilled the request and that there is no additional
   content to send in the response content.  Metadata in the response
   header fields refer to the target resource and its selected
   representation after the requested action was applied.

   For example, if a 204 status code is received in response to a PUT
   request and the response contains an ETag field, then the PUT was
   successful and the ETag field value contains the entity tag for the
   new representation of that target resource.

   The 204 response allows a server to indicate that the action has been
   successfully applied to the target resource, while implying that the
   user agent does not need to traverse away from its current "document
   view" (if any).  The server assumes that the user agent will provide
   some indication of the success to its user, in accord with its own
   interface, and apply any new or updated metadata in the response to
   its active representation.

   For example, a 204 status code is commonly used with document editing
   interfaces corresponding to a "save" action, such that the document
   being saved remains available to the user for editing.  It is also
   frequently used with interfaces that expect automated data transfers
   to be prevalent, such as within distributed version control systems.

   A 204 response is terminated by the end of the header section; it
   cannot contain content or trailers.

   A 204 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

15.3.6.  205 Reset Content

   The 205 (Reset Content) status code indicates that the server has
   fulfilled the request and desires that the user agent reset the
   "document view", which caused the request to be sent, to its original
   state as received from the origin server.

   This response is intended to support a common data entry use case
   where the user receives content that supports data entry (a form,
   notepad, canvas, etc.), enters or manipulates data in that space,
   causes the entered data to be submitted in a request, and then the
   data entry mechanism is reset for the next entry so that the user can
   easily initiate another input action.

   Since the 205 status code implies that no additional content will be
   provided, a server MUST NOT generate content in a 205 response.

15.3.7.  206 Partial Content

   The 206 (Partial Content) status code indicates that the server is
   successfully fulfilling a range request for the target resource by
   transferring one or more parts of the selected representation.

   A server that supports range requests (Section 14) will usually
   attempt to satisfy all of the requested ranges, since sending less
   data will likely result in another client request for the remainder.
   However, a server might want to send only a subset of the data
   requested for reasons of its own, such as temporary unavailability,
   cache efficiency, load balancing, etc.  Since a 206 response is self-
   descriptive, the client can still understand a response that only
   partially satisfies its range request.

   A client MUST inspect a 206 response's Content-Type and Content-Range
   field(s) to determine what parts are enclosed and whether additional
   requests are needed.

   A server that generates a 206 response MUST generate the following
   header fields, in addition to those required in the subsections
   below, if the field would have been sent in a 200 (OK) response to
   the same request: Date, Cache-Control, ETag, Expires,
   Content-Location, and Vary.

   A Content-Length header field present in a 206 response indicates the
   number of octets in the content of this message, which is usually not
   the complete length of the selected representation.  Each
   Content-Range header field includes information about the selected
   representation's complete length.

   A sender that generates a 206 response to a request with an If-Range
   header field SHOULD NOT generate other representation header fields
   beyond those required because the client already has a prior response
   containing those header fields.  Otherwise, a sender MUST generate
   all of the representation header fields that would have been sent in
   a 200 (OK) response to the same request.

   A 206 response is heuristically cacheable; i.e., unless otherwise
   indicated by explicit cache controls (see Section 4.2.2 of
   [CACHING]).

15.3.7.1.  Single Part

   If a single part is being transferred, the server generating the 206
   response MUST generate a Content-Range header field, describing what
   range of the selected representation is enclosed, and a content
   consisting of the range.  For example:

   HTTP/1.1 206 Partial Content
   Date: Wed, 15 Nov 1995 06:25:24 GMT
   Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
   Content-Range: bytes 21010-47021/47022
   Content-Length: 26012
   Content-Type: image/gif

   ... 26012 bytes of partial image data ...

15.3.7.2.  Multiple Parts

   If multiple parts are being transferred, the server generating the
   206 response MUST generate "multipart/byteranges" content, as defined
   in Section 14.6, and a Content-Type header field containing the
   "multipart/byteranges" media type and its required boundary
   parameter.  To avoid confusion with single-part responses, a server
   MUST NOT generate a Content-Range header field in the HTTP header
   section of a multiple part response (this field will be sent in each
   part instead).

   Within the header area of each body part in the multipart content,
   the server MUST generate a Content-Range header field corresponding
   to the range being enclosed in that body part.  If the selected
   representation would have had a Content-Type header field in a 200
   (OK) response, the server SHOULD generate that same Content-Type
   header field in the header area of each body part.  For example:

   HTTP/1.1 206 Partial Content
   Date: Wed, 15 Nov 1995 06:25:24 GMT
   Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
   Content-Length: 1741
   Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

   --THIS_STRING_SEPARATES
   Content-Type: application/pdf
   Content-Range: bytes 500-999/8000

   ...the first range...
   --THIS_STRING_SEPARATES
   Content-Type: application/pdf
   Content-Range: bytes 7000-7999/8000

   ...the second range
   --THIS_STRING_SEPARATES--

   When multiple ranges are requested, a server MAY coalesce any of the
   ranges that overlap, or that are separated by a gap that is smaller
   than the overhead of sending multiple parts, regardless of the order
   in which the corresponding range-spec appeared in the received Range
   header field.  Since the typical overhead between each part of a
   "multipart/byteranges" is around 80 bytes, depending on the selected
   representation's media type and the chosen boundary parameter length,
   it can be less efficient to transfer many small disjoint parts than
   it is to transfer the entire selected representation.

   A server MUST NOT generate a multipart response to a request for a
   single range, since a client that does not request multiple parts
   might not support multipart responses.  However, a server MAY
   generate a "multipart/byteranges" response with only a single body
   part if multiple ranges were requested and only one range was found
   to be satisfiable or only one range remained after coalescing.  A
   client that cannot process a "multipart/byteranges" response MUST NOT
   generate a request that asks for multiple ranges.

   A server that generates a multipart response SHOULD send the parts in
   the same order that the corresponding range-spec appeared in the
   received Range header field, excluding those ranges that were deemed
   unsatisfiable or that were coalesced into other ranges.  A client
   that receives a multipart response MUST inspect the Content-Range
   header field present in each body part in order to determine which
   range is contained in that body part; a client cannot rely on
   receiving the same ranges that it requested, nor the same order that
   it requested.

15.3.7.3.  Combining Parts

   A response might transfer only a subrange of a representation if the
   connection closed prematurely or if the request used one or more
   Range specifications.  After several such transfers, a client might
   have received several ranges of the same representation.  These
   ranges can only be safely combined if they all have in common the
   same strong validator (Section 8.8.1).

   A client that has received multiple partial responses to GET requests
   on a target resource MAY combine those responses into a larger
   continuous range if they share the same strong validator.

   If the most recent response is an incomplete 200 (OK) response, then
   the header fields of that response are used for any combined response
   and replace those of the matching stored responses.

   If the most recent response is a 206 (Partial Content) response and
   at least one of the matching stored responses is a 200 (OK), then the
   combined response header fields consist of the most recent 200
   response's header fields.  If all of the matching stored responses
   are 206 responses, then the stored response with the most recent
   header fields is used as the source of header fields for the combined
   response, except that the client MUST use other header fields
   provided in the new response, aside from Content-Range, to replace
   all instances of the corresponding header fields in the stored
   response.

   The combined response content consists of the union of partial
   content ranges within the new response and all of the matching stored
   responses.  If the union consists of the entire range of the
   representation, then the client MUST process the combined response as
   if it were a complete 200 (OK) response, including a Content-Length
   header field that reflects the complete length.  Otherwise, the
   client MUST process the set of continuous ranges as one of the
   following: an incomplete 200 (OK) response if the combined response
   is a prefix of the representation, a single 206 (Partial Content)
   response containing "multipart/byteranges" content, or multiple 206
   (Partial Content) responses, each with one continuous range that is
   indicated by a Content-Range header field.

15.4.  Redirection 3xx

   The 3xx (Redirection) class of status code indicates that further
   action needs to be taken by the user agent in order to fulfill the
   request.  There are several types of redirects:

   1.  Redirects that indicate this resource might be available at a
       different URI, as provided by the Location header field, as in
       the status codes 301 (Moved Permanently), 302 (Found), 307
       (Temporary Redirect), and 308 (Permanent Redirect).

   2.  Redirection that offers a choice among matching resources capable
       of representing this resource, as in the 300 (Multiple Choices)
       status code.

   3.  Redirection to a different resource, identified by the Location
       header field, that can represent an indirect response to the
       request, as in the 303 (See Other) status code.

   4.  Redirection to a previously stored result, as in the 304 (Not
       Modified) status code.

      |  *Note:* In HTTP/1.0, the status codes 301 (Moved Permanently)
      |  and 302 (Found) were originally defined as method-preserving
      |  ([HTTP/1.0], Section 9.3) to match their implementation at
      |  CERN; 303 (See Other) was defined for a redirection that
      |  changed its method to GET.  However, early user agents split on
      |  whether to redirect POST requests as POST (according to then-
      |  current specification) or as GET (the safer alternative when
      |  redirected to a different site).  Prevailing practice
      |  eventually converged on changing the method to GET.  307
      |  (Temporary Redirect) and 308 (Permanent Redirect) [RFC7538]
      |  were later added to unambiguously indicate method-preserving
      |  redirects, and status codes 301 and 302 have been adjusted to
      |  allow a POST request to be redirected as GET.

   If a Location header field (Section 10.2.2) is provided, the user
   agent MAY automatically redirect its request to the URI referenced by
   the Location field value, even if the specific status code is not
   understood.  Automatic redirection needs to be done with care for
   methods not known to be safe, as defined in Section 9.2.1, since the
   user might not wish to redirect an unsafe request.

   When automatically following a redirected request, the user agent
   SHOULD resend the original request message with the following
   modifications:

   1.  Replace the target URI with the URI referenced by the redirection
       response's Location header field value after resolving it
       relative to the original request's target URI.

   2.  Remove header fields that were automatically generated by the
       implementation, replacing them with updated values as appropriate
       to the new request.  This includes:

       1.  Connection-specific header fields (see Section 7.6.1),

       2.  Header fields specific to the client's proxy configuration,
           including (but not limited to) Proxy-Authorization,

       3.  Origin-specific header fields (if any), including (but not
           limited to) Host,

       4.  Validating header fields that were added by the
           implementation's cache (e.g., If-None-Match,
           If-Modified-Since), and

       5.  Resource-specific header fields, including (but not limited
           to) Referer, Origin, Authorization, and Cookie.

   3.  Consider removing header fields that were not automatically
       generated by the implementation (i.e., those present in the
       request because they were added by the calling context) where
       there are security implications; this includes but is not limited
       to Authorization and Cookie.

   4.  Change the request method according to the redirecting status
       code's semantics, if applicable.

   5.  If the request method has been changed to GET or HEAD, remove
       content-specific header fields, including (but not limited to)
       Content-Encoding, Content-Language, Content-Location,
       Content-Type, Content-Length, Digest, Last-Modified.

   A client SHOULD detect and intervene in cyclical redirections (i.e.,
   "infinite" redirection loops).

      |  *Note:* An earlier version of this specification recommended a
      |  maximum of five redirections ([RFC2068], Section 10.3).
      |  Content developers need to be aware that some clients might
      |  implement such a fixed limitation.

15.4.1.  300 Multiple Choices

   The 300 (Multiple Choices) status code indicates that the target
   resource has more than one representation, each with its own more
   specific identifier, and information about the alternatives is being
   provided so that the user (or user agent) can select a preferred
   representation by redirecting its request to one or more of those
   identifiers.  In other words, the server desires that the user agent
   engage in reactive negotiation to select the most appropriate
   representation(s) for its needs (Section 12).

   If the server has a preferred choice, the server SHOULD generate a
   Location header field containing a preferred choice's URI reference.
   The user agent MAY use the Location field value for automatic
   redirection.

   For request methods other than HEAD, the server SHOULD generate
   content in the 300 response containing a list of representation
   metadata and URI reference(s) from which the user or user agent can
   choose the one most preferred.  The user agent MAY make a selection
   from that list automatically if it understands the provided media
   type.  A specific format for automatic selection is not defined by
   this specification because HTTP tries to remain orthogonal to the
   definition of its content.  In practice, the representation is
   provided in some easily parsed format believed to be acceptable to
   the user agent, as determined by shared design or content
   negotiation, or in some commonly accepted hypertext format.

   A 300 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

      |  *Note:* The original proposal for the 300 status code defined
      |  the URI header field as providing a list of alternative
      |  representations, such that it would be usable for 200, 300, and
      |  406 responses and be transferred in responses to the HEAD
      |  method.  However, lack of deployment and disagreement over
      |  syntax led to both URI and Alternates (a subsequent proposal)
      |  being dropped from this specification.  It is possible to
      |  communicate the list as a Link header field value [RFC8288]
      |  whose members have a relationship of "alternate", though
      |  deployment is a chicken-and-egg problem.

15.4.2.  301 Moved Permanently

   The 301 (Moved Permanently) status code indicates that the target
   resource has been assigned a new permanent URI and any future
   references to this resource ought to use one of the enclosed URIs.
   The server is suggesting that a user agent with link-editing
   capability can permanently replace references to the target URI with
   one of the new references sent by the server.  However, this
   suggestion is usually ignored unless the user agent is actively
   editing references (e.g., engaged in authoring content), the
   connection is secured, and the origin server is a trusted authority
   for the content being edited.

   The server SHOULD generate a Location header field in the response
   containing a preferred URI reference for the new permanent URI.  The
   user agent MAY use the Location field value for automatic
   redirection.  The server's response content usually contains a short
   hypertext note with a hyperlink to the new URI(s).

      |  *Note:* For historical reasons, a user agent MAY change the
      |  request method from POST to GET for the subsequent request.  If
      |  this behavior is undesired, the 308 (Permanent Redirect) status
      |  code can be used instead.

   A 301 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

15.4.3.  302 Found

   The 302 (Found) status code indicates that the target resource
   resides temporarily under a different URI.  Since the redirection
   might be altered on occasion, the client ought to continue to use the
   target URI for future requests.

   The server SHOULD generate a Location header field in the response
   containing a URI reference for the different URI.  The user agent MAY
   use the Location field value for automatic redirection.  The server's
   response content usually contains a short hypertext note with a
   hyperlink to the different URI(s).

      |  *Note:* For historical reasons, a user agent MAY change the
      |  request method from POST to GET for the subsequent request.  If
      |  this behavior is undesired, the 307 (Temporary Redirect) status
      |  code can be used instead.

15.4.4.  303 See Other

   The 303 (See Other) status code indicates that the server is
   redirecting the user agent to a different resource, as indicated by a
   URI in the Location header field, which is intended to provide an
   indirect response to the original request.  A user agent can perform
   a retrieval request targeting that URI (a GET or HEAD request if
   using HTTP), which might also be redirected, and present the eventual
   result as an answer to the original request.  Note that the new URI
   in the Location header field is not considered equivalent to the
   target URI.

   This status code is applicable to any HTTP method.  It is primarily
   used to allow the output of a POST action to redirect the user agent
   to a different resource, since doing so provides the information
   corresponding to the POST response as a resource that can be
   separately identified, bookmarked, and cached.

   A 303 response to a GET request indicates that the origin server does
   not have a representation of the target resource that can be
   transferred by the server over HTTP.  However, the Location field
   value refers to a resource that is descriptive of the target
   resource, such that making a retrieval request on that other resource
   might result in a representation that is useful to recipients without
   implying that it represents the original target resource.  Note that
   answers to the questions of what can be represented, what
   representations are adequate, and what might be a useful description
   are outside the scope of HTTP.

   Except for responses to a HEAD request, the representation of a 303
   response ought to contain a short hypertext note with a hyperlink to
   the same URI reference provided in the Location header field.

15.4.5.  304 Not Modified

   The 304 (Not Modified) status code indicates that a conditional GET
   or HEAD request has been received and would have resulted in a 200
   (OK) response if it were not for the fact that the condition
   evaluated to false.  In other words, there is no need for the server
   to transfer a representation of the target resource because the
   request indicates that the client, which made the request
   conditional, already has a valid representation; the server is
   therefore redirecting the client to make use of that stored
   representation as if it were the content of a 200 (OK) response.

   The server generating a 304 response MUST generate any of the
   following header fields that would have been sent in a 200 (OK)
   response to the same request:

   *  Content-Location, Date, ETag, and Vary

   *  Cache-Control and Expires (see [CACHING])

   Since the goal of a 304 response is to minimize information transfer
   when the recipient already has one or more cached representations, a
   sender SHOULD NOT generate representation metadata other than the
   above listed fields unless said metadata exists for the purpose of
   guiding cache updates (e.g., Last-Modified might be useful if the
   response does not have an ETag field).

   Requirements on a cache that receives a 304 response are defined in
   Section 4.3.4 of [CACHING].  If the conditional request originated
   with an outbound client, such as a user agent with its own cache
   sending a conditional GET to a shared proxy, then the proxy SHOULD
   forward the 304 response to that client.

   A 304 response is terminated by the end of the header section; it
   cannot contain content or trailers.

15.4.6.  305 Use Proxy

   The 305 (Use Proxy) status code was defined in a previous version of
   this specification and is now deprecated (Appendix B of [RFC7231]).

15.4.7.  306 (Unused)

   The 306 status code was defined in a previous version of this
   specification, is no longer used, and the code is reserved.

15.4.8.  307 Temporary Redirect

   The 307 (Temporary Redirect) status code indicates that the target
   resource resides temporarily under a different URI and the user agent
   MUST NOT change the request method if it performs an automatic
   redirection to that URI.  Since the redirection can change over time,
   the client ought to continue using the original target URI for future
   requests.

   The server SHOULD generate a Location header field in the response
   containing a URI reference for the different URI.  The user agent MAY
   use the Location field value for automatic redirection.  The server's
   response content usually contains a short hypertext note with a
   hyperlink to the different URI(s).

15.4.9.  308 Permanent Redirect

   The 308 (Permanent Redirect) status code indicates that the target
   resource has been assigned a new permanent URI and any future
   references to this resource ought to use one of the enclosed URIs.
   The server is suggesting that a user agent with link-editing
   capability can permanently replace references to the target URI with
   one of the new references sent by the server.  However, this
   suggestion is usually ignored unless the user agent is actively
   editing references (e.g., engaged in authoring content), the
   connection is secured, and the origin server is a trusted authority
   for the content being edited.

   The server SHOULD generate a Location header field in the response
   containing a preferred URI reference for the new permanent URI.  The
   user agent MAY use the Location field value for automatic
   redirection.  The server's response content usually contains a short
   hypertext note with a hyperlink to the new URI(s).

   A 308 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

      |  *Note:* This status code is much younger (June 2014) than its
      |  sibling codes and thus might not be recognized everywhere.  See
      |  Section 4 of [RFC7538] for deployment considerations.

15.5.  Client Error 4xx

   The 4xx (Client Error) class of status code indicates that the client
   seems to have erred.  Except when responding to a HEAD request, the
   server SHOULD send a representation containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition.  These status codes are applicable to any request method.
   User agents SHOULD display any included representation to the user.

15.5.1.  400 Bad Request

   The 400 (Bad Request) status code indicates that the server cannot or
   will not process the request due to something that is perceived to be
   a client error (e.g., malformed request syntax, invalid request
   message framing, or deceptive request routing).

15.5.2.  401 Unauthorized

   The 401 (Unauthorized) status code indicates that the request has not
   been applied because it lacks valid authentication credentials for
   the target resource.  The server generating a 401 response MUST send
   a WWW-Authenticate header field (Section 11.6.1) containing at least
   one challenge applicable to the target resource.

   If the request included authentication credentials, then the 401
   response indicates that authorization has been refused for those
   credentials.  The user agent MAY repeat the request with a new or
   replaced Authorization header field (Section 11.6.2).  If the 401
   response contains the same challenge as the prior response, and the
   user agent has already attempted authentication at least once, then
   the user agent SHOULD present the enclosed representation to the
   user, since it usually contains relevant diagnostic information.

15.5.3.  402 Payment Required

   The 402 (Payment Required) status code is reserved for future use.

15.5.4.  403 Forbidden

   The 403 (Forbidden) status code indicates that the server understood
   the request but refuses to fulfill it.  A server that wishes to make
   public why the request has been forbidden can describe that reason in
   the response content (if any).

   If authentication credentials were provided in the request, the
   server considers them insufficient to grant access.  The client
   SHOULD NOT automatically repeat the request with the same
   credentials.  The client MAY repeat the request with new or different
   credentials.  However, a request might be forbidden for reasons
   unrelated to the credentials.

   An origin server that wishes to "hide" the current existence of a
   forbidden target resource MAY instead respond with a status code of
   404 (Not Found).

15.5.5.  404 Not Found

   The 404 (Not Found) status code indicates that the origin server did
   not find a current representation for the target resource or is not
   willing to disclose that one exists.  A 404 status code does not
   indicate whether this lack of representation is temporary or
   permanent; the 410 (Gone) status code is preferred over 404 if the
   origin server knows, presumably through some configurable means, that
   the condition is likely to be permanent.

   A 404 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

15.5.6.  405 Method Not Allowed

   The 405 (Method Not Allowed) status code indicates that the method
   received in the request-line is known by the origin server but not
   supported by the target resource.  The origin server MUST generate an
   Allow header field in a 405 response containing a list of the target
   resource's currently supported methods.

   A 405 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

15.5.7.  406 Not Acceptable

   The 406 (Not Acceptable) status code indicates that the target
   resource does not have a current representation that would be
   acceptable to the user agent, according to the proactive negotiation
   header fields received in the request (Section 12.1), and the server
   is unwilling to supply a default representation.

   The server SHOULD generate content containing a list of available
   representation characteristics and corresponding resource identifiers
   from which the user or user agent can choose the one most
   appropriate.  A user agent MAY automatically select the most
   appropriate choice from that list.  However, this specification does
   not define any standard for such automatic selection, as described in
   Section 15.4.1.

15.5.8.  407 Proxy Authentication Required

   The 407 (Proxy Authentication Required) status code is similar to 401
   (Unauthorized), but it indicates that the client needs to
   authenticate itself in order to use a proxy for this request.  The
   proxy MUST send a Proxy-Authenticate header field (Section 11.7.1)
   containing a challenge applicable to that proxy for the request.  The
   client MAY repeat the request with a new or replaced
   Proxy-Authorization header field (Section 11.7.2).

15.5.9.  408 Request Timeout

   The 408 (Request Timeout) status code indicates that the server did
   not receive a complete request message within the time that it was
   prepared to wait.

   If the client has an outstanding request in transit, it MAY repeat
   that request.  If the current connection is not usable (e.g., as it
   would be in HTTP/1.1 because request delimitation is lost), a new
   connection will be used.

15.5.10.  409 Conflict

   The 409 (Conflict) status code indicates that the request could not
   be completed due to a conflict with the current state of the target
   resource.  This code is used in situations where the user might be
   able to resolve the conflict and resubmit the request.  The server
   SHOULD generate content that includes enough information for a user
   to recognize the source of the conflict.

   Conflicts are most likely to occur in response to a PUT request.  For
   example, if versioning were being used and the representation being
   PUT included changes to a resource that conflict with those made by
   an earlier (third-party) request, the origin server might use a 409
   response to indicate that it can't complete the request.  In this
   case, the response representation would likely contain information
   useful for merging the differences based on the revision history.

15.5.11.  410 Gone

   The 410 (Gone) status code indicates that access to the target
   resource is no longer available at the origin server and that this
   condition is likely to be permanent.  If the origin server does not
   know, or has no facility to determine, whether or not the condition
   is permanent, the status code 404 (Not Found) ought to be used
   instead.

   The 410 response is primarily intended to assist the task of web
   maintenance by notifying the recipient that the resource is
   intentionally unavailable and that the server owners desire that
   remote links to that resource be removed.  Such an event is common
   for limited-time, promotional services and for resources belonging to
   individuals no longer associated with the origin server's site.  It
   is not necessary to mark all permanently unavailable resources as
   "gone" or to keep the mark for any length of time -- that is left to
   the discretion of the server owner.

   A 410 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

15.5.12.  411 Length Required

   The 411 (Length Required) status code indicates that the server
   refuses to accept the request without a defined Content-Length
   (Section 8.6).  The client MAY repeat the request if it adds a valid
   Content-Length header field containing the length of the request
   content.

15.5.13.  412 Precondition Failed

   The 412 (Precondition Failed) status code indicates that one or more
   conditions given in the request header fields evaluated to false when
   tested on the server (Section 13).  This response status code allows
   the client to place preconditions on the current resource state (its
   current representations and metadata) and, thus, prevent the request
   method from being applied if the target resource is in an unexpected
   state.

15.5.14.  413 Content Too Large

   The 413 (Content Too Large) status code indicates that the server is
   refusing to process a request because the request content is larger
   than the server is willing or able to process.  The server MAY
   terminate the request, if the protocol version in use allows it;
   otherwise, the server MAY close the connection.

   If the condition is temporary, the server SHOULD generate a
   Retry-After header field to indicate that it is temporary and after
   what time the client MAY try again.

15.5.15.  414 URI Too Long

   The 414 (URI Too Long) status code indicates that the server is
   refusing to service the request because the target URI is longer than
   the server is willing to interpret.  This rare condition is only
   likely to occur when a client has improperly converted a POST request
   to a GET request with long query information, when the client has
   descended into an infinite loop of redirection (e.g., a redirected
   URI prefix that points to a suffix of itself) or when the server is
   under attack by a client attempting to exploit potential security
   holes.

   A 414 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

15.5.16.  415 Unsupported Media Type

   The 415 (Unsupported Media Type) status code indicates that the
   origin server is refusing to service the request because the content
   is in a format not supported by this method on the target resource.

   The format problem might be due to the request's indicated
   Content-Type or Content-Encoding, or as a result of inspecting the
   data directly.

   If the problem was caused by an unsupported content coding, the
   Accept-Encoding response header field (Section 12.5.3) ought to be
   used to indicate which (if any) content codings would have been
   accepted in the request.

   On the other hand, if the cause was an unsupported media type, the
   Accept response header field (Section 12.5.1) can be used to indicate
   which media types would have been accepted in the request.

15.5.17.  416 Range Not Satisfiable

   The 416 (Range Not Satisfiable) status code indicates that the set of
   ranges in the request's Range header field (Section 14.2) has been
   rejected either because none of the requested ranges are satisfiable
   or because the client has requested an excessive number of small or
   overlapping ranges (a potential denial of service attack).

   Each range unit defines what is required for its own range sets to be
   satisfiable.  For example, Section 14.1.2 defines what makes a bytes
   range set satisfiable.

   A server that generates a 416 response to a byte-range request SHOULD
   generate a Content-Range header field specifying the current length
   of the selected representation (Section 14.4).

   For example:

   HTTP/1.1 416 Range Not Satisfiable
   Date: Fri, 20 Jan 2012 15:41:54 GMT
   Content-Range: bytes */47022

      |  *Note:* Because servers are free to ignore Range, many
      |  implementations will respond with the entire selected
      |  representation in a 200 (OK) response.  That is partly because
      |  most clients are prepared to receive a 200 (OK) to complete the
      |  task (albeit less efficiently) and partly because clients might
      |  not stop making an invalid range request until they have
      |  received a complete representation.  Thus, clients cannot
      |  depend on receiving a 416 (Range Not Satisfiable) response even
      |  when it is most appropriate.

15.5.18.  417 Expectation Failed

   The 417 (Expectation Failed) status code indicates that the
   expectation given in the request's Expect header field
   (Section 10.1.1) could not be met by at least one of the inbound
   servers.

15.5.19.  418 (Unused)

   [RFC2324] was an April 1 RFC that lampooned the various ways HTTP was
   abused; one such abuse was the definition of an application-specific
   418 status code, which has been deployed as a joke often enough for
   the code to be unusable for any future use.

   Therefore, the 418 status code is reserved in the IANA HTTP Status
   Code Registry.  This indicates that the status code cannot be
   assigned to other applications currently.  If future circumstances
   require its use (e.g., exhaustion of 4NN status codes), it can be re-
   assigned to another use.

15.5.20.  421 Misdirected Request

   The 421 (Misdirected Request) status code indicates that the request
   was directed at a server that is unable or unwilling to produce an
   authoritative response for the target URI.  An origin server (or
   gateway acting on behalf of the origin server) sends 421 to reject a
   target URI that does not match an origin for which the server has
   been configured (Section 4.3.1) or does not match the connection
   context over which the request was received (Section 7.4).

   A client that receives a 421 (Misdirected Request) response MAY retry
   the request, whether or not the request method is idempotent, over a
   different connection, such as a fresh connection specific to the
   target resource's origin, or via an alternative service [ALTSVC].

   A proxy MUST NOT generate a 421 response.

15.5.21.  422 Unprocessable Content

   The 422 (Unprocessable Content) status code indicates that the server
   understands the content type of the request content (hence a 415
   (Unsupported Media Type) status code is inappropriate), and the
   syntax of the request content is correct, but it was unable to
   process the contained instructions.  For example, this status code
   can be sent if an XML request content contains well-formed (i.e.,
   syntactically correct), but semantically erroneous XML instructions.

15.5.22.  426 Upgrade Required

   The 426 (Upgrade Required) status code indicates that the server
   refuses to perform the request using the current protocol but might
   be willing to do so after the client upgrades to a different
   protocol.  The server MUST send an Upgrade header field in a 426
   response to indicate the required protocol(s) (Section 7.8).

   Example:

   HTTP/1.1 426 Upgrade Required
   Upgrade: HTTP/3.0
   Connection: Upgrade
   Content-Length: 53
   Content-Type: text/plain

   This service requires use of the HTTP/3.0 protocol.

15.6.  Server Error 5xx

   The 5xx (Server Error) class of status code indicates that the server
   is aware that it has erred or is incapable of performing the
   requested method.  Except when responding to a HEAD request, the
   server SHOULD send a representation containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition.  A user agent SHOULD display any included representation
   to the user.  These status codes are applicable to any request
   method.

15.6.1.  500 Internal Server Error

   The 500 (Internal Server Error) status code indicates that the server
   encountered an unexpected condition that prevented it from fulfilling
   the request.

15.6.2.  501 Not Implemented

   The 501 (Not Implemented) status code indicates that the server does
   not support the functionality required to fulfill the request.  This
   is the appropriate response when the server does not recognize the
   request method and is not capable of supporting it for any resource.

   A 501 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [CACHING]).

15.6.3.  502 Bad Gateway

   The 502 (Bad Gateway) status code indicates that the server, while
   acting as a gateway or proxy, received an invalid response from an
   inbound server it accessed while attempting to fulfill the request.

15.6.4.  503 Service Unavailable

   The 503 (Service Unavailable) status code indicates that the server
   is currently unable to handle the request due to a temporary overload
   or scheduled maintenance, which will likely be alleviated after some
   delay.  The server MAY send a Retry-After header field
   (Section 10.2.3) to suggest an appropriate amount of time for the
   client to wait before retrying the request.

      |  *Note:* The existence of the 503 status code does not imply
      |  that a server has to use it when becoming overloaded.  Some
      |  servers might simply refuse the connection.

15.6.5.  504 Gateway Timeout

   The 504 (Gateway Timeout) status code indicates that the server,
   while acting as a gateway or proxy, did not receive a timely response
   from an upstream server it needed to access in order to complete the
   request.

15.6.6.  505 HTTP Version Not Supported

   The 505 (HTTP Version Not Supported) status code indicates that the
   server does not support, or refuses to support, the major version of
   HTTP that was used in the request message.  The server is indicating
   that it is unable or unwilling to complete the request using the same
   major version as the client, as described in Section 2.5, other than
   with this error message.  The server SHOULD generate a representation
   for the 505 response that describes why that version is not supported
   and what other protocols are supported by that server.

16.  Extending HTTP

   HTTP defines a number of generic extension points that can be used to
   introduce capabilities to the protocol without introducing a new
   version, including methods, status codes, field names, and further
   extensibility points within defined fields, such as authentication
   schemes and cache directives (see Cache-Control extensions in
   Section 5.2.3 of [CACHING]).  Because the semantics of HTTP are not
   versioned, these extension points are persistent; the version of the
   protocol in use does not affect their semantics.

   Version-independent extensions are discouraged from depending on or
   interacting with the specific version of the protocol in use.  When
   this is unavoidable, careful consideration needs to be given to how
   the extension can interoperate across versions.

   Additionally, specific versions of HTTP might have their own
   extensibility points, such as transfer codings in HTTP/1.1
   (Section 6.1 of [HTTP/1.1]) and HTTP/2 SETTINGS or frame types
   ([HTTP/2]).  These extension points are specific to the version of
   the protocol they occur within.

   Version-specific extensions cannot override or modify the semantics
   of a version-independent mechanism or extension point (like a method
   or header field) without explicitly being allowed by that protocol
   element.  For example, the CONNECT method (Section 9.3.6) allows
   this.

   These guidelines assure that the protocol operates correctly and
   predictably, even when parts of the path implement different versions
   of HTTP.

16.1.  Method Extensibility

16.1.1.  Method Registry

   The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained
   by IANA at <https://www.iana.org/assignments/http-methods>, registers
   method names.

   HTTP method registrations MUST include the following fields:

   *  Method Name (see Section 9)

   *  Safe ("yes" or "no", see Section 9.2.1)

   *  Idempotent ("yes" or "no", see Section 9.2.2)

   *  Pointer to specification text

   Values to be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

16.1.2.  Considerations for New Methods

   Standardized methods are generic; that is, they are potentially
   applicable to any resource, not just one particular media type, kind
   of resource, or application.  As such, it is preferred that new
   methods be registered in a document that isn't specific to a single
   application or data format, since orthogonal technologies deserve
   orthogonal specification.

   Since message parsing (Section 6) needs to be independent of method
   semantics (aside from responses to HEAD), definitions of new methods
   cannot change the parsing algorithm or prohibit the presence of
   content on either the request or the response message.  Definitions
   of new methods can specify that only a zero-length content is allowed
   by requiring a Content-Length header field with a value of "0".

   Likewise, new methods cannot use the special host:port and asterisk
   forms of request target that are allowed for CONNECT and OPTIONS,
   respectively (Section 7.1).  A full URI in absolute form is needed
   for the target URI, which means either the request target needs to be
   sent in absolute form or the target URI will be reconstructed from
   the request context in the same way it is for other methods.

   A new method definition needs to indicate whether it is safe
   (Section 9.2.1), idempotent (Section 9.2.2), cacheable
   (Section 9.2.3), what semantics are to be associated with the request
   content (if any), and what refinements the method makes to header
   field or status code semantics.  If the new method is cacheable, its
   definition ought to describe how, and under what conditions, a cache
   can store a response and use it to satisfy a subsequent request.  The
   new method ought to describe whether it can be made conditional
   (Section 13.1) and, if so, how a server responds when the condition
   is false.  Likewise, if the new method might have some use for
   partial response semantics (Section 14.2), it ought to document this,
   too.

      |  *Note:* Avoid defining a method name that starts with "M-",
      |  since that prefix might be misinterpreted as having the
      |  semantics assigned to it by [RFC2774].

16.2.  Status Code Extensibility

16.2.1.  Status Code Registry

   The "Hypertext Transfer Protocol (HTTP) Status Code Registry",
   maintained by IANA at <https://www.iana.org/assignments/http-status-
   codes>, registers status code numbers.

   A registration MUST include the following fields:

   *  Status Code (3 digits)

   *  Short Description

   *  Pointer to specification text

   Values to be added to the HTTP status code namespace require IETF
   Review (see [RFC8126], Section 4.8).

16.2.2.  Considerations for New Status Codes

   When it is necessary to express semantics for a response that are not
   defined by current status codes, a new status code can be registered.
   Status codes are generic; they are potentially applicable to any
   resource, not just one particular media type, kind of resource, or
   application of HTTP.  As such, it is preferred that new status codes
   be registered in a document that isn't specific to a single
   application.

   New status codes are required to fall under one of the categories
   defined in Section 15.  To allow existing parsers to process the
   response message, new status codes cannot disallow content, although
   they can mandate a zero-length content.

   Proposals for new status codes that are not yet widely deployed ought
   to avoid allocating a specific number for the code until there is
   clear consensus that it will be registered; instead, early drafts can
   use a notation such as "4NN", or "3N0" .. "3N9", to indicate the
   class of the proposed status code(s) without consuming a number
   prematurely.

   The definition of a new status code ought to explain the request
   conditions that would cause a response containing that status code
   (e.g., combinations of request header fields and/or method(s)) along
   with any dependencies on response header fields (e.g., what fields
   are required, what fields can modify the semantics, and what field
   semantics are further refined when used with the new status code).

   By default, a status code applies only to the request corresponding
   to the response it occurs within.  If a status code applies to a
   larger scope of applicability -- for example, all requests to the
   resource in question or all requests to a server -- this must be
   explicitly specified.  When doing so, it should be noted that not all
   clients can be expected to consistently apply a larger scope because
   they might not understand the new status code.

   The definition of a new final status code ought to specify whether or
   not it is heuristically cacheable.  Note that any response with a
   final status code can be cached if the response has explicit
   freshness information.  A status code defined as heuristically
   cacheable is allowed to be cached without explicit freshness
   information.  Likewise, the definition of a status code can place
   constraints upon cache behavior if the must-understand cache
   directive is used.  See [CACHING] for more information.

   Finally, the definition of a new status code ought to indicate
   whether the content has any implied association with an identified
   resource (Section 6.4.2).

16.3.  Field Extensibility

   HTTP's most widely used extensibility point is the definition of new
   header and trailer fields.

   New fields can be defined such that, when they are understood by a
   recipient, they override or enhance the interpretation of previously
   defined fields, define preconditions on request evaluation, or refine
   the meaning of responses.

   However, defining a field doesn't guarantee its deployment or
   recognition by recipients.  Most fields are designed with the
   expectation that a recipient can safely ignore (but forward
   downstream) any field not recognized.  In other cases, the sender's
   ability to understand a given field might be indicated by its prior
   communication, perhaps in the protocol version or fields that it sent
   in prior messages, or its use of a specific media type.  Likewise,
   direct inspection of support might be possible through an OPTIONS
   request or by interacting with a defined well-known URI [RFC8615] if
   such inspection is defined along with the field being introduced.

16.3.1.  Field Name Registry

   The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines
   the namespace for HTTP field names.

   Any party can request registration of an HTTP field.  See
   Section 16.3.2 for considerations to take into account when creating
   a new HTTP field.

   The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is
   located at <https://www.iana.org/assignments/http-fields/>.
   Registration requests can be made by following the instructions
   located there or by sending an email to the "ietf-http-wg@w3.org"
   mailing list.

   Field names are registered on the advice of a designated expert
   (appointed by the IESG or their delegate).  Fields with the status
   'permanent' are Specification Required ([RFC8126], Section 4.6).

   Registration requests consist of the following information:

   Field name:
      The requested field name.  It MUST conform to the field-name
      syntax defined in Section 5.1, and it SHOULD be restricted to just
      letters, digits, and hyphen ('-') characters, with the first
      character being a letter.

   Status:
      "permanent", "provisional", "deprecated", or "obsoleted".

   Specification document(s):
      Reference to the document that specifies the field, preferably
      including a URI that can be used to retrieve a copy of the
      document.  Optional but encouraged for provisional registrations.
      An indication of the relevant section(s) can also be included, but
      is not required.

   And optionally:

   Comments:  Additional information, such as about reserved entries.

   The expert(s) can define additional fields to be collected in the
   registry, in consultation with the community.

   Standards-defined names have a status of "permanent".  Other names
   can also be registered as permanent if the expert(s) finds that they
   are in use, in consultation with the community.  Other names should
   be registered as "provisional".

   Provisional entries can be removed by the expert(s) if -- in
   consultation with the community -- the expert(s) find that they are
   not in use.  The expert(s) can change a provisional entry's status to
   permanent at any time.

   Note that names can be registered by third parties (including the
   expert(s)) if the expert(s) determines that an unregistered name is
   widely deployed and not likely to be registered in a timely manner
   otherwise.

16.3.2.  Considerations for New Fields

   HTTP header and trailer fields are a widely used extension point for
   the protocol.  While they can be used in an ad hoc fashion, fields
   that are intended for wider use need to be carefully documented to
   ensure interoperability.

   In particular, authors of specifications defining new fields are
   advised to consider and, where appropriate, document the following
   aspects:

   *  Under what conditions the field can be used; e.g., only in
      responses or requests, in all messages, only on responses to a
      particular request method, etc.

   *  Whether the field semantics are further refined by their context,
      such as their use with certain request methods or status codes.

   *  The scope of applicability for the information conveyed.  By
      default, fields apply only to the message they are associated
      with, but some response fields are designed to apply to all
      representations of a resource, the resource itself, or an even
      broader scope.  Specifications that expand the scope of a response
      field will need to carefully consider issues such as content
      negotiation, the time period of applicability, and (in some cases)
      multi-tenant server deployments.

   *  Under what conditions intermediaries are allowed to insert,
      delete, or modify the field's value.

   *  If the field is allowable in trailers; by default, it will not be
      (see Section 6.5.1).

   *  Whether it is appropriate or even required to list the field name
      in the Connection header field (i.e., if the field is to be hop-
      by-hop; see Section 7.6.1).

   *  Whether the field introduces any additional security
      considerations, such as disclosure of privacy-related data.

   Request header fields have additional considerations that need to be
   documented if the default behavior is not appropriate:

   *  If it is appropriate to list the field name in a Vary response
      header field (e.g., when the request header field is used by an
      origin server's content selection algorithm; see Section 12.5.5).

   *  If the field is intended to be stored when received in a PUT
      request (see Section 9.3.4).

   *  If the field ought to be removed when automatically redirecting a
      request due to security concerns (see Section 15.4).

16.3.2.1.  Considerations for New Field Names

   Authors of specifications defining new fields are advised to choose a
   short but descriptive field name.  Short names avoid needless data
   transmission; descriptive names avoid confusion and "squatting" on
   names that might have broader uses.

   To that end, limited-use fields (such as a header confined to a
   single application or use case) are encouraged to use a name that
   includes that use (or an abbreviation) as a prefix; for example, if
   the Foo Application needs a Description field, it might use "Foo-
   Desc"; "Description" is too generic, and "Foo-Description" is
   needlessly long.

   While the field-name syntax is defined to allow any token character,
   in practice some implementations place limits on the characters they
   accept in field-names.  To be interoperable, new field names SHOULD
   constrain themselves to alphanumeric characters, "-", and ".", and
   SHOULD begin with a letter.  For example, the underscore ("_")
   character can be problematic when passed through non-HTTP gateway
   interfaces (see Section 17.10).

   Field names ought not be prefixed with "X-"; see [BCP178] for further
   information.

   Other prefixes are sometimes used in HTTP field names; for example,
   "Accept-" is used in many content negotiation headers, and "Content-"
   is used as explained in Section 6.4.  These prefixes are only an aid
   to recognizing the purpose of a field and do not trigger automatic
   processing.

16.3.2.2.  Considerations for New Field Values

   A major task in the definition of a new HTTP field is the
   specification of the field value syntax: what senders should
   generate, and how recipients should infer semantics from what is
   received.

   Authors are encouraged (but not required) to use either the ABNF
   rules in this specification or those in [RFC8941] to define the
   syntax of new field values.

   Authors are advised to carefully consider how the combination of
   multiple field lines will impact them (see Section 5.3).  Because
   senders might erroneously send multiple values, and both
   intermediaries and HTTP libraries can perform combination
   automatically, this applies to all field values -- even when only a
   single value is anticipated.

   Therefore, authors are advised to delimit or encode values that
   contain commas (e.g., with the quoted-string rule of Section 5.6.4,
   the String data type of [RFC8941], or a field-specific encoding).
   This ensures that commas within field data are not confused with the
   commas that delimit a list value.

   For example, the Content-Type field value only allows commas inside
   quoted strings, which can be reliably parsed even when multiple
   values are present.  The Location field value provides a counter-
   example that should not be emulated: because URIs can include commas,
   it is not possible to reliably distinguish between a single value
   that includes a comma from two values.

   Authors of fields with a singleton value (see Section 5.5) are
   additionally advised to document how to treat messages where the
   multiple members are present (a sensible default would be to ignore
   the field, but this might not always be the right choice).

16.4.  Authentication Scheme Extensibility

16.4.1.  Authentication Scheme Registry

   The "Hypertext Transfer Protocol (HTTP) Authentication Scheme
   Registry" defines the namespace for the authentication schemes in
   challenges and credentials.  It is maintained at
   <https://www.iana.org/assignments/http-authschemes>.

   Registrations MUST include the following fields:

   *  Authentication Scheme Name

   *  Pointer to specification text

   *  Notes (optional)

   Values to be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

16.4.2.  Considerations for New Authentication Schemes

   There are certain aspects of the HTTP Authentication framework that
   put constraints on how new authentication schemes can work:

   *  HTTP authentication is presumed to be stateless: all of the
      information necessary to authenticate a request MUST be provided
      in the request, rather than be dependent on the server remembering
      prior requests.  Authentication based on, or bound to, the
      underlying connection is outside the scope of this specification
      and inherently flawed unless steps are taken to ensure that the
      connection cannot be used by any party other than the
      authenticated user (see Section 3.3).

   *  The authentication parameter "realm" is reserved for defining
      protection spaces as described in Section 11.5.  New schemes MUST
      NOT use it in a way incompatible with that definition.

   *  The "token68" notation was introduced for compatibility with
      existing authentication schemes and can only be used once per
      challenge or credential.  Thus, new schemes ought to use the auth-
      param syntax instead, because otherwise future extensions will be
      impossible.

   *  The parsing of challenges and credentials is defined by this
      specification and cannot be modified by new authentication
      schemes.  When the auth-param syntax is used, all parameters ought
      to support both token and quoted-string syntax, and syntactical
      constraints ought to be defined on the field value after parsing
      (i.e., quoted-string processing).  This is necessary so that
      recipients can use a generic parser that applies to all
      authentication schemes.

      *Note:* The fact that the value syntax for the "realm" parameter
      is restricted to quoted-string was a bad design choice not to be
      repeated for new parameters.

   *  Definitions of new schemes ought to define the treatment of
      unknown extension parameters.  In general, a "must-ignore" rule is
      preferable to a "must-understand" rule, because otherwise it will
      be hard to introduce new parameters in the presence of legacy
      recipients.  Furthermore, it's good to describe the policy for
      defining new parameters (such as "update the specification" or
      "use this registry").

   *  Authentication schemes need to document whether they are usable in
      origin-server authentication (i.e., using WWW-Authenticate), and/
      or proxy authentication (i.e., using Proxy-Authenticate).

   *  The credentials carried in an Authorization header field are
      specific to the user agent and, therefore, have the same effect on
      HTTP caches as the "private" cache response directive
      (Section 5.2.2.7 of [CACHING]), within the scope of the request in
      which they appear.

      Therefore, new authentication schemes that choose not to carry
      credentials in the Authorization header field (e.g., using a newly
      defined header field) will need to explicitly disallow caching, by
      mandating the use of cache response directives (e.g., "private").

   *  Schemes using Authentication-Info, Proxy-Authentication-Info, or
      any other authentication related response header field need to
      consider and document the related security considerations (see
      Section 17.16.4).

16.5.  Range Unit Extensibility

16.5.1.  Range Unit Registry

   The "HTTP Range Unit Registry" defines the namespace for the range
   unit names and refers to their corresponding specifications.  It is
   maintained at <https://www.iana.org/assignments/http-parameters>.

   Registration of an HTTP Range Unit MUST include the following fields:

   *  Name

   *  Description

   *  Pointer to specification text

   Values to be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

16.5.2.  Considerations for New Range Units

   Other range units, such as format-specific boundaries like pages,
   sections, records, rows, or time, are potentially usable in HTTP for
   application-specific purposes, but are not commonly used in practice.
   Implementors of alternative range units ought to consider how they
   would work with content codings and general-purpose intermediaries.

16.6.  Content Coding Extensibility

16.6.1.  Content Coding Registry

   The "HTTP Content Coding Registry", maintained by IANA at
   <https://www.iana.org/assignments/http-parameters/>, registers
   content-coding names.

   Content coding registrations MUST include the following fields:

   *  Name

   *  Description

   *  Pointer to specification text

   Names of content codings MUST NOT overlap with names of transfer
   codings (per the "HTTP Transfer Coding Registry" located at
   <https://www.iana.org/assignments/http-parameters/>) unless the
   encoding transformation is identical (as is the case for the
   compression codings defined in Section 8.4.1).

   Values to be added to this namespace require IETF Review (see
   Section 4.8 of [RFC8126]) and MUST conform to the purpose of content
   coding defined in Section 8.4.1.

16.6.2.  Considerations for New Content Codings

   New content codings ought to be self-descriptive whenever possible,
   with optional parameters discoverable within the coding format
   itself, rather than rely on external metadata that might be lost
   during transit.

16.7.  Upgrade Token Registry

   The "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry"
   defines the namespace for protocol-name tokens used to identify
   protocols in the Upgrade header field.  The registry is maintained at
   <https://www.iana.org/assignments/http-upgrade-tokens>.

   Each registered protocol name is associated with contact information
   and an optional set of specifications that details how the connection
   will be processed after it has been upgraded.

   Registrations happen on a "First Come First Served" basis (see
   Section 4.4 of [RFC8126]) and are subject to the following rules:

   1.  A protocol-name token, once registered, stays registered forever.

   2.  A protocol-name token is case-insensitive and registered with the
       preferred case to be generated by senders.

   3.  The registration MUST name a responsible party for the
       registration.

   4.  The registration MUST name a point of contact.

   5.  The registration MAY name a set of specifications associated with
       that token.  Such specifications need not be publicly available.

   6.  The registration SHOULD name a set of expected "protocol-version"
       tokens associated with that token at the time of registration.

   7.  The responsible party MAY change the registration at any time.
       The IANA will keep a record of all such changes, and make them
       available upon request.

   8.  The IESG MAY reassign responsibility for a protocol token.  This
       will normally only be used in the case when a responsible party
       cannot be contacted.

17.  Security Considerations

   This section is meant to inform developers, information providers,
   and users of known security concerns relevant to HTTP semantics and
   its use for transferring information over the Internet.
   Considerations related to caching are discussed in Section 7 of
   [CACHING], and considerations related to HTTP/1.1 message syntax and
   parsing are discussed in Section 11 of [HTTP/1.1].

   The list of considerations below is not exhaustive.  Most security
   concerns related to HTTP semantics are about securing server-side
   applications (code behind the HTTP interface), securing user agent
   processing of content received via HTTP, or secure use of the
   Internet in general, rather than security of the protocol.  The
   security considerations for URIs, which are fundamental to HTTP
   operation, are discussed in Section 7 of [URI].  Various
   organizations maintain topical information and links to current
   research on Web application security (e.g., [OWASP]).

17.1.  Establishing Authority

   HTTP relies on the notion of an "authoritative response": a response
   that has been determined by (or at the direction of) the origin
   server identified within the target URI to be the most appropriate
   response for that request given the state of the target resource at
   the time of response message origination.

   When a registered name is used in the authority component, the "http"
   URI scheme (Section 4.2.1) relies on the user's local name resolution
   service to determine where it can find authoritative responses.  This
   means that any attack on a user's network host table, cached names,
   or name resolution libraries becomes an avenue for attack on
   establishing authority for "http" URIs.  Likewise, the user's choice
   of server for Domain Name Service (DNS), and the hierarchy of servers
   from which it obtains resolution results, could impact the
   authenticity of address mappings; DNS Security Extensions (DNSSEC,
   [RFC4033]) are one way to improve authenticity, as are the various
   mechanisms for making DNS requests over more secure transfer
   protocols.

   Furthermore, after an IP address is obtained, establishing authority
   for an "http" URI is vulnerable to attacks on Internet Protocol
   routing.

   The "https" scheme (Section 4.2.2) is intended to prevent (or at
   least reveal) many of these potential attacks on establishing
   authority, provided that the negotiated connection is secured and the
   client properly verifies that the communicating server's identity
   matches the target URI's authority component (Section 4.3.4).
   Correctly implementing such verification can be difficult (see
   [Georgiev]).

   Authority for a given origin server can be delegated through protocol
   extensions; for example, [ALTSVC].  Likewise, the set of servers for
   which a connection is considered authoritative can be changed with a
   protocol extension like [RFC8336].

   Providing a response from a non-authoritative source, such as a
   shared proxy cache, is often useful to improve performance and
   availability, but only to the extent that the source can be trusted
   or the distrusted response can be safely used.

   Unfortunately, communicating authority to users can be difficult.
   For example, "phishing" is an attack on the user's perception of
   authority, where that perception can be misled by presenting similar
   branding in hypertext, possibly aided by userinfo obfuscating the
   authority component (see Section 4.2.1).  User agents can reduce the
   impact of phishing attacks by enabling users to easily inspect a
   target URI prior to making an action, by prominently distinguishing
   (or rejecting) userinfo when present, and by not sending stored
   credentials and cookies when the referring document is from an
   unknown or untrusted source.

17.2.  Risks of Intermediaries

   HTTP intermediaries are inherently situated for on-path attacks.
   Compromise of the systems on which the intermediaries run can result
   in serious security and privacy problems.  Intermediaries might have
   access to security-related information, personal information about
   individual users and organizations, and proprietary information
   belonging to users and content providers.  A compromised
   intermediary, or an intermediary implemented or configured without
   regard to security and privacy considerations, might be used in the
   commission of a wide range of potential attacks.

   Intermediaries that contain a shared cache are especially vulnerable
   to cache poisoning attacks, as described in Section 7 of [CACHING].

   Implementers need to consider the privacy and security implications
   of their design and coding decisions, and of the configuration
   options they provide to operators (especially the default
   configuration).

   Intermediaries are no more trustworthy than the people and policies
   under which they operate; HTTP cannot solve this problem.

17.3.  Attacks Based on File and Path Names

   Origin servers frequently make use of their local file system to
   manage the mapping from target URI to resource representations.  Most
   file systems are not designed to protect against malicious file or
   path names.  Therefore, an origin server needs to avoid accessing
   names that have a special significance to the system when mapping the
   target resource to files, folders, or directories.

   For example, UNIX, Microsoft Windows, and other operating systems use
   ".." as a path component to indicate a directory level above the
   current one, and they use specially named paths or file names to send
   data to system devices.  Similar naming conventions might exist
   within other types of storage systems.  Likewise, local storage
   systems have an annoying tendency to prefer user-friendliness over
   security when handling invalid or unexpected characters,
   recomposition of decomposed characters, and case-normalization of
   case-insensitive names.

   Attacks based on such special names tend to focus on either denial-
   of-service (e.g., telling the server to read from a COM port) or
   disclosure of configuration and source files that are not meant to be
   served.

17.4.  Attacks Based on Command, Code, or Query Injection

   Origin servers often use parameters within the URI as a means of
   identifying system services, selecting database entries, or choosing
   a data source.  However, data received in a request cannot be
   trusted.  An attacker could construct any of the request data
   elements (method, target URI, header fields, or content) to contain
   data that might be misinterpreted as a command, code, or query when
   passed through a command invocation, language interpreter, or
   database interface.

   For example, SQL injection is a common attack wherein additional
   query language is inserted within some part of the target URI or
   header fields (e.g., Host, Referer, etc.).  If the received data is
   used directly within a SELECT statement, the query language might be
   interpreted as a database command instead of a simple string value.
   This type of implementation vulnerability is extremely common, in
   spite of being easy to prevent.

   In general, resource implementations ought to avoid use of request
   data in contexts that are processed or interpreted as instructions.
   Parameters ought to be compared to fixed strings and acted upon as a
   result of that comparison, rather than passed through an interface
   that is not prepared for untrusted data.  Received data that isn't
   based on fixed parameters ought to be carefully filtered or encoded
   to avoid being misinterpreted.

   Similar considerations apply to request data when it is stored and
   later processed, such as within log files, monitoring tools, or when
   included within a data format that allows embedded scripts.

17.5.  Attacks via Protocol Element Length

   Because HTTP uses mostly textual, character-delimited fields, parsers
   are often vulnerable to attacks based on sending very long (or very
   slow) streams of data, particularly where an implementation is
   expecting a protocol element with no predefined length (Section 2.3).

   To promote interoperability, specific recommendations are made for
   minimum size limits on fields (Section 5.4).  These are minimum
   recommendations, chosen to be supportable even by implementations
   with limited resources; it is expected that most implementations will
   choose substantially higher limits.

   A server can reject a message that has a target URI that is too long
   (Section 15.5.15) or request content that is too large
   (Section 15.5.14).  Additional status codes related to capacity
   limits have been defined by extensions to HTTP [RFC6585].

   Recipients ought to carefully limit the extent to which they process
   other protocol elements, including (but not limited to) request
   methods, response status phrases, field names, numeric values, and
   chunk lengths.  Failure to limit such processing can result in
   arbitrary code execution due to buffer or arithmetic overflows, and
   increased vulnerability to denial-of-service attacks.

17.6.  Attacks Using Shared-Dictionary Compression

   Some attacks on encrypted protocols use the differences in size
   created by dynamic compression to reveal confidential information;
   for example, [BREACH].  These attacks rely on creating a redundancy
   between attacker-controlled content and the confidential information,
   such that a dynamic compression algorithm using the same dictionary
   for both content will compress more efficiently when the attacker-
   controlled content matches parts of the confidential content.

   HTTP messages can be compressed in a number of ways, including using
   TLS compression, content codings, transfer codings, and other
   extension or version-specific mechanisms.

   The most effective mitigation for this risk is to disable compression
   on sensitive data, or to strictly separate sensitive data from
   attacker-controlled data so that they cannot share the same
   compression dictionary.  With careful design, a compression scheme
   can be designed in a way that is not considered exploitable in
   limited use cases, such as HPACK ([HPACK]).

17.7.  Disclosure of Personal Information

   Clients are often privy to large amounts of personal information,
   including both information provided by the user to interact with
   resources (e.g., the user's name, location, mail address, passwords,
   encryption keys, etc.) and information about the user's browsing
   activity over time (e.g., history, bookmarks, etc.).  Implementations
   need to prevent unintentional disclosure of personal information.

17.8.  Privacy of Server Log Information

   A server is in the position to save personal data about a user's
   requests over time, which might identify their reading patterns or
   subjects of interest.  In particular, log information gathered at an
   intermediary often contains a history of user agent interaction,
   across a multitude of sites, that can be traced to individual users.

   HTTP log information is confidential in nature; its handling is often
   constrained by laws and regulations.  Log information needs to be
   securely stored and appropriate guidelines followed for its analysis.
   Anonymization of personal information within individual entries
   helps, but it is generally not sufficient to prevent real log traces
   from being re-identified based on correlation with other access
   characteristics.  As such, access traces that are keyed to a specific
   client are unsafe to publish even if the key is pseudonymous.

   To minimize the risk of theft or accidental publication, log
   information ought to be purged of personally identifiable
   information, including user identifiers, IP addresses, and user-
   provided query parameters, as soon as that information is no longer
   necessary to support operational needs for security, auditing, or
   fraud control.

17.9.  Disclosure of Sensitive Information in URIs

   URIs are intended to be shared, not secured, even when they identify
   secure resources.  URIs are often shown on displays, added to
   templates when a page is printed, and stored in a variety of
   unprotected bookmark lists.  Many servers, proxies, and user agents
   log or display the target URI in places where it might be visible to
   third parties.  It is therefore unwise to include information within
   a URI that is sensitive, personally identifiable, or a risk to
   disclose.

   When an application uses client-side mechanisms to construct a target
   URI out of user-provided information, such as the query fields of a
   form using GET, potentially sensitive data might be provided that
   would not be appropriate for disclosure within a URI.  POST is often
   preferred in such cases because it usually doesn't construct a URI;
   instead, POST of a form transmits the potentially sensitive data in
   the request content.  However, this hinders caching and uses an
   unsafe method for what would otherwise be a safe request.
   Alternative workarounds include transforming the user-provided data
   prior to constructing the URI or filtering the data to only include
   common values that are not sensitive.  Likewise, redirecting the
   result of a query to a different (server-generated) URI can remove
   potentially sensitive data from later links and provide a cacheable
   response for later reuse.

   Since the Referer header field tells a target site about the context
   that resulted in a request, it has the potential to reveal
   information about the user's immediate browsing history and any
   personal information that might be found in the referring resource's
   URI.  Limitations on the Referer header field are described in
   Section 10.1.3 to address some of its security considerations.

17.10.  Application Handling of Field Names

   Servers often use non-HTTP gateway interfaces and frameworks to
   process a received request and produce content for the response.  For
   historical reasons, such interfaces often pass received field names
   as external variable names, using a name mapping suitable for
   environment variables.

   For example, the Common Gateway Interface (CGI) mapping of protocol-
   specific meta-variables, defined by Section 4.1.18 of [RFC3875], is
   applied to received header fields that do not correspond to one of
   CGI's standard variables; the mapping consists of prepending "HTTP_"
   to each name and changing all instances of hyphen ("-") to underscore
   ("_").  This same mapping has been inherited by many other
   application frameworks in order to simplify moving applications from
   one platform to the next.

   In CGI, a received Content-Length field would be passed as the meta-
   variable "CONTENT_LENGTH" with a string value matching the received
   field's value.  In contrast, a received "Content_Length" header field
   would be passed as the protocol-specific meta-variable
   "HTTP_CONTENT_LENGTH", which might lead to some confusion if an
   application mistakenly reads the protocol-specific meta-variable
   instead of the default one.  (This historical practice is why
   Section 16.3.2.1 discourages the creation of new field names that
   contain an underscore.)

   Unfortunately, mapping field names to different interface names can
   lead to security vulnerabilities if the mapping is incomplete or
   ambiguous.  For example, if an attacker were to send a field named
   "Transfer_Encoding", a naive interface might map that to the same
   variable name as the "Transfer-Encoding" field, resulting in a
   potential request smuggling vulnerability (Section 11.2 of
   [HTTP/1.1]).

   To mitigate the associated risks, implementations that perform such
   mappings are advised to make the mapping unambiguous and complete for
   the full range of potential octets received as a name (including
   those that are discouraged or forbidden by the HTTP grammar).  For
   example, a field with an unusual name character might result in the
   request being blocked, the specific field being removed, or the name
   being passed with a different prefix to distinguish it from other
   fields.

17.11.  Disclosure of Fragment after Redirects

   Although fragment identifiers used within URI references are not sent
   in requests, implementers ought to be aware that they will be visible
   to the user agent and any extensions or scripts running as a result
   of the response.  In particular, when a redirect occurs and the
   original request's fragment identifier is inherited by the new
   reference in Location (Section 10.2.2), this might have the effect of
   disclosing one site's fragment to another site.  If the first site
   uses personal information in fragments, it ought to ensure that
   redirects to other sites include a (possibly empty) fragment
   component in order to block that inheritance.

17.12.  Disclosure of Product Information

   The User-Agent (Section 10.1.5), Via (Section 7.6.3), and Server
   (Section 10.2.4) header fields often reveal information about the
   respective sender's software systems.  In theory, this can make it
   easier for an attacker to exploit known security holes; in practice,
   attackers tend to try all potential holes regardless of the apparent
   software versions being used.

   Proxies that serve as a portal through a network firewall ought to
   take special precautions regarding the transfer of header information
   that might identify hosts behind the firewall.  The Via header field
   allows intermediaries to replace sensitive machine names with
   pseudonyms.

17.13.  Browser Fingerprinting

   Browser fingerprinting is a set of techniques for identifying a
   specific user agent over time through its unique set of
   characteristics.  These characteristics might include information
   related to how it uses the underlying transport protocol, feature
   capabilities, and scripting environment, though of particular
   interest here is the set of unique characteristics that might be
   communicated via HTTP.  Fingerprinting is considered a privacy
   concern because it enables tracking of a user agent's behavior over
   time ([Bujlow]) without the corresponding controls that the user
   might have over other forms of data collection (e.g., cookies).  Many
   general-purpose user agents (i.e., Web browsers) have taken steps to
   reduce their fingerprints.

   There are a number of request header fields that might reveal
   information to servers that is sufficiently unique to enable
   fingerprinting.  The From header field is the most obvious, though it
   is expected that From will only be sent when self-identification is
   desired by the user.  Likewise, Cookie header fields are deliberately
   designed to enable re-identification, so fingerprinting concerns only
   apply to situations where cookies are disabled or restricted by the
   user agent's configuration.

   The User-Agent header field might contain enough information to
   uniquely identify a specific device, usually when combined with other
   characteristics, particularly if the user agent sends excessive
   details about the user's system or extensions.  However, the source
   of unique information that is least expected by users is proactive
   negotiation (Section 12.1), including the Accept, Accept-Charset,
   Accept-Encoding, and Accept-Language header fields.

   In addition to the fingerprinting concern, detailed use of the
   Accept-Language header field can reveal information the user might
   consider to be of a private nature.  For example, understanding a
   given language set might be strongly correlated to membership in a
   particular ethnic group.  An approach that limits such loss of
   privacy would be for a user agent to omit the sending of Accept-
   Language except for sites that have been explicitly permitted,
   perhaps via interaction after detecting a Vary header field that
   indicates language negotiation might be useful.

   In environments where proxies are used to enhance privacy, user
   agents ought to be conservative in sending proactive negotiation
   header fields.  General-purpose user agents that provide a high
   degree of header field configurability ought to inform users about
   the loss of privacy that might result if too much detail is provided.
   As an extreme privacy measure, proxies could filter the proactive
   negotiation header fields in relayed requests.

17.14.  Validator Retention

   The validators defined by this specification are not intended to
   ensure the validity of a representation, guard against malicious
   changes, or detect on-path attacks.  At best, they enable more
   efficient cache updates and optimistic concurrent writes when all
   participants are behaving nicely.  At worst, the conditions will fail
   and the client will receive a response that is no more harmful than
   an HTTP exchange without conditional requests.

   An entity tag can be abused in ways that create privacy risks.  For
   example, a site might deliberately construct a semantically invalid
   entity tag that is unique to the user or user agent, send it in a
   cacheable response with a long freshness time, and then read that
   entity tag in later conditional requests as a means of re-identifying
   that user or user agent.  Such an identifying tag would become a
   persistent identifier for as long as the user agent retained the
   original cache entry.  User agents that cache representations ought
   to ensure that the cache is cleared or replaced whenever the user
   performs privacy-maintaining actions, such as clearing stored cookies
   or changing to a private browsing mode.

17.15.  Denial-of-Service Attacks Using Range

   Unconstrained multiple range requests are susceptible to denial-of-
   service attacks because the effort required to request many
   overlapping ranges of the same data is tiny compared to the time,
   memory, and bandwidth consumed by attempting to serve the requested
   data in many parts.  Servers ought to ignore, coalesce, or reject
   egregious range requests, such as requests for more than two
   overlapping ranges or for many small ranges in a single set,
   particularly when the ranges are requested out of order for no
   apparent reason.  Multipart range requests are not designed to
   support random access.

17.16.  Authentication Considerations

   Everything about the topic of HTTP authentication is a security
   consideration, so the list of considerations below is not exhaustive.
   Furthermore, it is limited to security considerations regarding the
   authentication framework, in general, rather than discussing all of
   the potential considerations for specific authentication schemes
   (which ought to be documented in the specifications that define those
   schemes).  Various organizations maintain topical information and
   links to current research on Web application security (e.g.,
   [OWASP]), including common pitfalls for implementing and using the
   authentication schemes found in practice.

17.16.1.  Confidentiality of Credentials

   The HTTP authentication framework does not define a single mechanism
   for maintaining the confidentiality of credentials; instead, each
   authentication scheme defines how the credentials are encoded prior
   to transmission.  While this provides flexibility for the development
   of future authentication schemes, it is inadequate for the protection
   of existing schemes that provide no confidentiality on their own, or
   that do not sufficiently protect against replay attacks.
   Furthermore, if the server expects credentials that are specific to
   each individual user, the exchange of those credentials will have the
   effect of identifying that user even if the content within
   credentials remains confidential.

   HTTP depends on the security properties of the underlying transport-
   or session-level connection to provide confidential transmission of
   fields.  Services that depend on individual user authentication
   require a secured connection prior to exchanging credentials
   (Section 4.2.2).

17.16.2.  Credentials and Idle Clients

   Existing HTTP clients and user agents typically retain authentication
   information indefinitely.  HTTP does not provide a mechanism for the
   origin server to direct clients to discard these cached credentials,
   since the protocol has no awareness of how credentials are obtained
   or managed by the user agent.  The mechanisms for expiring or
   revoking credentials can be specified as part of an authentication
   scheme definition.

   Circumstances under which credential caching can interfere with the
   application's security model include but are not limited to:

   *  Clients that have been idle for an extended period, following
      which the server might wish to cause the client to re-prompt the
      user for credentials.

   *  Applications that include a session termination indication (such
      as a "logout" or "commit" button on a page) after which the server
      side of the application "knows" that there is no further reason
      for the client to retain the credentials.

   User agents that cache credentials are encouraged to provide a
   readily accessible mechanism for discarding cached credentials under
   user control.

17.16.3.  Protection Spaces

   Authentication schemes that solely rely on the "realm" mechanism for
   establishing a protection space will expose credentials to all
   resources on an origin server.  Clients that have successfully made
   authenticated requests with a resource can use the same
   authentication credentials for other resources on the same origin
   server.  This makes it possible for a different resource to harvest
   authentication credentials for other resources.

   This is of particular concern when an origin server hosts resources
   for multiple parties under the same origin (Section 11.5).  Possible
   mitigation strategies include restricting direct access to
   authentication credentials (i.e., not making the content of the
   Authorization request header field available), and separating
   protection spaces by using a different host name (or port number) for
   each party.

17.16.4.  Additional Response Fields

   Adding information to responses that are sent over an unencrypted
   channel can affect security and privacy.  The presence of the
   Authentication-Info and Proxy-Authentication-Info header fields alone
   indicates that HTTP authentication is in use.  Additional information
   could be exposed by the contents of the authentication-scheme
   specific parameters; this will have to be considered in the
   definitions of these schemes.

18.  IANA Considerations

   The change controller for the following registrations is: "IETF
   (iesg@ietf.org) - Internet Engineering Task Force".

18.1.  URI Scheme Registration

   IANA has updated the "Uniform Resource Identifier (URI) Schemes"
   registry [BCP35] at <https://www.iana.org/assignments/uri-schemes/>
   with the permanent schemes listed in Table 2 in Section 4.2.

18.2.  Method Registration

   IANA has updated the "Hypertext Transfer Protocol (HTTP) Method
   Registry" at <https://www.iana.org/assignments/http-methods> with the
   registration procedure of Section 16.1.1 and the method names
   summarized in the following table.

                 +=========+======+============+=========+
                 | Method  | Safe | Idempotent | Section |
                 +=========+======+============+=========+
                 | CONNECT | no   | no         | 9.3.6   |
                 +---------+------+------------+---------+
                 | DELETE  | no   | yes        | 9.3.5   |
                 +---------+------+------------+---------+
                 | GET     | yes  | yes        | 9.3.1   |
                 +---------+------+------------+---------+
                 | HEAD    | yes  | yes        | 9.3.2   |
                 +---------+------+------------+---------+
                 | OPTIONS | yes  | yes        | 9.3.7   |
                 +---------+------+------------+---------+
                 | POST    | no   | no         | 9.3.3   |
                 +---------+------+------------+---------+
                 | PUT     | no   | yes        | 9.3.4   |
                 +---------+------+------------+---------+
                 | TRACE   | yes  | yes        | 9.3.8   |
                 +---------+------+------------+---------+
                 | *       | no   | no         | 18.2    |
                 +---------+------+------------+---------+

                                  Table 7

   The method name "*" is reserved because using "*" as a method name
   would conflict with its usage as a wildcard in some fields (e.g.,
   "Access-Control-Request-Method").

18.3.  Status Code Registration

   IANA has updated the "Hypertext Transfer Protocol (HTTP) Status Code
   Registry" at <https://www.iana.org/assignments/http-status-codes>
   with the registration procedure of Section 16.2.1 and the status code
   values summarized in the following table.

            +=======+===============================+=========+
            | Value | Description                   | Section |
            +=======+===============================+=========+
            | 100   | Continue                      | 15.2.1  |
            +-------+-------------------------------+---------+
            | 101   | Switching Protocols           | 15.2.2  |
            +-------+-------------------------------+---------+
            | 200   | OK                            | 15.3.1  |
            +-------+-------------------------------+---------+
            | 201   | Created                       | 15.3.2  |
            +-------+-------------------------------+---------+
            | 202   | Accepted                      | 15.3.3  |
            +-------+-------------------------------+---------+
            | 203   | Non-Authoritative Information | 15.3.4  |
            +-------+-------------------------------+---------+
            | 204   | No Content                    | 15.3.5  |
            +-------+-------------------------------+---------+
            | 205   | Reset Content                 | 15.3.6  |
            +-------+-------------------------------+---------+
            | 206   | Partial Content               | 15.3.7  |
            +-------+-------------------------------+---------+
            | 300   | Multiple Choices              | 15.4.1  |
            +-------+-------------------------------+---------+
            | 301   | Moved Permanently             | 15.4.2  |
            +-------+-------------------------------+---------+
            | 302   | Found                         | 15.4.3  |
            +-------+-------------------------------+---------+
            | 303   | See Other                     | 15.4.4  |
            +-------+-------------------------------+---------+
            | 304   | Not Modified                  | 15.4.5  |
            +-------+-------------------------------+---------+
            | 305   | Use Proxy                     | 15.4.6  |
            +-------+-------------------------------+---------+
            | 306   | (Unused)                      | 15.4.7  |
            +-------+-------------------------------+---------+
            | 307   | Temporary Redirect            | 15.4.8  |
            +-------+-------------------------------+---------+
            | 308   | Permanent Redirect            | 15.4.9  |
            +-------+-------------------------------+---------+
            | 400   | Bad Request                   | 15.5.1  |
            +-------+-------------------------------+---------+
            | 401   | Unauthorized                  | 15.5.2  |
            +-------+-------------------------------+---------+
            | 402   | Payment Required              | 15.5.3  |
            +-------+-------------------------------+---------+
            | 403   | Forbidden                     | 15.5.4  |
            +-------+-------------------------------+---------+
            | 404   | Not Found                     | 15.5.5  |
            +-------+-------------------------------+---------+
            | 405   | Method Not Allowed            | 15.5.6  |
            +-------+-------------------------------+---------+
            | 406   | Not Acceptable                | 15.5.7  |
            +-------+-------------------------------+---------+
            | 407   | Proxy Authentication Required | 15.5.8  |
            +-------+-------------------------------+---------+
            | 408   | Request Timeout               | 15.5.9  |
            +-------+-------------------------------+---------+
            | 409   | Conflict                      | 15.5.10 |
            +-------+-------------------------------+---------+
            | 410   | Gone                          | 15.5.11 |
            +-------+-------------------------------+---------+
            | 411   | Length Required               | 15.5.12 |
            +-------+-------------------------------+---------+
            | 412   | Precondition Failed           | 15.5.13 |
            +-------+-------------------------------+---------+
            | 413   | Content Too Large             | 15.5.14 |
            +-------+-------------------------------+---------+
            | 414   | URI Too Long                  | 15.5.15 |
            +-------+-------------------------------+---------+
            | 415   | Unsupported Media Type        | 15.5.16 |
            +-------+-------------------------------+---------+
            | 416   | Range Not Satisfiable         | 15.5.17 |
            +-------+-------------------------------+---------+
            | 417   | Expectation Failed            | 15.5.18 |
            +-------+-------------------------------+---------+
            | 418   | (Unused)                      | 15.5.19 |
            +-------+-------------------------------+---------+
            | 421   | Misdirected Request           | 15.5.20 |
            +-------+-------------------------------+---------+
            | 422   | Unprocessable Content         | 15.5.21 |
            +-------+-------------------------------+---------+
            | 426   | Upgrade Required              | 15.5.22 |
            +-------+-------------------------------+---------+
            | 500   | Internal Server Error         | 15.6.1  |
            +-------+-------------------------------+---------+
            | 501   | Not Implemented               | 15.6.2  |
            +-------+-------------------------------+---------+
            | 502   | Bad Gateway                   | 15.6.3  |
            +-------+-------------------------------+---------+
            | 503   | Service Unavailable           | 15.6.4  |
            +-------+-------------------------------+---------+
            | 504   | Gateway Timeout               | 15.6.5  |
            +-------+-------------------------------+---------+
            | 505   | HTTP Version Not Supported    | 15.6.6  |
            +-------+-------------------------------+---------+

                                  Table 8

18.4.  Field Name Registration

   This specification updates the HTTP-related aspects of the existing
   registration procedures for message header fields defined in
   [RFC3864].  It replaces the old procedures as they relate to HTTP by
   defining a new registration procedure and moving HTTP field
   definitions into a separate registry.

   IANA has created a new registry titled "Hypertext Transfer Protocol
   (HTTP) Field Name Registry" as outlined in Section 16.3.1.

   IANA has moved all entries in the "Permanent Message Header Field
   Names" and "Provisional Message Header Field Names" registries (see
   <https://www.iana.org/assignments/message-headers/>) with the
   protocol 'http' to this registry and has applied the following
   changes:

   1.  The 'Applicable Protocol' field has been omitted.

   2.  Entries that had a status of 'standard', 'experimental',
       'reserved', or 'informational' have been made to have a status of
       'permanent'.

   3.  Provisional entries without a status have been made to have a
       status of 'provisional'.

   4.  Permanent entries without a status (after confirmation that the
       registration document did not define one) have been made to have
       a status of 'provisional'.  The expert(s) can choose to update
       the entries' status if there is evidence that another is more
       appropriate.

   IANA has annotated the "Permanent Message Header Field Names" and
   "Provisional Message Header Field Names" registries with the
   following note to indicate that HTTP field name registrations have
   moved:

      |  *Note*
      |  
      |  HTTP field name registrations have been moved to
      |  [https://www.iana.org/assignments/http-fields] per [RFC9110].

   IANA has updated the "Hypertext Transfer Protocol (HTTP) Field Name
   Registry" with the field names listed in the following table.

   +===========================+============+=========+============+
   | Field Name                | Status     | Section | Comments   |
   +===========================+============+=========+============+
   | Accept                    | permanent  | 12.5.1  |            |
   +---------------------------+------------+---------+------------+
   | Accept-Charset            | deprecated | 12.5.2  |            |
   +---------------------------+------------+---------+------------+
   | Accept-Encoding           | permanent  | 12.5.3  |            |
   +---------------------------+------------+---------+------------+
   | Accept-Language           | permanent  | 12.5.4  |            |
   +---------------------------+------------+---------+------------+
   | Accept-Ranges             | permanent  | 14.3    |            |
   +---------------------------+------------+---------+------------+
   | Allow                     | permanent  | 10.2.1  |            |
   +---------------------------+------------+---------+------------+
   | Authentication-Info       | permanent  | 11.6.3  |            |
   +---------------------------+------------+---------+------------+
   | Authorization             | permanent  | 11.6.2  |            |
   +---------------------------+------------+---------+------------+
   | Connection                | permanent  | 7.6.1   |            |
   +---------------------------+------------+---------+------------+
   | Content-Encoding          | permanent  | 8.4     |            |
   +---------------------------+------------+---------+------------+
   | Content-Language          | permanent  | 8.5     |            |
   +---------------------------+------------+---------+------------+
   | Content-Length            | permanent  | 8.6     |            |
   +---------------------------+------------+---------+------------+
   | Content-Location          | permanent  | 8.7     |            |
   +---------------------------+------------+---------+------------+
   | Content-Range             | permanent  | 14.4    |            |
   +---------------------------+------------+---------+------------+
   | Content-Type              | permanent  | 8.3     |            |
   +---------------------------+------------+---------+------------+
   | Date                      | permanent  | 6.6.1   |            |
   +---------------------------+------------+---------+------------+
   | ETag                      | permanent  | 8.8.3   |            |
   +---------------------------+------------+---------+------------+
   | Expect                    | permanent  | 10.1.1  |            |
   +---------------------------+------------+---------+------------+
   | From                      | permanent  | 10.1.2  |            |
   +---------------------------+------------+---------+------------+
   | Host                      | permanent  | 7.2     |            |
   +---------------------------+------------+---------+------------+
   | If-Match                  | permanent  | 13.1.1  |            |
   +---------------------------+------------+---------+------------+
   | If-Modified-Since         | permanent  | 13.1.3  |            |
   +---------------------------+------------+---------+------------+
   | If-None-Match             | permanent  | 13.1.2  |            |
   +---------------------------+------------+---------+------------+
   | If-Range                  | permanent  | 13.1.5  |            |
   +---------------------------+------------+---------+------------+
   | If-Unmodified-Since       | permanent  | 13.1.4  |            |
   +---------------------------+------------+---------+------------+
   | Last-Modified             | permanent  | 8.8.2   |            |
   +---------------------------+------------+---------+------------+
   | Location                  | permanent  | 10.2.2  |            |
   +---------------------------+------------+---------+------------+
   | Max-Forwards              | permanent  | 7.6.2   |            |
   +---------------------------+------------+---------+------------+
   | Proxy-Authenticate        | permanent  | 11.7.1  |            |
   +---------------------------+------------+---------+------------+
   | Proxy-Authentication-Info | permanent  | 11.7.3  |            |
   +---------------------------+------------+---------+------------+
   | Proxy-Authorization       | permanent  | 11.7.2  |            |
   +---------------------------+------------+---------+------------+
   | Range                     | permanent  | 14.2    |            |
   +---------------------------+------------+---------+------------+
   | Referer                   | permanent  | 10.1.3  |            |
   +---------------------------+------------+---------+------------+
   | Retry-After               | permanent  | 10.2.3  |            |
   +---------------------------+------------+---------+------------+
   | Server                    | permanent  | 10.2.4  |            |
   +---------------------------+------------+---------+------------+
   | TE                        | permanent  | 10.1.4  |            |
   +---------------------------+------------+---------+------------+
   | Trailer                   | permanent  | 6.6.2   |            |
   +---------------------------+------------+---------+------------+
   | Upgrade                   | permanent  | 7.8     |            |
   +---------------------------+------------+---------+------------+
   | User-Agent                | permanent  | 10.1.5  |            |
   +---------------------------+------------+---------+------------+
   | Vary                      | permanent  | 12.5.5  |            |
   +---------------------------+------------+---------+------------+
   | Via                       | permanent  | 7.6.3   |            |
   +---------------------------+------------+---------+------------+
   | WWW-Authenticate          | permanent  | 11.6.1  |            |
   +---------------------------+------------+---------+------------+
   | *                         | permanent  | 12.5.5  | (reserved) |
   +---------------------------+------------+---------+------------+

                                Table 9

   The field name "*" is reserved because using that name as an HTTP
   header field might conflict with its special semantics in the Vary
   header field (Section 12.5.5).

   IANA has updated the "Content-MD5" entry in the new registry to have
   a status of 'obsoleted' with references to Section 14.15 of [RFC2616]
   (for the definition of the header field) and Appendix B of [RFC7231]
   (which removed the field definition from the updated specification).

18.5.  Authentication Scheme Registration

   IANA has updated the "Hypertext Transfer Protocol (HTTP)
   Authentication Scheme Registry" at <https://www.iana.org/assignments/
   http-authschemes> with the registration procedure of Section 16.4.1.
   No authentication schemes are defined in this document.

18.6.  Content Coding Registration

   IANA has updated the "HTTP Content Coding Registry" at
   <https://www.iana.org/assignments/http-parameters/> with the
   registration procedure of Section 16.6.1 and the content coding names
   summarized in the table below.

   +============+===========================================+=========+
   | Name       | Description                               | Section |
   +============+===========================================+=========+
   | compress   | UNIX "compress" data format [Welch]       | 8.4.1.1 |
   +------------+-------------------------------------------+---------+
   | deflate    | "deflate" compressed data ([RFC1951])     | 8.4.1.2 |
   |            | inside the "zlib" data format ([RFC1950]) |         |
   +------------+-------------------------------------------+---------+
   | gzip       | GZIP file format [RFC1952]                | 8.4.1.3 |
   +------------+-------------------------------------------+---------+
   | identity   | Reserved                                  | 12.5.3  |
   +------------+-------------------------------------------+---------+
   | x-compress | Deprecated (alias for compress)           | 8.4.1.1 |
   +------------+-------------------------------------------+---------+
   | x-gzip     | Deprecated (alias for gzip)               | 8.4.1.3 |
   +------------+-------------------------------------------+---------+

                                 Table 10

18.7.  Range Unit Registration

   IANA has updated the "HTTP Range Unit Registry" at
   <https://www.iana.org/assignments/http-parameters/> with the
   registration procedure of Section 16.5.1 and the range unit names
   summarized in the table below.

   +=================+==================================+=========+
   | Range Unit Name | Description                      | Section |
   +=================+==================================+=========+
   | bytes           | a range of octets                | 14.1.2  |
   +-----------------+----------------------------------+---------+
   | none            | reserved as keyword to indicate  | 14.3    |
   |                 | range requests are not supported |         |
   +-----------------+----------------------------------+---------+

                               Table 11

18.8.  Media Type Registration

   IANA has updated the "Media Types" registry at
   <https://www.iana.org/assignments/media-types> with the registration
   information in Section 14.6 for the media type "multipart/
   byteranges".

   IANA has updated the registry note about "q" parameters with a link
   to Section 12.5.1 of this document.

18.9.  Port Registration

   IANA has updated the "Service Name and Transport Protocol Port Number
   Registry" at <https://www.iana.org/assignments/service-names-port-
   numbers/> for the services on ports 80 and 443 that use UDP or TCP
   to:

   1.  use this document as "Reference", and

   2.  when currently unspecified, set "Assignee" to "IESG" and
       "Contact" to "IETF_Chair".

18.10.  Upgrade Token Registration

   IANA has updated the "Hypertext Transfer Protocol (HTTP) Upgrade
   Token Registry" at <https://www.iana.org/assignments/http-upgrade-
   tokens> with the registration procedure described in Section 16.7 and
   the upgrade token names summarized in the following table.

   +======+===================+=========================+=========+
   | Name | Description       | Expected Version Tokens | Section |
   +======+===================+=========================+=========+
   | HTTP | Hypertext         | any DIGIT.DIGIT (e.g.,  | 2.5     |
   |      | Transfer Protocol | "2.0")                  |         |
   +------+-------------------+-------------------------+---------+

                               Table 12

19.  References

19.1.  Normative References

   [CACHING]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", STD 98, RFC 9111,
              DOI 10.17487/RFC9111, June 2022,
              <https://www.rfc-editor.org/info/rfc9111>.

   [RFC1950]  Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
              Specification version 3.3", RFC 1950,
              DOI 10.17487/RFC1950, May 1996,
              <https://www.rfc-editor.org/info/rfc1950>.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
              <https://www.rfc-editor.org/info/rfc1951>.

   [RFC1952]  Deutsch, P., "GZIP file format specification version 4.3",
              RFC 1952, DOI 10.17487/RFC1952, May 1996,
              <https://www.rfc-editor.org/info/rfc1952>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4647]  Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
              Tags", BCP 47, RFC 4647, DOI 10.17487/RFC4647, September
              2006, <https://www.rfc-editor.org/info/rfc4647>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              DOI 10.17487/RFC6365, September 2011,
              <https://www.rfc-editor.org/info/rfc6365>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [USASCII]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

   [Welch]    Welch, T., "A Technique for High-Performance Data
              Compression", IEEE Computer 17(6),
              DOI 10.1109/MC.1984.1659158, June 1984,
              <https://ieeexplore.ieee.org/document/1659158/>.

19.2.  Informative References

   [ALTSVC]   Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/info/rfc7838>.

   [BCP13]    Freed, N. and J. Klensin, "Multipurpose Internet Mail
              Extensions (MIME) Part Four: Registration Procedures",
              BCP 13, RFC 4289, December 2005.

              Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, January 2013.

              <https://www.rfc-editor.org/info/bcp13>

   [BCP178]   Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648, June 2012.

              <https://www.rfc-editor.org/info/bcp178>

   [BCP35]    Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, June 2015.

              <https://www.rfc-editor.org/info/bcp35>

   [BREACH]   Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving the
              CRIME Attack", July 2013,
              <http://breachattack.com/resources/
              BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.

   [Bujlow]   Bujlow, T., Carela-Español, V., Solé-Pareta, J., and P.
              Barlet-Ros, "A Survey on Web Tracking: Mechanisms,
              Implications, and Defenses", In Proceedings of the IEEE
              105(8), DOI 10.1109/JPROC.2016.2637878, August 2017,
              <https://doi.org/10.1109/JPROC.2016.2637878>.

   [COOKIE]   Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [Err1912]  RFC Errata, Erratum ID 1912, RFC 2978,
              <https://www.rfc-editor.org/errata/eid1912>.

   [Err5433]  RFC Errata, Erratum ID 5433, RFC 2978,
              <https://www.rfc-editor.org/errata/eid5433>.

   [Georgiev] Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
              D., and V. Shmatikov, "The Most Dangerous Code in the
              World: Validating SSL Certificates in Non-Browser
              Software", In Proceedings of the 2012 ACM Conference on
              Computer and Communications Security (CCS '12), pp. 38-49,
              DOI 10.1145/2382196.2382204, October 2012,
              <https://doi.org/10.1145/2382196.2382204>.

   [HPACK]    Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
              <https://www.rfc-editor.org/info/rfc7541>.

   [HTTP/1.0] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945,
              DOI 10.17487/RFC1945, May 1996,
              <https://www.rfc-editor.org/info/rfc1945>.

   [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <https://www.rfc-editor.org/info/rfc9112>.

   [HTTP/2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,
              <https://www.rfc-editor.org/info/rfc9113>.

   [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/info/rfc9114>.

   [ISO-8859-1]
              International Organization for Standardization,
              "Information technology -- 8-bit single-byte coded graphic
              character sets -- Part 1: Latin alphabet No. 1", ISO/
              IEC 8859-1:1998, 1998.

   [Kri2001]  Kristol, D., "HTTP Cookies: Standards, Privacy, and
              Politics", ACM Transactions on Internet Technology 1(2),
              November 2001, <http://arxiv.org/abs/cs.SE/0105018>.

   [OWASP]    The Open Web Application Security Project,
              <https://www.owasp.org/>.

   [REST]     Fielding, R.T., "Architectural Styles and the Design of
              Network-based Software Architectures", Doctoral
              Dissertation, University of California, Irvine, September
              2000, <https://roy.gbiv.com/pubs/dissertation/top.htm>.

   [RFC1919]  Chatel, M., "Classical versus Transparent IP Proxies",
              RFC 1919, DOI 10.17487/RFC1919, March 1996,
              <https://www.rfc-editor.org/info/rfc1919>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/info/rfc2047>.

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, DOI 10.17487/RFC2068, January 1997,
              <https://www.rfc-editor.org/info/rfc2068>.

   [RFC2145]  Mogul, J. C., Fielding, R., Gettys, J., and H. Frystyk,
              "Use and Interpretation of HTTP Version Numbers",
              RFC 2145, DOI 10.17487/RFC2145, May 1997,
              <https://www.rfc-editor.org/info/rfc2145>.

   [RFC2295]  Holtman, K. and A. Mutz, "Transparent Content Negotiation
              in HTTP", RFC 2295, DOI 10.17487/RFC2295, March 1998,
              <https://www.rfc-editor.org/info/rfc2295>.

   [RFC2324]  Masinter, L., "Hyper Text Coffee Pot Control Protocol
              (HTCPCP/1.0)", RFC 2324, DOI 10.17487/RFC2324, 1 April
              1998, <https://www.rfc-editor.org/info/rfc2324>.

   [RFC2557]  Palme, J., Hopmann, A., and N. Shelness, "MIME
              Encapsulation of Aggregate Documents, such as HTML
              (MHTML)", RFC 2557, DOI 10.17487/RFC2557, March 1999,
              <https://www.rfc-editor.org/info/rfc2557>.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, DOI 10.17487/RFC2617, June 1999,
              <https://www.rfc-editor.org/info/rfc2617>.

   [RFC2774]  Nielsen, H., Leach, P., and S. Lawrence, "An HTTP
              Extension Framework", RFC 2774, DOI 10.17487/RFC2774,
              February 2000, <https://www.rfc-editor.org/info/rfc2774>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <https://www.rfc-editor.org/info/rfc2978>.

   [RFC3040]  Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
              Replication and Caching Taxonomy", RFC 3040,
              DOI 10.17487/RFC3040, January 2001,
              <https://www.rfc-editor.org/info/rfc3040>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC3875]  Robinson, D. and K. Coar, "The Common Gateway Interface
              (CGI) Version 1.1", RFC 3875, DOI 10.17487/RFC3875,
              October 2004, <https://www.rfc-editor.org/info/rfc3875>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,
              <https://www.rfc-editor.org/info/rfc4559>.

   [RFC5789]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",
              RFC 5789, DOI 10.17487/RFC5789, March 2010,
              <https://www.rfc-editor.org/info/rfc5789>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
              Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
              <https://www.rfc-editor.org/info/rfc6585>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <https://www.rfc-editor.org/info/rfc7232>.

   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              RFC 7233, DOI 10.17487/RFC7233, June 2014,
              <https://www.rfc-editor.org/info/rfc7233>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [RFC7538]  Reschke, J., "The Hypertext Transfer Protocol Status Code
              308 (Permanent Redirect)", RFC 7538, DOI 10.17487/RFC7538,
              April 2015, <https://www.rfc-editor.org/info/rfc7538>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [RFC7578]  Masinter, L., "Returning Values from Forms: multipart/
              form-data", RFC 7578, DOI 10.17487/RFC7578, July 2015,
              <https://www.rfc-editor.org/info/rfc7578>.

   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-
              Authentication-Info Response Header Fields", RFC 7615,
              DOI 10.17487/RFC7615, September 2015,
              <https://www.rfc-editor.org/info/rfc7615>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <https://www.rfc-editor.org/info/rfc7616>.

   [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              RFC 7617, DOI 10.17487/RFC7617, September 2015,
              <https://www.rfc-editor.org/info/rfc7617>.

   [RFC7694]  Reschke, J., "Hypertext Transfer Protocol (HTTP) Client-
              Initiated Content-Encoding", RFC 7694,
              DOI 10.17487/RFC7694, November 2015,
              <https://www.rfc-editor.org/info/rfc7694>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8187]  Reschke, J., "Indicating Character Encoding and Language
              for HTTP Header Field Parameters", RFC 8187,
              DOI 10.17487/RFC8187, September 2017,
              <https://www.rfc-editor.org/info/rfc8187>.

   [RFC8246]  McManus, P., "HTTP Immutable Responses", RFC 8246,
              DOI 10.17487/RFC8246, September 2017,
              <https://www.rfc-editor.org/info/rfc8246>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [RFC8336]  Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
              RFC 8336, DOI 10.17487/RFC8336, March 2018,
              <https://www.rfc-editor.org/info/rfc8336>.

   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
              <https://www.rfc-editor.org/info/rfc8615>.

   [RFC8941]  Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/info/rfc8941>.

   [Sniffing] WHATWG, "MIME Sniffing",
              <https://mimesniff.spec.whatwg.org>.

   [WEBDAV]   Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918,
              DOI 10.17487/RFC4918, June 2007,
              <https://www.rfc-editor.org/info/rfc4918>.

Appendix A.  Collected ABNF

   In the collected ABNF below, list rules are expanded per
   Section 5.6.1.

   Accept = [ ( media-range [ weight ] ) *( OWS "," OWS ( media-range [
    weight ] ) ) ]
   Accept-Charset = [ ( ( token / "*" ) [ weight ] ) *( OWS "," OWS ( (
    token / "*" ) [ weight ] ) ) ]
   Accept-Encoding = [ ( codings [ weight ] ) *( OWS "," OWS ( codings [
    weight ] ) ) ]
   Accept-Language = [ ( language-range [ weight ] ) *( OWS "," OWS (
    language-range [ weight ] ) ) ]
   Accept-Ranges = acceptable-ranges
   Allow = [ method *( OWS "," OWS method ) ]
   Authentication-Info = [ auth-param *( OWS "," OWS auth-param ) ]
   Authorization = credentials

   BWS = OWS

   Connection = [ connection-option *( OWS "," OWS connection-option )
    ]
   Content-Encoding = [ content-coding *( OWS "," OWS content-coding )
    ]
   Content-Language = [ language-tag *( OWS "," OWS language-tag ) ]
   Content-Length = 1*DIGIT
   Content-Location = absolute-URI / partial-URI
   Content-Range = range-unit SP ( range-resp / unsatisfied-range )
   Content-Type = media-type

   Date = HTTP-date

   ETag = entity-tag
   Expect = [ expectation *( OWS "," OWS expectation ) ]

   From = mailbox

   GMT = %x47.4D.54 ; GMT

   HTTP-date = IMF-fixdate / obs-date
   Host = uri-host [ ":" port ]

   IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT
   If-Match = "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]
   If-Modified-Since = HTTP-date
   If-None-Match = "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]
   If-Range = entity-tag / HTTP-date
   If-Unmodified-Since = HTTP-date

   Last-Modified = HTTP-date
   Location = URI-reference

   Max-Forwards = 1*DIGIT

   OWS = *( SP / HTAB )

   Proxy-Authenticate = [ challenge *( OWS "," OWS challenge ) ]
   Proxy-Authentication-Info = [ auth-param *( OWS "," OWS auth-param )
    ]
   Proxy-Authorization = credentials

   RWS = 1*( SP / HTAB )
   Range = ranges-specifier
   Referer = absolute-URI / partial-URI
   Retry-After = HTTP-date / delay-seconds

   Server = product *( RWS ( product / comment ) )

   TE = [ t-codings *( OWS "," OWS t-codings ) ]
   Trailer = [ field-name *( OWS "," OWS field-name ) ]

   URI-reference = <URI-reference, see [URI], Section 4.1>
   Upgrade = [ protocol *( OWS "," OWS protocol ) ]
   User-Agent = product *( RWS ( product / comment ) )

   Vary = [ ( "*" / field-name ) *( OWS "," OWS ( "*" / field-name ) )
    ]
   Via = [ ( received-protocol RWS received-by [ RWS comment ] ) *( OWS
    "," OWS ( received-protocol RWS received-by [ RWS comment ] ) ) ]

   WWW-Authenticate = [ challenge *( OWS "," OWS challenge ) ]

   absolute-URI = <absolute-URI, see [URI], Section 4.3>
   absolute-path = 1*( "/" segment )
   acceptable-ranges = range-unit *( OWS "," OWS range-unit )
   asctime-date = day-name SP date3 SP time-of-day SP year
   auth-param = token BWS "=" BWS ( token / quoted-string )
   auth-scheme = token
   authority = <authority, see [URI], Section 3.2>

   challenge = auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","
    OWS auth-param ) ] ) ]
   codings = content-coding / "identity" / "*"
   comment = "(" *( ctext / quoted-pair / comment ) ")"
   complete-length = 1*DIGIT
   connection-option = token
   content-coding = token
   credentials = auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","
    OWS auth-param ) ] ) ]
   ctext = HTAB / SP / %x21-27 ; '!'-'''
    / %x2A-5B ; '*'-'['
    / %x5D-7E ; ']'-'~'
    / obs-text

   date1 = day SP month SP year
   date2 = day "-" month "-" 2DIGIT
   date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
   day = 2DIGIT
   day-name = %x4D.6F.6E ; Mon
    / %x54.75.65 ; Tue
    / %x57.65.64 ; Wed
    / %x54.68.75 ; Thu
    / %x46.72.69 ; Fri
    / %x53.61.74 ; Sat
    / %x53.75.6E ; Sun
   day-name-l = %x4D.6F.6E.64.61.79 ; Monday
    / %x54.75.65.73.64.61.79 ; Tuesday
    / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
    / %x54.68.75.72.73.64.61.79 ; Thursday
    / %x46.72.69.64.61.79 ; Friday
    / %x53.61.74.75.72.64.61.79 ; Saturday
    / %x53.75.6E.64.61.79 ; Sunday
   delay-seconds = 1*DIGIT

   entity-tag = [ weak ] opaque-tag
   etagc = "!" / %x23-7E ; '#'-'~'
    / obs-text
   expectation = token [ "=" ( token / quoted-string ) parameters ]

   field-content = field-vchar [ 1*( SP / HTAB / field-vchar )
    field-vchar ]
   field-name = token
   field-value = *field-content
   field-vchar = VCHAR / obs-text
   first-pos = 1*DIGIT

   hour = 2DIGIT
   http-URI = "http://" authority path-abempty [ "?" query ]
   https-URI = "https://" authority path-abempty [ "?" query ]

   incl-range = first-pos "-" last-pos
   int-range = first-pos "-" [ last-pos ]

   language-range = <language-range, see [RFC4647], Section 2.1>
   language-tag = <Language-Tag, see [RFC5646], Section 2.1>
   last-pos = 1*DIGIT

   mailbox = <mailbox, see [RFC5322], Section 3.4>
   media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) )
    parameters
   media-type = type "/" subtype parameters
   method = token
   minute = 2DIGIT
   month = %x4A.61.6E ; Jan
    / %x46.65.62 ; Feb
    / %x4D.61.72 ; Mar
    / %x41.70.72 ; Apr
    / %x4D.61.79 ; May
    / %x4A.75.6E ; Jun
    / %x4A.75.6C ; Jul
    / %x41.75.67 ; Aug
    / %x53.65.70 ; Sep
    / %x4F.63.74 ; Oct
    / %x4E.6F.76 ; Nov
    / %x44.65.63 ; Dec

   obs-date = rfc850-date / asctime-date
   obs-text = %x80-FF
   opaque-tag = DQUOTE *etagc DQUOTE
   other-range = 1*( %x21-2B ; '!'-'+'
    / %x2D-7E ; '-'-'~'
    )

   parameter = parameter-name "=" parameter-value
   parameter-name = token
   parameter-value = ( token / quoted-string )
   parameters = *( OWS ";" OWS [ parameter ] )
   partial-URI = relative-part [ "?" query ]
   path-abempty = <path-abempty, see [URI], Section 3.3>
   port = <port, see [URI], Section 3.2.3>
   product = token [ "/" product-version ]
   product-version = token
   protocol = protocol-name [ "/" protocol-version ]
   protocol-name = token
   protocol-version = token
   pseudonym = token

   qdtext = HTAB / SP / "!" / %x23-5B ; '#'-'['
    / %x5D-7E ; ']'-'~'
    / obs-text
   query = <query, see [URI], Section 3.4>
   quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
   quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
   qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )

   range-resp = incl-range "/" ( complete-length / "*" )
   range-set = range-spec *( OWS "," OWS range-spec )
   range-spec = int-range / suffix-range / other-range
   range-unit = token
   ranges-specifier = range-unit "=" range-set
   received-by = pseudonym [ ":" port ]
   received-protocol = [ protocol-name "/" ] protocol-version
   relative-part = <relative-part, see [URI], Section 4.2>
   rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT

   second = 2DIGIT
   segment = <segment, see [URI], Section 3.3>
   subtype = token
   suffix-length = 1*DIGIT
   suffix-range = "-" suffix-length

   t-codings = "trailers" / ( transfer-coding [ weight ] )
   tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
    "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
   time-of-day = hour ":" minute ":" second
   token = 1*tchar
   token68 = 1*( ALPHA / DIGIT / "-" / "." / "_" / "~" / "+" / "/" )
    *"="
   transfer-coding = token *( OWS ";" OWS transfer-parameter )
   transfer-parameter = token BWS "=" BWS ( token / quoted-string )
   type = token

   unsatisfied-range = "*/" complete-length
   uri-host = <host, see [URI], Section 3.2.2>

   weak = %x57.2F ; W/
   weight = OWS ";" OWS "q=" qvalue

   year = 4DIGIT

Appendix B.  Changes from Previous RFCs

B.1.  Changes from RFC 2818

   None.

B.2.  Changes from RFC 7230

   The sections introducing HTTP's design goals, history, architecture,
   conformance criteria, protocol versioning, URIs, message routing, and
   header fields have been moved here.

   The requirement on semantic conformance has been replaced with
   permission to ignore or work around implementation-specific failures.
   (Section 2.2)

   The description of an origin and authoritative access to origin
   servers has been extended for both "http" and "https" URIs to account
   for alternative services and secured connections that are not
   necessarily based on TCP.  (Sections 4.2.1, 4.2.2, 4.3.1, and 7.3.3)

   Explicit requirements have been added to check the target URI
   scheme's semantics and reject requests that don't meet any associated
   requirements.  (Section 7.4)

   Parameters in media type, media range, and expectation can be empty
   via one or more trailing semicolons.  (Section 5.6.6)

   "Field value" now refers to the value after multiple field lines are
   combined with commas -- by far the most common use.  To refer to a
   single header line's value, use "field line value".  (Section 6.3)

   Trailer field semantics now transcend the specifics of chunked
   transfer coding.  The use of trailer fields has been further limited
   to allow generation as a trailer field only when the sender knows the
   field defines that usage and to allow merging into the header section
   only if the recipient knows the corresponding field definition
   permits and defines how to merge.  In all other cases,
   implementations are encouraged either to store the trailer fields
   separately or to discard them instead of merging.  (Section 6.5.1)

   The priority of the absolute form of the request URI over the Host
   header field by origin servers has been made explicit to align with
   proxy handling.  (Section 7.2)

   The grammar definition for the Via field's "received-by" was expanded
   in RFC 7230 due to changes in the URI grammar for host [URI] that are
   not desirable for Via. For simplicity, we have removed uri-host from
   the received-by production because it can be encompassed by the
   existing grammar for pseudonym.  In particular, this change removed
   comma from the allowed set of characters for a host name in received-
   by.  (Section 7.6.3)

B.3.  Changes from RFC 7231

   Minimum URI lengths to be supported by implementations are now
   recommended.  (Section 4.1)

   The following have been clarified: CR and NUL in field values are to
   be rejected or mapped to SP, and leading and trailing whitespace
   needs to be stripped from field values before they are consumed.
   (Section 5.5)

   Parameters in media type, media range, and expectation can be empty
   via one or more trailing semicolons.  (Section 5.6.6)

   An abstract data type for HTTP messages has been introduced to define
   the components of a message and their semantics as an abstraction
   across multiple HTTP versions, rather than in terms of the specific
   syntax form of HTTP/1.1 in [HTTP/1.1], and reflect the contents after
   the message is parsed.  This makes it easier to distinguish between
   requirements on the content (what is conveyed) versus requirements on
   the messaging syntax (how it is conveyed) and avoids baking
   limitations of early protocol versions into the future of HTTP.
   (Section 6)

   The terms "payload" and "payload body" have been replaced with
   "content", to better align with its usage elsewhere (e.g., in field
   names) and to avoid confusion with frame payloads in HTTP/2 and
   HTTP/3.  (Section 6.4)

   The term "effective request URI" has been replaced with "target URI".
   (Section 7.1)

   Restrictions on client retries have been loosened to reflect
   implementation behavior.  (Section 9.2.2)

   The fact that request bodies on GET, HEAD, and DELETE are not
   interoperable has been clarified.  (Sections 9.3.1, 9.3.2, and 9.3.5)

   The use of the Content-Range header field (Section 14.4) as a request
   modifier on PUT is allowed.  (Section 9.3.4)

   A superfluous requirement about setting Content-Length has been
   removed from the description of the OPTIONS method.  (Section 9.3.7)

   The normative requirement to use the "message/http" media type in
   TRACE responses has been removed.  (Section 9.3.8)

   List-based grammar for Expect has been restored for compatibility
   with RFC 2616.  (Section 10.1.1)

   Accept and Accept-Encoding are allowed in response messages; the
   latter was introduced by [RFC7694].  (Section 12.3)

   "Accept Parameters" (accept-params and accept-ext ABNF production)
   have been removed from the definition of the Accept field.
   (Section 12.5.1)

   The Accept-Charset field is now deprecated.  (Section 12.5.2)

   The semantics of "*" in the Vary header field when other values are
   present was clarified.  (Section 12.5.5)

   Range units are compared in a case-insensitive fashion.
   (Section 14.1)

   The use of the Accept-Ranges field is not restricted to origin
   servers.  (Section 14.3)

   The process of creating a redirected request has been clarified.
   (Section 15.4)

   Status code 308 (previously defined in [RFC7538]) has been added so
   that it's defined closer to status codes 301, 302, and 307.
   (Section 15.4.9)

   Status code 421 (previously defined in Section 9.1.2 of [RFC7540])
   has been added because of its general applicability. 421 is no longer
   defined as heuristically cacheable since the response is specific to
   the connection (not the target resource).  (Section 15.5.20)

   Status code 422 (previously defined in Section 11.2 of [WEBDAV]) has
   been added because of its general applicability.  (Section 15.5.21)

B.4.  Changes from RFC 7232

   Previous revisions of HTTP imposed an arbitrary 60-second limit on
   the determination of whether Last-Modified was a strong validator to
   guard against the possibility that the Date and Last-Modified values
   are generated from different clocks or at somewhat different times
   during the preparation of the response.  This specification has
   relaxed that to allow reasonable discretion.  (Section 8.8.2.2)

   An edge-case requirement on If-Match and If-Unmodified-Since has been
   removed that required a validator not to be sent in a 2xx response if
   validation fails because the change request has already been applied.
   (Sections 13.1.1 and 13.1.4)

   The fact that If-Unmodified-Since does not apply to a resource
   without a concept of modification time has been clarified.
   (Section 13.1.4)

   Preconditions can now be evaluated before the request content is
   processed rather than waiting until the response would otherwise be
   successful.  (Section 13.2)

B.5.  Changes from RFC 7233

   Refactored the range-unit and ranges-specifier grammars to simplify
   and reduce artificial distinctions between bytes and other
   (extension) range units, removing the overlapping grammar of other-
   range-unit by defining range units generically as a token and placing
   extensions within the scope of a range-spec (other-range).  This
   disambiguates the role of list syntax (commas) in all range sets,
   including extension range units, for indicating a range-set of more
   than one range.  Moving the extension grammar into range specifiers
   also allows protocol specific to byte ranges to be specified
   separately.

   It is now possible to define Range handling on extension methods.
   (Section 14.2)

   Described use of the Content-Range header field (Section 14.4) as a
   request modifier to perform a partial PUT.  (Section 14.5)

B.6.  Changes from RFC 7235

   None.

B.7.  Changes from RFC 7538

   None.

B.8.  Changes from RFC 7615

   None.

B.9.  Changes from RFC 7694

   This specification includes the extension defined in [RFC7694] but
   leaves out examples and deployment considerations.

Acknowledgements

   Aside from the current editors, the following individuals deserve
   special recognition for their contributions to early aspects of HTTP
   and its core specifications: Marc Andreessen, Tim Berners-Lee, Robert
   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jim Gettys,
   Jean-François Groff, Phillip M. Hallam-Baker, Koen Holtman, Jeffery
   L. Hostetler, Shel Kaphan, Dave Kristol, Yves Lafon, Scott
   D. Lawrence, Paul J. Leach, Håkon W. Lie, Ari Luotonen, Larry
   Masinter, Rob McCool, Jeffrey C. Mogul, Lou Montulli, David Morris,
   Henrik Frystyk Nielsen, Dave Raggett, Eric Rescorla, Tony Sanders,
   Lawrence C. Stewart, Marc VanHeyningen, and Steve Zilles.

   This document builds on the many contributions that went into past
   specifications of HTTP, including [HTTP/1.0], [RFC2068], [RFC2145],
   [RFC2616], [RFC2617], [RFC2818], [RFC7230], [RFC7231], [RFC7232],
   [RFC7233], [RFC7234], and [RFC7235].  The acknowledgements within
   those documents still apply.

   Since 2014, the following contributors have helped improve this
   specification by reporting bugs, asking smart questions, drafting or
   reviewing text, and evaluating issues:

   Alan Egerton, Alex Rousskov, Amichai Rothman, Amos Jeffries, Anders
   Kaseorg, Andreas Gebhardt, Anne van Kesteren, Armin Abfalterer, Aron
   Duby, Asanka Herath, Asbjørn Ulsberg, Asta Olofsson, Attila Gulyas,
   Austin Wright, Barry Pollard, Ben Burkert, Benjamin Kaduk, Björn
   Höhrmann, Brad Fitzpatrick, Chris Pacejo, Colin Bendell, Cory
   Benfield, Cory Nelson, Daisuke Miyakawa, Dale Worley, Daniel
   Stenberg, Danil Suits, David Benjamin, David Matson, David Schinazi,
   Дилян Палаузов (Dilyan Palauzov), Eric Anderson, Eric Rescorla, Éric
   Vyncke, Erik Kline, Erwin Pe, Etan Kissling, Evert Pot, Evgeny
   Vrublevsky, Florian Best, Francesca Palombini, Igor Lubashev, James
   Callahan, James Peach, Jeffrey Yasskin, Kalin Gyokov, Kannan Goundan,
   奥 一穂 (Kazuho Oku), Ken Murchison, Krzysztof Maczyński, Lars Eggert,
   Lucas Pardue, Martin Duke, Martin Dürst, Martin Thomson, Martynas
   Jusevičius, Matt Menke, Matthias Pigulla, Mattias Grenfeldt, Michael
   Osipov, Mike Bishop, Mike Pennisi, Mike Taylor, Mike West, Mohit
   Sethi, Murray Kucherawy, Nathaniel J. Smith, Nicholas Hurley, Nikita
   Prokhorov, Patrick McManus, Piotr Sikora, Poul-Henning Kamp, Rick van
   Rein, Robert Wilton, Roberto Polli, Roman Danyliw, Samuel Williams,
   Semyon Kholodnov, Simon Pieters, Simon Schüppel, Stefan Eissing,
   Taylor Hunt, Todd Greer, Tommy Pauly, Vasiliy Faronov, Vladimir
   Lashchev, Wenbo Zhu, William A. Rowe Jr., Willy Tarreau, Xingwei Liu,
   Yishuai Li, and Zaheduzzaman Sarker.

Index

   1 2 3 4 5 A B C D E F G H I L M N O P R S T U V W X

      1

         100 Continue (status code)  *_Section 15.2.1_*
         100-continue (expect value)  *_Section 10.1.1_*
         101 Switching Protocols (status code)  *_Section 15.2.2_*
         1xx Informational (status code class)  *_Section 15.2_*

      2

         200 OK (status code)  *_Section 15.3.1_*
         201 Created (status code)  *_Section 15.3.2_*
         202 Accepted (status code)  *_Section 15.3.3_*
         203 Non-Authoritative Information (status code)  *_Section 15.3
            .4_*
         204 No Content (status code)  *_Section 15.3.5_*
         205 Reset Content (status code)  *_Section 15.3.6_*
         206 Partial Content (status code)  *_Section 15.3.7_*
         2xx Successful (status code class)  *_Section 15.3_*

      3

         300 Multiple Choices (status code)  *_Section 15.4.1_*
         301 Moved Permanently (status code)  *_Section 15.4.2_*
         302 Found (status code)  *_Section 15.4.3_*
         303 See Other (status code)  *_Section 15.4.4_*
         304 Not Modified (status code)  *_Section 15.4.5_*
         305 Use Proxy (status code)  *_Section 15.4.6_*
         306 (Unused) (status code)  *_Section 15.4.7_*
         307 Temporary Redirect (status code)  *_Section 15.4.8_*
         308 Permanent Redirect (status code)  *_Section 15.4.9_*
         3xx Redirection (status code class)  *_Section 15.4_*

      4

         400 Bad Request (status code)  *_Section 15.5.1_*
         401 Unauthorized (status code)  *_Section 15.5.2_*
         402 Payment Required (status code)  *_Section 15.5.3_*
         403 Forbidden (status code)  *_Section 15.5.4_*
         404 Not Found (status code)  *_Section 15.5.5_*
         405 Method Not Allowed (status code)  *_Section 15.5.6_*
         406 Not Acceptable (status code)  *_Section 15.5.7_*
         407 Proxy Authentication Required (status code)  *_Section 15.5
            .8_*
         408 Request Timeout (status code)  *_Section 15.5.9_*
         409 Conflict (status code)  *_Section 15.5.10_*
         410 Gone (status code)  *_Section 15.5.11_*
         411 Length Required (status code)  *_Section 15.5.12_*
         412 Precondition Failed (status code)  *_Section 15.5.13_*
         413 Content Too Large (status code)  *_Section 15.5.14_*
         414 URI Too Long (status code)  *_Section 15.5.15_*
         415 Unsupported Media Type (status code)  *_Section 15.5.16_*
         416 Range Not Satisfiable (status code)  *_Section 15.5.17_*
         417 Expectation Failed (status code)  *_Section 15.5.18_*
         418 (Unused) (status code)  *_Section 15.5.19_*
         421 Misdirected Request (status code)  *_Section 15.5.20_*
         422 Unprocessable Content (status code)  *_Section 15.5.21_*
         426 Upgrade Required (status code)  *_Section 15.5.22_*
         4xx Client Error (status code class)  *_Section 15.5_*

      5

         500 Internal Server Error (status code)  *_Section 15.6.1_*
         501 Not Implemented (status code)  *_Section 15.6.2_*
         502 Bad Gateway (status code)  *_Section 15.6.3_*
         503 Service Unavailable (status code)  *_Section 15.6.4_*
         504 Gateway Timeout (status code)  *_Section 15.6.5_*
         505 HTTP Version Not Supported (status code)  *_Section 15.6.6_
            *
         5xx Server Error (status code class)  *_Section 15.6_*

      A

         accelerator  *_Section 3.7, Paragraph 6_*
         Accept header field  *_Section 12.5.1_*
         Accept-Charset header field  *_Section 12.5.2_*
         Accept-Encoding header field  *_Section 12.5.3_*
         Accept-Language header field  *_Section 12.5.4_*
         Accept-Ranges header field  *_Section 14.3_*
         Allow header field  *_Section 10.2.1_*
         Authentication-Info header field  *_Section 11.6.3_*
         authoritative response  *_Section 17.1_*
         Authorization header field  *_Section 11.6.2_*

      B

         browser  *_Section 3.5_*

      C

         cache  *_Section 3.8_*
         cacheable  *_Section 3.8, Paragraph 4_*
         client  *_Section 3.3_*
         clock  *_Section 5.6.7_*
         complete  *_Section 6.1_*
         compress (Coding Format)  Section 8.4.1.1
         compress (content coding)  *_Section 8.4.1_*
         conditional request  *_Section 13_*
         CONNECT method  *_Section 9.3.6_*
         connection  *_Section 3.3_*
         Connection header field  *_Section 7.6.1_*
         content  Section 6.4
         content coding  *_Section 8.4.1_*
         content negotiation  Section 1.3, Paragraph 4
         Content-Encoding header field  *_Section 8.4_*
         Content-Language header field  *_Section 8.5_*
         Content-Length header field  *_Section 8.6_*
         Content-Location header field  *_Section 8.7_*
         Content-MD5 header field  *_Section 18.4, Paragraph 10_*
         Content-Range header field  *_Section 14.4_*; Section 14.5
         Content-Type header field  *_Section 8.3_*
         control data  *_Section 6.2_*

      D

         Date header field  *_Section 6.6.1_*
         deflate (Coding Format)  Section 8.4.1.2
         deflate (content coding)  *_Section 8.4.1_*
         DELETE method  *_Section 9.3.5_*
         Delimiters  Section 5.6.2, Paragraph 3
         downstream  *_Section 3.7, Paragraph 4_*

      E

         effective request URI  *_Section 7.1, Paragraph 8.1_*
         ETag field  *_Section 8.8.3_*
         Expect header field  *_Section 10.1.1_*

      F

         field  *_Section 5_*; Section 6.3
         field line  Section 5.2, Paragraph 1
         field line value  Section 5.2, Paragraph 1
         field name  Section 5.2, Paragraph 1
         field value  Section 5.2, Paragraph 2
         Fields
            *  *_Section 18.4, Paragraph 9_*
            Accept  *_Section 12.5.1_*
            Accept-Charset  *_Section 12.5.2_*
            Accept-Encoding  *_Section 12.5.3_*
            Accept-Language  *_Section 12.5.4_*
            Accept-Ranges  *_Section 14.3_*
            Allow  *_Section 10.2.1_*
            Authentication-Info  *_Section 11.6.3_*
            Authorization  *_Section 11.6.2_*
            Connection  *_Section 7.6.1_*
            Content-Encoding  *_Section 8.4_*
            Content-Language  *_Section 8.5_*
            Content-Length  *_Section 8.6_*
            Content-Location  *_Section 8.7_*
            Content-MD5  *_Section 18.4, Paragraph 10_*
            Content-Range  *_Section 14.4_*; Section 14.5
            Content-Type  *_Section 8.3_*
            Date  *_Section 6.6.1_*
            ETag  *_Section 8.8.3_*
            Expect  *_Section 10.1.1_*
            From  *_Section 10.1.2_*
            Host  *_Section 7.2_*
            If-Match  *_Section 13.1.1_*
            If-Modified-Since  *_Section 13.1.3_*
            If-None-Match  *_Section 13.1.2_*
            If-Range  *_Section 13.1.5_*
            If-Unmodified-Since  *_Section 13.1.4_*
            Last-Modified  *_Section 8.8.2_*
            Location  *_Section 10.2.2_*
            Max-Forwards  *_Section 7.6.2_*
            Proxy-Authenticate  *_Section 11.7.1_*
            Proxy-Authentication-Info  *_Section 11.7.3_*
            Proxy-Authorization  *_Section 11.7.2_*
            Range  *_Section 14.2_*
            Referer  *_Section 10.1.3_*
            Retry-After  *_Section 10.2.3_*
            Server  *_Section 10.2.4_*
            TE  *_Section 10.1.4_*
            Trailer  *_Section 6.6.2_*
            Upgrade  *_Section 7.8_*
            User-Agent  *_Section 10.1.5_*
            Vary  *_Section 12.5.5_*
            Via  *_Section 7.6.3_*
            WWW-Authenticate  *_Section 11.6.1_*
         Fragment Identifiers  Section 4.2.5
         From header field  *_Section 10.1.2_*

      G

         gateway  *_Section 3.7, Paragraph 6_*
         GET method  *_Section 9.3.1_*
         Grammar
            ALPHA  *_Section 2.1_*
            Accept  *_Section 12.5.1_*
            Accept-Charset  *_Section 12.5.2_*
            Accept-Encoding  *_Section 12.5.3_*
            Accept-Language  *_Section 12.5.4_*
            Accept-Ranges  *_Section 14.3_*
            Allow  *_Section 10.2.1_*
            Authentication-Info  *_Section 11.6.3_*
            Authorization  *_Section 11.6.2_*
            BWS  *_Section 5.6.3_*
            CR  *_Section 2.1_*
            CRLF  *_Section 2.1_*
            CTL  *_Section 2.1_*
            Connection  *_Section 7.6.1_*
            Content-Encoding  *_Section 8.4_*
            Content-Language  *_Section 8.5_*
            Content-Length  *_Section 8.6_*
            Content-Location  *_Section 8.7_*
            Content-Range  *_Section 14.4_*
            Content-Type  *_Section 8.3_*
            DIGIT  *_Section 2.1_*
            DQUOTE  *_Section 2.1_*
            Date  *_Section 6.6.1_*
            ETag  *_Section 8.8.3_*
            Expect  *_Section 10.1.1_*
            From  *_Section 10.1.2_*
            GMT  *_Section 5.6.7_*
            HEXDIG  *_Section 2.1_*
            HTAB  *_Section 2.1_*
            HTTP-date  *_Section 5.6.7_*
            Host  *_Section 7.2_*
            IMF-fixdate  *_Section 5.6.7_*
            If-Match  *_Section 13.1.1_*
            If-Modified-Since  *_Section 13.1.3_*
            If-None-Match  *_Section 13.1.2_*
            If-Range  *_Section 13.1.5_*
            If-Unmodified-Since  *_Section 13.1.4_*
            LF  *_Section 2.1_*
            Last-Modified  *_Section 8.8.2_*
            Location  *_Section 10.2.2_*
            Max-Forwards  *_Section 7.6.2_*
            OCTET  *_Section 2.1_*
            OWS  *_Section 5.6.3_*
            Proxy-Authenticate  *_Section 11.7.1_*
            Proxy-Authentication-Info  *_Section 11.7.3_*
            Proxy-Authorization  *_Section 11.7.2_*
            RWS  *_Section 5.6.3_*
            Range  *_Section 14.2_*
            Referer  *_Section 10.1.3_*
            Retry-After  *_Section 10.2.3_*
            SP  *_Section 2.1_*
            Server  *_Section 10.2.4_*
            TE  *_Section 10.1.4_*
            Trailer  *_Section 6.6.2_*
            URI-reference  *_Section 4.1_*
            Upgrade  *_Section 7.8_*
            User-Agent  *_Section 10.1.5_*
            VCHAR  *_Section 2.1_*
            Vary  *_Section 12.5.5_*
            Via  *_Section 7.6.3_*
            WWW-Authenticate  *_Section 11.6.1_*
            absolute-URI  *_Section 4.1_*
            absolute-path  *_Section 4.1_*
            acceptable-ranges  *_Section 14.3_*
            asctime-date  *_Section 5.6.7_*
            auth-param  *_Section 11.2_*
            auth-scheme  *_Section 11.1_*
            authority  *_Section 4.1_*
            challenge  *_Section 11.3_*
            codings  *_Section 12.5.3_*
            comment  *_Section 5.6.5_*
            complete-length  *_Section 14.4_*
            connection-option  *_Section 7.6.1_*
            content-coding  *_Section 8.4.1_*
            credentials  *_Section 11.4_*
            ctext  *_Section 5.6.5_*
            date1  *_Section 5.6.7_*
            day  *_Section 5.6.7_*
            day-name  *_Section 5.6.7_*
            day-name-l  *_Section 5.6.7_*
            delay-seconds  *_Section 10.2.3_*
            entity-tag  *_Section 8.8.3_*
            etagc  *_Section 8.8.3_*
            field-content  *_Section 5.5_*
            field-name  *_Section 5.1_*; Section 6.6.2
            field-value  *_Section 5.5_*
            field-vchar  *_Section 5.5_*
            first-pos  *_Section 14.1.1_*; Section 14.4
            hour  *_Section 5.6.7_*
            http-URI  *_Section 4.2.1_*
            https-URI  *_Section 4.2.2_*
            incl-range  *_Section 14.4_*
            int-range  *_Section 14.1.1_*
            language-range  *_Section 12.5.4_*
            language-tag  *_Section 8.5.1_*
            last-pos  *_Section 14.1.1_*; Section 14.4
            media-range  *_Section 12.5.1_*
            media-type  *_Section 8.3.1_*
            method  *_Section 9.1_*
            minute  *_Section 5.6.7_*
            month  *_Section 5.6.7_*
            obs-date  *_Section 5.6.7_*
            obs-text  *_Section 5.5_*
            opaque-tag  *_Section 8.8.3_*
            other-range  *_Section 14.1.1_*
            parameter  *_Section 5.6.6_*
            parameter-name  *_Section 5.6.6_*
            parameter-value  *_Section 5.6.6_*
            parameters  *_Section 5.6.6_*
            partial-URI  *_Section 4.1_*
            port  *_Section 4.1_*
            product  *_Section 10.1.5_*
            product-version  *_Section 10.1.5_*
            protocol-name  *_Section 7.6.3_*
            protocol-version  *_Section 7.6.3_*
            pseudonym  *_Section 7.6.3_*
            qdtext  *_Section 5.6.4_*
            query  *_Section 4.1_*
            quoted-pair  *_Section 5.6.4_*
            quoted-string  *_Section 5.6.4_*
            qvalue  *_Section 12.4.2_*
            range-resp  *_Section 14.4_*
            range-set  *_Section 14.1.1_*
            range-spec  *_Section 14.1.1_*
            range-unit  *_Section 14.1_*
            ranges-specifier  *_Section 14.1.1_*
            received-by  *_Section 7.6.3_*
            received-protocol  *_Section 7.6.3_*
            rfc850-date  *_Section 5.6.7_*
            second  *_Section 5.6.7_*
            segment  *_Section 4.1_*
            subtype  *_Section 8.3.1_*
            suffix-length  *_Section 14.1.1_*
            suffix-range  *_Section 14.1.1_*
            t-codings  *_Section 10.1.4_*
            tchar  *_Section 5.6.2_*
            time-of-day  *_Section 5.6.7_*
            token  *_Section 5.6.2_*
            token68  *_Section 11.2_*
            transfer-coding  *_Section 10.1.4_*
            transfer-parameter  *_Section 10.1.4_*
            type  *_Section 8.3.1_*
            unsatisfied-range  *_Section 14.4_*
            uri-host  *_Section 4.1_*
            weak  *_Section 8.8.3_*
            weight  *_Section 12.4.2_*
            year  *_Section 5.6.7_*
         gzip (Coding Format)  Section 8.4.1.3
         gzip (content coding)  *_Section 8.4.1_*

      H

         HEAD method  *_Section 9.3.2_*
         Header Fields
            Accept  *_Section 12.5.1_*
            Accept-Charset  *_Section 12.5.2_*
            Accept-Encoding  *_Section 12.5.3_*
            Accept-Language  *_Section 12.5.4_*
            Accept-Ranges  *_Section 14.3_*
            Allow  *_Section 10.2.1_*
            Authentication-Info  *_Section 11.6.3_*
            Authorization  *_Section 11.6.2_*
            Connection  *_Section 7.6.1_*
            Content-Encoding  *_Section 8.4_*
            Content-Language  *_Section 8.5_*
            Content-Length  *_Section 8.6_*
            Content-Location  *_Section 8.7_*
            Content-MD5  *_Section 18.4, Paragraph 10_*
            Content-Range  *_Section 14.4_*; Section 14.5
            Content-Type  *_Section 8.3_*
            Date  *_Section 6.6.1_*
            ETag  *_Section 8.8.3_*
            Expect  *_Section 10.1.1_*
            From  *_Section 10.1.2_*
            Host  *_Section 7.2_*
            If-Match  *_Section 13.1.1_*
            If-Modified-Since  *_Section 13.1.3_*
            If-None-Match  *_Section 13.1.2_*
            If-Range  *_Section 13.1.5_*
            If-Unmodified-Since  *_Section 13.1.4_*
            Last-Modified  *_Section 8.8.2_*
            Location  *_Section 10.2.2_*
            Max-Forwards  *_Section 7.6.2_*
            Proxy-Authenticate  *_Section 11.7.1_*
            Proxy-Authentication-Info  *_Section 11.7.3_*
            Proxy-Authorization  *_Section 11.7.2_*
            Range  *_Section 14.2_*
            Referer  *_Section 10.1.3_*
            Retry-After  *_Section 10.2.3_*
            Server  *_Section 10.2.4_*
            TE  *_Section 10.1.4_*
            Trailer  *_Section 6.6.2_*
            Upgrade  *_Section 7.8_*
            User-Agent  *_Section 10.1.5_*
            Vary  *_Section 12.5.5_*
            Via  *_Section 7.6.3_*
            WWW-Authenticate  *_Section 11.6.1_*
         header section  *_Section 6.3_*
         Host header field  *_Section 7.2_*
         http URI scheme  *_Section 4.2.1_*
         https URI scheme  *_Section 4.2.2_*

      I

         idempotent  *_Section 9.2.2_*
         If-Match header field  *_Section 13.1.1_*
         If-Modified-Since header field  *_Section 13.1.3_*
         If-None-Match header field  *_Section 13.1.2_*
         If-Range header field  *_Section 13.1.5_*
         If-Unmodified-Since header field  *_Section 13.1.4_*
         inbound  *_Section 3.7, Paragraph 4_*
         incomplete  *_Section 6.1_*
         interception proxy  *_Section 3.7, Paragraph 10_*
         intermediary  *_Section 3.7_*

      L

         Last-Modified header field  *_Section 8.8.2_*
         list-based field  Section 5.5, Paragraph 7
         Location header field  *_Section 10.2.2_*

      M

         Max-Forwards header field  *_Section 7.6.2_*
         Media Type
            multipart/byteranges  *_Section 14.6_*
            multipart/x-byteranges  Section 14.6, Paragraph 4, Item 3
         message  Section 3.4; *_Section 6_*
         message abstraction  *_Section 6_*
         messages  *_Section 3.4_*
         metadata  *_Section 8.8_*
         Method
            *  *_Section 18.2, Paragraph 3_*
            CONNECT  *_Section 9.3.6_*
            DELETE  *_Section 9.3.5_*
            GET  *_Section 9.3.1_*
            HEAD  *_Section 9.3.2_*
            OPTIONS  *_Section 9.3.7_*
            POST  *_Section 9.3.3_*
            PUT  *_Section 9.3.4_*
            TRACE  *_Section 9.3.8_*
         multipart/byteranges Media Type  *_Section 14.6_*
         multipart/x-byteranges Media Type  Section 14.6, Paragraph 4,
            Item 3

      N

         non-transforming proxy  *_Section 7.7_*

      O

         OPTIONS method  *_Section 9.3.7_*
         origin  *_Section 4.3.1_*; Section 11.5
         origin server  *_Section 3.6_*
         outbound  *_Section 3.7, Paragraph 4_*

      P

         phishing  *_Section 17.1_*
         POST method  *_Section 9.3.3_*
         Protection Space  Section 11.5
         proxy  *_Section 3.7, Paragraph 5_*
         Proxy-Authenticate header field  *_Section 11.7.1_*
         Proxy-Authentication-Info header field  *_Section 11.7.3_*
         Proxy-Authorization header field  *_Section 11.7.2_*
         PUT method  *_Section 9.3.4_*

      R

         Range header field  *_Section 14.2_*
         Realm  Section 11.5
         recipient  *_Section 3.4_*
         Referer header field  *_Section 10.1.3_*
         representation  *_Section 3.2_*
         request  *_Section 3.4_*
         request target  *_Section 7.1_*
         resource  *_Section 3.1_*; Section 4
         response  *_Section 3.4_*
         Retry-After header field  *_Section 10.2.3_*
         reverse proxy  *_Section 3.7, Paragraph 6_*

      S

         safe  *_Section 9.2.1_*
         satisfiable range  *_Section 14.1.1_*
         secured  *_Section 4.2.2_*
         selected representation  *_Section 3.2, Paragraph 4_*;
            Section 8.8; Section 13.1
         self-descriptive  *_Section 6_*
         sender  *_Section 3.4_*
         server  *_Section 3.3_*
         Server header field  *_Section 10.2.4_*
         singleton field  Section 5.5, Paragraph 6
         spider  *_Section 3.5_*
         Status Code  Section 15
         Status Codes
            Final  Section 15, Paragraph 7
            Informational  Section 15, Paragraph 7
            Interim  Section 15, Paragraph 7
         Status Codes Classes
            1xx Informational  *_Section 15.2_*
            2xx Successful  *_Section 15.3_*
            3xx Redirection  *_Section 15.4_*
            4xx Client Error  *_Section 15.5_*
            5xx Server Error  *_Section 15.6_*

      T

         target resource  *_Section 7.1_*
         target URI  *_Section 7.1_*
         TE header field  *_Section 10.1.4_*
         TRACE method  *_Section 9.3.8_*
         Trailer Fields  *_Section 6.5_*
            ETag  *_Section 8.8.3_*
         Trailer header field  *_Section 6.6.2_*
         trailer section  *_Section 6.5_*
         trailers  *_Section 6.5_*
         transforming proxy  *_Section 7.7_*
         transparent proxy  *_Section 3.7, Paragraph 10_*
         tunnel  *_Section 3.7, Paragraph 8_*

      U

         unsatisfiable range  *_Section 14.1.1_*
         Upgrade header field  *_Section 7.8_*
         upstream  *_Section 3.7, Paragraph 4_*
         URI  *_Section 4_*
            origin  *_Section 4.3.1_*
         URI reference  *_Section 4.1_*
         URI scheme
            http  *_Section 4.2.1_*
            https  *_Section 4.2.2_*
         user agent  *_Section 3.5_*
         User-Agent header field  *_Section 10.1.5_*

      V

         validator  *_Section 8.8_*
            strong  *_Section 8.8.1_*
            weak  *_Section 8.8.1_*
         Vary header field  *_Section 12.5.5_*
         Via header field  *_Section 7.6.3_*

      W

         WWW-Authenticate header field  *_Section 11.6.1_*

      X

         x-compress (content coding)  *_Section 8.4.1_*
         x-gzip (content coding)  *_Section 8.4.1_*

Authors' Addresses

   Roy T. Fielding (editor)
   Adobe
   345 Park Ave
   San Jose, CA 95110
   United States of America
   Email: fielding@gbiv.com
   URI:   https://roy.gbiv.com/


   Mark Nottingham (editor)
   Fastly
   Prahran
   Australia
   Email: mnot@mnot.net
   URI:   https://www.mnot.net/


   Julian Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   48155 Münster
   Germany
   Email: julian.reschke@greenbytes.de
   URI:   https://greenbytes.de/tech/webdav/