path: root/doc/rfc/rfc9292.txt

Internet Engineering Task Force (IETF)                        M. Thomson
Request for Comments: 9292                                       Mozilla
Category: Standards Track                                     C. A. Wood
ISSN: 2070-1721                                               Cloudflare
                                                             August 2022


                 Binary Representation of HTTP Messages

Abstract

   This document defines a binary format for representing HTTP messages.

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/rfc9292.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions and Definitions
   3.  Format
     3.1.  Known-Length Messages
     3.2.  Indeterminate-Length Messages
     3.3.  Framing Indicator
     3.4.  Request Control Data
     3.5.  Response Control Data
       3.5.1.  Informational Status Codes
     3.6.  Header and Trailer Field Lines
     3.7.  Content
     3.8.  Padding and Truncation
   4.  Invalid Messages
   5.  Examples
     5.1.  Request Example
     5.2.  Response Example
   6.  Notable Differences with HTTP Protocol Messages
   7.  "message/bhttp" Media Type
   8.  Security Considerations
   9.  IANA Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   This document defines a simple format for representing an HTTP
   message [HTTP], either request or response.  This allows for the
   encoding of HTTP messages that can be conveyed outside an HTTP
   protocol.  This enables the transformation of entire messages,
   including the application of authenticated encryption.

   The design of this format is informed by the framing structure of
   HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3].  Rules for constructing messages
   rely on the rules defined in HTTP/2, but the format itself is
   distinct; see Section 6.

   This format defines "message/bhttp", a binary alternative to the
   "message/http" content type defined in [HTTP/1.1].  A binary format
   permits more efficient encoding and processing of messages.  A binary
   format also reduces exposure to security problems related to
   processing of HTTP messages.

   Two modes for encoding are described:

   *  a known-length encoding includes length prefixes for all major
      message components, and

   *  an indeterminate-length encoding enables efficient generation of
      messages where lengths are not known when encoding starts.

   This format is designed to convey the semantics of valid HTTP
   messages as simply and efficiently as possible.  It is not designed
   to capture all of the details of the encoding of messages from
   specific HTTP versions [HTTP/1.1] [HTTP/2] [HTTP/3].  As such, this
   format is unlikely to be suitable for applications that depend on an
   exact recording of the encoding of messages.

2.  Conventions and Definitions

   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 document uses terminology from HTTP [HTTP] and notation from
   QUIC (Section 1.3 of [QUIC]).

3.  Format

   Section 6 of [HTTP] defines the general structure of HTTP messages
   and composes those messages into distinct parts.  This format
   describes how those parts are composed into a sequence of bytes.  At
   a high level, binary messages are comprised of:

   1.  Framing indicator.  This format uses a single integer to describe
       framing, which describes whether the message is a request or
       response and how subsequent sections are formatted; see
       Section 3.3.

   2.  For a response, zero or more informational responses.  Each
       informational response consists of an informational status code
       and header section.

   3.  Control data.  For a request, this contains the request method
       and target.  For a response, this contains the status code.

   4.  Header section.  This contains zero or more header fields.

   5.  Content.  This is a sequence of zero or more bytes.

   6.  Trailer section.  This contains zero or more trailer fields.

   7.  Optional padding.  Any amount of zero-valued bytes.

   All lengths and numeric values are encoded using the variable-length
   integer encoding from Section 16 of [QUIC].  Integer values do not
   need to be encoded on the minimum number of bytes necessary.

3.1.  Known-Length Messages

   A request or response that has a known length at the time of
   construction uses the format shown in Figure 1.

   Known-Length Request {
     Framing Indicator (i) = 0,
     Request Control Data (..),
     Known-Length Field Section (..),
     Known-Length Content (..),
     Known-Length Field Section (..),
     Padding (..),
   }

   Known-Length Response {
     Framing Indicator (i) = 1,
     Known-Length Informational Response (..) ...,
     Final Response Control Data (..),
     Known-Length Field Section (..),
     Known-Length Content (..),
     Known-Length Field Section (..),
     Padding (..),
   }

   Known-Length Field Section {
     Length (i),
     Field Line (..) ...,
   }

   Known-Length Content {
     Content Length (i),
     Content (..),
   }

   Known-Length Informational Response {
     Informational Response Control Data (..),
     Known-Length Field Section (..),
   }

                       Figure 1: Known-Length Message

   A known-length request consists of a framing indicator (Section 3.3),
   request control data (Section 3.4), a header section with a length
   prefix, binary content with a length prefix, a trailer section with a
   length prefix, and padding.

   A known-length response contains the same fields, with the exception
   that request control data is replaced by zero or more informational
   responses (Section 3.5.1) followed by response control data
   (Section 3.5).

   For a known-length encoding, the length prefix on field sections and
   content is a variable-length encoding of an integer.  This integer is
   the number of bytes in the field section or content, not including
   the length field itself.

   Fields in the header and trailer sections consist of a length-
   prefixed name and length-prefixed value; see Section 3.6.

   The format allows for the message to be truncated before any of the
   length prefixes that precede the field sections or content; see
   Section 3.8.

   The variable-length integer encoding means that there is a limit of
   2^62-1 bytes for each field section and the message content.

3.2.  Indeterminate-Length Messages

   A request or response that is constructed without encoding a known
   length for each section uses the format shown in Figure 2:

   Indeterminate-Length Request  {
     Framing Indicator (i) = 2,
     Request Control Data (..),
     Indeterminate-Length Field Section (..),
     Indeterminate-Length Content (..),
     Indeterminate-Length Field Section (..),
     Padding (..),
   }

   Indeterminate-Length Response  {
     Framing Indicator (i) = 3,
     Indeterminate-Length Informational Response (..) ...,
     Final Response Control Data (..),
     Indeterminate-Length Field Section (..),
     Indeterminate-Length Content (..),
     Indeterminate-Length Field Section (..),
     Padding (..),
   }

   Indeterminate-Length Content {
     Indeterminate-Length Content Chunk (..) ...,
     Content Terminator (i) = 0,
   }

   Indeterminate-Length Content Chunk {
     Chunk Length (i) = 1..,
     Chunk (..),
   }

   Indeterminate-Length Field Section {
     Field Line (..) ...,
     Content Terminator (i) = 0,
   }

   Indeterminate-Length Informational Response {
     Informational Response Control Data (..),
     Indeterminate-Length Field Section (..),
   }

                   Figure 2: Indeterminate-Length Message

   An indeterminate-length request consists of a framing indicator
   (Section 3.3), request control data (Section 3.4), a header section
   that is terminated by a zero value, any number of non-zero-length
   chunks of binary content, a zero value, a trailer section that is
   terminated by a zero value, and padding.

   An indeterminate-length response contains the same fields, with the
   exception that request control data is replaced by zero or more
   informational responses (Section 3.5.1) and response control data
   (Section 3.5).

   The indeterminate-length encoding only uses length prefixes for
   content blocks.  Multiple length-prefixed portions of content can be
   included, each prefixed by a non-zero Chunk Length integer describing
   the number of bytes in the block.  The Chunk Length is encoded as a
   variable-length integer.

   Each Field Line in an Indeterminate-Length Field Section starts with
   a Name Length field.  An Indeterminate-Length Field Section ends with
   a Content Terminator field.  The zero value of the Content Terminator
   distinguishes it from the Name Length field, which cannot contain a
   value of 0.

   Indeterminate-length messages can be truncated in a way similar to
   that for known-length messages; see Section 3.8.

   Indeterminate-length messages use the same encoding for Field Line as
   known-length messages; see Section 3.6.

3.3.  Framing Indicator

   The start of each binary message is a framing indicator that is a
   single integer that describes the structure of the subsequent
   sections.  The framing indicator can take just four values:

   *  A value of 0 describes a request of known length.

   *  A value of 1 describes a response of known length.

   *  A value of 2 describes a request of indeterminate length.

   *  A value of 3 describes a response of indeterminate length.

   Other values cause the message to be invalid; see Section 4.

3.4.  Request Control Data

   The control data for a request message contains the method and
   request target.  That information is encoded as an ordered sequence
   of fields: Method, Scheme, Authority, Path.  Each of these fields is
   prefixed with a length.

   The values of these fields follow the rules in HTTP/2 (Section 8.3.1
   of [HTTP/2]) that apply to the ":method", ":scheme", ":authority",
   and ":path" pseudo-header fields, respectively.  However, where the
   ":authority" pseudo-header field might be omitted in HTTP/2, a zero-
   length value is encoded instead.

   The format of request control data is shown in Figure 3.

   Request Control Data {
     Method Length (i),
     Method (..),
     Scheme Length (i),
     Scheme (..),
     Authority Length (i),
     Authority (..),
     Path Length (i),
     Path (..),
   }

                  Figure 3: Format of Request Control Data

3.5.  Response Control Data

   The control data for a response message consists of the status code.
   The status code (Section 15 of [HTTP]) is encoded as a variable-
   length integer, not a length-prefixed decimal string.

   The format of final response control data is shown in Figure 4.

   Final Response Control Data {
     Status Code (i) = 200..599,
   }

              Figure 4: Format of Final Response Control Data

3.5.1.  Informational Status Codes

   Responses that include informational status codes (see Section 15.2
   of [HTTP]) are encoded by repeating the response control data and
   associated header section until a final status code is encoded; that
   is, a Status Code field with a value from 200 to 599 (inclusive).
   The status code distinguishes between informational and final
   responses.

   The format of the informational response control data is shown in
   Figure 5.

   Informational Response Control Data {
     Status Code (i) = 100..199,
   }

          Figure 5: Format of Informational Response Control Data

   A response message can include any number of informational responses
   that precede a final status code.  These convey an informational
   status code and a header block.

   If the response control data includes an informational status code
   (that is, a value between 100 and 199 inclusive), the control data is
   followed by a header section (encoded with known length or
   indeterminate length according to the framing indicator) and another
   block of control data.  This pattern repeats until the control data
   contains a final status code (200 to 599 inclusive).

3.6.  Header and Trailer Field Lines

   Header and trailer sections consist of zero or more field lines; see
   Section 5 of [HTTP].  The format of a field section depends on
   whether the message is of known length or indeterminate length.

   Each Field Line encoding includes a name and a value.  Both the name
   and value are length-prefixed sequences of bytes.  The Name field is
   a minimum of one byte.  The format of a Field Line is shown in
   Figure 6.

   Field Line {
     Name Length (i) = 1..,
     Name (..),
     Value Length (i),
     Value (..),
   }

                      Figure 6: Format of a Field Line

   For field names, byte values that are not permitted in an HTTP field
   name cause the message to be invalid; see Section 5.1 of [HTTP] for a
   definition of what is valid and Section 4 regarding the handling of
   invalid messages.  A recipient MUST treat a message that contains
   field values that would cause an HTTP/2 message to be malformed
   according to Section 8.2.1 of [HTTP/2] as invalid; see Section 4.

   The same field name can be repeated over more than one field line;
   see Section 5.2 of [HTTP] for the semantics of repeated field names
   and rules for combining values.

   Messages are invalid (Section 4) if they contain fields named
   ":method", ":scheme", ":authority", ":path", or ":status".  Other
   pseudo-fields that are defined by protocol extensions MAY be
   included; pseudo-fields cannot be included in trailers (see
   Section 8.1 of [HTTP/2]).  A Field Line containing pseudo-fields MUST
   precede other Field Line values.  A message that contains a pseudo-
   field after any other field is invalid; see Section 4.

   Fields that relate to connections (Section 7.6.1 of [HTTP]) cannot be
   used to produce the effect on a connection in this context.  These
   fields SHOULD be removed when constructing a binary message.
   However, they do not cause a message to be invalid (Section 4);
   permitting these fields allows a binary message to capture messages
   that are exchanged in a protocol context.

   Like HTTP/2 or HTTP/3, this format has an exception for the
   combination of multiple instances of the Cookie field.  Instances of
   fields with the ASCII-encoded value of "cookie" are combined using a
   semicolon octet (0x3b) rather than a comma; see Section 8.2.3 of
   [HTTP/2].

3.7.  Content

   The content of messages is a sequence of bytes of any length.  Though
   a known-length message has a limit, this limit is large enough that
   it is unlikely to be a practical limitation.  There is no limit to
   the size of content in an indeterminate-length message.

3.8.  Padding and Truncation

   Messages can be padded with any number of zero-valued bytes.  Non-
   zero padding bytes cause a message to be invalid (see Section 4).
   Unlike other parts of a message, a processor MAY decide not to
   validate the value of padding bytes.

   Truncation can be used to reduce the size of messages that have no
   data in trailing field sections or content.  If the trailers of a
   message are empty, they MAY be omitted by the encoder in place of
   adding a length field equal to zero.  An encoder MAY omit empty
   content in the same way if the trailers are also empty.  A message
   that is truncated at any other point is invalid; see Section 4.

   Decoders MUST treat missing truncated fields as equivalent to having
   been sent with the length field set to zero.

   Padding is compatible with truncation of empty parts of the messages.
   Zero-valued bytes will be interpreted as a zero-length part, which is
   semantically equivalent to the part being absent.

4.  Invalid Messages

   This document describes a number of ways that a message can be
   invalid.  Invalid messages MUST NOT be processed further except to
   log an error and produce an error response.

   The format is designed to allow incremental processing.
   Implementations need to be aware of the possibility that an error
   might be detected after performing incremental processing.

5.  Examples

   This section includes example requests and responses encoded in both
   known-length and indeterminate-length forms.

5.1.  Request Example

   The example HTTP/1.1 message in Figure 7 shows the content in the
   "message/http" format.

   Valid HTTP/1.1 messages require lines terminated with CRLF (the two
   bytes 0x0d and 0x0a).  For simplicity and consistency, the content of
   these examples is limited to text, which also uses CRLF for line
   endings.

   GET /hello.txt HTTP/1.1
   User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
   Host: www.example.com
   Accept-Language: en, mi

                       Figure 7: Sample HTTP Request

   This can be expressed as a binary message (type "message/bhttp")
   using a known-length encoding as shown in hexadecimal in Figure 8.
   Figure 8 includes text alongside to show that most of the content is
   not modified.

   00034745 54056874 74707300 0a2f6865  ..GET.https../he
   6c6c6f2e 74787440 6c0a7573 65722d61  llo.txt@l.user-a
   67656e74 34637572 6c2f372e 31362e33  gent4curl/7.16.3
   206c6962 6375726c 2f372e31 362e3320   libcurl/7.16.3
   4f70656e 53534c2f 302e392e 376c207a  OpenSSL/0.9.7l z
   6c69622f 312e322e 3304686f 73740f77  lib/1.2.3.host.w
   77772e65 78616d70 6c652e63 6f6d0f61  ww.example.com.a
   63636570 742d6c61 6e677561 67650665  ccept-language.e
   6e2c206d 690000                      n, mi..

             Figure 8: Known-Length Binary Encoding of Request

   This example shows that the Host header field is not replicated in
   the ":authority" field, as is required for ensuring that the request
   is reproduced accurately; see Section 8.3.1 of [HTTP/2].

   The same message can be truncated with no effect on interpretation.
   In this case, the last two bytes -- corresponding to content and a
   trailer section -- can each be removed without altering the semantics
   of the message.

   The same message, encoded using an indeterminate-length encoding, is
   shown in Figure 9.  As the content of this message is empty, the
   difference in formats is negligible.

   02034745 54056874 74707300 0a2f6865  ..GET.https../he
   6c6c6f2e 7478740a 75736572 2d616765  llo.txt.user-age
   6e743463 75726c2f 372e3136 2e33206c  nt4curl/7.16.3 l
   69626375 726c2f37 2e31362e 33204f70  ibcurl/7.16.3 Op
   656e5353 4c2f302e 392e376c 207a6c69  enSSL/0.9.7l zli
   622f312e 322e3304 686f7374 0f777777  b/1.2.3.host.www
   2e657861 6d706c65 2e636f6d 0f616363  .example.com.acc
   6570742d 6c616e67 75616765 06656e2c  ept-language.en,
   206d6900 00000000 00000000 00000000   mi.............

         Figure 9: Indeterminate-Length Binary Encoding of Request

   This indeterminate-length encoding contains 10 bytes of padding.  As
   two additional bytes can be truncated in the same way as the known-
   length example, anything up to 12 bytes can be removed from this
   message without affecting its meaning.

5.2.  Response Example

   Response messages can contain interim (1xx) status codes, as the
   message in Figure 10 shows.  Figure 10 includes examples of
   informational status codes 102 and 103, as defined in [RFC2518] (now
   obsolete but defines status code 102) and [RFC8297], respectively.

   HTTP/1.1 102 Processing
   Running: "sleep 15"

   HTTP/1.1 103 Early Hints
   Link: </style.css>; rel=preload; as=style
   Link: </script.js>; rel=preload; as=script

   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.

                      Figure 10: Sample HTTP Response

   As this is a longer example, only the indeterminate-length encoding
   is shown in Figure 11.  Note here that the specific text used in the
   reason phrase is not retained by this encoding.

   03406607 72756e6e 696e670a 22736c65  .@f.running."sle
   65702031 35220040 67046c69 6e6b233c  ep 15".@g.link#<
   2f737479 6c652e63 73733e3b 2072656c  /style.css>; rel
   3d707265 6c6f6164 3b206173 3d737479  =preload; as=sty
   6c65046c 696e6b24 3c2f7363 72697074  le.link$</script
   2e6a733e 3b207265 6c3d7072 656c6f61  .js>; rel=preloa
   643b2061 733d7363 72697074 0040c804  d; as=script.@..
   64617465 1d4d6f6e 2c203237 204a756c  date.Mon, 27 Jul
   20323030 39203132 3a32383a 35332047   2009 12:28:53 G
   4d540673 65727665 72064170 61636865  MT.server.Apache
   0d6c6173 742d6d6f 64696669 65641d57  .last-modified.W
   65642c20 3232204a 756c2032 30303920  ed, 22 Jul 2009
   31393a31 353a3536 20474d54 04657461  19:15:56 GMT.eta
   67142233 34616133 38372d64 2d313536  g."34aa387-d-156
   38656230 30220d61 63636570 742d7261  8eb00".accept-ra
   6e676573 05627974 65730e63 6f6e7465  nges.bytes.conte
   6e742d6c 656e6774 68023531 04766172  nt-length.51.var
   790f4163 63657074 2d456e63 6f64696e  y.Accept-Encodin
   670c636f 6e74656e 742d7479 70650a74  g.content-type.t
   6578742f 706c6169 6e003348 656c6c6f  ext/plain.3Hello
   20576f72 6c642120 4d792063 6f6e7465   World! My conte
   6e742069 6e636c75 64657320 61207472  nt includes a tr
   61696c69 6e672043 524c462e 0d0a0000  ailing CRLF.....

       Figure 11: Binary Response, including Informational Responses

   A response that uses the chunked encoding (see Section 7.1 of
   [HTTP/1.1]) as shown in Figure 12 can be encoded using indeterminate-
   length encoding, which minimizes buffering needed to translate into
   the binary format.  However, chunk boundaries do not need to be
   retained, and any chunk extensions cannot be conveyed using the
   binary format; see Section 6.

   HTTP/1.1 200 OK
   Transfer-Encoding: chunked

   4
   This
   6
    conte
   13;chunk-extension=foo
   nt contains CRLF.

   0
   Trailer: text

                    Figure 12: Chunked Encoding Example

   Figure 13 shows this message using the known-length encoding.  Note
   that the Transfer-Encoding header field is removed.

   0140c800 1d546869 7320636f 6e74656e  .@...This conten
   7420636f 6e746169 6e732043 524c462e  t contains CRLF.
   0d0a0d07 74726169 6c657204 74657874  ....trailer.text

                Figure 13: Known-Length Encoding of Response

6.  Notable Differences with HTTP Protocol Messages

   This format is designed to carry HTTP semantics just like HTTP/1.1
   [HTTP/1.1], HTTP/2 [HTTP/2], or HTTP/3 [HTTP/3].  However, there are
   some notable differences between this format and the format used in
   an interactive protocol version.

   In particular, as a standalone representation, this format lacks the
   following features of the formats used in those protocols:

   *  chunk extensions (Section 7.1.1 of [HTTP/1.1]) and transfer
      encoding (Section 6.1 of [HTTP/1.1])

   *  generic framing and extensibility capabilities

   *  field blocks other than a single header and trailer field block

   *  carrying reason phrases in responses (Section 4 of [HTTP/1.1])

   *  header compression [HPACK] [QPACK]

   *  response framing that depends on the corresponding request (such
      as HEAD) or the value of the status code (such as 204 or 304);
      these responses use the same framing as all other messages

   Some of these features are also absent in HTTP/2 and HTTP/3.

   Unlike HTTP/2 and HTTP/3, this format uses a fixed format for control
   data rather than using pseudo-fields.

   Note that while some messages -- CONNECT or upgrade requests in
   particular -- can be represented using this format, doing so serves
   no purpose, as these requests are used to affect protocol behavior,
   which this format cannot do without additional mechanisms.

7.  "message/bhttp" Media Type

   The "message/bhttp" media type can be used to enclose a single HTTP
   request or response message, provided that it obeys the MIME
   restrictions for all "message" types regarding line length and
   encodings.

   Type name:  message
   Subtype name:  bhttp
   Required parameters:  N/A
   Optional parameters:  N/A
   Encoding considerations:  Only "8bit" or "binary" is permitted.
   Security considerations:  See Section 8.
   Interoperability considerations:  N/A
   Published specification:  RFC 9292
   Applications that use this media type:  Applications seeking to
      convey HTTP semantics that are independent of a specific protocol.
   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 & email address to contact for further information:  See the
      Authors' Addresses section.
   Intended usage:  COMMON
   Restrictions on usage:  N/A
   Author:  See the Authors' Addresses section.
   Change controller:  IESG

8.  Security Considerations

   Many of the considerations that apply to HTTP message handling apply
   to this format; see Section 17 of [HTTP] and Section 11 of [HTTP/1.1]
   for common issues in handling HTTP messages.

   Strict parsing of the format with no tolerance for errors can help
   avoid a number of attacks.  However, implementations still need to be
   aware of the possibility of resource exhaustion attacks that might
   arise from receiving large messages, particularly those with large
   numbers of fields.

   Implementations need to ensure that they aren't subject to resource
   exhaustion attacks from maliciously crafted messages.  Overall, the
   format is designed to allow for minimal state when processing
   messages.  However, producing a combined field value (Section 5.2 of
   [HTTP]) for fields might require the commitment of resources.  In
   particular, combining might be necessary for the Cookie field when
   translating this format for use in other contexts, such as use in an
   API or translation to HTTP/1.1 [HTTP/1.1], where the recipient of the
   field might not expect multiple values.

9.  IANA Considerations

   IANA has added the media type "message/bhttp" to the "Media Types"
   registry at <https://www.iana.org/assignments/media-types>.  See
   Section 7 for registration information.

10.  References

10.1.  Normative References

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [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>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [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>.

   [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>.

10.2.  Informative References

   [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/1.1] 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/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/info/rfc9114>.

   [QPACK]    Krasic, C., Bishop, M., and A. Frindell, Ed., "QPACK:
              Field Compression for HTTP/3", RFC 9204,
              DOI 10.17487/RFC9204, June 2022,
              <https://www.rfc-editor.org/info/rfc9204>.

   [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
              Jensen, "HTTP Extensions for Distributed Authoring --
              WEBDAV", RFC 2518, DOI 10.17487/RFC2518, February 1999,
              <https://www.rfc-editor.org/info/rfc2518>.

   [RFC8297]  Oku, K., "An HTTP Status Code for Indicating Hints",
              RFC 8297, DOI 10.17487/RFC8297, December 2017,
              <https://www.rfc-editor.org/info/rfc8297>.

Acknowledgments

   Julian Reschke, David Schinazi, Lucas Pardue, and Tommy Pauly
   provided excellent feedback on both the design and its documentation.

Authors' Addresses

   Martin Thomson
   Mozilla
   Email: mt@lowentropy.net


   Christopher A. Wood
   Cloudflare
   Email: caw@heapingbits.net