path: root/doc/rfc/rfc9000.txt

Internet Engineering Task Force (IETF)                   J. Iyengar, Ed.
Request for Comments: 9000                                        Fastly
Category: Standards Track                                M. Thomson, Ed.
ISSN: 2070-1721                                                  Mozilla
                                                                May 2021


           QUIC: A UDP-Based Multiplexed and Secure Transport

Abstract

   This document defines the core of the QUIC transport protocol.  QUIC
   provides applications with flow-controlled streams for structured
   communication, low-latency connection establishment, and network path
   migration.  QUIC includes security measures that ensure
   confidentiality, integrity, and availability in a range of deployment
   circumstances.  Accompanying documents describe the integration of
   TLS for key negotiation, loss detection, and an exemplary congestion
   control algorithm.

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/rfc9000.

Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Overview
     1.1.  Document Structure
     1.2.  Terms and Definitions
     1.3.  Notational Conventions
   2.  Streams
     2.1.  Stream Types and Identifiers
     2.2.  Sending and Receiving Data
     2.3.  Stream Prioritization
     2.4.  Operations on Streams
   3.  Stream States
     3.1.  Sending Stream States
     3.2.  Receiving Stream States
     3.3.  Permitted Frame Types
     3.4.  Bidirectional Stream States
     3.5.  Solicited State Transitions
   4.  Flow Control
     4.1.  Data Flow Control
     4.2.  Increasing Flow Control Limits
     4.3.  Flow Control Performance
     4.4.  Handling Stream Cancellation
     4.5.  Stream Final Size
     4.6.  Controlling Concurrency
   5.  Connections
     5.1.  Connection ID
       5.1.1.  Issuing Connection IDs
       5.1.2.  Consuming and Retiring Connection IDs
     5.2.  Matching Packets to Connections
       5.2.1.  Client Packet Handling
       5.2.2.  Server Packet Handling
       5.2.3.  Considerations for Simple Load Balancers
     5.3.  Operations on Connections
   6.  Version Negotiation
     6.1.  Sending Version Negotiation Packets
     6.2.  Handling Version Negotiation Packets
     6.3.  Using Reserved Versions
   7.  Cryptographic and Transport Handshake
     7.1.  Example Handshake Flows
     7.2.  Negotiating Connection IDs
     7.3.  Authenticating Connection IDs
     7.4.  Transport Parameters
       7.4.1.  Values of Transport Parameters for 0-RTT
       7.4.2.  New Transport Parameters
     7.5.  Cryptographic Message Buffering
   8.  Address Validation
     8.1.  Address Validation during Connection Establishment
       8.1.1.  Token Construction
       8.1.2.  Address Validation Using Retry Packets
       8.1.3.  Address Validation for Future Connections
       8.1.4.  Address Validation Token Integrity
     8.2.  Path Validation
       8.2.1.  Initiating Path Validation
       8.2.2.  Path Validation Responses
       8.2.3.  Successful Path Validation
       8.2.4.  Failed Path Validation
   9.  Connection Migration
     9.1.  Probing a New Path
     9.2.  Initiating Connection Migration
     9.3.  Responding to Connection Migration
       9.3.1.  Peer Address Spoofing
       9.3.2.  On-Path Address Spoofing
       9.3.3.  Off-Path Packet Forwarding
     9.4.  Loss Detection and Congestion Control
     9.5.  Privacy Implications of Connection Migration
     9.6.  Server's Preferred Address
       9.6.1.  Communicating a Preferred Address
       9.6.2.  Migration to a Preferred Address
       9.6.3.  Interaction of Client Migration and Preferred Address
     9.7.  Use of IPv6 Flow Label and Migration
   10. Connection Termination
     10.1.  Idle Timeout
       10.1.1.  Liveness Testing
       10.1.2.  Deferring Idle Timeout
     10.2.  Immediate Close
       10.2.1.  Closing Connection State
       10.2.2.  Draining Connection State
       10.2.3.  Immediate Close during the Handshake
     10.3.  Stateless Reset
       10.3.1.  Detecting a Stateless Reset
       10.3.2.  Calculating a Stateless Reset Token
       10.3.3.  Looping
   11. Error Handling
     11.1.  Connection Errors
     11.2.  Stream Errors
   12. Packets and Frames
     12.1.  Protected Packets
     12.2.  Coalescing Packets
     12.3.  Packet Numbers
     12.4.  Frames and Frame Types
     12.5.  Frames and Number Spaces
   13. Packetization and Reliability
     13.1.  Packet Processing
     13.2.  Generating Acknowledgments
       13.2.1.  Sending ACK Frames
       13.2.2.  Acknowledgment Frequency
       13.2.3.  Managing ACK Ranges
       13.2.4.  Limiting Ranges by Tracking ACK Frames
       13.2.5.  Measuring and Reporting Host Delay
       13.2.6.  ACK Frames and Packet Protection
       13.2.7.  PADDING Frames Consume Congestion Window
     13.3.  Retransmission of Information
     13.4.  Explicit Congestion Notification
       13.4.1.  Reporting ECN Counts
       13.4.2.  ECN Validation
   14. Datagram Size
     14.1.  Initial Datagram Size
     14.2.  Path Maximum Transmission Unit
       14.2.1.  Handling of ICMP Messages by PMTUD
     14.3.  Datagram Packetization Layer PMTU Discovery
       14.3.1.  DPLPMTUD and Initial Connectivity
       14.3.2.  Validating the Network Path with DPLPMTUD
       14.3.3.  Handling of ICMP Messages by DPLPMTUD
     14.4.  Sending QUIC PMTU Probes
       14.4.1.  PMTU Probes Containing Source Connection ID
   15. Versions
   16. Variable-Length Integer Encoding
   17. Packet Formats
     17.1.  Packet Number Encoding and Decoding
     17.2.  Long Header Packets
       17.2.1.  Version Negotiation Packet
       17.2.2.  Initial Packet
       17.2.3.  0-RTT
       17.2.4.  Handshake Packet
       17.2.5.  Retry Packet
     17.3.  Short Header Packets
       17.3.1.  1-RTT Packet
     17.4.  Latency Spin Bit
   18. Transport Parameter Encoding
     18.1.  Reserved Transport Parameters
     18.2.  Transport Parameter Definitions
   19. Frame Types and Formats
     19.1.  PADDING Frames
     19.2.  PING Frames
     19.3.  ACK Frames
       19.3.1.  ACK Ranges
       19.3.2.  ECN Counts
     19.4.  RESET_STREAM Frames
     19.5.  STOP_SENDING Frames
     19.6.  CRYPTO Frames
     19.7.  NEW_TOKEN Frames
     19.8.  STREAM Frames
     19.9.  MAX_DATA Frames
     19.10. MAX_STREAM_DATA Frames
     19.11. MAX_STREAMS Frames
     19.12. DATA_BLOCKED Frames
     19.13. STREAM_DATA_BLOCKED Frames
     19.14. STREAMS_BLOCKED Frames
     19.15. NEW_CONNECTION_ID Frames
     19.16. RETIRE_CONNECTION_ID Frames
     19.17. PATH_CHALLENGE Frames
     19.18. PATH_RESPONSE Frames
     19.19. CONNECTION_CLOSE Frames
     19.20. HANDSHAKE_DONE Frames
     19.21. Extension Frames
   20. Error Codes
     20.1.  Transport Error Codes
     20.2.  Application Protocol Error Codes
   21. Security Considerations
     21.1.  Overview of Security Properties
       21.1.1.  Handshake
       21.1.2.  Protected Packets
       21.1.3.  Connection Migration
     21.2.  Handshake Denial of Service
     21.3.  Amplification Attack
     21.4.  Optimistic ACK Attack
     21.5.  Request Forgery Attacks
       21.5.1.  Control Options for Endpoints
       21.5.2.  Request Forgery with Client Initial Packets
       21.5.3.  Request Forgery with Preferred Addresses
       21.5.4.  Request Forgery with Spoofed Migration
       21.5.5.  Request Forgery with Version Negotiation
       21.5.6.  Generic Request Forgery Countermeasures
     21.6.  Slowloris Attacks
     21.7.  Stream Fragmentation and Reassembly Attacks
     21.8.  Stream Commitment Attack
     21.9.  Peer Denial of Service
     21.10. Explicit Congestion Notification Attacks
     21.11. Stateless Reset Oracle
     21.12. Version Downgrade
     21.13. Targeted Attacks by Routing
     21.14. Traffic Analysis
   22. IANA Considerations
     22.1.  Registration Policies for QUIC Registries
       22.1.1.  Provisional Registrations
       22.1.2.  Selecting Codepoints
       22.1.3.  Reclaiming Provisional Codepoints
       22.1.4.  Permanent Registrations
     22.2.  QUIC Versions Registry
     22.3.  QUIC Transport Parameters Registry
     22.4.  QUIC Frame Types Registry
     22.5.  QUIC Transport Error Codes Registry
   23. References
     23.1.  Normative References
     23.2.  Informative References
   Appendix A.  Pseudocode
     A.1.  Sample Variable-Length Integer Decoding
     A.2.  Sample Packet Number Encoding Algorithm
     A.3.  Sample Packet Number Decoding Algorithm
     A.4.  Sample ECN Validation Algorithm
   Contributors
   Authors' Addresses

1.  Overview

   QUIC is a secure general-purpose transport protocol.  This document
   defines version 1 of QUIC, which conforms to the version-independent
   properties of QUIC defined in [QUIC-INVARIANTS].

   QUIC is a connection-oriented protocol that creates a stateful
   interaction between a client and server.

   The QUIC handshake combines negotiation of cryptographic and
   transport parameters.  QUIC integrates the TLS handshake [TLS13],
   although using a customized framing for protecting packets.  The
   integration of TLS and QUIC is described in more detail in
   [QUIC-TLS].  The handshake is structured to permit the exchange of
   application data as soon as possible.  This includes an option for
   clients to send data immediately (0-RTT), which requires some form of
   prior communication or configuration to enable.

   Endpoints communicate in QUIC by exchanging QUIC packets.  Most
   packets contain frames, which carry control information and
   application data between endpoints.  QUIC authenticates the entirety
   of each packet and encrypts as much of each packet as is practical.
   QUIC packets are carried in UDP datagrams [UDP] to better facilitate
   deployment in existing systems and networks.

   Application protocols exchange information over a QUIC connection via
   streams, which are ordered sequences of bytes.  Two types of streams
   can be created: bidirectional streams, which allow both endpoints to
   send data; and unidirectional streams, which allow a single endpoint
   to send data.  A credit-based scheme is used to limit stream creation
   and to bound the amount of data that can be sent.

   QUIC provides the necessary feedback to implement reliable delivery
   and congestion control.  An algorithm for detecting and recovering
   from loss of data is described in Section 6 of [QUIC-RECOVERY].  QUIC
   depends on congestion control to avoid network congestion.  An
   exemplary congestion control algorithm is described in Section 7 of
   [QUIC-RECOVERY].

   QUIC connections are not strictly bound to a single network path.
   Connection migration uses connection identifiers to allow connections
   to transfer to a new network path.  Only clients are able to migrate
   in this version of QUIC.  This design also allows connections to
   continue after changes in network topology or address mappings, such
   as might be caused by NAT rebinding.

   Once established, multiple options are provided for connection
   termination.  Applications can manage a graceful shutdown, endpoints
   can negotiate a timeout period, errors can cause immediate connection
   teardown, and a stateless mechanism provides for termination of
   connections after one endpoint has lost state.

1.1.  Document Structure

   This document describes the core QUIC protocol and is structured as
   follows:

   *  Streams are the basic service abstraction that QUIC provides.

      -  Section 2 describes core concepts related to streams,

      -  Section 3 provides a reference model for stream states, and

      -  Section 4 outlines the operation of flow control.

   *  Connections are the context in which QUIC endpoints communicate.

      -  Section 5 describes core concepts related to connections,

      -  Section 6 describes version negotiation,

      -  Section 7 details the process for establishing connections,

      -  Section 8 describes address validation and critical denial-of-
         service mitigations,

      -  Section 9 describes how endpoints migrate a connection to a new
         network path,

      -  Section 10 lists the options for terminating an open
         connection, and

      -  Section 11 provides guidance for stream and connection error
         handling.

   *  Packets and frames are the basic unit used by QUIC to communicate.

      -  Section 12 describes concepts related to packets and frames,

      -  Section 13 defines models for the transmission, retransmission,
         and acknowledgment of data, and

      -  Section 14 specifies rules for managing the size of datagrams
         carrying QUIC packets.

   *  Finally, encoding details of QUIC protocol elements are described
      in:

      -  Section 15 (versions),

      -  Section 16 (integer encoding),

      -  Section 17 (packet headers),

      -  Section 18 (transport parameters),

      -  Section 19 (frames), and

      -  Section 20 (errors).

   Accompanying documents describe QUIC's loss detection and congestion
   control [QUIC-RECOVERY], and the use of TLS and other cryptographic
   mechanisms [QUIC-TLS].

   This document defines QUIC version 1, which conforms to the protocol
   invariants in [QUIC-INVARIANTS].

   To refer to QUIC version 1, cite this document.  References to the
   limited set of version-independent properties of QUIC can cite
   [QUIC-INVARIANTS].

1.2.  Terms 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.

   Commonly used terms in this document are described below.

   QUIC:  The transport protocol described by this document.  QUIC is a
      name, not an acronym.

   Endpoint:  An entity that can participate in a QUIC connection by
      generating, receiving, and processing QUIC packets.  There are
      only two types of endpoints in QUIC: client and server.

   Client:  The endpoint that initiates a QUIC connection.

   Server:  The endpoint that accepts a QUIC connection.

   QUIC packet:  A complete processable unit of QUIC that can be
      encapsulated in a UDP datagram.  One or more QUIC packets can be
      encapsulated in a single UDP datagram.

   Ack-eliciting packet:  A QUIC packet that contains frames other than
      ACK, PADDING, and CONNECTION_CLOSE.  These cause a recipient to
      send an acknowledgment; see Section 13.2.1.

   Frame:  A unit of structured protocol information.  There are
      multiple frame types, each of which carries different information.
      Frames are contained in QUIC packets.

   Address:  When used without qualification, the tuple of IP version,
      IP address, and UDP port number that represents one end of a
      network path.

   Connection ID:  An identifier that is used to identify a QUIC
      connection at an endpoint.  Each endpoint selects one or more
      connection IDs for its peer to include in packets sent towards the
      endpoint.  This value is opaque to the peer.

   Stream:  A unidirectional or bidirectional channel of ordered bytes
      within a QUIC connection.  A QUIC connection can carry multiple
      simultaneous streams.

   Application:  An entity that uses QUIC to send and receive data.

   This document uses the terms "QUIC packets", "UDP datagrams", and "IP
   packets" to refer to the units of the respective protocols.  That is,
   one or more QUIC packets can be encapsulated in a UDP datagram, which
   is in turn encapsulated in an IP packet.

1.3.  Notational Conventions

   Packet and frame diagrams in this document use a custom format.  The
   purpose of this format is to summarize, not define, protocol
   elements.  Prose defines the complete semantics and details of
   structures.

   Complex fields are named and then followed by a list of fields
   surrounded by a pair of matching braces.  Each field in this list is
   separated by commas.

   Individual fields include length information, plus indications about
   fixed value, optionality, or repetitions.  Individual fields use the
   following notational conventions, with all lengths in bits:

   x (A):  Indicates that x is A bits long

   x (i):  Indicates that x holds an integer value using the variable-
      length encoding described in Section 16

   x (A..B):  Indicates that x can be any length from A to B; A can be
      omitted to indicate a minimum of zero bits, and B can be omitted
      to indicate no set upper limit; values in this format always end
      on a byte boundary

   x (L) = C:  Indicates that x has a fixed value of C; the length of x
      is described by L, which can use any of the length forms above

   x (L) = C..D:  Indicates that x has a value in the range from C to D,
      inclusive, with the length described by L, as above

   [x (L)]:  Indicates that x is optional and has a length of L

   x (L) ...:  Indicates that x is repeated zero or more times and that
      each instance has a length of L

   This document uses network byte order (that is, big endian) values.
   Fields are placed starting from the high-order bits of each byte.

   By convention, individual fields reference a complex field by using
   the name of the complex field.

   Figure 1 provides an example:

   Example Structure {
     One-bit Field (1),
     7-bit Field with Fixed Value (7) = 61,
     Field with Variable-Length Integer (i),
     Arbitrary-Length Field (..),
     Variable-Length Field (8..24),
     Field With Minimum Length (16..),
     Field With Maximum Length (..128),
     [Optional Field (64)],
     Repeated Field (8) ...,
   }

                          Figure 1: Example Format

   When a single-bit field is referenced in prose, the position of that
   field can be clarified by using the value of the byte that carries
   the field with the field's value set.  For example, the value 0x80
   could be used to refer to the single-bit field in the most
   significant bit of the byte, such as One-bit Field in Figure 1.

2.  Streams

   Streams in QUIC provide a lightweight, ordered byte-stream
   abstraction to an application.  Streams can be unidirectional or
   bidirectional.

   Streams can be created by sending data.  Other processes associated
   with stream management -- ending, canceling, and managing flow
   control -- are all designed to impose minimal overheads.  For
   instance, a single STREAM frame (Section 19.8) can open, carry data
   for, and close a stream.  Streams can also be long-lived and can last
   the entire duration of a connection.

   Streams can be created by either endpoint, can concurrently send data
   interleaved with other streams, and can be canceled.  QUIC does not
   provide any means of ensuring ordering between bytes on different
   streams.

   QUIC allows for an arbitrary number of streams to operate
   concurrently and for an arbitrary amount of data to be sent on any
   stream, subject to flow control constraints and stream limits; see
   Section 4.

2.1.  Stream Types and Identifiers

   Streams can be unidirectional or bidirectional.  Unidirectional
   streams carry data in one direction: from the initiator of the stream
   to its peer.  Bidirectional streams allow for data to be sent in both
   directions.

   Streams are identified within a connection by a numeric value,
   referred to as the stream ID.  A stream ID is a 62-bit integer (0 to
   2^62-1) that is unique for all streams on a connection.  Stream IDs
   are encoded as variable-length integers; see Section 16.  A QUIC
   endpoint MUST NOT reuse a stream ID within a connection.

   The least significant bit (0x01) of the stream ID identifies the
   initiator of the stream.  Client-initiated streams have even-numbered
   stream IDs (with the bit set to 0), and server-initiated streams have
   odd-numbered stream IDs (with the bit set to 1).

   The second least significant bit (0x02) of the stream ID
   distinguishes between bidirectional streams (with the bit set to 0)
   and unidirectional streams (with the bit set to 1).

   The two least significant bits from a stream ID therefore identify a
   stream as one of four types, as summarized in Table 1.

                +======+==================================+
                | Bits | Stream Type                      |
                +======+==================================+
                | 0x00 | Client-Initiated, Bidirectional  |
                +------+----------------------------------+
                | 0x01 | Server-Initiated, Bidirectional  |
                +------+----------------------------------+
                | 0x02 | Client-Initiated, Unidirectional |
                +------+----------------------------------+
                | 0x03 | Server-Initiated, Unidirectional |
                +------+----------------------------------+

                          Table 1: Stream ID Types

   The stream space for each type begins at the minimum value (0x00
   through 0x03, respectively); successive streams of each type are
   created with numerically increasing stream IDs.  A stream ID that is
   used out of order results in all streams of that type with lower-
   numbered stream IDs also being opened.

2.2.  Sending and Receiving Data

   STREAM frames (Section 19.8) encapsulate data sent by an application.
   An endpoint uses the Stream ID and Offset fields in STREAM frames to
   place data in order.

   Endpoints MUST be able to deliver stream data to an application as an
   ordered byte stream.  Delivering an ordered byte stream requires that
   an endpoint buffer any data that is received out of order, up to the
   advertised flow control limit.

   QUIC makes no specific allowances for delivery of stream data out of
   order.  However, implementations MAY choose to offer the ability to
   deliver data out of order to a receiving application.

   An endpoint could receive data for a stream at the same stream offset
   multiple times.  Data that has already been received can be
   discarded.  The data at a given offset MUST NOT change if it is sent
   multiple times; an endpoint MAY treat receipt of different data at
   the same offset within a stream as a connection error of type
   PROTOCOL_VIOLATION.

   Streams are an ordered byte-stream abstraction with no other
   structure visible to QUIC.  STREAM frame boundaries are not expected
   to be preserved when data is transmitted, retransmitted after packet
   loss, or delivered to the application at a receiver.

   An endpoint MUST NOT send data on any stream without ensuring that it
   is within the flow control limits set by its peer.  Flow control is
   described in detail in Section 4.

2.3.  Stream Prioritization

   Stream multiplexing can have a significant effect on application
   performance if resources allocated to streams are correctly
   prioritized.

   QUIC does not provide a mechanism for exchanging prioritization
   information.  Instead, it relies on receiving priority information
   from the application.

   A QUIC implementation SHOULD provide ways in which an application can
   indicate the relative priority of streams.  An implementation uses
   information provided by the application to determine how to allocate
   resources to active streams.

2.4.  Operations on Streams

   This document does not define an API for QUIC; it instead defines a
   set of functions on streams that application protocols can rely upon.
   An application protocol can assume that a QUIC implementation
   provides an interface that includes the operations described in this
   section.  An implementation designed for use with a specific
   application protocol might provide only those operations that are
   used by that protocol.

   On the sending part of a stream, an application protocol can:

   *  write data, understanding when stream flow control credit
      (Section 4.1) has successfully been reserved to send the written
      data;

   *  end the stream (clean termination), resulting in a STREAM frame
      (Section 19.8) with the FIN bit set; and

   *  reset the stream (abrupt termination), resulting in a RESET_STREAM
      frame (Section 19.4) if the stream was not already in a terminal
      state.

   On the receiving part of a stream, an application protocol can:

   *  read data; and

   *  abort reading of the stream and request closure, possibly
      resulting in a STOP_SENDING frame (Section 19.5).

   An application protocol can also request to be informed of state
   changes on streams, including when the peer has opened or reset a
   stream, when a peer aborts reading on a stream, when new data is
   available, and when data can or cannot be written to the stream due
   to flow control.

3.  Stream States

   This section describes streams in terms of their send or receive
   components.  Two state machines are described: one for the streams on
   which an endpoint transmits data (Section 3.1) and another for
   streams on which an endpoint receives data (Section 3.2).

   Unidirectional streams use either the sending or receiving state
   machine, depending on the stream type and endpoint role.
   Bidirectional streams use both state machines at both endpoints.  For
   the most part, the use of these state machines is the same whether
   the stream is unidirectional or bidirectional.  The conditions for
   opening a stream are slightly more complex for a bidirectional stream
   because the opening of either the send or receive side causes the
   stream to open in both directions.

   The state machines shown in this section are largely informative.
   This document uses stream states to describe rules for when and how
   different types of frames can be sent and the reactions that are
   expected when different types of frames are received.  Though these
   state machines are intended to be useful in implementing QUIC, these
   states are not intended to constrain implementations.  An
   implementation can define a different state machine as long as its
   behavior is consistent with an implementation that implements these
   states.

      |  Note: In some cases, a single event or action can cause a
      |  transition through multiple states.  For instance, sending
      |  STREAM with a FIN bit set can cause two state transitions for a
      |  sending stream: from the "Ready" state to the "Send" state, and
      |  from the "Send" state to the "Data Sent" state.

3.1.  Sending Stream States

   Figure 2 shows the states for the part of a stream that sends data to
   a peer.

          o
          | Create Stream (Sending)
          | Peer Creates Bidirectional Stream
          v
      +-------+
      | Ready | Send RESET_STREAM
      |       |-----------------------.
      +-------+                       |
          |                           |
          | Send STREAM /             |
          |      STREAM_DATA_BLOCKED  |
          v                           |
      +-------+                       |
      | Send  | Send RESET_STREAM     |
      |       |---------------------->|
      +-------+                       |
          |                           |
          | Send STREAM + FIN         |
          v                           v
      +-------+                   +-------+
      | Data  | Send RESET_STREAM | Reset |
      | Sent  |------------------>| Sent  |
      +-------+                   +-------+
          |                           |
          | Recv All ACKs             | Recv ACK
          v                           v
      +-------+                   +-------+
      | Data  |                   | Reset |
      | Recvd |                   | Recvd |
      +-------+                   +-------+

               Figure 2: States for Sending Parts of Streams

   The sending part of a stream that the endpoint initiates (types 0 and
   2 for clients, 1 and 3 for servers) is opened by the application.
   The "Ready" state represents a newly created stream that is able to
   accept data from the application.  Stream data might be buffered in
   this state in preparation for sending.

   Sending the first STREAM or STREAM_DATA_BLOCKED frame causes a
   sending part of a stream to enter the "Send" state.  An
   implementation might choose to defer allocating a stream ID to a
   stream until it sends the first STREAM frame and enters this state,
   which can allow for better stream prioritization.

   The sending part of a bidirectional stream initiated by a peer (type
   0 for a server, type 1 for a client) starts in the "Ready" state when
   the receiving part is created.

   In the "Send" state, an endpoint transmits -- and retransmits as
   necessary -- stream data in STREAM frames.  The endpoint respects the
   flow control limits set by its peer and continues to accept and
   process MAX_STREAM_DATA frames.  An endpoint in the "Send" state
   generates STREAM_DATA_BLOCKED frames if it is blocked from sending by
   stream flow control limits (Section 4.1).

   After the application indicates that all stream data has been sent
   and a STREAM frame containing the FIN bit is sent, the sending part
   of the stream enters the "Data Sent" state.  From this state, the
   endpoint only retransmits stream data as necessary.  The endpoint
   does not need to check flow control limits or send
   STREAM_DATA_BLOCKED frames for a stream in this state.
   MAX_STREAM_DATA frames might be received until the peer receives the
   final stream offset.  The endpoint can safely ignore any
   MAX_STREAM_DATA frames it receives from its peer for a stream in this
   state.

   Once all stream data has been successfully acknowledged, the sending
   part of the stream enters the "Data Recvd" state, which is a terminal
   state.

   From any state that is one of "Ready", "Send", or "Data Sent", an
   application can signal that it wishes to abandon transmission of
   stream data.  Alternatively, an endpoint might receive a STOP_SENDING
   frame from its peer.  In either case, the endpoint sends a
   RESET_STREAM frame, which causes the stream to enter the "Reset Sent"
   state.

   An endpoint MAY send a RESET_STREAM as the first frame that mentions
   a stream; this causes the sending part of that stream to open and
   then immediately transition to the "Reset Sent" state.

   Once a packet containing a RESET_STREAM has been acknowledged, the
   sending part of the stream enters the "Reset Recvd" state, which is a
   terminal state.

3.2.  Receiving Stream States

   Figure 3 shows the states for the part of a stream that receives data
   from a peer.  The states for a receiving part of a stream mirror only
   some of the states of the sending part of the stream at the peer.
   The receiving part of a stream does not track states on the sending
   part that cannot be observed, such as the "Ready" state.  Instead,
   the receiving part of a stream tracks the delivery of data to the
   application, some of which cannot be observed by the sender.

          o
          | Recv STREAM / STREAM_DATA_BLOCKED / RESET_STREAM
          | Create Bidirectional Stream (Sending)
          | Recv MAX_STREAM_DATA / STOP_SENDING (Bidirectional)
          | Create Higher-Numbered Stream
          v
      +-------+
      | Recv  | Recv RESET_STREAM
      |       |-----------------------.
      +-------+                       |
          |                           |
          | Recv STREAM + FIN         |
          v                           |
      +-------+                       |
      | Size  | Recv RESET_STREAM     |
      | Known |---------------------->|
      +-------+                       |
          |                           |
          | Recv All Data             |
          v                           v
      +-------+ Recv RESET_STREAM +-------+
      | Data  |--- (optional) --->| Reset |
      | Recvd |  Recv All Data    | Recvd |
      +-------+<-- (optional) ----+-------+
          |                           |
          | App Read All Data         | App Read Reset
          v                           v
      +-------+                   +-------+
      | Data  |                   | Reset |
      | Read  |                   | Read  |
      +-------+                   +-------+

              Figure 3: States for Receiving Parts of Streams

   The receiving part of a stream initiated by a peer (types 1 and 3 for
   a client, or 0 and 2 for a server) is created when the first STREAM,
   STREAM_DATA_BLOCKED, or RESET_STREAM frame is received for that
   stream.  For bidirectional streams initiated by a peer, receipt of a
   MAX_STREAM_DATA or STOP_SENDING frame for the sending part of the
   stream also creates the receiving part.  The initial state for the
   receiving part of a stream is "Recv".

   For a bidirectional stream, the receiving part enters the "Recv"
   state when the sending part initiated by the endpoint (type 0 for a
   client, type 1 for a server) enters the "Ready" state.

   An endpoint opens a bidirectional stream when a MAX_STREAM_DATA or
   STOP_SENDING frame is received from the peer for that stream.
   Receiving a MAX_STREAM_DATA frame for an unopened stream indicates
   that the remote peer has opened the stream and is providing flow
   control credit.  Receiving a STOP_SENDING frame for an unopened
   stream indicates that the remote peer no longer wishes to receive
   data on this stream.  Either frame might arrive before a STREAM or
   STREAM_DATA_BLOCKED frame if packets are lost or reordered.

   Before a stream is created, all streams of the same type with lower-
   numbered stream IDs MUST be created.  This ensures that the creation
   order for streams is consistent on both endpoints.

   In the "Recv" state, the endpoint receives STREAM and
   STREAM_DATA_BLOCKED frames.  Incoming data is buffered and can be
   reassembled into the correct order for delivery to the application.
   As data is consumed by the application and buffer space becomes
   available, the endpoint sends MAX_STREAM_DATA frames to allow the
   peer to send more data.

   When a STREAM frame with a FIN bit is received, the final size of the
   stream is known; see Section 4.5.  The receiving part of the stream
   then enters the "Size Known" state.  In this state, the endpoint no
   longer needs to send MAX_STREAM_DATA frames; it only receives any
   retransmissions of stream data.

   Once all data for the stream has been received, the receiving part
   enters the "Data Recvd" state.  This might happen as a result of
   receiving the same STREAM frame that causes the transition to "Size
   Known".  After all data has been received, any STREAM or
   STREAM_DATA_BLOCKED frames for the stream can be discarded.

   The "Data Recvd" state persists until stream data has been delivered
   to the application.  Once stream data has been delivered, the stream
   enters the "Data Read" state, which is a terminal state.

   Receiving a RESET_STREAM frame in the "Recv" or "Size Known" state
   causes the stream to enter the "Reset Recvd" state.  This might cause
   the delivery of stream data to the application to be interrupted.

   It is possible that all stream data has already been received when a
   RESET_STREAM is received (that is, in the "Data Recvd" state).
   Similarly, it is possible for remaining stream data to arrive after
   receiving a RESET_STREAM frame (the "Reset Recvd" state).  An
   implementation is free to manage this situation as it chooses.

   Sending a RESET_STREAM means that an endpoint cannot guarantee
   delivery of stream data; however, there is no requirement that stream
   data not be delivered if a RESET_STREAM is received.  An
   implementation MAY interrupt delivery of stream data, discard any
   data that was not consumed, and signal the receipt of the
   RESET_STREAM.  A RESET_STREAM signal might be suppressed or withheld
   if stream data is completely received and is buffered to be read by
   the application.  If the RESET_STREAM is suppressed, the receiving
   part of the stream remains in "Data Recvd".

   Once the application receives the signal indicating that the stream
   was reset, the receiving part of the stream transitions to the "Reset
   Read" state, which is a terminal state.

3.3.  Permitted Frame Types

   The sender of a stream sends just three frame types that affect the
   state of a stream at either the sender or the receiver: STREAM
   (Section 19.8), STREAM_DATA_BLOCKED (Section 19.13), and RESET_STREAM
   (Section 19.4).

   A sender MUST NOT send any of these frames from a terminal state
   ("Data Recvd" or "Reset Recvd").  A sender MUST NOT send a STREAM or
   STREAM_DATA_BLOCKED frame for a stream in the "Reset Sent" state or
   any terminal state -- that is, after sending a RESET_STREAM frame.  A
   receiver could receive any of these three frames in any state, due to
   the possibility of delayed delivery of packets carrying them.

   The receiver of a stream sends MAX_STREAM_DATA frames (Section 19.10)
   and STOP_SENDING frames (Section 19.5).

   The receiver only sends MAX_STREAM_DATA frames in the "Recv" state.
   A receiver MAY send a STOP_SENDING frame in any state where it has
   not received a RESET_STREAM frame -- that is, states other than
   "Reset Recvd" or "Reset Read".  However, there is little value in
   sending a STOP_SENDING frame in the "Data Recvd" state, as all stream
   data has been received.  A sender could receive either of these two
   types of frames in any state as a result of delayed delivery of
   packets.

3.4.  Bidirectional Stream States

   A bidirectional stream is composed of sending and receiving parts.
   Implementations can represent states of the bidirectional stream as
   composites of sending and receiving stream states.  The simplest
   model presents the stream as "open" when either sending or receiving
   parts are in a non-terminal state and "closed" when both sending and
   receiving streams are in terminal states.

   Table 2 shows a more complex mapping of bidirectional stream states
   that loosely correspond to the stream states defined in HTTP/2
   [HTTP2].  This shows that multiple states on sending or receiving
   parts of streams are mapped to the same composite state.  Note that
   this is just one possibility for such a mapping; this mapping
   requires that data be acknowledged before the transition to a
   "closed" or "half-closed" state.

      +===================+=======================+=================+
      | Sending Part      | Receiving Part        | Composite State |
      +===================+=======================+=================+
      | No Stream / Ready | No Stream / Recv (*1) | idle            |
      +-------------------+-----------------------+-----------------+
      | Ready / Send /    | Recv / Size Known     | open            |
      | Data Sent         |                       |                 |
      +-------------------+-----------------------+-----------------+
      | Ready / Send /    | Data Recvd / Data     | half-closed     |
      | Data Sent         | Read                  | (remote)        |
      +-------------------+-----------------------+-----------------+
      | Ready / Send /    | Reset Recvd / Reset   | half-closed     |
      | Data Sent         | Read                  | (remote)        |
      +-------------------+-----------------------+-----------------+
      | Data Recvd        | Recv / Size Known     | half-closed     |
      |                   |                       | (local)         |
      +-------------------+-----------------------+-----------------+
      | Reset Sent /      | Recv / Size Known     | half-closed     |
      | Reset Recvd       |                       | (local)         |
      +-------------------+-----------------------+-----------------+
      | Reset Sent /      | Data Recvd / Data     | closed          |
      | Reset Recvd       | Read                  |                 |
      +-------------------+-----------------------+-----------------+
      | Reset Sent /      | Reset Recvd / Reset   | closed          |
      | Reset Recvd       | Read                  |                 |
      +-------------------+-----------------------+-----------------+
      | Data Recvd        | Data Recvd / Data     | closed          |
      |                   | Read                  |                 |
      +-------------------+-----------------------+-----------------+
      | Data Recvd        | Reset Recvd / Reset   | closed          |
      |                   | Read                  |                 |
      +-------------------+-----------------------+-----------------+

            Table 2: Possible Mapping of Stream States to HTTP/2

      |  Note (*1): A stream is considered "idle" if it has not yet been
      |  created or if the receiving part of the stream is in the "Recv"
      |  state without yet having received any frames.

3.5.  Solicited State Transitions

   If an application is no longer interested in the data it is receiving
   on a stream, it can abort reading the stream and specify an
   application error code.

   If the stream is in the "Recv" or "Size Known" state, the transport
   SHOULD signal this by sending a STOP_SENDING frame to prompt closure
   of the stream in the opposite direction.  This typically indicates
   that the receiving application is no longer reading data it receives
   from the stream, but it is not a guarantee that incoming data will be
   ignored.

   STREAM frames received after sending a STOP_SENDING frame are still
   counted toward connection and stream flow control, even though these
   frames can be discarded upon receipt.

   A STOP_SENDING frame requests that the receiving endpoint send a
   RESET_STREAM frame.  An endpoint that receives a STOP_SENDING frame
   MUST send a RESET_STREAM frame if the stream is in the "Ready" or
   "Send" state.  If the stream is in the "Data Sent" state, the
   endpoint MAY defer sending the RESET_STREAM frame until the packets
   containing outstanding data are acknowledged or declared lost.  If
   any outstanding data is declared lost, the endpoint SHOULD send a
   RESET_STREAM frame instead of retransmitting the data.

   An endpoint SHOULD copy the error code from the STOP_SENDING frame to
   the RESET_STREAM frame it sends, but it can use any application error
   code.  An endpoint that sends a STOP_SENDING frame MAY ignore the
   error code in any RESET_STREAM frames subsequently received for that
   stream.

   STOP_SENDING SHOULD only be sent for a stream that has not been reset
   by the peer.  STOP_SENDING is most useful for streams in the "Recv"
   or "Size Known" state.

   An endpoint is expected to send another STOP_SENDING frame if a
   packet containing a previous STOP_SENDING is lost.  However, once
   either all stream data or a RESET_STREAM frame has been received for
   the stream -- that is, the stream is in any state other than "Recv"
   or "Size Known" -- sending a STOP_SENDING frame is unnecessary.

   An endpoint that wishes to terminate both directions of a
   bidirectional stream can terminate one direction by sending a
   RESET_STREAM frame, and it can encourage prompt termination in the
   opposite direction by sending a STOP_SENDING frame.

4.  Flow Control

   Receivers need to limit the amount of data that they are required to
   buffer, in order to prevent a fast sender from overwhelming them or a
   malicious sender from consuming a large amount of memory.  To enable
   a receiver to limit memory commitments for a connection, streams are
   flow controlled both individually and across a connection as a whole.
   A QUIC receiver controls the maximum amount of data the sender can
   send on a stream as well as across all streams at any time, as
   described in Sections 4.1 and 4.2.

   Similarly, to limit concurrency within a connection, a QUIC endpoint
   controls the maximum cumulative number of streams that its peer can
   initiate, as described in Section 4.6.

   Data sent in CRYPTO frames is not flow controlled in the same way as
   stream data.  QUIC relies on the cryptographic protocol
   implementation to avoid excessive buffering of data; see [QUIC-TLS].
   To avoid excessive buffering at multiple layers, QUIC implementations
   SHOULD provide an interface for the cryptographic protocol
   implementation to communicate its buffering limits.

4.1.  Data Flow Control

   QUIC employs a limit-based flow control scheme where a receiver
   advertises the limit of total bytes it is prepared to receive on a
   given stream or for the entire connection.  This leads to two levels
   of data flow control in QUIC:

   *  Stream flow control, which prevents a single stream from consuming
      the entire receive buffer for a connection by limiting the amount
      of data that can be sent on each stream.

   *  Connection flow control, which prevents senders from exceeding a
      receiver's buffer capacity for the connection by limiting the
      total bytes of stream data sent in STREAM frames on all streams.

   Senders MUST NOT send data in excess of either limit.

   A receiver sets initial limits for all streams through transport
   parameters during the handshake (Section 7.4).  Subsequently, a
   receiver sends MAX_STREAM_DATA frames (Section 19.10) or MAX_DATA
   frames (Section 19.9) to the sender to advertise larger limits.

   A receiver can advertise a larger limit for a stream by sending a
   MAX_STREAM_DATA frame with the corresponding stream ID.  A
   MAX_STREAM_DATA frame indicates the maximum absolute byte offset of a
   stream.  A receiver could determine the flow control offset to be
   advertised based on the current offset of data consumed on that
   stream.

   A receiver can advertise a larger limit for a connection by sending a
   MAX_DATA frame, which indicates the maximum of the sum of the
   absolute byte offsets of all streams.  A receiver maintains a
   cumulative sum of bytes received on all streams, which is used to
   check for violations of the advertised connection or stream data
   limits.  A receiver could determine the maximum data limit to be
   advertised based on the sum of bytes consumed on all streams.

   Once a receiver advertises a limit for the connection or a stream, it
   is not an error to advertise a smaller limit, but the smaller limit
   has no effect.

   A receiver MUST close the connection with an error of type
   FLOW_CONTROL_ERROR if the sender violates the advertised connection
   or stream data limits; see Section 11 for details on error handling.

   A sender MUST ignore any MAX_STREAM_DATA or MAX_DATA frames that do
   not increase flow control limits.

   If a sender has sent data up to the limit, it will be unable to send
   new data and is considered blocked.  A sender SHOULD send a
   STREAM_DATA_BLOCKED or DATA_BLOCKED frame to indicate to the receiver
   that it has data to write but is blocked by flow control limits.  If
   a sender is blocked for a period longer than the idle timeout
   (Section 10.1), the receiver might close the connection even when the
   sender has data that is available for transmission.  To keep the
   connection from closing, a sender that is flow control limited SHOULD
   periodically send a STREAM_DATA_BLOCKED or DATA_BLOCKED frame when it
   has no ack-eliciting packets in flight.

4.2.  Increasing Flow Control Limits

   Implementations decide when and how much credit to advertise in
   MAX_STREAM_DATA and MAX_DATA frames, but this section offers a few
   considerations.

   To avoid blocking a sender, a receiver MAY send a MAX_STREAM_DATA or
   MAX_DATA frame multiple times within a round trip or send it early
   enough to allow time for loss of the frame and subsequent recovery.

   Control frames contribute to connection overhead.  Therefore,
   frequently sending MAX_STREAM_DATA and MAX_DATA frames with small
   changes is undesirable.  On the other hand, if updates are less
   frequent, larger increments to limits are necessary to avoid blocking
   a sender, requiring larger resource commitments at the receiver.
   There is a trade-off between resource commitment and overhead when
   determining how large a limit is advertised.

   A receiver can use an autotuning mechanism to tune the frequency and
   amount of advertised additional credit based on a round-trip time
   estimate and the rate at which the receiving application consumes
   data, similar to common TCP implementations.  As an optimization, an
   endpoint could send frames related to flow control only when there
   are other frames to send, ensuring that flow control does not cause
   extra packets to be sent.

   A blocked sender is not required to send STREAM_DATA_BLOCKED or
   DATA_BLOCKED frames.  Therefore, a receiver MUST NOT wait for a
   STREAM_DATA_BLOCKED or DATA_BLOCKED frame before sending a
   MAX_STREAM_DATA or MAX_DATA frame; doing so could result in the
   sender being blocked for the rest of the connection.  Even if the
   sender sends these frames, waiting for them will result in the sender
   being blocked for at least an entire round trip.

   When a sender receives credit after being blocked, it might be able
   to send a large amount of data in response, resulting in short-term
   congestion; see Section 7.7 of [QUIC-RECOVERY] for a discussion of
   how a sender can avoid this congestion.

4.3.  Flow Control Performance

   If an endpoint cannot ensure that its peer always has available flow
   control credit that is greater than the peer's bandwidth-delay
   product on this connection, its receive throughput will be limited by
   flow control.

   Packet loss can cause gaps in the receive buffer, preventing the
   application from consuming data and freeing up receive buffer space.

   Sending timely updates of flow control limits can improve
   performance.  Sending packets only to provide flow control updates
   can increase network load and adversely affect performance.  Sending
   flow control updates along with other frames, such as ACK frames,
   reduces the cost of those updates.

4.4.  Handling Stream Cancellation

   Endpoints need to eventually agree on the amount of flow control
   credit that has been consumed on every stream, to be able to account
   for all bytes for connection-level flow control.

   On receipt of a RESET_STREAM frame, an endpoint will tear down state
   for the matching stream and ignore further data arriving on that
   stream.

   RESET_STREAM terminates one direction of a stream abruptly.  For a
   bidirectional stream, RESET_STREAM has no effect on data flow in the
   opposite direction.  Both endpoints MUST maintain flow control state
   for the stream in the unterminated direction until that direction
   enters a terminal state.

4.5.  Stream Final Size

   The final size is the amount of flow control credit that is consumed
   by a stream.  Assuming that every contiguous byte on the stream was
   sent once, the final size is the number of bytes sent.  More
   generally, this is one higher than the offset of the byte with the
   largest offset sent on the stream, or zero if no bytes were sent.

   A sender always communicates the final size of a stream to the
   receiver reliably, no matter how the stream is terminated.  The final
   size is the sum of the Offset and Length fields of a STREAM frame
   with a FIN flag, noting that these fields might be implicit.
   Alternatively, the Final Size field of a RESET_STREAM frame carries
   this value.  This guarantees that both endpoints agree on how much
   flow control credit was consumed by the sender on that stream.

   An endpoint will know the final size for a stream when the receiving
   part of the stream enters the "Size Known" or "Reset Recvd" state
   (Section 3).  The receiver MUST use the final size of the stream to
   account for all bytes sent on the stream in its connection-level flow
   controller.

   An endpoint MUST NOT send data on a stream at or beyond the final
   size.

   Once a final size for a stream is known, it cannot change.  If a
   RESET_STREAM or STREAM frame is received indicating a change in the
   final size for the stream, an endpoint SHOULD respond with an error
   of type FINAL_SIZE_ERROR; see Section 11 for details on error
   handling.  A receiver SHOULD treat receipt of data at or beyond the
   final size as an error of type FINAL_SIZE_ERROR, even after a stream
   is closed.  Generating these errors is not mandatory, because
   requiring that an endpoint generate these errors also means that the
   endpoint needs to maintain the final size state for closed streams,
   which could mean a significant state commitment.

4.6.  Controlling Concurrency

   An endpoint limits the cumulative number of incoming streams a peer
   can open.  Only streams with a stream ID less than "(max_streams * 4
   + first_stream_id_of_type)" can be opened; see Table 1.  Initial
   limits are set in the transport parameters; see Section 18.2.
   Subsequent limits are advertised using MAX_STREAMS frames; see
   Section 19.11.  Separate limits apply to unidirectional and
   bidirectional streams.

   If a max_streams transport parameter or a MAX_STREAMS frame is
   received with a value greater than 2^60, this would allow a maximum
   stream ID that cannot be expressed as a variable-length integer; see
   Section 16.  If either is received, the connection MUST be closed
   immediately with a connection error of type TRANSPORT_PARAMETER_ERROR
   if the offending value was received in a transport parameter or of
   type FRAME_ENCODING_ERROR if it was received in a frame; see
   Section 10.2.

   Endpoints MUST NOT exceed the limit set by their peer.  An endpoint
   that receives a frame with a stream ID exceeding the limit it has
   sent MUST treat this as a connection error of type
   STREAM_LIMIT_ERROR; see Section 11 for details on error handling.

   Once a receiver advertises a stream limit using the MAX_STREAMS
   frame, advertising a smaller limit has no effect.  MAX_STREAMS frames
   that do not increase the stream limit MUST be ignored.

   As with stream and connection flow control, this document leaves
   implementations to decide when and how many streams should be
   advertised to a peer via MAX_STREAMS.  Implementations might choose
   to increase limits as streams are closed, to keep the number of
   streams available to peers roughly consistent.

   An endpoint that is unable to open a new stream due to the peer's
   limits SHOULD send a STREAMS_BLOCKED frame (Section 19.14).  This
   signal is considered useful for debugging.  An endpoint MUST NOT wait
   to receive this signal before advertising additional credit, since
   doing so will mean that the peer will be blocked for at least an
   entire round trip, and potentially indefinitely if the peer chooses
   not to send STREAMS_BLOCKED frames.

5.  Connections

   A QUIC connection is shared state between a client and a server.

   Each connection starts with a handshake phase, during which the two
   endpoints establish a shared secret using the cryptographic handshake
   protocol [QUIC-TLS] and negotiate the application protocol.  The
   handshake (Section 7) confirms that both endpoints are willing to
   communicate (Section 8.1) and establishes parameters for the
   connection (Section 7.4).

   An application protocol can use the connection during the handshake
   phase with some limitations.  0-RTT allows application data to be
   sent by a client before receiving a response from the server.
   However, 0-RTT provides no protection against replay attacks; see
   Section 9.2 of [QUIC-TLS].  A server can also send application data
   to a client before it receives the final cryptographic handshake
   messages that allow it to confirm the identity and liveness of the
   client.  These capabilities allow an application protocol to offer
   the option of trading some security guarantees for reduced latency.

   The use of connection IDs (Section 5.1) allows connections to migrate
   to a new network path, both as a direct choice of an endpoint and
   when forced by a change in a middlebox.  Section 9 describes
   mitigations for the security and privacy issues associated with
   migration.

   For connections that are no longer needed or desired, there are
   several ways for a client and server to terminate a connection, as
   described in Section 10.

5.1.  Connection ID

   Each connection possesses a set of connection identifiers, or
   connection IDs, each of which can identify the connection.
   Connection IDs are independently selected by endpoints; each endpoint
   selects the connection IDs that its peer uses.

   The primary function of a connection ID is to ensure that changes in
   addressing at lower protocol layers (UDP, IP) do not cause packets
   for a QUIC connection to be delivered to the wrong endpoint.  Each
   endpoint selects connection IDs using an implementation-specific (and
   perhaps deployment-specific) method that will allow packets with that
   connection ID to be routed back to the endpoint and to be identified
   by the endpoint upon receipt.

   Multiple connection IDs are used so that endpoints can send packets
   that cannot be identified by an observer as being for the same
   connection without cooperation from an endpoint; see Section 9.5.

   Connection IDs MUST NOT contain any information that can be used by
   an external observer (that is, one that does not cooperate with the
   issuer) to correlate them with other connection IDs for the same
   connection.  As a trivial example, this means the same connection ID
   MUST NOT be issued more than once on the same connection.

   Packets with long headers include Source Connection ID and
   Destination Connection ID fields.  These fields are used to set the
   connection IDs for new connections; see Section 7.2 for details.

   Packets with short headers (Section 17.3) only include the
   Destination Connection ID and omit the explicit length.  The length
   of the Destination Connection ID field is expected to be known to
   endpoints.  Endpoints using a load balancer that routes based on
   connection ID could agree with the load balancer on a fixed length
   for connection IDs or agree on an encoding scheme.  A fixed portion
   could encode an explicit length, which allows the entire connection
   ID to vary in length and still be used by the load balancer.

   A Version Negotiation (Section 17.2.1) packet echoes the connection
   IDs selected by the client, both to ensure correct routing toward the
   client and to demonstrate that the packet is in response to a packet
   sent by the client.

   A zero-length connection ID can be used when a connection ID is not
   needed to route to the correct endpoint.  However, multiplexing
   connections on the same local IP address and port while using zero-
   length connection IDs will cause failures in the presence of peer
   connection migration, NAT rebinding, and client port reuse.  An
   endpoint MUST NOT use the same IP address and port for multiple
   concurrent connections with zero-length connection IDs, unless it is
   certain that those protocol features are not in use.

   When an endpoint uses a non-zero-length connection ID, it needs to
   ensure that the peer has a supply of connection IDs from which to
   choose for packets sent to the endpoint.  These connection IDs are
   supplied by the endpoint using the NEW_CONNECTION_ID frame
   (Section 19.15).

5.1.1.  Issuing Connection IDs

   Each connection ID has an associated sequence number to assist in
   detecting when NEW_CONNECTION_ID or RETIRE_CONNECTION_ID frames refer
   to the same value.  The initial connection ID issued by an endpoint
   is sent in the Source Connection ID field of the long packet header
   (Section 17.2) during the handshake.  The sequence number of the
   initial connection ID is 0.  If the preferred_address transport
   parameter is sent, the sequence number of the supplied connection ID
   is 1.

   Additional connection IDs are communicated to the peer using
   NEW_CONNECTION_ID frames (Section 19.15).  The sequence number on
   each newly issued connection ID MUST increase by 1.  The connection
   ID that a client selects for the first Destination Connection ID
   field it sends and any connection ID provided by a Retry packet are
   not assigned sequence numbers.

   When an endpoint issues a connection ID, it MUST accept packets that
   carry this connection ID for the duration of the connection or until
   its peer invalidates the connection ID via a RETIRE_CONNECTION_ID
   frame (Section 19.16).  Connection IDs that are issued and not
   retired are considered active; any active connection ID is valid for
   use with the current connection at any time, in any packet type.
   This includes the connection ID issued by the server via the
   preferred_address transport parameter.

   An endpoint SHOULD ensure that its peer has a sufficient number of
   available and unused connection IDs.  Endpoints advertise the number
   of active connection IDs they are willing to maintain using the
   active_connection_id_limit transport parameter.  An endpoint MUST NOT
   provide more connection IDs than the peer's limit.  An endpoint MAY
   send connection IDs that temporarily exceed a peer's limit if the
   NEW_CONNECTION_ID frame also requires the retirement of any excess,
   by including a sufficiently large value in the Retire Prior To field.

   A NEW_CONNECTION_ID frame might cause an endpoint to add some active
   connection IDs and retire others based on the value of the Retire
   Prior To field.  After processing a NEW_CONNECTION_ID frame and
   adding and retiring active connection IDs, if the number of active
   connection IDs exceeds the value advertised in its
   active_connection_id_limit transport parameter, an endpoint MUST
   close the connection with an error of type CONNECTION_ID_LIMIT_ERROR.

   An endpoint SHOULD supply a new connection ID when the peer retires a
   connection ID.  If an endpoint provided fewer connection IDs than the
   peer's active_connection_id_limit, it MAY supply a new connection ID
   when it receives a packet with a previously unused connection ID.  An
   endpoint MAY limit the total number of connection IDs issued for each
   connection to avoid the risk of running out of connection IDs; see
   Section 10.3.2.  An endpoint MAY also limit the issuance of
   connection IDs to reduce the amount of per-path state it maintains,
   such as path validation status, as its peer might interact with it
   over as many paths as there are issued connection IDs.

   An endpoint that initiates migration and requires non-zero-length
   connection IDs SHOULD ensure that the pool of connection IDs
   available to its peer allows the peer to use a new connection ID on
   migration, as the peer will be unable to respond if the pool is
   exhausted.

   An endpoint that selects a zero-length connection ID during the
   handshake cannot issue a new connection ID.  A zero-length
   Destination Connection ID field is used in all packets sent toward
   such an endpoint over any network path.

5.1.2.  Consuming and Retiring Connection IDs

   An endpoint can change the connection ID it uses for a peer to
   another available one at any time during the connection.  An endpoint
   consumes connection IDs in response to a migrating peer; see
   Section 9.5 for more details.

   An endpoint maintains a set of connection IDs received from its peer,
   any of which it can use when sending packets.  When the endpoint
   wishes to remove a connection ID from use, it sends a
   RETIRE_CONNECTION_ID frame to its peer.  Sending a
   RETIRE_CONNECTION_ID frame indicates that the connection ID will not
   be used again and requests that the peer replace it with a new
   connection ID using a NEW_CONNECTION_ID frame.

   As discussed in Section 9.5, endpoints limit the use of a connection
   ID to packets sent from a single local address to a single
   destination address.  Endpoints SHOULD retire connection IDs when
   they are no longer actively using either the local or destination
   address for which the connection ID was used.

   An endpoint might need to stop accepting previously issued connection
   IDs in certain circumstances.  Such an endpoint can cause its peer to
   retire connection IDs by sending a NEW_CONNECTION_ID frame with an
   increased Retire Prior To field.  The endpoint SHOULD continue to
   accept the previously issued connection IDs until they are retired by
   the peer.  If the endpoint can no longer process the indicated
   connection IDs, it MAY close the connection.

   Upon receipt of an increased Retire Prior To field, the peer MUST
   stop using the corresponding connection IDs and retire them with
   RETIRE_CONNECTION_ID frames before adding the newly provided
   connection ID to the set of active connection IDs.  This ordering
   allows an endpoint to replace all active connection IDs without the
   possibility of a peer having no available connection IDs and without
   exceeding the limit the peer sets in the active_connection_id_limit
   transport parameter; see Section 18.2.  Failure to cease using the
   connection IDs when requested can result in connection failures, as
   the issuing endpoint might be unable to continue using the connection
   IDs with the active connection.

   An endpoint SHOULD limit the number of connection IDs it has retired
   locally for which RETIRE_CONNECTION_ID frames have not yet been
   acknowledged.  An endpoint SHOULD allow for sending and tracking a
   number of RETIRE_CONNECTION_ID frames of at least twice the value of
   the active_connection_id_limit transport parameter.  An endpoint MUST
   NOT forget a connection ID without retiring it, though it MAY choose
   to treat having connection IDs in need of retirement that exceed this
   limit as a connection error of type CONNECTION_ID_LIMIT_ERROR.

   Endpoints SHOULD NOT issue updates of the Retire Prior To field
   before receiving RETIRE_CONNECTION_ID frames that retire all
   connection IDs indicated by the previous Retire Prior To value.

5.2.  Matching Packets to Connections

   Incoming packets are classified on receipt.  Packets can either be
   associated with an existing connection or -- for servers --
   potentially create a new connection.

   Endpoints try to associate a packet with an existing connection.  If
   the packet has a non-zero-length Destination Connection ID
   corresponding to an existing connection, QUIC processes that packet
   accordingly.  Note that more than one connection ID can be associated
   with a connection; see Section 5.1.

   If the Destination Connection ID is zero length and the addressing
   information in the packet matches the addressing information the
   endpoint uses to identify a connection with a zero-length connection
   ID, QUIC processes the packet as part of that connection.  An
   endpoint can use just destination IP and port or both source and
   destination addresses for identification, though this makes
   connections fragile as described in Section 5.1.

   Endpoints can send a Stateless Reset (Section 10.3) for any packets
   that cannot be attributed to an existing connection.  A Stateless
   Reset allows a peer to more quickly identify when a connection
   becomes unusable.

   Packets that are matched to an existing connection are discarded if
   the packets are inconsistent with the state of that connection.  For
   example, packets are discarded if they indicate a different protocol
   version than that of the connection or if the removal of packet
   protection is unsuccessful once the expected keys are available.

   Invalid packets that lack strong integrity protection, such as
   Initial, Retry, or Version Negotiation, MAY be discarded.  An
   endpoint MUST generate a connection error if processing the contents
   of these packets prior to discovering an error, or fully revert any
   changes made during that processing.

5.2.1.  Client Packet Handling

   Valid packets sent to clients always include a Destination Connection
   ID that matches a value the client selects.  Clients that choose to
   receive zero-length connection IDs can use the local address and port
   to identify a connection.  Packets that do not match an existing
   connection -- based on Destination Connection ID or, if this value is
   zero length, local IP address and port -- are discarded.

   Due to packet reordering or loss, a client might receive packets for
   a connection that are encrypted with a key it has not yet computed.
   The client MAY drop these packets, or it MAY buffer them in
   anticipation of later packets that allow it to compute the key.

   If a client receives a packet that uses a different version than it
   initially selected, it MUST discard that packet.

5.2.2.  Server Packet Handling

   If a server receives a packet that indicates an unsupported version
   and if the packet is large enough to initiate a new connection for
   any supported version, the server SHOULD send a Version Negotiation
   packet as described in Section 6.1.  A server MAY limit the number of
   packets to which it responds with a Version Negotiation packet.
   Servers MUST drop smaller packets that specify unsupported versions.

   The first packet for an unsupported version can use different
   semantics and encodings for any version-specific field.  In
   particular, different packet protection keys might be used for
   different versions.  Servers that do not support a particular version
   are unlikely to be able to decrypt the payload of the packet or
   properly interpret the result.  Servers SHOULD respond with a Version
   Negotiation packet, provided that the datagram is sufficiently long.

   Packets with a supported version, or no Version field, are matched to
   a connection using the connection ID or -- for packets with zero-
   length connection IDs -- the local address and port.  These packets
   are processed using the selected connection; otherwise, the server
   continues as described below.

   If the packet is an Initial packet fully conforming with the
   specification, the server proceeds with the handshake (Section 7).
   This commits the server to the version that the client selected.

   If a server refuses to accept a new connection, it SHOULD send an
   Initial packet containing a CONNECTION_CLOSE frame with error code
   CONNECTION_REFUSED.

   If the packet is a 0-RTT packet, the server MAY buffer a limited
   number of these packets in anticipation of a late-arriving Initial
   packet.  Clients are not able to send Handshake packets prior to
   receiving a server response, so servers SHOULD ignore any such
   packets.

   Servers MUST drop incoming packets under all other circumstances.

5.2.3.  Considerations for Simple Load Balancers

   A server deployment could load-balance among servers using only
   source and destination IP addresses and ports.  Changes to the
   client's IP address or port could result in packets being forwarded
   to the wrong server.  Such a server deployment could use one of the
   following methods for connection continuity when a client's address
   changes.

   *  Servers could use an out-of-band mechanism to forward packets to
      the correct server based on connection ID.

   *  If servers can use a dedicated server IP address or port, other
      than the one that the client initially connects to, they could use
      the preferred_address transport parameter to request that clients
      move connections to that dedicated address.  Note that clients
      could choose not to use the preferred address.

   A server in a deployment that does not implement a solution to
   maintain connection continuity when the client address changes SHOULD
   indicate that migration is not supported by using the
   disable_active_migration transport parameter.  The
   disable_active_migration transport parameter does not prohibit
   connection migration after a client has acted on a preferred_address
   transport parameter.

   Server deployments that use this simple form of load balancing MUST
   avoid the creation of a stateless reset oracle; see Section 21.11.

5.3.  Operations on Connections

   This document does not define an API for QUIC; it instead defines a
   set of functions for QUIC connections that application protocols can
   rely upon.  An application protocol can assume that an implementation
   of QUIC provides an interface that includes the operations described
   in this section.  An implementation designed for use with a specific
   application protocol might provide only those operations that are
   used by that protocol.

   When implementing the client role, an application protocol can:

   *  open a connection, which begins the exchange described in
      Section 7;

   *  enable Early Data when available; and

   *  be informed when Early Data has been accepted or rejected by a
      server.

   When implementing the server role, an application protocol can:

   *  listen for incoming connections, which prepares for the exchange
      described in Section 7;

   *  if Early Data is supported, embed application-controlled data in
      the TLS resumption ticket sent to the client; and

   *  if Early Data is supported, retrieve application-controlled data
      from the client's resumption ticket and accept or reject Early
      Data based on that information.

   In either role, an application protocol can:

   *  configure minimum values for the initial number of permitted
      streams of each type, as communicated in the transport parameters
      (Section 7.4);

   *  control resource allocation for receive buffers by setting flow
      control limits both for streams and for the connection;

   *  identify whether the handshake has completed successfully or is
      still ongoing;

   *  keep a connection from silently closing, by either generating PING
      frames (Section 19.2) or requesting that the transport send
      additional frames before the idle timeout expires (Section 10.1);
      and

   *  immediately close (Section 10.2) the connection.

6.  Version Negotiation

   Version negotiation allows a server to indicate that it does not
   support the version the client used.  A server sends a Version
   Negotiation packet in response to each packet that might initiate a
   new connection; see Section 5.2 for details.

   The size of the first packet sent by a client will determine whether
   a server sends a Version Negotiation packet.  Clients that support
   multiple QUIC versions SHOULD ensure that the first UDP datagram they
   send is sized to the largest of the minimum datagram sizes from all
   versions they support, using PADDING frames (Section 19.1) as
   necessary.  This ensures that the server responds if there is a
   mutually supported version.  A server might not send a Version
   Negotiation packet if the datagram it receives is smaller than the
   minimum size specified in a different version; see Section 14.1.

6.1.  Sending Version Negotiation Packets

   If the version selected by the client is not acceptable to the
   server, the server responds with a Version Negotiation packet; see
   Section 17.2.1.  This includes a list of versions that the server
   will accept.  An endpoint MUST NOT send a Version Negotiation packet
   in response to receiving a Version Negotiation packet.

   This system allows a server to process packets with unsupported
   versions without retaining state.  Though either the Initial packet
   or the Version Negotiation packet that is sent in response could be
   lost, the client will send new packets until it successfully receives
   a response or it abandons the connection attempt.

   A server MAY limit the number of Version Negotiation packets it
   sends.  For instance, a server that is able to recognize packets as
   0-RTT might choose not to send Version Negotiation packets in
   response to 0-RTT packets with the expectation that it will
   eventually receive an Initial packet.

6.2.  Handling Version Negotiation Packets

   Version Negotiation packets are designed to allow for functionality
   to be defined in the future that allows QUIC to negotiate the version
   of QUIC to use for a connection.  Future Standards Track
   specifications might change how implementations that support multiple
   versions of QUIC react to Version Negotiation packets received in
   response to an attempt to establish a connection using this version.

   A client that supports only this version of QUIC MUST abandon the
   current connection attempt if it receives a Version Negotiation
   packet, with the following two exceptions.  A client MUST discard any
   Version Negotiation packet if it has received and successfully
   processed any other packet, including an earlier Version Negotiation
   packet.  A client MUST discard a Version Negotiation packet that
   lists the QUIC version selected by the client.

   How to perform version negotiation is left as future work defined by
   future Standards Track specifications.  In particular, that future
   work will ensure robustness against version downgrade attacks; see
   Section 21.12.

6.3.  Using Reserved Versions

   For a server to use a new version in the future, clients need to
   correctly handle unsupported versions.  Some version numbers
   (0x?a?a?a?a, as defined in Section 15) are reserved for inclusion in
   fields that contain version numbers.

   Endpoints MAY add reserved versions to any field where unknown or
   unsupported versions are ignored to test that a peer correctly
   ignores the value.  For instance, an endpoint could include a
   reserved version in a Version Negotiation packet; see Section 17.2.1.
   Endpoints MAY send packets with a reserved version to test that a
   peer correctly discards the packet.

7.  Cryptographic and Transport Handshake

   QUIC relies on a combined cryptographic and transport handshake to
   minimize connection establishment latency.  QUIC uses the CRYPTO
   frame (Section 19.6) to transmit the cryptographic handshake.  The
   version of QUIC defined in this document is identified as 0x00000001
   and uses TLS as described in [QUIC-TLS]; a different QUIC version
   could indicate that a different cryptographic handshake protocol is
   in use.

   QUIC provides reliable, ordered delivery of the cryptographic
   handshake data.  QUIC packet protection is used to encrypt as much of
   the handshake protocol as possible.  The cryptographic handshake MUST
   provide the following properties:

   *  authenticated key exchange, where

      -  a server is always authenticated,

      -  a client is optionally authenticated,

      -  every connection produces distinct and unrelated keys, and

      -  keying material is usable for packet protection for both 0-RTT
         and 1-RTT packets.

   *  authenticated exchange of values for transport parameters of both
      endpoints, and confidentiality protection for server transport
      parameters (see Section 7.4).

   *  authenticated negotiation of an application protocol (TLS uses
      Application-Layer Protocol Negotiation (ALPN) [ALPN] for this
      purpose).

   The CRYPTO frame can be sent in different packet number spaces
   (Section 12.3).  The offsets used by CRYPTO frames to ensure ordered
   delivery of cryptographic handshake data start from zero in each
   packet number space.

   Figure 4 shows a simplified handshake and the exchange of packets and
   frames that are used to advance the handshake.  Exchange of
   application data during the handshake is enabled where possible,
   shown with an asterisk ("*").  Once the handshake is complete,
   endpoints are able to exchange application data freely.

   Client                                               Server

   Initial (CRYPTO)
   0-RTT (*)              ---------->
                                              Initial (CRYPTO)
                                            Handshake (CRYPTO)
                          <----------                1-RTT (*)
   Handshake (CRYPTO)
   1-RTT (*)              ---------->
                          <----------   1-RTT (HANDSHAKE_DONE)

   1-RTT                  <=========>                    1-RTT

                    Figure 4: Simplified QUIC Handshake

   Endpoints can use packets sent during the handshake to test for
   Explicit Congestion Notification (ECN) support; see Section 13.4.  An
   endpoint validates support for ECN by observing whether the ACK
   frames acknowledging the first packets it sends carry ECN counts, as
   described in Section 13.4.2.

   Endpoints MUST explicitly negotiate an application protocol.  This
   avoids situations where there is a disagreement about the protocol
   that is in use.

7.1.  Example Handshake Flows

   Details of how TLS is integrated with QUIC are provided in
   [QUIC-TLS], but some examples are provided here.  An extension of
   this exchange to support client address validation is shown in
   Section 8.1.2.

   Once any address validation exchanges are complete, the cryptographic
   handshake is used to agree on cryptographic keys.  The cryptographic
   handshake is carried in Initial (Section 17.2.2) and Handshake
   (Section 17.2.4) packets.

   Figure 5 provides an overview of the 1-RTT handshake.  Each line
   shows a QUIC packet with the packet type and packet number shown
   first, followed by the frames that are typically contained in those
   packets.  For instance, the first packet is of type Initial, with
   packet number 0, and contains a CRYPTO frame carrying the
   ClientHello.

   Multiple QUIC packets -- even of different packet types -- can be
   coalesced into a single UDP datagram; see Section 12.2.  As a result,
   this handshake could consist of as few as four UDP datagrams, or any
   number more (subject to limits inherent to the protocol, such as
   congestion control and anti-amplification).  For instance, the
   server's first flight contains Initial packets, Handshake packets,
   and "0.5-RTT data" in 1-RTT packets.

   Client                                                  Server

   Initial[0]: CRYPTO[CH] ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                          Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                    <- 1-RTT[0]: STREAM[1, "..."]

   Initial[1]: ACK[0]
   Handshake[0]: CRYPTO[FIN], ACK[0]
   1-RTT[0]: STREAM[0, "..."], ACK[0] ->

                                             Handshake[1]: ACK[0]
            <- 1-RTT[1]: HANDSHAKE_DONE, STREAM[3, "..."], ACK[0]

                     Figure 5: Example 1-RTT Handshake

   Figure 6 shows an example of a connection with a 0-RTT handshake and
   a single packet of 0-RTT data.  Note that as described in
   Section 12.3, the server acknowledges 0-RTT data in 1-RTT packets,
   and the client sends 1-RTT packets in the same packet number space.

   Client                                                  Server

   Initial[0]: CRYPTO[CH]
   0-RTT[0]: STREAM[0, "..."] ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                                     Handshake[0] CRYPTO[EE, FIN]
                             <- 1-RTT[0]: STREAM[1, "..."] ACK[0]

   Initial[1]: ACK[0]
   Handshake[0]: CRYPTO[FIN], ACK[0]
   1-RTT[1]: STREAM[0, "..."] ACK[0] ->

                                             Handshake[1]: ACK[0]
            <- 1-RTT[1]: HANDSHAKE_DONE, STREAM[3, "..."], ACK[1]

                     Figure 6: Example 0-RTT Handshake

7.2.  Negotiating Connection IDs

   A connection ID is used to ensure consistent routing of packets, as
   described in Section 5.1.  The long header contains two connection
   IDs: the Destination Connection ID is chosen by the recipient of the
   packet and is used to provide consistent routing; the Source
   Connection ID is used to set the Destination Connection ID used by
   the peer.

   During the handshake, packets with the long header (Section 17.2) are
   used to establish the connection IDs used by both endpoints.  Each
   endpoint uses the Source Connection ID field to specify the
   connection ID that is used in the Destination Connection ID field of
   packets being sent to them.  After processing the first Initial
   packet, each endpoint sets the Destination Connection ID field in
   subsequent packets it sends to the value of the Source Connection ID
   field that it received.

   When an Initial packet is sent by a client that has not previously
   received an Initial or Retry packet from the server, the client
   populates the Destination Connection ID field with an unpredictable
   value.  This Destination Connection ID MUST be at least 8 bytes in
   length.  Until a packet is received from the server, the client MUST
   use the same Destination Connection ID value on all packets in this
   connection.

   The Destination Connection ID field from the first Initial packet
   sent by a client is used to determine packet protection keys for
   Initial packets.  These keys change after receiving a Retry packet;
   see Section 5.2 of [QUIC-TLS].

   The client populates the Source Connection ID field with a value of
   its choosing and sets the Source Connection ID Length field to
   indicate the length.

   0-RTT packets in the first flight use the same Destination Connection
   ID and Source Connection ID values as the client's first Initial
   packet.

   Upon first receiving an Initial or Retry packet from the server, the
   client uses the Source Connection ID supplied by the server as the
   Destination Connection ID for subsequent packets, including any 0-RTT
   packets.  This means that a client might have to change the
   connection ID it sets in the Destination Connection ID field twice
   during connection establishment: once in response to a Retry packet
   and once in response to an Initial packet from the server.  Once a
   client has received a valid Initial packet from the server, it MUST
   discard any subsequent packet it receives on that connection with a
   different Source Connection ID.

   A client MUST change the Destination Connection ID it uses for
   sending packets in response to only the first received Initial or
   Retry packet.  A server MUST set the Destination Connection ID it
   uses for sending packets based on the first received Initial packet.
   Any further changes to the Destination Connection ID are only
   permitted if the values are taken from NEW_CONNECTION_ID frames; if
   subsequent Initial packets include a different Source Connection ID,
   they MUST be discarded.  This avoids unpredictable outcomes that
   might otherwise result from stateless processing of multiple Initial
   packets with different Source Connection IDs.

   The Destination Connection ID that an endpoint sends can change over
   the lifetime of a connection, especially in response to connection
   migration (Section 9); see Section 5.1.1 for details.

7.3.  Authenticating Connection IDs

   The choice each endpoint makes about connection IDs during the
   handshake is authenticated by including all values in transport
   parameters; see Section 7.4.  This ensures that all connection IDs
   used for the handshake are also authenticated by the cryptographic
   handshake.

   Each endpoint includes the value of the Source Connection ID field
   from the first Initial packet it sent in the
   initial_source_connection_id transport parameter; see Section 18.2.
   A server includes the Destination Connection ID field from the first
   Initial packet it received from the client in the
   original_destination_connection_id transport parameter; if the server
   sent a Retry packet, this refers to the first Initial packet received
   before sending the Retry packet.  If it sends a Retry packet, a
   server also includes the Source Connection ID field from the Retry
   packet in the retry_source_connection_id transport parameter.

   The values provided by a peer for these transport parameters MUST
   match the values that an endpoint used in the Destination and Source
   Connection ID fields of Initial packets that it sent (and received,
   for servers).  Endpoints MUST validate that received transport
   parameters match received connection ID values.  Including connection
   ID values in transport parameters and verifying them ensures that an
   attacker cannot influence the choice of connection ID for a
   successful connection by injecting packets carrying attacker-chosen
   connection IDs during the handshake.

   An endpoint MUST treat the absence of the
   initial_source_connection_id transport parameter from either endpoint
   or the absence of the original_destination_connection_id transport
   parameter from the server as a connection error of type
   TRANSPORT_PARAMETER_ERROR.

   An endpoint MUST treat the following as a connection error of type
   TRANSPORT_PARAMETER_ERROR or PROTOCOL_VIOLATION:

   *  absence of the retry_source_connection_id transport parameter from
      the server after receiving a Retry packet,

   *  presence of the retry_source_connection_id transport parameter
      when no Retry packet was received, or

   *  a mismatch between values received from a peer in these transport
      parameters and the value sent in the corresponding Destination or
      Source Connection ID fields of Initial packets.

   If a zero-length connection ID is selected, the corresponding
   transport parameter is included with a zero-length value.

   Figure 7 shows the connection IDs (with DCID=Destination Connection
   ID, SCID=Source Connection ID) that are used in a complete handshake.
   The exchange of Initial packets is shown, plus the later exchange of
   1-RTT packets that includes the connection ID established during the
   handshake.

   Client                                                  Server

   Initial: DCID=S1, SCID=C1 ->
                                     <- Initial: DCID=C1, SCID=S3
                                ...
   1-RTT: DCID=S3 ->
                                                <- 1-RTT: DCID=C1

               Figure 7: Use of Connection IDs in a Handshake

   Figure 8 shows a similar handshake that includes a Retry packet.

   Client                                                  Server

   Initial: DCID=S1, SCID=C1 ->
                                       <- Retry: DCID=C1, SCID=S2
   Initial: DCID=S2, SCID=C1 ->
                                     <- Initial: DCID=C1, SCID=S3
                                ...
   1-RTT: DCID=S3 ->
                                                <- 1-RTT: DCID=C1

         Figure 8: Use of Connection IDs in a Handshake with Retry

   In both cases (Figures 7 and 8), the client sets the value of the
   initial_source_connection_id transport parameter to "C1".

   When the handshake does not include a Retry (Figure 7), the server
   sets original_destination_connection_id to "S1" (note that this value
   is chosen by the client) and initial_source_connection_id to "S3".
   In this case, the server does not include a
   retry_source_connection_id transport parameter.

   When the handshake includes a Retry (Figure 8), the server sets
   original_destination_connection_id to "S1",
   retry_source_connection_id to "S2", and initial_source_connection_id
   to "S3".

7.4.  Transport Parameters

   During connection establishment, both endpoints make authenticated
   declarations of their transport parameters.  Endpoints are required
   to comply with the restrictions that each parameter defines; the
   description of each parameter includes rules for its handling.

   Transport parameters are declarations that are made unilaterally by
   each endpoint.  Each endpoint can choose values for transport
   parameters independent of the values chosen by its peer.

   The encoding of the transport parameters is detailed in Section 18.

   QUIC includes the encoded transport parameters in the cryptographic
   handshake.  Once the handshake completes, the transport parameters
   declared by the peer are available.  Each endpoint validates the
   values provided by its peer.

   Definitions for each of the defined transport parameters are included
   in Section 18.2.

   An endpoint MUST treat receipt of a transport parameter with an
   invalid value as a connection error of type
   TRANSPORT_PARAMETER_ERROR.

   An endpoint MUST NOT send a parameter more than once in a given
   transport parameters extension.  An endpoint SHOULD treat receipt of
   duplicate transport parameters as a connection error of type
   TRANSPORT_PARAMETER_ERROR.

   Endpoints use transport parameters to authenticate the negotiation of
   connection IDs during the handshake; see Section 7.3.

   ALPN (see [ALPN]) allows clients to offer multiple application
   protocols during connection establishment.  The transport parameters
   that a client includes during the handshake apply to all application
   protocols that the client offers.  Application protocols can
   recommend values for transport parameters, such as the initial flow
   control limits.  However, application protocols that set constraints
   on values for transport parameters could make it impossible for a
   client to offer multiple application protocols if these constraints
   conflict.

7.4.1.  Values of Transport Parameters for 0-RTT

   Using 0-RTT depends on both client and server using protocol
   parameters that were negotiated from a previous connection.  To
   enable 0-RTT, endpoints store the values of the server transport
   parameters with any session tickets it receives on the connection.
   Endpoints also store any information required by the application
   protocol or cryptographic handshake; see Section 4.6 of [QUIC-TLS].
   The values of stored transport parameters are used when attempting
   0-RTT using the session tickets.

   Remembered transport parameters apply to the new connection until the
   handshake completes and the client starts sending 1-RTT packets.
   Once the handshake completes, the client uses the transport
   parameters established in the handshake.  Not all transport
   parameters are remembered, as some do not apply to future connections
   or they have no effect on the use of 0-RTT.

   The definition of a new transport parameter (Section 7.4.2) MUST
   specify whether storing the transport parameter for 0-RTT is
   mandatory, optional, or prohibited.  A client need not store a
   transport parameter it cannot process.

   A client MUST NOT use remembered values for the following parameters:
   ack_delay_exponent, max_ack_delay, initial_source_connection_id,
   original_destination_connection_id, preferred_address,
   retry_source_connection_id, and stateless_reset_token.  The client
   MUST use the server's new values in the handshake instead; if the
   server does not provide new values, the default values are used.

   A client that attempts to send 0-RTT data MUST remember all other
   transport parameters used by the server that it is able to process.
   The server can remember these transport parameters or can store an
   integrity-protected copy of the values in the ticket and recover the
   information when accepting 0-RTT data.  A server uses the transport
   parameters in determining whether to accept 0-RTT data.

   If 0-RTT data is accepted by the server, the server MUST NOT reduce
   any limits or alter any values that might be violated by the client
   with its 0-RTT data.  In particular, a server that accepts 0-RTT data
   MUST NOT set values for the following parameters (Section 18.2) that
   are smaller than the remembered values of the parameters.

   *  active_connection_id_limit

   *  initial_max_data

   *  initial_max_stream_data_bidi_local

   *  initial_max_stream_data_bidi_remote

   *  initial_max_stream_data_uni

   *  initial_max_streams_bidi

   *  initial_max_streams_uni

   Omitting or setting a zero value for certain transport parameters can
   result in 0-RTT data being enabled but not usable.  The applicable
   subset of transport parameters that permit the sending of application
   data SHOULD be set to non-zero values for 0-RTT.  This includes
   initial_max_data and either (1) initial_max_streams_bidi and
   initial_max_stream_data_bidi_remote or (2) initial_max_streams_uni
   and initial_max_stream_data_uni.

   A server might provide larger initial stream flow control limits for
   streams than the remembered values that a client applies when sending
   0-RTT.  Once the handshake completes, the client updates the flow
   control limits on all sending streams using the updated values of
   initial_max_stream_data_bidi_remote and initial_max_stream_data_uni.

   A server MAY store and recover the previously sent values of the
   max_idle_timeout, max_udp_payload_size, and disable_active_migration
   parameters and reject 0-RTT if it selects smaller values.  Lowering
   the values of these parameters while also accepting 0-RTT data could
   degrade the performance of the connection.  Specifically, lowering
   the max_udp_payload_size could result in dropped packets, leading to
   worse performance compared to rejecting 0-RTT data outright.

   A server MUST reject 0-RTT data if the restored values for transport
   parameters cannot be supported.

   When sending frames in 0-RTT packets, a client MUST only use
   remembered transport parameters; importantly, it MUST NOT use updated
   values that it learns from the server's updated transport parameters
   or from frames received in 1-RTT packets.  Updated values of
   transport parameters from the handshake apply only to 1-RTT packets.
   For instance, flow control limits from remembered transport
   parameters apply to all 0-RTT packets even if those values are
   increased by the handshake or by frames sent in 1-RTT packets.  A
   server MAY treat the use of updated transport parameters in 0-RTT as
   a connection error of type PROTOCOL_VIOLATION.

7.4.2.  New Transport Parameters

   New transport parameters can be used to negotiate new protocol
   behavior.  An endpoint MUST ignore transport parameters that it does
   not support.  The absence of a transport parameter therefore disables
   any optional protocol feature that is negotiated using the parameter.
   As described in Section 18.1, some identifiers are reserved in order
   to exercise this requirement.

   A client that does not understand a transport parameter can discard
   it and attempt 0-RTT on subsequent connections.  However, if the
   client adds support for a discarded transport parameter, it risks
   violating the constraints that the transport parameter establishes if
   it attempts 0-RTT.  New transport parameters can avoid this problem
   by setting a default of the most conservative value.  Clients can
   avoid this problem by remembering all parameters, even those not
   currently supported.

   New transport parameters can be registered according to the rules in
   Section 22.3.

7.5.  Cryptographic Message Buffering

   Implementations need to maintain a buffer of CRYPTO data received out
   of order.  Because there is no flow control of CRYPTO frames, an
   endpoint could potentially force its peer to buffer an unbounded
   amount of data.

   Implementations MUST support buffering at least 4096 bytes of data
   received in out-of-order CRYPTO frames.  Endpoints MAY choose to
   allow more data to be buffered during the handshake.  A larger limit
   during the handshake could allow for larger keys or credentials to be
   exchanged.  An endpoint's buffer size does not need to remain
   constant during the life of the connection.

   Being unable to buffer CRYPTO frames during the handshake can lead to
   a connection failure.  If an endpoint's buffer is exceeded during the
   handshake, it can expand its buffer temporarily to complete the
   handshake.  If an endpoint does not expand its buffer, it MUST close
   the connection with a CRYPTO_BUFFER_EXCEEDED error code.

   Once the handshake completes, if an endpoint is unable to buffer all
   data in a CRYPTO frame, it MAY discard that CRYPTO frame and all
   CRYPTO frames received in the future, or it MAY close the connection
   with a CRYPTO_BUFFER_EXCEEDED error code.  Packets containing
   discarded CRYPTO frames MUST be acknowledged because the packet has
   been received and processed by the transport even though the CRYPTO
   frame was discarded.

8.  Address Validation

   Address validation ensures that an endpoint cannot be used for a
   traffic amplification attack.  In such an attack, a packet is sent to
   a server with spoofed source address information that identifies a
   victim.  If a server generates more or larger packets in response to
   that packet, the attacker can use the server to send more data toward
   the victim than it would be able to send on its own.

   The primary defense against amplification attacks is verifying that a
   peer is able to receive packets at the transport address that it
   claims.  Therefore, after receiving packets from an address that is
   not yet validated, an endpoint MUST limit the amount of data it sends
   to the unvalidated address to three times the amount of data received
   from that address.  This limit on the size of responses is known as
   the anti-amplification limit.

   Address validation is performed both during connection establishment
   (see Section 8.1) and during connection migration (see Section 8.2).

8.1.  Address Validation during Connection Establishment

   Connection establishment implicitly provides address validation for
   both endpoints.  In particular, receipt of a packet protected with
   Handshake keys confirms that the peer successfully processed an
   Initial packet.  Once an endpoint has successfully processed a
   Handshake packet from the peer, it can consider the peer address to
   have been validated.

   Additionally, an endpoint MAY consider the peer address validated if
   the peer uses a connection ID chosen by the endpoint and the
   connection ID contains at least 64 bits of entropy.

   For the client, the value of the Destination Connection ID field in
   its first Initial packet allows it to validate the server address as
   a part of successfully processing any packet.  Initial packets from
   the server are protected with keys that are derived from this value
   (see Section 5.2 of [QUIC-TLS]).  Alternatively, the value is echoed
   by the server in Version Negotiation packets (Section 6) or included
   in the Integrity Tag in Retry packets (Section 5.8 of [QUIC-TLS]).

   Prior to validating the client address, servers MUST NOT send more
   than three times as many bytes as the number of bytes they have
   received.  This limits the magnitude of any amplification attack that
   can be mounted using spoofed source addresses.  For the purposes of
   avoiding amplification prior to address validation, servers MUST
   count all of the payload bytes received in datagrams that are
   uniquely attributed to a single connection.  This includes datagrams
   that contain packets that are successfully processed and datagrams
   that contain packets that are all discarded.

   Clients MUST ensure that UDP datagrams containing Initial packets
   have UDP payloads of at least 1200 bytes, adding PADDING frames as
   necessary.  A client that sends padded datagrams allows the server to
   send more data prior to completing address validation.

   Loss of an Initial or Handshake packet from the server can cause a
   deadlock if the client does not send additional Initial or Handshake
   packets.  A deadlock could occur when the server reaches its anti-
   amplification limit and the client has received acknowledgments for
   all the data it has sent.  In this case, when the client has no
   reason to send additional packets, the server will be unable to send
   more data because it has not validated the client's address.  To
   prevent this deadlock, clients MUST send a packet on a Probe Timeout
   (PTO); see Section 6.2 of [QUIC-RECOVERY].  Specifically, the client
   MUST send an Initial packet in a UDP datagram that contains at least
   1200 bytes if it does not have Handshake keys, and otherwise send a
   Handshake packet.

   A server might wish to validate the client address before starting
   the cryptographic handshake.  QUIC uses a token in the Initial packet
   to provide address validation prior to completing the handshake.
   This token is delivered to the client during connection establishment
   with a Retry packet (see Section 8.1.2) or in a previous connection
   using the NEW_TOKEN frame (see Section 8.1.3).

   In addition to sending limits imposed prior to address validation,
   servers are also constrained in what they can send by the limits set
   by the congestion controller.  Clients are only constrained by the
   congestion controller.

8.1.1.  Token Construction

   A token sent in a NEW_TOKEN frame or a Retry packet MUST be
   constructed in a way that allows the server to identify how it was
   provided to a client.  These tokens are carried in the same field but
   require different handling from servers.

8.1.2.  Address Validation Using Retry Packets

   Upon receiving the client's Initial packet, the server can request
   address validation by sending a Retry packet (Section 17.2.5)
   containing a token.  This token MUST be repeated by the client in all
   Initial packets it sends for that connection after it receives the
   Retry packet.

   In response to processing an Initial packet containing a token that
   was provided in a Retry packet, a server cannot send another Retry
   packet; it can only refuse the connection or permit it to proceed.

   As long as it is not possible for an attacker to generate a valid
   token for its own address (see Section 8.1.4) and the client is able
   to return that token, it proves to the server that it received the
   token.

   A server can also use a Retry packet to defer the state and
   processing costs of connection establishment.  Requiring the server
   to provide a different connection ID, along with the
   original_destination_connection_id transport parameter defined in
   Section 18.2, forces the server to demonstrate that it, or an entity
   it cooperates with, received the original Initial packet from the
   client.  Providing a different connection ID also grants a server
   some control over how subsequent packets are routed.  This can be
   used to direct connections to a different server instance.

   If a server receives a client Initial that contains an invalid Retry
   token but is otherwise valid, it knows the client will not accept
   another Retry token.  The server can discard such a packet and allow
   the client to time out to detect handshake failure, but that could
   impose a significant latency penalty on the client.  Instead, the
   server SHOULD immediately close (Section 10.2) the connection with an
   INVALID_TOKEN error.  Note that a server has not established any
   state for the connection at this point and so does not enter the
   closing period.

   A flow showing the use of a Retry packet is shown in Figure 9.

   Client                                                  Server

   Initial[0]: CRYPTO[CH] ->

                                                   <- Retry+Token

   Initial+Token[1]: CRYPTO[CH] ->

                                    Initial[0]: CRYPTO[SH] ACK[1]
                          Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                    <- 1-RTT[0]: STREAM[1, "..."]

                   Figure 9: Example Handshake with Retry

8.1.3.  Address Validation for Future Connections

   A server MAY provide clients with an address validation token during
   one connection that can be used on a subsequent connection.  Address
   validation is especially important with 0-RTT because a server
   potentially sends a significant amount of data to a client in
   response to 0-RTT data.

   The server uses the NEW_TOKEN frame (Section 19.7) to provide the
   client with an address validation token that can be used to validate
   future connections.  In a future connection, the client includes this
   token in Initial packets to provide address validation.  The client
   MUST include the token in all Initial packets it sends, unless a
   Retry replaces the token with a newer one.  The client MUST NOT use
   the token provided in a Retry for future connections.  Servers MAY
   discard any Initial packet that does not carry the expected token.

   Unlike the token that is created for a Retry packet, which is used
   immediately, the token sent in the NEW_TOKEN frame can be used after
   some period of time has passed.  Thus, a token SHOULD have an
   expiration time, which could be either an explicit expiration time or
   an issued timestamp that can be used to dynamically calculate the
   expiration time.  A server can store the expiration time or include
   it in an encrypted form in the token.

   A token issued with NEW_TOKEN MUST NOT include information that would
   allow values to be linked by an observer to the connection on which
   it was issued.  For example, it cannot include the previous
   connection ID or addressing information, unless the values are
   encrypted.  A server MUST ensure that every NEW_TOKEN frame it sends
   is unique across all clients, with the exception of those sent to
   repair losses of previously sent NEW_TOKEN frames.  Information that
   allows the server to distinguish between tokens from Retry and
   NEW_TOKEN MAY be accessible to entities other than the server.

   It is unlikely that the client port number is the same on two
   different connections; validating the port is therefore unlikely to
   be successful.

   A token received in a NEW_TOKEN frame is applicable to any server
   that the connection is considered authoritative for (e.g., server
   names included in the certificate).  When connecting to a server for
   which the client retains an applicable and unused token, it SHOULD
   include that token in the Token field of its Initial packet.
   Including a token might allow the server to validate the client
   address without an additional round trip.  A client MUST NOT include
   a token that is not applicable to the server that it is connecting
   to, unless the client has the knowledge that the server that issued
   the token and the server the client is connecting to are jointly
   managing the tokens.  A client MAY use a token from any previous
   connection to that server.

   A token allows a server to correlate activity between the connection
   where the token was issued and any connection where it is used.
   Clients that want to break continuity of identity with a server can
   discard tokens provided using the NEW_TOKEN frame.  In comparison, a
   token obtained in a Retry packet MUST be used immediately during the
   connection attempt and cannot be used in subsequent connection
   attempts.

   A client SHOULD NOT reuse a token from a NEW_TOKEN frame for
   different connection attempts.  Reusing a token allows connections to
   be linked by entities on the network path; see Section 9.5.

   Clients might receive multiple tokens on a single connection.  Aside
   from preventing linkability, any token can be used in any connection
   attempt.  Servers can send additional tokens to either enable address
   validation for multiple connection attempts or replace older tokens
   that might become invalid.  For a client, this ambiguity means that
   sending the most recent unused token is most likely to be effective.
   Though saving and using older tokens have no negative consequences,
   clients can regard older tokens as being less likely to be useful to
   the server for address validation.

   When a server receives an Initial packet with an address validation
   token, it MUST attempt to validate the token, unless it has already
   completed address validation.  If the token is invalid, then the
   server SHOULD proceed as if the client did not have a validated
   address, including potentially sending a Retry packet.  Tokens
   provided with NEW_TOKEN frames and Retry packets can be distinguished
   by servers (see Section 8.1.1), and the latter can be validated more
   strictly.  If the validation succeeds, the server SHOULD then allow
   the handshake to proceed.

      |  Note: The rationale for treating the client as unvalidated
      |  rather than discarding the packet is that the client might have
      |  received the token in a previous connection using the NEW_TOKEN
      |  frame, and if the server has lost state, it might be unable to
      |  validate the token at all, leading to connection failure if the
      |  packet is discarded.

   In a stateless design, a server can use encrypted and authenticated
   tokens to pass information to clients that the server can later
   recover and use to validate a client address.  Tokens are not
   integrated into the cryptographic handshake, and so they are not
   authenticated.  For instance, a client might be able to reuse a
   token.  To avoid attacks that exploit this property, a server can
   limit its use of tokens to only the information needed to validate
   client addresses.

   Clients MAY use tokens obtained on one connection for any connection
   attempt using the same version.  When selecting a token to use,
   clients do not need to consider other properties of the connection
   that is being attempted, including the choice of possible application
   protocols, session tickets, or other connection properties.

8.1.4.  Address Validation Token Integrity

   An address validation token MUST be difficult to guess.  Including a
   random value with at least 128 bits of entropy in the token would be
   sufficient, but this depends on the server remembering the value it
   sends to clients.

   A token-based scheme allows the server to offload any state
   associated with validation to the client.  For this design to work,
   the token MUST be covered by integrity protection against
   modification or falsification by clients.  Without integrity
   protection, malicious clients could generate or guess values for
   tokens that would be accepted by the server.  Only the server
   requires access to the integrity protection key for tokens.

   There is no need for a single well-defined format for the token
   because the server that generates the token also consumes it.  Tokens
   sent in Retry packets SHOULD include information that allows the
   server to verify that the source IP address and port in client
   packets remain constant.

   Tokens sent in NEW_TOKEN frames MUST include information that allows
   the server to verify that the client IP address has not changed from
   when the token was issued.  Servers can use tokens from NEW_TOKEN
   frames in deciding not to send a Retry packet, even if the client
   address has changed.  If the client IP address has changed, the
   server MUST adhere to the anti-amplification limit; see Section 8.
   Note that in the presence of NAT, this requirement might be
   insufficient to protect other hosts that share the NAT from
   amplification attacks.

   Attackers could replay tokens to use servers as amplifiers in DDoS
   attacks.  To protect against such attacks, servers MUST ensure that
   replay of tokens is prevented or limited.  Servers SHOULD ensure that
   tokens sent in Retry packets are only accepted for a short time, as
   they are returned immediately by clients.  Tokens that are provided
   in NEW_TOKEN frames (Section 19.7) need to be valid for longer but
   SHOULD NOT be accepted multiple times.  Servers are encouraged to
   allow tokens to be used only once, if possible; tokens MAY include
   additional information about clients to further narrow applicability
   or reuse.

8.2.  Path Validation

   Path validation is used by both peers during connection migration
   (see Section 9) to verify reachability after a change of address.  In
   path validation, endpoints test reachability between a specific local
   address and a specific peer address, where an address is the 2-tuple
   of IP address and port.

   Path validation tests that packets sent on a path to a peer are
   received by that peer.  Path validation is used to ensure that
   packets received from a migrating peer do not carry a spoofed source
   address.

   Path validation does not validate that a peer can send in the return
   direction.  Acknowledgments cannot be used for return path validation
   because they contain insufficient entropy and might be spoofed.
   Endpoints independently determine reachability on each direction of a
   path, and therefore return reachability can only be established by
   the peer.

   Path validation can be used at any time by either endpoint.  For
   instance, an endpoint might check that a peer is still in possession
   of its address after a period of quiescence.

   Path validation is not designed as a NAT traversal mechanism.  Though
   the mechanism described here might be effective for the creation of
   NAT bindings that support NAT traversal, the expectation is that one
   endpoint is able to receive packets without first having sent a
   packet on that path.  Effective NAT traversal needs additional
   synchronization mechanisms that are not provided here.

   An endpoint MAY include other frames with the PATH_CHALLENGE and
   PATH_RESPONSE frames used for path validation.  In particular, an
   endpoint can include PADDING frames with a PATH_CHALLENGE frame for
   Path Maximum Transmission Unit Discovery (PMTUD); see Section 14.2.1.
   An endpoint can also include its own PATH_CHALLENGE frame when
   sending a PATH_RESPONSE frame.

   An endpoint uses a new connection ID for probes sent from a new local
   address; see Section 9.5.  When probing a new path, an endpoint can
   ensure that its peer has an unused connection ID available for
   responses.  Sending NEW_CONNECTION_ID and PATH_CHALLENGE frames in
   the same packet, if the peer's active_connection_id_limit permits,
   ensures that an unused connection ID will be available to the peer
   when sending a response.

   An endpoint can choose to simultaneously probe multiple paths.  The
   number of simultaneous paths used for probes is limited by the number
   of extra connection IDs its peer has previously supplied, since each
   new local address used for a probe requires a previously unused
   connection ID.

8.2.1.  Initiating Path Validation

   To initiate path validation, an endpoint sends a PATH_CHALLENGE frame
   containing an unpredictable payload on the path to be validated.

   An endpoint MAY send multiple PATH_CHALLENGE frames to guard against
   packet loss.  However, an endpoint SHOULD NOT send multiple
   PATH_CHALLENGE frames in a single packet.

   An endpoint SHOULD NOT probe a new path with packets containing a
   PATH_CHALLENGE frame more frequently than it would send an Initial
   packet.  This ensures that connection migration is no more load on a
   new path than establishing a new connection.

   The endpoint MUST use unpredictable data in every PATH_CHALLENGE
   frame so that it can associate the peer's response with the
   corresponding PATH_CHALLENGE.

   An endpoint MUST expand datagrams that contain a PATH_CHALLENGE frame
   to at least the smallest allowed maximum datagram size of 1200 bytes,
   unless the anti-amplification limit for the path does not permit
   sending a datagram of this size.  Sending UDP datagrams of this size
   ensures that the network path from the endpoint to the peer can be
   used for QUIC; see Section 14.

   When an endpoint is unable to expand the datagram size to 1200 bytes
   due to the anti-amplification limit, the path MTU will not be
   validated.  To ensure that the path MTU is large enough, the endpoint
   MUST perform a second path validation by sending a PATH_CHALLENGE
   frame in a datagram of at least 1200 bytes.  This additional
   validation can be performed after a PATH_RESPONSE is successfully
   received or when enough bytes have been received on the path that
   sending the larger datagram will not result in exceeding the anti-
   amplification limit.

   Unlike other cases where datagrams are expanded, endpoints MUST NOT
   discard datagrams that appear to be too small when they contain
   PATH_CHALLENGE or PATH_RESPONSE.

8.2.2.  Path Validation Responses

   On receiving a PATH_CHALLENGE frame, an endpoint MUST respond by
   echoing the data contained in the PATH_CHALLENGE frame in a
   PATH_RESPONSE frame.  An endpoint MUST NOT delay transmission of a
   packet containing a PATH_RESPONSE frame unless constrained by
   congestion control.

   A PATH_RESPONSE frame MUST be sent on the network path where the
   PATH_CHALLENGE frame was received.  This ensures that path validation
   by a peer only succeeds if the path is functional in both directions.
   This requirement MUST NOT be enforced by the endpoint that initiates
   path validation, as that would enable an attack on migration; see
   Section 9.3.3.

   An endpoint MUST expand datagrams that contain a PATH_RESPONSE frame
   to at least the smallest allowed maximum datagram size of 1200 bytes.
   This verifies that the path is able to carry datagrams of this size
   in both directions.  However, an endpoint MUST NOT expand the
   datagram containing the PATH_RESPONSE if the resulting data exceeds
   the anti-amplification limit.  This is expected to only occur if the
   received PATH_CHALLENGE was not sent in an expanded datagram.

   An endpoint MUST NOT send more than one PATH_RESPONSE frame in
   response to one PATH_CHALLENGE frame; see Section 13.3.  The peer is
   expected to send more PATH_CHALLENGE frames as necessary to evoke
   additional PATH_RESPONSE frames.

8.2.3.  Successful Path Validation

   Path validation succeeds when a PATH_RESPONSE frame is received that
   contains the data that was sent in a previous PATH_CHALLENGE frame.
   A PATH_RESPONSE frame received on any network path validates the path
   on which the PATH_CHALLENGE was sent.

   If an endpoint sends a PATH_CHALLENGE frame in a datagram that is not
   expanded to at least 1200 bytes and if the response to it validates
   the peer address, the path is validated but not the path MTU.  As a
   result, the endpoint can now send more than three times the amount of
   data that has been received.  However, the endpoint MUST initiate
   another path validation with an expanded datagram to verify that the
   path supports the required MTU.

   Receipt of an acknowledgment for a packet containing a PATH_CHALLENGE
   frame is not adequate validation, since the acknowledgment can be
   spoofed by a malicious peer.

8.2.4.  Failed Path Validation

   Path validation only fails when the endpoint attempting to validate
   the path abandons its attempt to validate the path.

   Endpoints SHOULD abandon path validation based on a timer.  When
   setting this timer, implementations are cautioned that the new path
   could have a longer round-trip time than the original.  A value of
   three times the larger of the current PTO or the PTO for the new path
   (using kInitialRtt, as defined in [QUIC-RECOVERY]) is RECOMMENDED.

   This timeout allows for multiple PTOs to expire prior to failing path
   validation, so that loss of a single PATH_CHALLENGE or PATH_RESPONSE
   frame does not cause path validation failure.

   Note that the endpoint might receive packets containing other frames
   on the new path, but a PATH_RESPONSE frame with appropriate data is
   required for path validation to succeed.

   When an endpoint abandons path validation, it determines that the
   path is unusable.  This does not necessarily imply a failure of the
   connection -- endpoints can continue sending packets over other paths
   as appropriate.  If no paths are available, an endpoint can wait for
   a new path to become available or close the connection.  An endpoint
   that has no valid network path to its peer MAY signal this using the
   NO_VIABLE_PATH connection error, noting that this is only possible if
   the network path exists but does not support the required MTU
   (Section 14).

   A path validation might be abandoned for other reasons besides
   failure.  Primarily, this happens if a connection migration to a new
   path is initiated while a path validation on the old path is in
   progress.

9.  Connection Migration

   The use of a connection ID allows connections to survive changes to
   endpoint addresses (IP address and port), such as those caused by an
   endpoint migrating to a new network.  This section describes the
   process by which an endpoint migrates to a new address.

   The design of QUIC relies on endpoints retaining a stable address for
   the duration of the handshake.  An endpoint MUST NOT initiate
   connection migration before the handshake is confirmed, as defined in
   Section 4.1.2 of [QUIC-TLS].

   If the peer sent the disable_active_migration transport parameter, an
   endpoint also MUST NOT send packets (including probing packets; see
   Section 9.1) from a different local address to the address the peer
   used during the handshake, unless the endpoint has acted on a
   preferred_address transport parameter from the peer.  If the peer
   violates this requirement, the endpoint MUST either drop the incoming
   packets on that path without generating a Stateless Reset or proceed
   with path validation and allow the peer to migrate.  Generating a
   Stateless Reset or closing the connection would allow third parties
   in the network to cause connections to close by spoofing or otherwise
   manipulating observed traffic.

   Not all changes of peer address are intentional, or active,
   migrations.  The peer could experience NAT rebinding: a change of
   address due to a middlebox, usually a NAT, allocating a new outgoing
   port or even a new outgoing IP address for a flow.  An endpoint MUST
   perform path validation (Section 8.2) if it detects any change to a
   peer's address, unless it has previously validated that address.

   When an endpoint has no validated path on which to send packets, it
   MAY discard connection state.  An endpoint capable of connection
   migration MAY wait for a new path to become available before
   discarding connection state.

   This document limits migration of connections to new client
   addresses, except as described in Section 9.6.  Clients are
   responsible for initiating all migrations.  Servers do not send non-
   probing packets (see Section 9.1) toward a client address until they
   see a non-probing packet from that address.  If a client receives
   packets from an unknown server address, the client MUST discard these
   packets.

9.1.  Probing a New Path

   An endpoint MAY probe for peer reachability from a new local address
   using path validation (Section 8.2) prior to migrating the connection
   to the new local address.  Failure of path validation simply means
   that the new path is not usable for this connection.  Failure to
   validate a path does not cause the connection to end unless there are
   no valid alternative paths available.

   PATH_CHALLENGE, PATH_RESPONSE, NEW_CONNECTION_ID, and PADDING frames
   are "probing frames", and all other frames are "non-probing frames".
   A packet containing only probing frames is a "probing packet", and a
   packet containing any other frame is a "non-probing packet".

9.2.  Initiating Connection Migration

   An endpoint can migrate a connection to a new local address by
   sending packets containing non-probing frames from that address.

   Each endpoint validates its peer's address during connection
   establishment.  Therefore, a migrating endpoint can send to its peer
   knowing that the peer is willing to receive at the peer's current
   address.  Thus, an endpoint can migrate to a new local address
   without first validating the peer's address.

   To establish reachability on the new path, an endpoint initiates path
   validation (Section 8.2) on the new path.  An endpoint MAY defer path
   validation until after a peer sends the next non-probing frame to its
   new address.

   When migrating, the new path might not support the endpoint's current
   sending rate.  Therefore, the endpoint resets its congestion
   controller and RTT estimate, as described in Section 9.4.

   The new path might not have the same ECN capability.  Therefore, the
   endpoint validates ECN capability as described in Section 13.4.

9.3.  Responding to Connection Migration

   Receiving a packet from a new peer address containing a non-probing
   frame indicates that the peer has migrated to that address.

   If the recipient permits the migration, it MUST send subsequent
   packets to the new peer address and MUST initiate path validation
   (Section 8.2) to verify the peer's ownership of the address if
   validation is not already underway.  If the recipient has no unused
   connection IDs from the peer, it will not be able to send anything on
   the new path until the peer provides one; see Section 9.5.

   An endpoint only changes the address to which it sends packets in
   response to the highest-numbered non-probing packet.  This ensures
   that an endpoint does not send packets to an old peer address in the
   case that it receives reordered packets.

   An endpoint MAY send data to an unvalidated peer address, but it MUST
   protect against potential attacks as described in Sections 9.3.1 and
   9.3.2.  An endpoint MAY skip validation of a peer address if that
   address has been seen recently.  In particular, if an endpoint
   returns to a previously validated path after detecting some form of
   spurious migration, skipping address validation and restoring loss
   detection and congestion state can reduce the performance impact of
   the attack.

   After changing the address to which it sends non-probing packets, an
   endpoint can abandon any path validation for other addresses.

   Receiving a packet from a new peer address could be the result of a
   NAT rebinding at the peer.

   After verifying a new client address, the server SHOULD send new
   address validation tokens (Section 8) to the client.

9.3.1.  Peer Address Spoofing

   It is possible that a peer is spoofing its source address to cause an
   endpoint to send excessive amounts of data to an unwilling host.  If
   the endpoint sends significantly more data than the spoofing peer,
   connection migration might be used to amplify the volume of data that
   an attacker can generate toward a victim.

   As described in Section 9.3, an endpoint is required to validate a
   peer's new address to confirm the peer's possession of the new
   address.  Until a peer's address is deemed valid, an endpoint limits
   the amount of data it sends to that address; see Section 8.  In the
   absence of this limit, an endpoint risks being used for a denial-of-
   service attack against an unsuspecting victim.

   If an endpoint skips validation of a peer address as described above,
   it does not need to limit its sending rate.

9.3.2.  On-Path Address Spoofing

   An on-path attacker could cause a spurious connection migration by
   copying and forwarding a packet with a spoofed address such that it
   arrives before the original packet.  The packet with the spoofed
   address will be seen to come from a migrating connection, and the
   original packet will be seen as a duplicate and dropped.  After a
   spurious migration, validation of the source address will fail
   because the entity at the source address does not have the necessary
   cryptographic keys to read or respond to the PATH_CHALLENGE frame
   that is sent to it even if it wanted to.

   To protect the connection from failing due to such a spurious
   migration, an endpoint MUST revert to using the last validated peer
   address when validation of a new peer address fails.  Additionally,
   receipt of packets with higher packet numbers from the legitimate
   peer address will trigger another connection migration.  This will
   cause the validation of the address of the spurious migration to be
   abandoned, thus containing migrations initiated by the attacker
   injecting a single packet.

   If an endpoint has no state about the last validated peer address, it
   MUST close the connection silently by discarding all connection
   state.  This results in new packets on the connection being handled
   generically.  For instance, an endpoint MAY send a Stateless Reset in
   response to any further incoming packets.

9.3.3.  Off-Path Packet Forwarding

   An off-path attacker that can observe packets might forward copies of
   genuine packets to endpoints.  If the copied packet arrives before
   the genuine packet, this will appear as a NAT rebinding.  Any genuine
   packet will be discarded as a duplicate.  If the attacker is able to
   continue forwarding packets, it might be able to cause migration to a
   path via the attacker.  This places the attacker on-path, giving it
   the ability to observe or drop all subsequent packets.

   This style of attack relies on the attacker using a path that has
   approximately the same characteristics as the direct path between
   endpoints.  The attack is more reliable if relatively few packets are
   sent or if packet loss coincides with the attempted attack.

   A non-probing packet received on the original path that increases the
   maximum received packet number will cause the endpoint to move back
   to that path.  Eliciting packets on this path increases the
   likelihood that the attack is unsuccessful.  Therefore, mitigation of
   this attack relies on triggering the exchange of packets.

   In response to an apparent migration, endpoints MUST validate the
   previously active path using a PATH_CHALLENGE frame.  This induces
   the sending of new packets on that path.  If the path is no longer
   viable, the validation attempt will time out and fail; if the path is
   viable but no longer desired, the validation will succeed but only
   results in probing packets being sent on the path.

   An endpoint that receives a PATH_CHALLENGE on an active path SHOULD
   send a non-probing packet in response.  If the non-probing packet
   arrives before any copy made by an attacker, this results in the
   connection being migrated back to the original path.  Any subsequent
   migration to another path restarts this entire process.

   This defense is imperfect, but this is not considered a serious
   problem.  If the path via the attack is reliably faster than the
   original path despite multiple attempts to use that original path, it
   is not possible to distinguish between an attack and an improvement
   in routing.

   An endpoint could also use heuristics to improve detection of this
   style of attack.  For instance, NAT rebinding is improbable if
   packets were recently received on the old path; similarly, rebinding
   is rare on IPv6 paths.  Endpoints can also look for duplicated
   packets.  Conversely, a change in connection ID is more likely to
   indicate an intentional migration rather than an attack.

9.4.  Loss Detection and Congestion Control

   The capacity available on the new path might not be the same as the
   old path.  Packets sent on the old path MUST NOT contribute to
   congestion control or RTT estimation for the new path.

   On confirming a peer's ownership of its new address, an endpoint MUST
   immediately reset the congestion controller and round-trip time
   estimator for the new path to initial values (see Appendices A.3 and
   B.3 of [QUIC-RECOVERY]) unless the only change in the peer's address
   is its port number.  Because port-only changes are commonly the
   result of NAT rebinding or other middlebox activity, the endpoint MAY
   instead retain its congestion control state and round-trip estimate
   in those cases instead of reverting to initial values.  In cases
   where congestion control state retained from an old path is used on a
   new path with substantially different characteristics, a sender could
   transmit too aggressively until the congestion controller and the RTT
   estimator have adapted.  Generally, implementations are advised to be
   cautious when using previous values on a new path.

   There could be apparent reordering at the receiver when an endpoint
   sends data and probes from/to multiple addresses during the migration
   period, since the two resulting paths could have different round-trip
   times.  A receiver of packets on multiple paths will still send ACK
   frames covering all received packets.

   While multiple paths might be used during connection migration, a
   single congestion control context and a single loss recovery context
   (as described in [QUIC-RECOVERY]) could be adequate.  For instance,
   an endpoint might delay switching to a new congestion control context
   until it is confirmed that an old path is no longer needed (such as
   the case described in Section 9.3.3).

   A sender can make exceptions for probe packets so that their loss
   detection is independent and does not unduly cause the congestion
   controller to reduce its sending rate.  An endpoint might set a
   separate timer when a PATH_CHALLENGE is sent, which is canceled if
   the corresponding PATH_RESPONSE is received.  If the timer fires
   before the PATH_RESPONSE is received, the endpoint might send a new
   PATH_CHALLENGE and restart the timer for a longer period of time.
   This timer SHOULD be set as described in Section 6.2.1 of
   [QUIC-RECOVERY] and MUST NOT be more aggressive.

9.5.  Privacy Implications of Connection Migration

   Using a stable connection ID on multiple network paths would allow a
   passive observer to correlate activity between those paths.  An
   endpoint that moves between networks might not wish to have their
   activity correlated by any entity other than their peer, so different
   connection IDs are used when sending from different local addresses,
   as discussed in Section 5.1.  For this to be effective, endpoints
   need to ensure that connection IDs they provide cannot be linked by
   any other entity.

   At any time, endpoints MAY change the Destination Connection ID they
   transmit with to a value that has not been used on another path.

   An endpoint MUST NOT reuse a connection ID when sending from more
   than one local address -- for example, when initiating connection
   migration as described in Section 9.2 or when probing a new network
   path as described in Section 9.1.

   Similarly, an endpoint MUST NOT reuse a connection ID when sending to
   more than one destination address.  Due to network changes outside
   the control of its peer, an endpoint might receive packets from a new
   source address with the same Destination Connection ID field value,
   in which case it MAY continue to use the current connection ID with
   the new remote address while still sending from the same local
   address.

   These requirements regarding connection ID reuse apply only to the
   sending of packets, as unintentional changes in path without a change
   in connection ID are possible.  For example, after a period of
   network inactivity, NAT rebinding might cause packets to be sent on a
   new path when the client resumes sending.  An endpoint responds to
   such an event as described in Section 9.3.

   Using different connection IDs for packets sent in both directions on
   each new network path eliminates the use of the connection ID for
   linking packets from the same connection across different network
   paths.  Header protection ensures that packet numbers cannot be used
   to correlate activity.  This does not prevent other properties of
   packets, such as timing and size, from being used to correlate
   activity.

   An endpoint SHOULD NOT initiate migration with a peer that has
   requested a zero-length connection ID, because traffic over the new
   path might be trivially linkable to traffic over the old one.  If the
   server is able to associate packets with a zero-length connection ID
   to the right connection, it means that the server is using other
   information to demultiplex packets.  For example, a server might
   provide a unique address to every client -- for instance, using HTTP
   alternative services [ALTSVC].  Information that might allow correct
   routing of packets across multiple network paths will also allow
   activity on those paths to be linked by entities other than the peer.

   A client might wish to reduce linkability by switching to a new
   connection ID, source UDP port, or IP address (see [RFC8981]) when
   sending traffic after a period of inactivity.  Changing the address
   from which it sends packets at the same time might cause the server
   to detect a connection migration.  This ensures that the mechanisms
   that support migration are exercised even for clients that do not
   experience NAT rebindings or genuine migrations.  Changing address
   can cause a peer to reset its congestion control state (see
   Section 9.4), so addresses SHOULD only be changed infrequently.

   An endpoint that exhausts available connection IDs cannot probe new
   paths or initiate migration, nor can it respond to probes or attempts
   by its peer to migrate.  To ensure that migration is possible and
   packets sent on different paths cannot be correlated, endpoints
   SHOULD provide new connection IDs before peers migrate; see
   Section 5.1.1.  If a peer might have exhausted available connection
   IDs, a migrating endpoint could include a NEW_CONNECTION_ID frame in
   all packets sent on a new network path.

9.6.  Server's Preferred Address

   QUIC allows servers to accept connections on one IP address and
   attempt to transfer these connections to a more preferred address
   shortly after the handshake.  This is particularly useful when
   clients initially connect to an address shared by multiple servers
   but would prefer to use a unicast address to ensure connection
   stability.  This section describes the protocol for migrating a
   connection to a preferred server address.

   Migrating a connection to a new server address mid-connection is not
   supported by the version of QUIC specified in this document.  If a
   client receives packets from a new server address when the client has
   not initiated a migration to that address, the client SHOULD discard
   these packets.

9.6.1.  Communicating a Preferred Address

   A server conveys a preferred address by including the
   preferred_address transport parameter in the TLS handshake.

   Servers MAY communicate a preferred address of each address family
   (IPv4 and IPv6) to allow clients to pick the one most suited to their
   network attachment.

   Once the handshake is confirmed, the client SHOULD select one of the
   two addresses provided by the server and initiate path validation
   (see Section 8.2).  A client constructs packets using any previously
   unused active connection ID, taken from either the preferred_address
   transport parameter or a NEW_CONNECTION_ID frame.

   As soon as path validation succeeds, the client SHOULD begin sending
   all future packets to the new server address using the new connection
   ID and discontinue use of the old server address.  If path validation
   fails, the client MUST continue sending all future packets to the
   server's original IP address.

9.6.2.  Migration to a Preferred Address

   A client that migrates to a preferred address MUST validate the
   address it chooses before migrating; see Section 21.5.3.

   A server might receive a packet addressed to its preferred IP address
   at any time after it accepts a connection.  If this packet contains a
   PATH_CHALLENGE frame, the server sends a packet containing a
   PATH_RESPONSE frame as per Section 8.2.  The server MUST send non-
   probing packets from its original address until it receives a non-
   probing packet from the client at its preferred address and until the
   server has validated the new path.

   The server MUST probe on the path toward the client from its
   preferred address.  This helps to guard against spurious migration
   initiated by an attacker.

   Once the server has completed its path validation and has received a
   non-probing packet with a new largest packet number on its preferred
   address, the server begins sending non-probing packets to the client
   exclusively from its preferred IP address.  The server SHOULD drop
   newer packets for this connection that are received on the old IP
   address.  The server MAY continue to process delayed packets that are
   received on the old IP address.

   The addresses that a server provides in the preferred_address
   transport parameter are only valid for the connection in which they
   are provided.  A client MUST NOT use these for other connections,
   including connections that are resumed from the current connection.

9.6.3.  Interaction of Client Migration and Preferred Address

   A client might need to perform a connection migration before it has
   migrated to the server's preferred address.  In this case, the client
   SHOULD perform path validation to both the original and preferred
   server address from the client's new address concurrently.

   If path validation of the server's preferred address succeeds, the
   client MUST abandon validation of the original address and migrate to
   using the server's preferred address.  If path validation of the
   server's preferred address fails but validation of the server's
   original address succeeds, the client MAY migrate to its new address
   and continue sending to the server's original address.

   If packets received at the server's preferred address have a
   different source address than observed from the client during the
   handshake, the server MUST protect against potential attacks as
   described in Sections 9.3.1 and 9.3.2.  In addition to intentional
   simultaneous migration, this might also occur because the client's
   access network used a different NAT binding for the server's
   preferred address.

   Servers SHOULD initiate path validation to the client's new address
   upon receiving a probe packet from a different address; see
   Section 8.

   A client that migrates to a new address SHOULD use a preferred
   address from the same address family for the server.

   The connection ID provided in the preferred_address transport
   parameter is not specific to the addresses that are provided.  This
   connection ID is provided to ensure that the client has a connection
   ID available for migration, but the client MAY use this connection ID
   on any path.

9.7.  Use of IPv6 Flow Label and Migration

   Endpoints that send data using IPv6 SHOULD apply an IPv6 flow label
   in compliance with [RFC6437], unless the local API does not allow
   setting IPv6 flow labels.

   The flow label generation MUST be designed to minimize the chances of
   linkability with a previously used flow label, as a stable flow label
   would enable correlating activity on multiple paths; see Section 9.5.

   [RFC6437] suggests deriving values using a pseudorandom function to
   generate flow labels.  Including the Destination Connection ID field
   in addition to source and destination addresses when generating flow
   labels ensures that changes are synchronized with changes in other
   observable identifiers.  A cryptographic hash function that combines
   these inputs with a local secret is one way this might be
   implemented.

10.  Connection Termination

   An established QUIC connection can be terminated in one of three
   ways:

   *  idle timeout (Section 10.1)

   *  immediate close (Section 10.2)

   *  stateless reset (Section 10.3)

   An endpoint MAY discard connection state if it does not have a
   validated path on which it can send packets; see Section 8.2.

10.1.  Idle Timeout

   If a max_idle_timeout is specified by either endpoint in its
   transport parameters (Section 18.2), the connection is silently
   closed and its state is discarded when it remains idle for longer
   than the minimum of the max_idle_timeout value advertised by both
   endpoints.

   Each endpoint advertises a max_idle_timeout, but the effective value
   at an endpoint is computed as the minimum of the two advertised
   values (or the sole advertised value, if only one endpoint advertises
   a non-zero value).  By announcing a max_idle_timeout, an endpoint
   commits to initiating an immediate close (Section 10.2) if it
   abandons the connection prior to the effective value.

   An endpoint restarts its idle timer when a packet from its peer is
   received and processed successfully.  An endpoint also restarts its
   idle timer when sending an ack-eliciting packet if no other ack-
   eliciting packets have been sent since last receiving and processing
   a packet.  Restarting this timer when sending a packet ensures that
   connections are not closed after new activity is initiated.

   To avoid excessively small idle timeout periods, endpoints MUST
   increase the idle timeout period to be at least three times the
   current Probe Timeout (PTO).  This allows for multiple PTOs to
   expire, and therefore multiple probes to be sent and lost, prior to
   idle timeout.

10.1.1.  Liveness Testing

   An endpoint that sends packets close to the effective timeout risks
   having them be discarded at the peer, since the idle timeout period
   might have expired at the peer before these packets arrive.

   An endpoint can send a PING or another ack-eliciting frame to test
   the connection for liveness if the peer could time out soon, such as
   within a PTO; see Section 6.2 of [QUIC-RECOVERY].  This is especially
   useful if any available application data cannot be safely retried.
   Note that the application determines what data is safe to retry.

10.1.2.  Deferring Idle Timeout

   An endpoint might need to send ack-eliciting packets to avoid an idle
   timeout if it is expecting response data but does not have or is
   unable to send application data.

   An implementation of QUIC might provide applications with an option
   to defer an idle timeout.  This facility could be used when the
   application wishes to avoid losing state that has been associated
   with an open connection but does not expect to exchange application
   data for some time.  With this option, an endpoint could send a PING
   frame (Section 19.2) periodically, which will cause the peer to
   restart its idle timeout period.  Sending a packet containing a PING
   frame restarts the idle timeout for this endpoint also if this is the
   first ack-eliciting packet sent since receiving a packet.  Sending a
   PING frame causes the peer to respond with an acknowledgment, which
   also restarts the idle timeout for the endpoint.

   Application protocols that use QUIC SHOULD provide guidance on when
   deferring an idle timeout is appropriate.  Unnecessary sending of
   PING frames could have a detrimental effect on performance.

   A connection will time out if no packets are sent or received for a
   period longer than the time negotiated using the max_idle_timeout
   transport parameter; see Section 10.  However, state in middleboxes
   might time out earlier than that.  Though REQ-5 in [RFC4787]
   recommends a 2-minute timeout interval, experience shows that sending
   packets every 30 seconds is necessary to prevent the majority of
   middleboxes from losing state for UDP flows [GATEWAY].

10.2.  Immediate Close

   An endpoint sends a CONNECTION_CLOSE frame (Section 19.19) to
   terminate the connection immediately.  A CONNECTION_CLOSE frame
   causes all streams to immediately become closed; open streams can be
   assumed to be implicitly reset.

   After sending a CONNECTION_CLOSE frame, an endpoint immediately
   enters the closing state; see Section 10.2.1.  After receiving a
   CONNECTION_CLOSE frame, endpoints enter the draining state; see
   Section 10.2.2.

   Violations of the protocol lead to an immediate close.

   An immediate close can be used after an application protocol has
   arranged to close a connection.  This might be after the application
   protocol negotiates a graceful shutdown.  The application protocol
   can exchange messages that are needed for both application endpoints
   to agree that the connection can be closed, after which the
   application requests that QUIC close the connection.  When QUIC
   consequently closes the connection, a CONNECTION_CLOSE frame with an
   application-supplied error code will be used to signal closure to the
   peer.

   The closing and draining connection states exist to ensure that
   connections close cleanly and that delayed or reordered packets are
   properly discarded.  These states SHOULD persist for at least three
   times the current PTO interval as defined in [QUIC-RECOVERY].

   Disposing of connection state prior to exiting the closing or
   draining state could result in an endpoint generating a Stateless
   Reset unnecessarily when it receives a late-arriving packet.
   Endpoints that have some alternative means to ensure that late-
   arriving packets do not induce a response, such as those that are
   able to close the UDP socket, MAY end these states earlier to allow
   for faster resource recovery.  Servers that retain an open socket for
   accepting new connections SHOULD NOT end the closing or draining
   state early.

   Once its closing or draining state ends, an endpoint SHOULD discard
   all connection state.  The endpoint MAY send a Stateless Reset in
   response to any further incoming packets belonging to this
   connection.

10.2.1.  Closing Connection State

   An endpoint enters the closing state after initiating an immediate
   close.

   In the closing state, an endpoint retains only enough information to
   generate a packet containing a CONNECTION_CLOSE frame and to identify
   packets as belonging to the connection.  An endpoint in the closing
   state sends a packet containing a CONNECTION_CLOSE frame in response
   to any incoming packet that it attributes to the connection.

   An endpoint SHOULD limit the rate at which it generates packets in
   the closing state.  For instance, an endpoint could wait for a
   progressively increasing number of received packets or amount of time
   before responding to received packets.

   An endpoint's selected connection ID and the QUIC version are
   sufficient information to identify packets for a closing connection;
   the endpoint MAY discard all other connection state.  An endpoint
   that is closing is not required to process any received frame.  An
   endpoint MAY retain packet protection keys for incoming packets to
   allow it to read and process a CONNECTION_CLOSE frame.

   An endpoint MAY drop packet protection keys when entering the closing
   state and send a packet containing a CONNECTION_CLOSE frame in
   response to any UDP datagram that is received.  However, an endpoint
   that discards packet protection keys cannot identify and discard
   invalid packets.  To avoid being used for an amplification attack,
   such endpoints MUST limit the cumulative size of packets it sends to
   three times the cumulative size of the packets that are received and
   attributed to the connection.  To minimize the state that an endpoint
   maintains for a closing connection, endpoints MAY send the exact same
   packet in response to any received packet.

      |  Note: Allowing retransmission of a closing packet is an
      |  exception to the requirement that a new packet number be used
      |  for each packet; see Section 12.3.  Sending new packet numbers
      |  is primarily of advantage to loss recovery and congestion
      |  control, which are not expected to be relevant for a closed
      |  connection.  Retransmitting the final packet requires less
      |  state.

   While in the closing state, an endpoint could receive packets from a
   new source address, possibly indicating a connection migration; see
   Section 9.  An endpoint in the closing state MUST either discard
   packets received from an unvalidated address or limit the cumulative
   size of packets it sends to an unvalidated address to three times the
   size of packets it receives from that address.

   An endpoint is not expected to handle key updates when it is closing
   (Section 6 of [QUIC-TLS]).  A key update might prevent the endpoint
   from moving from the closing state to the draining state, as the
   endpoint will not be able to process subsequently received packets,
   but it otherwise has no impact.

10.2.2.  Draining Connection State

   The draining state is entered once an endpoint receives a
   CONNECTION_CLOSE frame, which indicates that its peer is closing or
   draining.  While otherwise identical to the closing state, an
   endpoint in the draining state MUST NOT send any packets.  Retaining
   packet protection keys is unnecessary once a connection is in the
   draining state.

   An endpoint that receives a CONNECTION_CLOSE frame MAY send a single
   packet containing a CONNECTION_CLOSE frame before entering the
   draining state, using a NO_ERROR code if appropriate.  An endpoint
   MUST NOT send further packets.  Doing so could result in a constant
   exchange of CONNECTION_CLOSE frames until one of the endpoints exits
   the closing state.

   An endpoint MAY enter the draining state from the closing state if it
   receives a CONNECTION_CLOSE frame, which indicates that the peer is
   also closing or draining.  In this case, the draining state ends when
   the closing state would have ended.  In other words, the endpoint
   uses the same end time but ceases transmission of any packets on this
   connection.

10.2.3.  Immediate Close during the Handshake

   When sending a CONNECTION_CLOSE frame, the goal is to ensure that the
   peer will process the frame.  Generally, this means sending the frame
   in a packet with the highest level of packet protection to avoid the
   packet being discarded.  After the handshake is confirmed (see
   Section 4.1.2 of [QUIC-TLS]), an endpoint MUST send any
   CONNECTION_CLOSE frames in a 1-RTT packet.  However, prior to
   confirming the handshake, it is possible that more advanced packet
   protection keys are not available to the peer, so another
   CONNECTION_CLOSE frame MAY be sent in a packet that uses a lower
   packet protection level.  More specifically:

   *  A client will always know whether the server has Handshake keys
      (see Section 17.2.2.1), but it is possible that a server does not
      know whether the client has Handshake keys.  Under these
      circumstances, a server SHOULD send a CONNECTION_CLOSE frame in
      both Handshake and Initial packets to ensure that at least one of
      them is processable by the client.

   *  A client that sends a CONNECTION_CLOSE frame in a 0-RTT packet
      cannot be assured that the server has accepted 0-RTT.  Sending a
      CONNECTION_CLOSE frame in an Initial packet makes it more likely
      that the server can receive the close signal, even if the
      application error code might not be received.

   *  Prior to confirming the handshake, a peer might be unable to
      process 1-RTT packets, so an endpoint SHOULD send a
      CONNECTION_CLOSE frame in both Handshake and 1-RTT packets.  A
      server SHOULD also send a CONNECTION_CLOSE frame in an Initial
      packet.

   Sending a CONNECTION_CLOSE of type 0x1d in an Initial or Handshake
   packet could expose application state or be used to alter application
   state.  A CONNECTION_CLOSE of type 0x1d MUST be replaced by a
   CONNECTION_CLOSE of type 0x1c when sending the frame in Initial or
   Handshake packets.  Otherwise, information about the application
   state might be revealed.  Endpoints MUST clear the value of the
   Reason Phrase field and SHOULD use the APPLICATION_ERROR code when
   converting to a CONNECTION_CLOSE of type 0x1c.

   CONNECTION_CLOSE frames sent in multiple packet types can be
   coalesced into a single UDP datagram; see Section 12.2.

   An endpoint can send a CONNECTION_CLOSE frame in an Initial packet.
   This might be in response to unauthenticated information received in
   Initial or Handshake packets.  Such an immediate close might expose
   legitimate connections to a denial of service.  QUIC does not include
   defensive measures for on-path attacks during the handshake; see
   Section 21.2.  However, at the cost of reducing feedback about errors
   for legitimate peers, some forms of denial of service can be made
   more difficult for an attacker if endpoints discard illegal packets
   rather than terminating a connection with CONNECTION_CLOSE.  For this
   reason, endpoints MAY discard packets rather than immediately close
   if errors are detected in packets that lack authentication.

   An endpoint that has not established state, such as a server that
   detects an error in an Initial packet, does not enter the closing
   state.  An endpoint that has no state for the connection does not
   enter a closing or draining period on sending a CONNECTION_CLOSE
   frame.

10.3.  Stateless Reset

   A stateless reset is provided as an option of last resort for an
   endpoint that does not have access to the state of a connection.  A
   crash or outage might result in peers continuing to send data to an
   endpoint that is unable to properly continue the connection.  An
   endpoint MAY send a Stateless Reset in response to receiving a packet
   that it cannot associate with an active connection.

   A stateless reset is not appropriate for indicating errors in active
   connections.  An endpoint that wishes to communicate a fatal
   connection error MUST use a CONNECTION_CLOSE frame if it is able.

   To support this process, an endpoint issues a stateless reset token,
   which is a 16-byte value that is hard to guess.  If the peer
   subsequently receives a Stateless Reset, which is a UDP datagram that
   ends in that stateless reset token, the peer will immediately end the
   connection.

   A stateless reset token is specific to a connection ID.  An endpoint
   issues a stateless reset token by including the value in the
   Stateless Reset Token field of a NEW_CONNECTION_ID frame.  Servers
   can also issue a stateless_reset_token transport parameter during the
   handshake that applies to the connection ID that it selected during
   the handshake.  These exchanges are protected by encryption, so only
   client and server know their value.  Note that clients cannot use the
   stateless_reset_token transport parameter because their transport
   parameters do not have confidentiality protection.

   Tokens are invalidated when their associated connection ID is retired
   via a RETIRE_CONNECTION_ID frame (Section 19.16).

   An endpoint that receives packets that it cannot process sends a
   packet in the following layout (see Section 1.3):

   Stateless Reset {
     Fixed Bits (2) = 1,
     Unpredictable Bits (38..),
     Stateless Reset Token (128),
   }

                         Figure 10: Stateless Reset

   This design ensures that a Stateless Reset is -- to the extent
   possible -- indistinguishable from a regular packet with a short
   header.

   A Stateless Reset uses an entire UDP datagram, starting with the
   first two bits of the packet header.  The remainder of the first byte
   and an arbitrary number of bytes following it are set to values that
   SHOULD be indistinguishable from random.  The last 16 bytes of the
   datagram contain a stateless reset token.

   To entities other than its intended recipient, a Stateless Reset will
   appear to be a packet with a short header.  For the Stateless Reset
   to appear as a valid QUIC packet, the Unpredictable Bits field needs
   to include at least 38 bits of data (or 5 bytes, less the two fixed
   bits).

   The resulting minimum size of 21 bytes does not guarantee that a
   Stateless Reset is difficult to distinguish from other packets if the
   recipient requires the use of a connection ID.  To achieve that end,
   the endpoint SHOULD ensure that all packets it sends are at least 22
   bytes longer than the minimum connection ID length that it requests
   the peer to include in its packets, adding PADDING frames as
   necessary.  This ensures that any Stateless Reset sent by the peer is
   indistinguishable from a valid packet sent to the endpoint.  An
   endpoint that sends a Stateless Reset in response to a packet that is
   43 bytes or shorter SHOULD send a Stateless Reset that is one byte
   shorter than the packet it responds to.

   These values assume that the stateless reset token is the same length
   as the minimum expansion of the packet protection AEAD.  Additional
   unpredictable bytes are necessary if the endpoint could have
   negotiated a packet protection scheme with a larger minimum
   expansion.

   An endpoint MUST NOT send a Stateless Reset that is three times or
   more larger than the packet it receives to avoid being used for
   amplification.  Section 10.3.3 describes additional limits on
   Stateless Reset size.

   Endpoints MUST discard packets that are too small to be valid QUIC
   packets.  To give an example, with the set of AEAD functions defined
   in [QUIC-TLS], short header packets that are smaller than 21 bytes
   are never valid.

   Endpoints MUST send Stateless Resets formatted as a packet with a
   short header.  However, endpoints MUST treat any packet ending in a
   valid stateless reset token as a Stateless Reset, as other QUIC
   versions might allow the use of a long header.

   An endpoint MAY send a Stateless Reset in response to a packet with a
   long header.  Sending a Stateless Reset is not effective prior to the
   stateless reset token being available to a peer.  In this QUIC
   version, packets with a long header are only used during connection
   establishment.  Because the stateless reset token is not available
   until connection establishment is complete or near completion,
   ignoring an unknown packet with a long header might be as effective
   as sending a Stateless Reset.

   An endpoint cannot determine the Source Connection ID from a packet
   with a short header; therefore, it cannot set the Destination
   Connection ID in the Stateless Reset.  The Destination Connection ID
   will therefore differ from the value used in previous packets.  A
   random Destination Connection ID makes the connection ID appear to be
   the result of moving to a new connection ID that was provided using a
   NEW_CONNECTION_ID frame; see Section 19.15.

   Using a randomized connection ID results in two problems:

   *  The packet might not reach the peer.  If the Destination
      Connection ID is critical for routing toward the peer, then this
      packet could be incorrectly routed.  This might also trigger
      another Stateless Reset in response; see Section 10.3.3.  A
      Stateless Reset that is not correctly routed is an ineffective
      error detection and recovery mechanism.  In this case, endpoints
      will need to rely on other methods -- such as timers -- to detect
      that the connection has failed.

   *  The randomly generated connection ID can be used by entities other
      than the peer to identify this as a potential Stateless Reset.  An
      endpoint that occasionally uses different connection IDs might
      introduce some uncertainty about this.

   This stateless reset design is specific to QUIC version 1.  An
   endpoint that supports multiple versions of QUIC needs to generate a
   Stateless Reset that will be accepted by peers that support any
   version that the endpoint might support (or might have supported
   prior to losing state).  Designers of new versions of QUIC need to be
   aware of this and either (1) reuse this design or (2) use a portion
   of the packet other than the last 16 bytes for carrying data.

10.3.1.  Detecting a Stateless Reset

   An endpoint detects a potential Stateless Reset using the trailing 16
   bytes of the UDP datagram.  An endpoint remembers all stateless reset
   tokens associated with the connection IDs and remote addresses for
   datagrams it has recently sent.  This includes Stateless Reset Token
   field values from NEW_CONNECTION_ID frames and the server's transport
   parameters but excludes stateless reset tokens associated with
   connection IDs that are either unused or retired.  The endpoint
   identifies a received datagram as a Stateless Reset by comparing the
   last 16 bytes of the datagram with all stateless reset tokens
   associated with the remote address on which the datagram was
   received.

   This comparison can be performed for every inbound datagram.
   Endpoints MAY skip this check if any packet from a datagram is
   successfully processed.  However, the comparison MUST be performed
   when the first packet in an incoming datagram either cannot be
   associated with a connection or cannot be decrypted.

   An endpoint MUST NOT check for any stateless reset tokens associated
   with connection IDs it has not used or for connection IDs that have
   been retired.

   When comparing a datagram to stateless reset token values, endpoints
   MUST perform the comparison without leaking information about the
   value of the token.  For example, performing this comparison in
   constant time protects the value of individual stateless reset tokens
   from information leakage through timing side channels.  Another
   approach would be to store and compare the transformed values of
   stateless reset tokens instead of the raw token values, where the
   transformation is defined as a cryptographically secure pseudorandom
   function using a secret key (e.g., block cipher, Hashed Message
   Authentication Code (HMAC) [RFC2104]).  An endpoint is not expected
   to protect information about whether a packet was successfully
   decrypted or the number of valid stateless reset tokens.

   If the last 16 bytes of the datagram are identical in value to a
   stateless reset token, the endpoint MUST enter the draining period
   and not send any further packets on this connection.

10.3.2.  Calculating a Stateless Reset Token

   The stateless reset token MUST be difficult to guess.  In order to
   create a stateless reset token, an endpoint could randomly generate
   [RANDOM] a secret for every connection that it creates.  However,
   this presents a coordination problem when there are multiple
   instances in a cluster or a storage problem for an endpoint that
   might lose state.  Stateless reset specifically exists to handle the
   case where state is lost, so this approach is suboptimal.

   A single static key can be used across all connections to the same
   endpoint by generating the proof using a pseudorandom function that
   takes a static key and the connection ID chosen by the endpoint (see
   Section 5.1) as input.  An endpoint could use HMAC [RFC2104] (for
   example, HMAC(static_key, connection_id)) or the HMAC-based Key
   Derivation Function (HKDF) [RFC5869] (for example, using the static
   key as input keying material, with the connection ID as salt).  The
   output of this function is truncated to 16 bytes to produce the
   stateless reset token for that connection.

   An endpoint that loses state can use the same method to generate a
   valid stateless reset token.  The connection ID comes from the packet
   that the endpoint receives.

   This design relies on the peer always sending a connection ID in its
   packets so that the endpoint can use the connection ID from a packet
   to reset the connection.  An endpoint that uses this design MUST
   either use the same connection ID length for all connections or
   encode the length of the connection ID such that it can be recovered
   without state.  In addition, it cannot provide a zero-length
   connection ID.

   Revealing the stateless reset token allows any entity to terminate
   the connection, so a value can only be used once.  This method for
   choosing the stateless reset token means that the combination of
   connection ID and static key MUST NOT be used for another connection.
   A denial-of-service attack is possible if the same connection ID is
   used by instances that share a static key or if an attacker can cause
   a packet to be routed to an instance that has no state but the same
   static key; see Section 21.11.  A connection ID from a connection
   that is reset by revealing the stateless reset token MUST NOT be
   reused for new connections at nodes that share a static key.

   The same stateless reset token MUST NOT be used for multiple
   connection IDs.  Endpoints are not required to compare new values
   against all previous values, but a duplicate value MAY be treated as
   a connection error of type PROTOCOL_VIOLATION.

   Note that Stateless Resets do not have any cryptographic protection.

10.3.3.  Looping

   The design of a Stateless Reset is such that without knowing the
   stateless reset token it is indistinguishable from a valid packet.
   For instance, if a server sends a Stateless Reset to another server,
   it might receive another Stateless Reset in response, which could
   lead to an infinite exchange.

   An endpoint MUST ensure that every Stateless Reset that it sends is
   smaller than the packet that triggered it, unless it maintains state
   sufficient to prevent looping.  In the event of a loop, this results
   in packets eventually being too small to trigger a response.

   An endpoint can remember the number of Stateless Resets that it has
   sent and stop generating new Stateless Resets once a limit is
   reached.  Using separate limits for different remote addresses will
   ensure that Stateless Resets can be used to close connections when
   other peers or connections have exhausted limits.

   A Stateless Reset that is smaller than 41 bytes might be identifiable
   as a Stateless Reset by an observer, depending upon the length of the
   peer's connection IDs.  Conversely, not sending a Stateless Reset in
   response to a small packet might result in Stateless Resets not being
   useful in detecting cases of broken connections where only very small
   packets are sent; such failures might only be detected by other
   means, such as timers.

11.  Error Handling

   An endpoint that detects an error SHOULD signal the existence of that
   error to its peer.  Both transport-level and application-level errors
   can affect an entire connection; see Section 11.1.  Only application-
   level errors can be isolated to a single stream; see Section 11.2.

   The most appropriate error code (Section 20) SHOULD be included in
   the frame that signals the error.  Where this specification
   identifies error conditions, it also identifies the error code that
   is used; though these are worded as requirements, different
   implementation strategies might lead to different errors being
   reported.  In particular, an endpoint MAY use any applicable error
   code when it detects an error condition; a generic error code (such
   as PROTOCOL_VIOLATION or INTERNAL_ERROR) can always be used in place
   of specific error codes.

   A stateless reset (Section 10.3) is not suitable for any error that
   can be signaled with a CONNECTION_CLOSE or RESET_STREAM frame.  A
   stateless reset MUST NOT be used by an endpoint that has the state
   necessary to send a frame on the connection.

11.1.  Connection Errors

   Errors that result in the connection being unusable, such as an
   obvious violation of protocol semantics or corruption of state that
   affects an entire connection, MUST be signaled using a
   CONNECTION_CLOSE frame (Section 19.19).

   Application-specific protocol errors are signaled using the
   CONNECTION_CLOSE frame with a frame type of 0x1d.  Errors that are
   specific to the transport, including all those described in this
   document, are carried in the CONNECTION_CLOSE frame with a frame type
   of 0x1c.

   A CONNECTION_CLOSE frame could be sent in a packet that is lost.  An
   endpoint SHOULD be prepared to retransmit a packet containing a
   CONNECTION_CLOSE frame if it receives more packets on a terminated
   connection.  Limiting the number of retransmissions and the time over
   which this final packet is sent limits the effort expended on
   terminated connections.

   An endpoint that chooses not to retransmit packets containing a
   CONNECTION_CLOSE frame risks a peer missing the first such packet.
   The only mechanism available to an endpoint that continues to receive
   data for a terminated connection is to attempt the stateless reset
   process (Section 10.3).

   As the AEAD for Initial packets does not provide strong
   authentication, an endpoint MAY discard an invalid Initial packet.
   Discarding an Initial packet is permitted even where this
   specification otherwise mandates a connection error.  An endpoint can
   only discard a packet if it does not process the frames in the packet
   or reverts the effects of any processing.  Discarding invalid Initial
   packets might be used to reduce exposure to denial of service; see
   Section 21.2.

11.2.  Stream Errors

   If an application-level error affects a single stream but otherwise
   leaves the connection in a recoverable state, the endpoint can send a
   RESET_STREAM frame (Section 19.4) with an appropriate error code to
   terminate just the affected stream.

   Resetting a stream without the involvement of the application
   protocol could cause the application protocol to enter an
   unrecoverable state.  RESET_STREAM MUST only be instigated by the
   application protocol that uses QUIC.

   The semantics of the application error code carried in RESET_STREAM
   are defined by the application protocol.  Only the application
   protocol is able to cause a stream to be terminated.  A local
   instance of the application protocol uses a direct API call, and a
   remote instance uses the STOP_SENDING frame, which triggers an
   automatic RESET_STREAM.

   Application protocols SHOULD define rules for handling streams that
   are prematurely canceled by either endpoint.

12.  Packets and Frames

   QUIC endpoints communicate by exchanging packets.  Packets have
   confidentiality and integrity protection; see Section 12.1.  Packets
   are carried in UDP datagrams; see Section 12.2.

   This version of QUIC uses the long packet header during connection
   establishment; see Section 17.2.  Packets with the long header are
   Initial (Section 17.2.2), 0-RTT (Section 17.2.3), Handshake
   (Section 17.2.4), and Retry (Section 17.2.5).  Version negotiation
   uses a version-independent packet with a long header; see
   Section 17.2.1.

   Packets with the short header are designed for minimal overhead and
   are used after a connection is established and 1-RTT keys are
   available; see Section 17.3.

12.1.  Protected Packets

   QUIC packets have different levels of cryptographic protection based
   on the type of packet.  Details of packet protection are found in
   [QUIC-TLS]; this section includes an overview of the protections that
   are provided.

   Version Negotiation packets have no cryptographic protection; see
   [QUIC-INVARIANTS].

   Retry packets use an AEAD function [AEAD] to protect against
   accidental modification.

   Initial packets use an AEAD function, the keys for which are derived
   using a value that is visible on the wire.  Initial packets therefore
   do not have effective confidentiality protection.  Initial protection
   exists to ensure that the sender of the packet is on the network
   path.  Any entity that receives an Initial packet from a client can
   recover the keys that will allow them to both read the contents of
   the packet and generate Initial packets that will be successfully
   authenticated at either endpoint.  The AEAD also protects Initial
   packets against accidental modification.

   All other packets are protected with keys derived from the
   cryptographic handshake.  The cryptographic handshake ensures that
   only the communicating endpoints receive the corresponding keys for
   Handshake, 0-RTT, and 1-RTT packets.  Packets protected with 0-RTT
   and 1-RTT keys have strong confidentiality and integrity protection.

   The Packet Number field that appears in some packet types has
   alternative confidentiality protection that is applied as part of
   header protection; see Section 5.4 of [QUIC-TLS] for details.  The
   underlying packet number increases with each packet sent in a given
   packet number space; see Section 12.3 for details.

12.2.  Coalescing Packets

   Initial (Section 17.2.2), 0-RTT (Section 17.2.3), and Handshake
   (Section 17.2.4) packets contain a Length field that determines the
   end of the packet.  The length includes both the Packet Number and
   Payload fields, both of which are confidentiality protected and
   initially of unknown length.  The length of the Payload field is
   learned once header protection is removed.

   Using the Length field, a sender can coalesce multiple QUIC packets
   into one UDP datagram.  This can reduce the number of UDP datagrams
   needed to complete the cryptographic handshake and start sending
   data.  This can also be used to construct Path Maximum Transmission
   Unit (PMTU) probes; see Section 14.4.1.  Receivers MUST be able to
   process coalesced packets.

   Coalescing packets in order of increasing encryption levels (Initial,
   0-RTT, Handshake, 1-RTT; see Section 4.1.4 of [QUIC-TLS]) makes it
   more likely that the receiver will be able to process all the packets
   in a single pass.  A packet with a short header does not include a
   length, so it can only be the last packet included in a UDP datagram.
   An endpoint SHOULD include multiple frames in a single packet if they
   are to be sent at the same encryption level, instead of coalescing
   multiple packets at the same encryption level.

   Receivers MAY route based on the information in the first packet
   contained in a UDP datagram.  Senders MUST NOT coalesce QUIC packets
   with different connection IDs into a single UDP datagram.  Receivers
   SHOULD ignore any subsequent packets with a different Destination
   Connection ID than the first packet in the datagram.

   Every QUIC packet that is coalesced into a single UDP datagram is
   separate and complete.  The receiver of coalesced QUIC packets MUST
   individually process each QUIC packet and separately acknowledge
   them, as if they were received as the payload of different UDP
   datagrams.  For example, if decryption fails (because the keys are
   not available or for any other reason), the receiver MAY either
   discard or buffer the packet for later processing and MUST attempt to
   process the remaining packets.

   Retry packets (Section 17.2.5), Version Negotiation packets
   (Section 17.2.1), and packets with a short header (Section 17.3) do
   not contain a Length field and so cannot be followed by other packets
   in the same UDP datagram.  Note also that there is no situation where
   a Retry or Version Negotiation packet is coalesced with another
   packet.

12.3.  Packet Numbers

   The packet number is an integer in the range 0 to 2^62-1.  This
   number is used in determining the cryptographic nonce for packet
   protection.  Each endpoint maintains a separate packet number for
   sending and receiving.

   Packet numbers are limited to this range because they need to be
   representable in whole in the Largest Acknowledged field of an ACK
   frame (Section 19.3).  When present in a long or short header,
   however, packet numbers are reduced and encoded in 1 to 4 bytes; see
   Section 17.1.

   Version Negotiation (Section 17.2.1) and Retry (Section 17.2.5)
   packets do not include a packet number.

   Packet numbers are divided into three spaces in QUIC:

   Initial space:  All Initial packets (Section 17.2.2) are in this
      space.

   Handshake space:  All Handshake packets (Section 17.2.4) are in this
      space.

   Application data space:  All 0-RTT (Section 17.2.3) and 1-RTT
      (Section 17.3.1) packets are in this space.

   As described in [QUIC-TLS], each packet type uses different
   protection keys.

   Conceptually, a packet number space is the context in which a packet
   can be processed and acknowledged.  Initial packets can only be sent
   with Initial packet protection keys and acknowledged in packets that
   are also Initial packets.  Similarly, Handshake packets are sent at
   the Handshake encryption level and can only be acknowledged in
   Handshake packets.

   This enforces cryptographic separation between the data sent in the
   different packet number spaces.  Packet numbers in each space start
   at packet number 0.  Subsequent packets sent in the same packet
   number space MUST increase the packet number by at least one.

   0-RTT and 1-RTT data exist in the same packet number space to make
   loss recovery algorithms easier to implement between the two packet
   types.

   A QUIC endpoint MUST NOT reuse a packet number within the same packet
   number space in one connection.  If the packet number for sending
   reaches 2^62-1, the sender MUST close the connection without sending
   a CONNECTION_CLOSE frame or any further packets; an endpoint MAY send
   a Stateless Reset (Section 10.3) in response to further packets that
   it receives.

   A receiver MUST discard a newly unprotected packet unless it is
   certain that it has not processed another packet with the same packet
   number from the same packet number space.  Duplicate suppression MUST
   happen after removing packet protection for the reasons described in
   Section 9.5 of [QUIC-TLS].

   Endpoints that track all individual packets for the purposes of
   detecting duplicates are at risk of accumulating excessive state.
   The data required for detecting duplicates can be limited by
   maintaining a minimum packet number below which all packets are
   immediately dropped.  Any minimum needs to account for large
   variations in round-trip time, which includes the possibility that a
   peer might probe network paths with much larger round-trip times; see
   Section 9.

   Packet number encoding at a sender and decoding at a receiver are
   described in Section 17.1.

12.4.  Frames and Frame Types

   The payload of QUIC packets, after removing packet protection,
   consists of a sequence of complete frames, as shown in Figure 11.
   Version Negotiation, Stateless Reset, and Retry packets do not
   contain frames.

   Packet Payload {
     Frame (8..) ...,
   }

                          Figure 11: QUIC Payload

   The payload of a packet that contains frames MUST contain at least
   one frame, and MAY contain multiple frames and multiple frame types.
   An endpoint MUST treat receipt of a packet containing no frames as a
   connection error of type PROTOCOL_VIOLATION.  Frames always fit
   within a single QUIC packet and cannot span multiple packets.

   Each frame begins with a Frame Type, indicating its type, followed by
   additional type-dependent fields:

   Frame {
     Frame Type (i),
     Type-Dependent Fields (..),
   }

                      Figure 12: Generic Frame Layout

   Table 3 lists and summarizes information about each frame type that
   is defined in this specification.  A description of this summary is
   included after the table.

    +============+======================+===============+======+======+
    | Type Value | Frame Type Name      | Definition    | Pkts | Spec |
    +============+======================+===============+======+======+
    | 0x00       | PADDING              | Section 19.1  | IH01 | NP   |
    +------------+----------------------+---------------+------+------+
    | 0x01       | PING                 | Section 19.2  | IH01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x02-0x03  | ACK                  | Section 19.3  | IH_1 | NC   |
    +------------+----------------------+---------------+------+------+
    | 0x04       | RESET_STREAM         | Section 19.4  | __01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x05       | STOP_SENDING         | Section 19.5  | __01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x06       | CRYPTO               | Section 19.6  | IH_1 |      |
    +------------+----------------------+---------------+------+------+
    | 0x07       | NEW_TOKEN            | Section 19.7  | ___1 |      |
    +------------+----------------------+---------------+------+------+
    | 0x08-0x0f  | STREAM               | Section 19.8  | __01 | F    |
    +------------+----------------------+---------------+------+------+
    | 0x10       | MAX_DATA             | Section 19.9  | __01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x11       | MAX_STREAM_DATA      | Section 19.10 | __01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x12-0x13  | MAX_STREAMS          | Section 19.11 | __01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x14       | DATA_BLOCKED         | Section 19.12 | __01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x15       | STREAM_DATA_BLOCKED  | Section 19.13 | __01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x16-0x17  | STREAMS_BLOCKED      | Section 19.14 | __01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x18       | NEW_CONNECTION_ID    | Section 19.15 | __01 | P    |
    +------------+----------------------+---------------+------+------+
    | 0x19       | RETIRE_CONNECTION_ID | Section 19.16 | __01 |      |
    +------------+----------------------+---------------+------+------+
    | 0x1a       | PATH_CHALLENGE       | Section 19.17 | __01 | P    |
    +------------+----------------------+---------------+------+------+
    | 0x1b       | PATH_RESPONSE        | Section 19.18 | ___1 | P    |
    +------------+----------------------+---------------+------+------+
    | 0x1c-0x1d  | CONNECTION_CLOSE     | Section 19.19 | ih01 | N    |
    +------------+----------------------+---------------+------+------+
    | 0x1e       | HANDSHAKE_DONE       | Section 19.20 | ___1 |      |
    +------------+----------------------+---------------+------+------+

                            Table 3: Frame Types

   The format and semantics of each frame type are explained in more
   detail in Section 19.  The remainder of this section provides a
   summary of important and general information.

   The Frame Type in ACK, STREAM, MAX_STREAMS, STREAMS_BLOCKED, and
   CONNECTION_CLOSE frames is used to carry other frame-specific flags.
   For all other frames, the Frame Type field simply identifies the
   frame.

   The "Pkts" column in Table 3 lists the types of packets that each
   frame type could appear in, indicated by the following characters:

   I:   Initial (Section 17.2.2)

   H:   Handshake (Section 17.2.4)

   0:   0-RTT (Section 17.2.3)

   1:   1-RTT (Section 17.3.1)

   ih:  Only a CONNECTION_CLOSE frame of type 0x1c can appear in Initial
        or Handshake packets.

   For more details about these restrictions, see Section 12.5.  Note
   that all frames can appear in 1-RTT packets.  An endpoint MUST treat
   receipt of a frame in a packet type that is not permitted as a
   connection error of type PROTOCOL_VIOLATION.

   The "Spec" column in Table 3 summarizes any special rules governing
   the processing or generation of the frame type, as indicated by the
   following characters:

   N:   Packets containing only frames with this marking are not ack-
        eliciting; see Section 13.2.

   C:   Packets containing only frames with this marking do not count
        toward bytes in flight for congestion control purposes; see
        [QUIC-RECOVERY].

   P:   Packets containing only frames with this marking can be used to
        probe new network paths during connection migration; see
        Section 9.1.

   F:   The contents of frames with this marking are flow controlled;
        see Section 4.

   The "Pkts" and "Spec" columns in Table 3 do not form part of the IANA
   registry; see Section 22.4.

   An endpoint MUST treat the receipt of a frame of unknown type as a
   connection error of type FRAME_ENCODING_ERROR.

   All frames are idempotent in this version of QUIC.  That is, a valid
   frame does not cause undesirable side effects or errors when received
   more than once.

   The Frame Type field uses a variable-length integer encoding (see
   Section 16), with one exception.  To ensure simple and efficient
   implementations of frame parsing, a frame type MUST use the shortest
   possible encoding.  For frame types defined in this document, this
   means a single-byte encoding, even though it is possible to encode
   these values as a two-, four-, or eight-byte variable-length integer.
   For instance, though 0x4001 is a legitimate two-byte encoding for a
   variable-length integer with a value of 1, PING frames are always
   encoded as a single byte with the value 0x01.  This rule applies to
   all current and future QUIC frame types.  An endpoint MAY treat the
   receipt of a frame type that uses a longer encoding than necessary as
   a connection error of type PROTOCOL_VIOLATION.

12.5.  Frames and Number Spaces

   Some frames are prohibited in different packet number spaces.  The
   rules here generalize those of TLS, in that frames associated with
   establishing the connection can usually appear in packets in any
   packet number space, whereas those associated with transferring data
   can only appear in the application data packet number space:

   *  PADDING, PING, and CRYPTO frames MAY appear in any packet number
      space.

   *  CONNECTION_CLOSE frames signaling errors at the QUIC layer (type
      0x1c) MAY appear in any packet number space.  CONNECTION_CLOSE
      frames signaling application errors (type 0x1d) MUST only appear
      in the application data packet number space.

   *  ACK frames MAY appear in any packet number space but can only
      acknowledge packets that appeared in that packet number space.
      However, as noted below, 0-RTT packets cannot contain ACK frames.

   *  All other frame types MUST only be sent in the application data
      packet number space.

   Note that it is not possible to send the following frames in 0-RTT
   packets for various reasons: ACK, CRYPTO, HANDSHAKE_DONE, NEW_TOKEN,
   PATH_RESPONSE, and RETIRE_CONNECTION_ID.  A server MAY treat receipt
   of these frames in 0-RTT packets as a connection error of type
   PROTOCOL_VIOLATION.

13.  Packetization and Reliability

   A sender sends one or more frames in a QUIC packet; see Section 12.4.

   A sender can minimize per-packet bandwidth and computational costs by
   including as many frames as possible in each QUIC packet.  A sender
   MAY wait for a short period of time to collect multiple frames before
   sending a packet that is not maximally packed, to avoid sending out
   large numbers of small packets.  An implementation MAY use knowledge
   about application sending behavior or heuristics to determine whether
   and for how long to wait.  This waiting period is an implementation
   decision, and an implementation should be careful to delay
   conservatively, since any delay is likely to increase application-
   visible latency.

   Stream multiplexing is achieved by interleaving STREAM frames from
   multiple streams into one or more QUIC packets.  A single QUIC packet
   can include multiple STREAM frames from one or more streams.

   One of the benefits of QUIC is avoidance of head-of-line blocking
   across multiple streams.  When a packet loss occurs, only streams
   with data in that packet are blocked waiting for a retransmission to
   be received, while other streams can continue making progress.  Note
   that when data from multiple streams is included in a single QUIC
   packet, loss of that packet blocks all those streams from making
   progress.  Implementations are advised to include as few streams as
   necessary in outgoing packets without losing transmission efficiency
   to underfilled packets.

13.1.  Packet Processing

   A packet MUST NOT be acknowledged until packet protection has been
   successfully removed and all frames contained in the packet have been
   processed.  For STREAM frames, this means the data has been enqueued
   in preparation to be received by the application protocol, but it
   does not require that data be delivered and consumed.

   Once the packet has been fully processed, a receiver acknowledges
   receipt by sending one or more ACK frames containing the packet
   number of the received packet.

   An endpoint SHOULD treat receipt of an acknowledgment for a packet it
   did not send as a connection error of type PROTOCOL_VIOLATION, if it
   is able to detect the condition.  For further discussion of how this
   might be achieved, see Section 21.4.

13.2.  Generating Acknowledgments

   Endpoints acknowledge all packets they receive and process.  However,
   only ack-eliciting packets cause an ACK frame to be sent within the
   maximum ack delay.  Packets that are not ack-eliciting are only
   acknowledged when an ACK frame is sent for other reasons.

   When sending a packet for any reason, an endpoint SHOULD attempt to
   include an ACK frame if one has not been sent recently.  Doing so
   helps with timely loss detection at the peer.

   In general, frequent feedback from a receiver improves loss and
   congestion response, but this has to be balanced against excessive
   load generated by a receiver that sends an ACK frame in response to
   every ack-eliciting packet.  The guidance offered below seeks to
   strike this balance.

13.2.1.  Sending ACK Frames

   Every packet SHOULD be acknowledged at least once, and ack-eliciting
   packets MUST be acknowledged at least once within the maximum delay
   an endpoint communicated using the max_ack_delay transport parameter;
   see Section 18.2.  max_ack_delay declares an explicit contract: an
   endpoint promises to never intentionally delay acknowledgments of an
   ack-eliciting packet by more than the indicated value.  If it does,
   any excess accrues to the RTT estimate and could result in spurious
   or delayed retransmissions from the peer.  A sender uses the
   receiver's max_ack_delay value in determining timeouts for timer-
   based retransmission, as detailed in Section 6.2 of [QUIC-RECOVERY].

   An endpoint MUST acknowledge all ack-eliciting Initial and Handshake
   packets immediately and all ack-eliciting 0-RTT and 1-RTT packets
   within its advertised max_ack_delay, with the following exception.
   Prior to handshake confirmation, an endpoint might not have packet
   protection keys for decrypting Handshake, 0-RTT, or 1-RTT packets
   when they are received.  It might therefore buffer them and
   acknowledge them when the requisite keys become available.

   Since packets containing only ACK frames are not congestion
   controlled, an endpoint MUST NOT send more than one such packet in
   response to receiving an ack-eliciting packet.

   An endpoint MUST NOT send a non-ack-eliciting packet in response to a
   non-ack-eliciting packet, even if there are packet gaps that precede
   the received packet.  This avoids an infinite feedback loop of
   acknowledgments, which could prevent the connection from ever
   becoming idle.  Non-ack-eliciting packets are eventually acknowledged
   when the endpoint sends an ACK frame in response to other events.

   An endpoint that is only sending ACK frames will not receive
   acknowledgments from its peer unless those acknowledgments are
   included in packets with ack-eliciting frames.  An endpoint SHOULD
   send an ACK frame with other frames when there are new ack-eliciting
   packets to acknowledge.  When only non-ack-eliciting packets need to
   be acknowledged, an endpoint MAY choose not to send an ACK frame with
   outgoing frames until an ack-eliciting packet has been received.

   An endpoint that is only sending non-ack-eliciting packets might
   choose to occasionally add an ack-eliciting frame to those packets to
   ensure that it receives an acknowledgment; see Section 13.2.4.  In
   that case, an endpoint MUST NOT send an ack-eliciting frame in all
   packets that would otherwise be non-ack-eliciting, to avoid an
   infinite feedback loop of acknowledgments.

   In order to assist loss detection at the sender, an endpoint SHOULD
   generate and send an ACK frame without delay when it receives an ack-
   eliciting packet either:

   *  when the received packet has a packet number less than another
      ack-eliciting packet that has been received, or

   *  when the packet has a packet number larger than the highest-
      numbered ack-eliciting packet that has been received and there are
      missing packets between that packet and this packet.

   Similarly, packets marked with the ECN Congestion Experienced (CE)
   codepoint in the IP header SHOULD be acknowledged immediately, to
   reduce the peer's response time to congestion events.

   The algorithms in [QUIC-RECOVERY] are expected to be resilient to
   receivers that do not follow the guidance offered above.  However, an
   implementation should only deviate from these requirements after
   careful consideration of the performance implications of a change,
   for connections made by the endpoint and for other users of the
   network.

13.2.2.  Acknowledgment Frequency

   A receiver determines how frequently to send acknowledgments in
   response to ack-eliciting packets.  This determination involves a
   trade-off.

   Endpoints rely on timely acknowledgment to detect loss; see Section 6
   of [QUIC-RECOVERY].  Window-based congestion controllers, such as the
   one described in Section 7 of [QUIC-RECOVERY], rely on
   acknowledgments to manage their congestion window.  In both cases,
   delaying acknowledgments can adversely affect performance.

   On the other hand, reducing the frequency of packets that carry only
   acknowledgments reduces packet transmission and processing cost at
   both endpoints.  It can improve connection throughput on severely
   asymmetric links and reduce the volume of acknowledgment traffic
   using return path capacity; see Section 3 of [RFC3449].

   A receiver SHOULD send an ACK frame after receiving at least two ack-
   eliciting packets.  This recommendation is general in nature and
   consistent with recommendations for TCP endpoint behavior [RFC5681].
   Knowledge of network conditions, knowledge of the peer's congestion
   controller, or further research and experimentation might suggest
   alternative acknowledgment strategies with better performance
   characteristics.

   A receiver MAY process multiple available packets before determining
   whether to send an ACK frame in response.

13.2.3.  Managing ACK Ranges

   When an ACK frame is sent, one or more ranges of acknowledged packets
   are included.  Including acknowledgments for older packets reduces
   the chance of spurious retransmissions caused by losing previously
   sent ACK frames, at the cost of larger ACK frames.

   ACK frames SHOULD always acknowledge the most recently received
   packets, and the more out of order the packets are, the more
   important it is to send an updated ACK frame quickly, to prevent the
   peer from declaring a packet as lost and spuriously retransmitting
   the frames it contains.  An ACK frame is expected to fit within a
   single QUIC packet.  If it does not, then older ranges (those with
   the smallest packet numbers) are omitted.

   A receiver limits the number of ACK Ranges (Section 19.3.1) it
   remembers and sends in ACK frames, both to limit the size of ACK
   frames and to avoid resource exhaustion.  After receiving
   acknowledgments for an ACK frame, the receiver SHOULD stop tracking
   those acknowledged ACK Ranges.  Senders can expect acknowledgments
   for most packets, but QUIC does not guarantee receipt of an
   acknowledgment for every packet that the receiver processes.

   It is possible that retaining many ACK Ranges could cause an ACK
   frame to become too large.  A receiver can discard unacknowledged ACK
   Ranges to limit ACK frame size, at the cost of increased
   retransmissions from the sender.  This is necessary if an ACK frame
   would be too large to fit in a packet.  Receivers MAY also limit ACK
   frame size further to preserve space for other frames or to limit the
   capacity that acknowledgments consume.

   A receiver MUST retain an ACK Range unless it can ensure that it will
   not subsequently accept packets with numbers in that range.
   Maintaining a minimum packet number that increases as ranges are
   discarded is one way to achieve this with minimal state.

   Receivers can discard all ACK Ranges, but they MUST retain the
   largest packet number that has been successfully processed, as that
   is used to recover packet numbers from subsequent packets; see
   Section 17.1.

   A receiver SHOULD include an ACK Range containing the largest
   received packet number in every ACK frame.  The Largest Acknowledged
   field is used in ECN validation at a sender, and including a lower
   value than what was included in a previous ACK frame could cause ECN
   to be unnecessarily disabled; see Section 13.4.2.

   Section 13.2.4 describes an exemplary approach for determining what
   packets to acknowledge in each ACK frame.  Though the goal of this
   algorithm is to generate an acknowledgment for every packet that is
   processed, it is still possible for acknowledgments to be lost.

13.2.4.  Limiting Ranges by Tracking ACK Frames

   When a packet containing an ACK frame is sent, the Largest
   Acknowledged field in that frame can be saved.  When a packet
   containing an ACK frame is acknowledged, the receiver can stop
   acknowledging packets less than or equal to the Largest Acknowledged
   field in the sent ACK frame.

   A receiver that sends only non-ack-eliciting packets, such as ACK
   frames, might not receive an acknowledgment for a long period of
   time.  This could cause the receiver to maintain state for a large
   number of ACK frames for a long period of time, and ACK frames it
   sends could be unnecessarily large.  In such a case, a receiver could
   send a PING or other small ack-eliciting frame occasionally, such as
   once per round trip, to elicit an ACK from the peer.

   In cases without ACK frame loss, this algorithm allows for a minimum
   of 1 RTT of reordering.  In cases with ACK frame loss and reordering,
   this approach does not guarantee that every acknowledgment is seen by
   the sender before it is no longer included in the ACK frame.  Packets
   could be received out of order, and all subsequent ACK frames
   containing them could be lost.  In this case, the loss recovery
   algorithm could cause spurious retransmissions, but the sender will
   continue making forward progress.

13.2.5.  Measuring and Reporting Host Delay

   An endpoint measures the delays intentionally introduced between the
   time the packet with the largest packet number is received and the
   time an acknowledgment is sent.  The endpoint encodes this
   acknowledgment delay in the ACK Delay field of an ACK frame; see
   Section 19.3.  This allows the receiver of the ACK frame to adjust
   for any intentional delays, which is important for getting a better
   estimate of the path RTT when acknowledgments are delayed.

   A packet might be held in the OS kernel or elsewhere on the host
   before being processed.  An endpoint MUST NOT include delays that it
   does not control when populating the ACK Delay field in an ACK frame.
   However, endpoints SHOULD include buffering delays caused by
   unavailability of decryption keys, since these delays can be large
   and are likely to be non-repeating.

   When the measured acknowledgment delay is larger than its
   max_ack_delay, an endpoint SHOULD report the measured delay.  This
   information is especially useful during the handshake when delays
   might be large; see Section 13.2.1.

13.2.6.  ACK Frames and Packet Protection

   ACK frames MUST only be carried in a packet that has the same packet
   number space as the packet being acknowledged; see Section 12.1.  For
   instance, packets that are protected with 1-RTT keys MUST be
   acknowledged in packets that are also protected with 1-RTT keys.

   Packets that a client sends with 0-RTT packet protection MUST be
   acknowledged by the server in packets protected by 1-RTT keys.  This
   can mean that the client is unable to use these acknowledgments if
   the server cryptographic handshake messages are delayed or lost.
   Note that the same limitation applies to other data sent by the
   server protected by the 1-RTT keys.

13.2.7.  PADDING Frames Consume Congestion Window

   Packets containing PADDING frames are considered to be in flight for
   congestion control purposes [QUIC-RECOVERY].  Packets containing only
   PADDING frames therefore consume congestion window but do not
   generate acknowledgments that will open the congestion window.  To
   avoid a deadlock, a sender SHOULD ensure that other frames are sent
   periodically in addition to PADDING frames to elicit acknowledgments
   from the receiver.

13.3.  Retransmission of Information

   QUIC packets that are determined to be lost are not retransmitted
   whole.  The same applies to the frames that are contained within lost
   packets.  Instead, the information that might be carried in frames is
   sent again in new frames as needed.

   New frames and packets are used to carry information that is
   determined to have been lost.  In general, information is sent again
   when a packet containing that information is determined to be lost,
   and sending ceases when a packet containing that information is
   acknowledged.

   *  Data sent in CRYPTO frames is retransmitted according to the rules
      in [QUIC-RECOVERY], until all data has been acknowledged.  Data in
      CRYPTO frames for Initial and Handshake packets is discarded when
      keys for the corresponding packet number space are discarded.

   *  Application data sent in STREAM frames is retransmitted in new
      STREAM frames unless the endpoint has sent a RESET_STREAM for that
      stream.  Once an endpoint sends a RESET_STREAM frame, no further
      STREAM frames are needed.

   *  ACK frames carry the most recent set of acknowledgments and the
      acknowledgment delay from the largest acknowledged packet, as
      described in Section 13.2.1.  Delaying the transmission of packets
      containing ACK frames or resending old ACK frames can cause the
      peer to generate an inflated RTT sample or unnecessarily disable
      ECN.

   *  Cancellation of stream transmission, as carried in a RESET_STREAM
      frame, is sent until acknowledged or until all stream data is
      acknowledged by the peer (that is, either the "Reset Recvd" or
      "Data Recvd" state is reached on the sending part of the stream).
      The content of a RESET_STREAM frame MUST NOT change when it is
      sent again.

   *  Similarly, a request to cancel stream transmission, as encoded in
      a STOP_SENDING frame, is sent until the receiving part of the
      stream enters either a "Data Recvd" or "Reset Recvd" state; see
      Section 3.5.

   *  Connection close signals, including packets that contain
      CONNECTION_CLOSE frames, are not sent again when packet loss is
      detected.  Resending these signals is described in Section 10.

   *  The current connection maximum data is sent in MAX_DATA frames.
      An updated value is sent in a MAX_DATA frame if the packet
      containing the most recently sent MAX_DATA frame is declared lost
      or when the endpoint decides to update the limit.  Care is
      necessary to avoid sending this frame too often, as the limit can
      increase frequently and cause an unnecessarily large number of
      MAX_DATA frames to be sent; see Section 4.2.

   *  The current maximum stream data offset is sent in MAX_STREAM_DATA
      frames.  Like MAX_DATA, an updated value is sent when the packet
      containing the most recent MAX_STREAM_DATA frame for a stream is
      lost or when the limit is updated, with care taken to prevent the
      frame from being sent too often.  An endpoint SHOULD stop sending
      MAX_STREAM_DATA frames when the receiving part of the stream
      enters a "Size Known" or "Reset Recvd" state.

   *  The limit on streams of a given type is sent in MAX_STREAMS
      frames.  Like MAX_DATA, an updated value is sent when a packet
      containing the most recent MAX_STREAMS for a stream type frame is
      declared lost or when the limit is updated, with care taken to
      prevent the frame from being sent too often.

   *  Blocked signals are carried in DATA_BLOCKED, STREAM_DATA_BLOCKED,
      and STREAMS_BLOCKED frames.  DATA_BLOCKED frames have connection
      scope, STREAM_DATA_BLOCKED frames have stream scope, and
      STREAMS_BLOCKED frames are scoped to a specific stream type.  A
      new frame is sent if a packet containing the most recent frame for
      a scope is lost, but only while the endpoint is blocked on the
      corresponding limit.  These frames always include the limit that
      is causing blocking at the time that they are transmitted.

   *  A liveness or path validation check using PATH_CHALLENGE frames is
      sent periodically until a matching PATH_RESPONSE frame is received
      or until there is no remaining need for liveness or path
      validation checking.  PATH_CHALLENGE frames include a different
      payload each time they are sent.

   *  Responses to path validation using PATH_RESPONSE frames are sent
      just once.  The peer is expected to send more PATH_CHALLENGE
      frames as necessary to evoke additional PATH_RESPONSE frames.

   *  New connection IDs are sent in NEW_CONNECTION_ID frames and
      retransmitted if the packet containing them is lost.
      Retransmissions of this frame carry the same sequence number
      value.  Likewise, retired connection IDs are sent in
      RETIRE_CONNECTION_ID frames and retransmitted if the packet
      containing them is lost.

   *  NEW_TOKEN frames are retransmitted if the packet containing them
      is lost.  No special support is made for detecting reordered and
      duplicated NEW_TOKEN frames other than a direct comparison of the
      frame contents.

   *  PING and PADDING frames contain no information, so lost PING or
      PADDING frames do not require repair.

   *  The HANDSHAKE_DONE frame MUST be retransmitted until it is
      acknowledged.

   Endpoints SHOULD prioritize retransmission of data over sending new
   data, unless priorities specified by the application indicate
   otherwise; see Section 2.3.

   Even though a sender is encouraged to assemble frames containing up-
   to-date information every time it sends a packet, it is not forbidden
   to retransmit copies of frames from lost packets.  A sender that
   retransmits copies of frames needs to handle decreases in available
   payload size due to changes in packet number length, connection ID
   length, and path MTU.  A receiver MUST accept packets containing an
   outdated frame, such as a MAX_DATA frame carrying a smaller maximum
   data value than one found in an older packet.

   A sender SHOULD avoid retransmitting information from packets once
   they are acknowledged.  This includes packets that are acknowledged
   after being declared lost, which can happen in the presence of
   network reordering.  Doing so requires senders to retain information
   about packets after they are declared lost.  A sender can discard
   this information after a period of time elapses that adequately
   allows for reordering, such as a PTO (Section 6.2 of
   [QUIC-RECOVERY]), or based on other events, such as reaching a memory
   limit.

   Upon detecting losses, a sender MUST take appropriate congestion
   control action.  The details of loss detection and congestion control
   are described in [QUIC-RECOVERY].

13.4.  Explicit Congestion Notification

   QUIC endpoints can use ECN [RFC3168] to detect and respond to network
   congestion.  ECN allows an endpoint to set an ECN-Capable Transport
   (ECT) codepoint in the ECN field of an IP packet.  A network node can
   then indicate congestion by setting the ECN-CE codepoint in the ECN
   field instead of dropping the packet [RFC8087].  Endpoints react to
   reported congestion by reducing their sending rate in response, as
   described in [QUIC-RECOVERY].

   To enable ECN, a sending QUIC endpoint first determines whether a
   path supports ECN marking and whether the peer reports the ECN values
   in received IP headers; see Section 13.4.2.

13.4.1.  Reporting ECN Counts

   The use of ECN requires the receiving endpoint to read the ECN field
   from an IP packet, which is not possible on all platforms.  If an
   endpoint does not implement ECN support or does not have access to
   received ECN fields, it does not report ECN counts for packets it
   receives.

   Even if an endpoint does not set an ECT field in packets it sends,
   the endpoint MUST provide feedback about ECN markings it receives, if
   these are accessible.  Failing to report the ECN counts will cause
   the sender to disable the use of ECN for this connection.

   On receiving an IP packet with an ECT(0), ECT(1), or ECN-CE
   codepoint, an ECN-enabled endpoint accesses the ECN field and
   increases the corresponding ECT(0), ECT(1), or ECN-CE count.  These
   ECN counts are included in subsequent ACK frames; see Sections 13.2
   and 19.3.

   Each packet number space maintains separate acknowledgment state and
   separate ECN counts.  Coalesced QUIC packets (see Section 12.2) share
   the same IP header so the ECN counts are incremented once for each
   coalesced QUIC packet.

   For example, if one each of an Initial, Handshake, and 1-RTT QUIC
   packet are coalesced into a single UDP datagram, the ECN counts for
   all three packet number spaces will be incremented by one each, based
   on the ECN field of the single IP header.

   ECN counts are only incremented when QUIC packets from the received
   IP packet are processed.  As such, duplicate QUIC packets are not
   processed and do not increase ECN counts; see Section 21.10 for
   relevant security concerns.

13.4.2.  ECN Validation

   It is possible for faulty network devices to corrupt or erroneously
   drop packets that carry a non-zero ECN codepoint.  To ensure
   connectivity in the presence of such devices, an endpoint validates
   the ECN counts for each network path and disables the use of ECN on
   that path if errors are detected.

   To perform ECN validation for a new path:

   *  The endpoint sets an ECT(0) codepoint in the IP header of early
      outgoing packets sent on a new path to the peer [RFC8311].

   *  The endpoint monitors whether all packets sent with an ECT
      codepoint are eventually deemed lost (Section 6 of
      [QUIC-RECOVERY]), indicating that ECN validation has failed.

   If an endpoint has cause to expect that IP packets with an ECT
   codepoint might be dropped by a faulty network element, the endpoint
   could set an ECT codepoint for only the first ten outgoing packets on
   a path, or for a period of three PTOs (see Section 6.2 of
   [QUIC-RECOVERY]).  If all packets marked with non-zero ECN codepoints
   are subsequently lost, it can disable marking on the assumption that
   the marking caused the loss.

   An endpoint thus attempts to use ECN and validates this for each new
   connection, when switching to a server's preferred address, and on
   active connection migration to a new path.  Appendix A.4 describes
   one possible algorithm.

   Other methods of probing paths for ECN support are possible, as are
   different marking strategies.  Implementations MAY use other methods
   defined in RFCs; see [RFC8311].  Implementations that use the ECT(1)
   codepoint need to perform ECN validation using the reported ECT(1)
   counts.

13.4.2.1.  Receiving ACK Frames with ECN Counts

   Erroneous application of ECN-CE markings by the network can result in
   degraded connection performance.  An endpoint that receives an ACK
   frame with ECN counts therefore validates the counts before using
   them.  It performs this validation by comparing newly received counts
   against those from the last successfully processed ACK frame.  Any
   increase in the ECN counts is validated based on the ECN markings
   that were applied to packets that are newly acknowledged in the ACK
   frame.

   If an ACK frame newly acknowledges a packet that the endpoint sent
   with either the ECT(0) or ECT(1) codepoint set, ECN validation fails
   if the corresponding ECN counts are not present in the ACK frame.
   This check detects a network element that zeroes the ECN field or a
   peer that does not report ECN markings.

   ECN validation also fails if the sum of the increase in ECT(0) and
   ECN-CE counts is less than the number of newly acknowledged packets
   that were originally sent with an ECT(0) marking.  Similarly, ECN
   validation fails if the sum of the increases to ECT(1) and ECN-CE
   counts is less than the number of newly acknowledged packets sent
   with an ECT(1) marking.  These checks can detect remarking of ECN-CE
   markings by the network.

   An endpoint could miss acknowledgments for a packet when ACK frames
   are lost.  It is therefore possible for the total increase in ECT(0),
   ECT(1), and ECN-CE counts to be greater than the number of packets
   that are newly acknowledged by an ACK frame.  This is why ECN counts
   are permitted to be larger than the total number of packets that are
   acknowledged.

   Validating ECN counts from reordered ACK frames can result in
   failure.  An endpoint MUST NOT fail ECN validation as a result of
   processing an ACK frame that does not increase the largest
   acknowledged packet number.

   ECN validation can fail if the received total count for either ECT(0)
   or ECT(1) exceeds the total number of packets sent with each
   corresponding ECT codepoint.  In particular, validation will fail
   when an endpoint receives a non-zero ECN count corresponding to an
   ECT codepoint that it never applied.  This check detects when packets
   are remarked to ECT(0) or ECT(1) in the network.

13.4.2.2.  ECN Validation Outcomes

   If validation fails, then the endpoint MUST disable ECN.  It stops
   setting the ECT codepoint in IP packets that it sends, assuming that
   either the network path or the peer does not support ECN.

   Even if validation fails, an endpoint MAY revalidate ECN for the same
   path at any later time in the connection.  An endpoint could continue
   to periodically attempt validation.

   Upon successful validation, an endpoint MAY continue to set an ECT
   codepoint in subsequent packets it sends, with the expectation that
   the path is ECN capable.  Network routing and path elements can
   change mid-connection; an endpoint MUST disable ECN if validation
   later fails.

14.  Datagram Size

   A UDP datagram can include one or more QUIC packets.  The datagram
   size refers to the total UDP payload size of a single UDP datagram
   carrying QUIC packets.  The datagram size includes one or more QUIC
   packet headers and protected payloads, but not the UDP or IP headers.

   The maximum datagram size is defined as the largest size of UDP
   payload that can be sent across a network path using a single UDP
   datagram.  QUIC MUST NOT be used if the network path cannot support a
   maximum datagram size of at least 1200 bytes.

   QUIC assumes a minimum IP packet size of at least 1280 bytes.  This
   is the IPv6 minimum size [IPv6] and is also supported by most modern
   IPv4 networks.  Assuming the minimum IP header size of 40 bytes for
   IPv6 and 20 bytes for IPv4 and a UDP header size of 8 bytes, this
   results in a maximum datagram size of 1232 bytes for IPv6 and 1252
   bytes for IPv4.  Thus, modern IPv4 and all IPv6 network paths are
   expected to be able to support QUIC.

      |  Note: This requirement to support a UDP payload of 1200 bytes
      |  limits the space available for IPv6 extension headers to 32
      |  bytes or IPv4 options to 52 bytes if the path only supports the
      |  IPv6 minimum MTU of 1280 bytes.  This affects Initial packets
      |  and path validation.

   Any maximum datagram size larger than 1200 bytes can be discovered
   using Path Maximum Transmission Unit Discovery (PMTUD) (see
   Section 14.2.1) or Datagram Packetization Layer PMTU Discovery
   (DPLPMTUD) (see Section 14.3).

   Enforcement of the max_udp_payload_size transport parameter
   (Section 18.2) might act as an additional limit on the maximum
   datagram size.  A sender can avoid exceeding this limit, once the
   value is known.  However, prior to learning the value of the
   transport parameter, endpoints risk datagrams being lost if they send
   datagrams larger than the smallest allowed maximum datagram size of
   1200 bytes.

   UDP datagrams MUST NOT be fragmented at the IP layer.  In IPv4
   [IPv4], the Don't Fragment (DF) bit MUST be set if possible, to
   prevent fragmentation on the path.

   QUIC sometimes requires datagrams to be no smaller than a certain
   size; see Section 8.1 as an example.  However, the size of a datagram
   is not authenticated.  That is, if an endpoint receives a datagram of
   a certain size, it cannot know that the sender sent the datagram at
   the same size.  Therefore, an endpoint MUST NOT close a connection
   when it receives a datagram that does not meet size constraints; the
   endpoint MAY discard such datagrams.

14.1.  Initial Datagram Size

   A client MUST expand the payload of all UDP datagrams carrying
   Initial packets to at least the smallest allowed maximum datagram
   size of 1200 bytes by adding PADDING frames to the Initial packet or
   by coalescing the Initial packet; see Section 12.2.  Initial packets
   can even be coalesced with invalid packets, which a receiver will
   discard.  Similarly, a server MUST expand the payload of all UDP
   datagrams carrying ack-eliciting Initial packets to at least the
   smallest allowed maximum datagram size of 1200 bytes.

   Sending UDP datagrams of this size ensures that the network path
   supports a reasonable Path Maximum Transmission Unit (PMTU), in both
   directions.  Additionally, a client that expands Initial packets
   helps reduce the amplitude of amplification attacks caused by server
   responses toward an unverified client address; see Section 8.

   Datagrams containing Initial packets MAY exceed 1200 bytes if the
   sender believes that the network path and peer both support the size
   that it chooses.

   A server MUST discard an Initial packet that is carried in a UDP
   datagram with a payload that is smaller than the smallest allowed
   maximum datagram size of 1200 bytes.  A server MAY also immediately
   close the connection by sending a CONNECTION_CLOSE frame with an
   error code of PROTOCOL_VIOLATION; see Section 10.2.3.

   The server MUST also limit the number of bytes it sends before
   validating the address of the client; see Section 8.

14.2.  Path Maximum Transmission Unit

   The PMTU is the maximum size of the entire IP packet, including the
   IP header, UDP header, and UDP payload.  The UDP payload includes one
   or more QUIC packet headers and protected payloads.  The PMTU can
   depend on path characteristics and can therefore change over time.
   The largest UDP payload an endpoint sends at any given time is
   referred to as the endpoint's maximum datagram size.

   An endpoint SHOULD use DPLPMTUD (Section 14.3) or PMTUD
   (Section 14.2.1) to determine whether the path to a destination will
   support a desired maximum datagram size without fragmentation.  In
   the absence of these mechanisms, QUIC endpoints SHOULD NOT send
   datagrams larger than the smallest allowed maximum datagram size.

   Both DPLPMTUD and PMTUD send datagrams that are larger than the
   current maximum datagram size, referred to as PMTU probes.  All QUIC
   packets that are not sent in a PMTU probe SHOULD be sized to fit
   within the maximum datagram size to avoid the datagram being
   fragmented or dropped [RFC8085].

   If a QUIC endpoint determines that the PMTU between any pair of local
   and remote IP addresses cannot support the smallest allowed maximum
   datagram size of 1200 bytes, it MUST immediately cease sending QUIC
   packets, except for those in PMTU probes or those containing
   CONNECTION_CLOSE frames, on the affected path.  An endpoint MAY
   terminate the connection if an alternative path cannot be found.

   Each pair of local and remote addresses could have a different PMTU.
   QUIC implementations that implement any kind of PMTU discovery
   therefore SHOULD maintain a maximum datagram size for each
   combination of local and remote IP addresses.

   A QUIC implementation MAY be more conservative in computing the
   maximum datagram size to allow for unknown tunnel overheads or IP
   header options/extensions.

14.2.1.  Handling of ICMP Messages by PMTUD

   PMTUD [RFC1191] [RFC8201] relies on reception of ICMP messages (that
   is, IPv6 Packet Too Big (PTB) messages) that indicate when an IP
   packet is dropped because it is larger than the local router MTU.
   DPLPMTUD can also optionally use these messages.  This use of ICMP
   messages is potentially vulnerable to attacks by entities that cannot
   observe packets but might successfully guess the addresses used on
   the path.  These attacks could reduce the PMTU to a bandwidth-
   inefficient value.

   An endpoint MUST ignore an ICMP message that claims the PMTU has
   decreased below QUIC's smallest allowed maximum datagram size.

   The requirements for generating ICMP [RFC1812] [RFC4443] state that
   the quoted packet should contain as much of the original packet as
   possible without exceeding the minimum MTU for the IP version.  The
   size of the quoted packet can actually be smaller, or the information
   unintelligible, as described in Section 1.1 of [DPLPMTUD].

   QUIC endpoints using PMTUD SHOULD validate ICMP messages to protect
   from packet injection as specified in [RFC8201] and Section 5.2 of
   [RFC8085].  This validation SHOULD use the quoted packet supplied in
   the payload of an ICMP message to associate the message with a
   corresponding transport connection (see Section 4.6.1 of [DPLPMTUD]).
   ICMP message validation MUST include matching IP addresses and UDP
   ports [RFC8085] and, when possible, connection IDs to an active QUIC
   session.  The endpoint SHOULD ignore all ICMP messages that fail
   validation.

   An endpoint MUST NOT increase the PMTU based on ICMP messages; see
   Item 6 in Section 3 of [DPLPMTUD].  Any reduction in QUIC's maximum
   datagram size in response to ICMP messages MAY be provisional until
   QUIC's loss detection algorithm determines that the quoted packet has
   actually been lost.

14.3.  Datagram Packetization Layer PMTU Discovery

   DPLPMTUD [DPLPMTUD] relies on tracking loss or acknowledgment of QUIC
   packets that are carried in PMTU probes.  PMTU probes for DPLPMTUD
   that use the PADDING frame implement "Probing using padding data", as
   defined in Section 4.1 of [DPLPMTUD].

   Endpoints SHOULD set the initial value of BASE_PLPMTU (Section 5.1 of
   [DPLPMTUD]) to be consistent with QUIC's smallest allowed maximum
   datagram size.  The MIN_PLPMTU is the same as the BASE_PLPMTU.

   QUIC endpoints implementing DPLPMTUD maintain a DPLPMTUD Maximum
   Packet Size (MPS) (Section 4.4 of [DPLPMTUD]) for each combination of
   local and remote IP addresses.  This corresponds to the maximum
   datagram size.

14.3.1.  DPLPMTUD and Initial Connectivity

   From the perspective of DPLPMTUD, QUIC is an acknowledged
   Packetization Layer (PL).  A QUIC sender can therefore enter the
   DPLPMTUD BASE state (Section 5.2 of [DPLPMTUD]) when the QUIC
   connection handshake has been completed.

14.3.2.  Validating the Network Path with DPLPMTUD

   QUIC is an acknowledged PL; therefore, a QUIC sender does not
   implement a DPLPMTUD CONFIRMATION_TIMER while in the SEARCH_COMPLETE
   state; see Section 5.2 of [DPLPMTUD].

14.3.3.  Handling of ICMP Messages by DPLPMTUD

   An endpoint using DPLPMTUD requires the validation of any received
   ICMP PTB message before using the PTB information, as defined in
   Section 4.6 of [DPLPMTUD].  In addition to UDP port validation, QUIC
   validates an ICMP message by using other PL information (e.g.,
   validation of connection IDs in the quoted packet of any received
   ICMP message).

   The considerations for processing ICMP messages described in
   Section 14.2.1 also apply if these messages are used by DPLPMTUD.

14.4.  Sending QUIC PMTU Probes

   PMTU probes are ack-eliciting packets.

   Endpoints could limit the content of PMTU probes to PING and PADDING
   frames, since packets that are larger than the current maximum
   datagram size are more likely to be dropped by the network.  Loss of
   a QUIC packet that is carried in a PMTU probe is therefore not a
   reliable indication of congestion and SHOULD NOT trigger a congestion
   control reaction; see Item 7 in Section 3 of [DPLPMTUD].  However,
   PMTU probes consume congestion window, which could delay subsequent
   transmission by an application.

14.4.1.  PMTU Probes Containing Source Connection ID

   Endpoints that rely on the Destination Connection ID field for
   routing incoming QUIC packets are likely to require that the
   connection ID be included in PMTU probes to route any resulting ICMP
   messages (Section 14.2.1) back to the correct endpoint.  However,
   only long header packets (Section 17.2) contain the Source Connection
   ID field, and long header packets are not decrypted or acknowledged
   by the peer once the handshake is complete.

   One way to construct a PMTU probe is to coalesce (see Section 12.2) a
   packet with a long header, such as a Handshake or 0-RTT packet
   (Section 17.2), with a short header packet in a single UDP datagram.
   If the resulting PMTU probe reaches the endpoint, the packet with the
   long header will be ignored, but the short header packet will be
   acknowledged.  If the PMTU probe causes an ICMP message to be sent,
   the first part of the probe will be quoted in that message.  If the
   Source Connection ID field is within the quoted portion of the probe,
   that could be used for routing or validation of the ICMP message.

      |  Note: The purpose of using a packet with a long header is only
      |  to ensure that the quoted packet contained in the ICMP message
      |  contains a Source Connection ID field.  This packet does not
      |  need to be a valid packet, and it can be sent even if there is
      |  no current use for packets of that type.

15.  Versions

   QUIC versions are identified using a 32-bit unsigned number.

   The version 0x00000000 is reserved to represent version negotiation.
   This version of the specification is identified by the number
   0x00000001.

   Other versions of QUIC might have different properties from this
   version.  The properties of QUIC that are guaranteed to be consistent
   across all versions of the protocol are described in
   [QUIC-INVARIANTS].

   Version 0x00000001 of QUIC uses TLS as a cryptographic handshake
   protocol, as described in [QUIC-TLS].

   Versions with the most significant 16 bits of the version number
   cleared are reserved for use in future IETF consensus documents.

   Versions that follow the pattern 0x?a?a?a?a are reserved for use in
   forcing version negotiation to be exercised -- that is, any version
   number where the low four bits of all bytes is 1010 (in binary).  A
   client or server MAY advertise support for any of these reserved
   versions.

   Reserved version numbers will never represent a real protocol; a
   client MAY use one of these version numbers with the expectation that
   the server will initiate version negotiation; a server MAY advertise
   support for one of these versions and can expect that clients ignore
   the value.

16.  Variable-Length Integer Encoding

   QUIC packets and frames commonly use a variable-length encoding for
   non-negative integer values.  This encoding ensures that smaller
   integer values need fewer bytes to encode.

   The QUIC variable-length integer encoding reserves the two most
   significant bits of the first byte to encode the base-2 logarithm of
   the integer encoding length in bytes.  The integer value is encoded
   on the remaining bits, in network byte order.

   This means that integers are encoded on 1, 2, 4, or 8 bytes and can
   encode 6-, 14-, 30-, or 62-bit values, respectively.  Table 4
   summarizes the encoding properties.

          +======+========+=============+=======================+
          | 2MSB | Length | Usable Bits | Range                 |
          +======+========+=============+=======================+
          | 00   | 1      | 6           | 0-63                  |
          +------+--------+-------------+-----------------------+
          | 01   | 2      | 14          | 0-16383               |
          +------+--------+-------------+-----------------------+
          | 10   | 4      | 30          | 0-1073741823          |
          +------+--------+-------------+-----------------------+
          | 11   | 8      | 62          | 0-4611686018427387903 |
          +------+--------+-------------+-----------------------+

                   Table 4: Summary of Integer Encodings

   An example of a decoding algorithm and sample encodings are shown in
   Appendix A.1.

   Values do not need to be encoded on the minimum number of bytes
   necessary, with the sole exception of the Frame Type field; see
   Section 12.4.

   Versions (Section 15), packet numbers sent in the header
   (Section 17.1), and the length of connection IDs in long header
   packets (Section 17.2) are described using integers but do not use
   this encoding.

17.  Packet Formats

   All numeric values are encoded in network byte order (that is, big
   endian), and all field sizes are in bits.  Hexadecimal notation is
   used for describing the value of fields.

17.1.  Packet Number Encoding and Decoding

   Packet numbers are integers in the range 0 to 2^62-1 (Section 12.3).
   When present in long or short packet headers, they are encoded in 1
   to 4 bytes.  The number of bits required to represent the packet
   number is reduced by including only the least significant bits of the
   packet number.

   The encoded packet number is protected as described in Section 5.4 of
   [QUIC-TLS].

   Prior to receiving an acknowledgment for a packet number space, the
   full packet number MUST be included; it is not to be truncated, as
   described below.

   After an acknowledgment is received for a packet number space, the
   sender MUST use a packet number size able to represent more than
   twice as large a range as the difference between the largest
   acknowledged packet number and the packet number being sent.  A peer
   receiving the packet will then correctly decode the packet number,
   unless the packet is delayed in transit such that it arrives after
   many higher-numbered packets have been received.  An endpoint SHOULD
   use a large enough packet number encoding to allow the packet number
   to be recovered even if the packet arrives after packets that are
   sent afterwards.

   As a result, the size of the packet number encoding is at least one
   bit more than the base-2 logarithm of the number of contiguous
   unacknowledged packet numbers, including the new packet.  Pseudocode
   and an example for packet number encoding can be found in
   Appendix A.2.

   At a receiver, protection of the packet number is removed prior to
   recovering the full packet number.  The full packet number is then
   reconstructed based on the number of significant bits present, the
   value of those bits, and the largest packet number received in a
   successfully authenticated packet.  Recovering the full packet number
   is necessary to successfully complete the removal of packet
   protection.

   Once header protection is removed, the packet number is decoded by
   finding the packet number value that is closest to the next expected
   packet.  The next expected packet is the highest received packet
   number plus one.  Pseudocode and an example for packet number
   decoding can be found in Appendix A.3.

17.2.  Long Header Packets

   Long Header Packet {
     Header Form (1) = 1,
     Fixed Bit (1) = 1,
     Long Packet Type (2),
     Type-Specific Bits (4),
     Version (32),
     Destination Connection ID Length (8),
     Destination Connection ID (0..160),
     Source Connection ID Length (8),
     Source Connection ID (0..160),
     Type-Specific Payload (..),
   }

                    Figure 13: Long Header Packet Format

   Long headers are used for packets that are sent prior to the
   establishment of 1-RTT keys.  Once 1-RTT keys are available, a sender
   switches to sending packets using the short header (Section 17.3).
   The long form allows for special packets -- such as the Version
   Negotiation packet -- to be represented in this uniform fixed-length
   packet format.  Packets that use the long header contain the
   following fields:

   Header Form:  The most significant bit (0x80) of byte 0 (the first
      byte) is set to 1 for long headers.

   Fixed Bit:  The next bit (0x40) of byte 0 is set to 1, unless the
      packet is a Version Negotiation packet.  Packets containing a zero
      value for this bit are not valid packets in this version and MUST
      be discarded.  A value of 1 for this bit allows QUIC to coexist
      with other protocols; see [RFC7983].

   Long Packet Type:  The next two bits (those with a mask of 0x30) of
      byte 0 contain a packet type.  Packet types are listed in Table 5.

   Type-Specific Bits:  The semantics of the lower four bits (those with
      a mask of 0x0f) of byte 0 are determined by the packet type.

   Version:  The QUIC Version is a 32-bit field that follows the first
      byte.  This field indicates the version of QUIC that is in use and
      determines how the rest of the protocol fields are interpreted.

   Destination Connection ID Length:  The byte following the version
      contains the length in bytes of the Destination Connection ID
      field that follows it.  This length is encoded as an 8-bit
      unsigned integer.  In QUIC version 1, this value MUST NOT exceed
      20 bytes.  Endpoints that receive a version 1 long header with a
      value larger than 20 MUST drop the packet.  In order to properly
      form a Version Negotiation packet, servers SHOULD be able to read
      longer connection IDs from other QUIC versions.

   Destination Connection ID:  The Destination Connection ID field
      follows the Destination Connection ID Length field, which
      indicates the length of this field.  Section 7.2 describes the use
      of this field in more detail.

   Source Connection ID Length:  The byte following the Destination
      Connection ID contains the length in bytes of the Source
      Connection ID field that follows it.  This length is encoded as an
      8-bit unsigned integer.  In QUIC version 1, this value MUST NOT
      exceed 20 bytes.  Endpoints that receive a version 1 long header
      with a value larger than 20 MUST drop the packet.  In order to
      properly form a Version Negotiation packet, servers SHOULD be able
      to read longer connection IDs from other QUIC versions.

   Source Connection ID:  The Source Connection ID field follows the
      Source Connection ID Length field, which indicates the length of
      this field.  Section 7.2 describes the use of this field in more
      detail.

   Type-Specific Payload:  The remainder of the packet, if any, is type
      specific.

   In this version of QUIC, the following packet types with the long
   header are defined:

                   +======+===========+================+
                   | Type | Name      | Section        |
                   +======+===========+================+
                   | 0x00 | Initial   | Section 17.2.2 |
                   +------+-----------+----------------+
                   | 0x01 | 0-RTT     | Section 17.2.3 |
                   +------+-----------+----------------+
                   | 0x02 | Handshake | Section 17.2.4 |
                   +------+-----------+----------------+
                   | 0x03 | Retry     | Section 17.2.5 |
                   +------+-----------+----------------+

                     Table 5: Long Header Packet Types

   The header form bit, Destination and Source Connection ID lengths,
   Destination and Source Connection ID fields, and Version fields of a
   long header packet are version independent.  The other fields in the
   first byte are version specific.  See [QUIC-INVARIANTS] for details
   on how packets from different versions of QUIC are interpreted.

   The interpretation of the fields and the payload are specific to a
   version and packet type.  While type-specific semantics for this
   version are described in the following sections, several long header
   packets in this version of QUIC contain these additional fields:

   Reserved Bits:  Two bits (those with a mask of 0x0c) of byte 0 are
      reserved across multiple packet types.  These bits are protected
      using header protection; see Section 5.4 of [QUIC-TLS].  The value
      included prior to protection MUST be set to 0.  An endpoint MUST
      treat receipt of a packet that has a non-zero value for these bits
      after removing both packet and header protection as a connection
      error of type PROTOCOL_VIOLATION.  Discarding such a packet after
      only removing header protection can expose the endpoint to
      attacks; see Section 9.5 of [QUIC-TLS].

   Packet Number Length:  In packet types that contain a Packet Number
      field, the least significant two bits (those with a mask of 0x03)
      of byte 0 contain the length of the Packet Number field, encoded
      as an unsigned two-bit integer that is one less than the length of
      the Packet Number field in bytes.  That is, the length of the
      Packet Number field is the value of this field plus one.  These
      bits are protected using header protection; see Section 5.4 of
      [QUIC-TLS].

   Length:  This is the length of the remainder of the packet (that is,
      the Packet Number and Payload fields) in bytes, encoded as a
      variable-length integer (Section 16).

   Packet Number:  This field is 1 to 4 bytes long.  The packet number
      is protected using header protection; see Section 5.4 of
      [QUIC-TLS].  The length of the Packet Number field is encoded in
      the Packet Number Length bits of byte 0; see above.

   Packet Payload:  This is the payload of the packet -- containing a
      sequence of frames -- that is protected using packet protection.

17.2.1.  Version Negotiation Packet

   A Version Negotiation packet is inherently not version specific.
   Upon receipt by a client, it will be identified as a Version
   Negotiation packet based on the Version field having a value of 0.

   The Version Negotiation packet is a response to a client packet that
   contains a version that is not supported by the server.  It is only
   sent by servers.

   The layout of a Version Negotiation packet is:

   Version Negotiation Packet {
     Header Form (1) = 1,
     Unused (7),
     Version (32) = 0,
     Destination Connection ID Length (8),
     Destination Connection ID (0..2040),
     Source Connection ID Length (8),
     Source Connection ID (0..2040),
     Supported Version (32) ...,
   }

                   Figure 14: Version Negotiation Packet

   The value in the Unused field is set to an arbitrary value by the
   server.  Clients MUST ignore the value of this field.  Where QUIC
   might be multiplexed with other protocols (see [RFC7983]), servers
   SHOULD set the most significant bit of this field (0x40) to 1 so that
   Version Negotiation packets appear to have the Fixed Bit field.  Note
   that other versions of QUIC might not make a similar recommendation.

   The Version field of a Version Negotiation packet MUST be set to
   0x00000000.

   The server MUST include the value from the Source Connection ID field
   of the packet it receives in the Destination Connection ID field.
   The value for Source Connection ID MUST be copied from the
   Destination Connection ID of the received packet, which is initially
   randomly selected by a client.  Echoing both connection IDs gives
   clients some assurance that the server received the packet and that
   the Version Negotiation packet was not generated by an entity that
   did not observe the Initial packet.

   Future versions of QUIC could have different requirements for the
   lengths of connection IDs.  In particular, connection IDs might have
   a smaller minimum length or a greater maximum length.  Version-
   specific rules for the connection ID therefore MUST NOT influence a
   decision about whether to send a Version Negotiation packet.

   The remainder of the Version Negotiation packet is a list of 32-bit
   versions that the server supports.

   A Version Negotiation packet is not acknowledged.  It is only sent in
   response to a packet that indicates an unsupported version; see
   Section 5.2.2.

   The Version Negotiation packet does not include the Packet Number and
   Length fields present in other packets that use the long header form.
   Consequently, a Version Negotiation packet consumes an entire UDP
   datagram.

   A server MUST NOT send more than one Version Negotiation packet in
   response to a single UDP datagram.

   See Section 6 for a description of the version negotiation process.

17.2.2.  Initial Packet

   An Initial packet uses long headers with a type value of 0x00.  It
   carries the first CRYPTO frames sent by the client and server to
   perform key exchange, and it carries ACK frames in either direction.

   Initial Packet {
     Header Form (1) = 1,
     Fixed Bit (1) = 1,
     Long Packet Type (2) = 0,
     Reserved Bits (2),
     Packet Number Length (2),
     Version (32),
     Destination Connection ID Length (8),
     Destination Connection ID (0..160),
     Source Connection ID Length (8),
     Source Connection ID (0..160),
     Token Length (i),
     Token (..),
     Length (i),
     Packet Number (8..32),
     Packet Payload (8..),
   }

                         Figure 15: Initial Packet

   The Initial packet contains a long header as well as the Length and
   Packet Number fields; see Section 17.2.  The first byte contains the
   Reserved and Packet Number Length bits; see also Section 17.2.
   Between the Source Connection ID and Length fields, there are two
   additional fields specific to the Initial packet.

   Token Length:  A variable-length integer specifying the length of the
      Token field, in bytes.  This value is 0 if no token is present.
      Initial packets sent by the server MUST set the Token Length field
      to 0; clients that receive an Initial packet with a non-zero Token
      Length field MUST either discard the packet or generate a
      connection error of type PROTOCOL_VIOLATION.

   Token:  The value of the token that was previously provided in a
      Retry packet or NEW_TOKEN frame; see Section 8.1.

   In order to prevent tampering by version-unaware middleboxes, Initial
   packets are protected with connection- and version-specific keys
   (Initial keys) as described in [QUIC-TLS].  This protection does not
   provide confidentiality or integrity against attackers that can
   observe packets, but it does prevent attackers that cannot observe
   packets from spoofing Initial packets.

   The client and server use the Initial packet type for any packet that
   contains an initial cryptographic handshake message.  This includes
   all cases where a new packet containing the initial cryptographic
   message needs to be created, such as the packets sent after receiving
   a Retry packet; see Section 17.2.5.

   A server sends its first Initial packet in response to a client
   Initial.  A server MAY send multiple Initial packets.  The
   cryptographic key exchange could require multiple round trips or
   retransmissions of this data.

   The payload of an Initial packet includes a CRYPTO frame (or frames)
   containing a cryptographic handshake message, ACK frames, or both.
   PING, PADDING, and CONNECTION_CLOSE frames of type 0x1c are also
   permitted.  An endpoint that receives an Initial packet containing
   other frames can either discard the packet as spurious or treat it as
   a connection error.

   The first packet sent by a client always includes a CRYPTO frame that
   contains the start or all of the first cryptographic handshake
   message.  The first CRYPTO frame sent always begins at an offset of
   0; see Section 7.

   Note that if the server sends a TLS HelloRetryRequest (see
   Section 4.7 of [QUIC-TLS]), the client will send another series of
   Initial packets.  These Initial packets will continue the
   cryptographic handshake and will contain CRYPTO frames starting at an
   offset matching the size of the CRYPTO frames sent in the first
   flight of Initial packets.

17.2.2.1.  Abandoning Initial Packets

   A client stops both sending and processing Initial packets when it
   sends its first Handshake packet.  A server stops sending and
   processing Initial packets when it receives its first Handshake
   packet.  Though packets might still be in flight or awaiting
   acknowledgment, no further Initial packets need to be exchanged
   beyond this point.  Initial packet protection keys are discarded (see
   Section 4.9.1 of [QUIC-TLS]) along with any loss recovery and
   congestion control state; see Section 6.4 of [QUIC-RECOVERY].

   Any data in CRYPTO frames is discarded -- and no longer retransmitted
   -- when Initial keys are discarded.

17.2.3.  0-RTT

   A 0-RTT packet uses long headers with a type value of 0x01, followed
   by the Length and Packet Number fields; see Section 17.2.  The first
   byte contains the Reserved and Packet Number Length bits; see
   Section 17.2.  A 0-RTT packet is used to carry "early" data from the
   client to the server as part of the first flight, prior to handshake
   completion.  As part of the TLS handshake, the server can accept or
   reject this early data.

   See Section 2.3 of [TLS13] for a discussion of 0-RTT data and its
   limitations.

   0-RTT Packet {
     Header Form (1) = 1,
     Fixed Bit (1) = 1,
     Long Packet Type (2) = 1,
     Reserved Bits (2),
     Packet Number Length (2),
     Version (32),
     Destination Connection ID Length (8),
     Destination Connection ID (0..160),
     Source Connection ID Length (8),
     Source Connection ID (0..160),
     Length (i),
     Packet Number (8..32),
     Packet Payload (8..),
   }

                          Figure 16: 0-RTT Packet

   Packet numbers for 0-RTT protected packets use the same space as
   1-RTT protected packets.

   After a client receives a Retry packet, 0-RTT packets are likely to
   have been lost or discarded by the server.  A client SHOULD attempt
   to resend data in 0-RTT packets after it sends a new Initial packet.
   New packet numbers MUST be used for any new packets that are sent; as
   described in Section 17.2.5.3, reusing packet numbers could
   compromise packet protection.

   A client only receives acknowledgments for its 0-RTT packets once the
   handshake is complete, as defined in Section 4.1.1 of [QUIC-TLS].

   A client MUST NOT send 0-RTT packets once it starts processing 1-RTT
   packets from the server.  This means that 0-RTT packets cannot
   contain any response to frames from 1-RTT packets.  For instance, a
   client cannot send an ACK frame in a 0-RTT packet, because that can
   only acknowledge a 1-RTT packet.  An acknowledgment for a 1-RTT
   packet MUST be carried in a 1-RTT packet.

   A server SHOULD treat a violation of remembered limits
   (Section 7.4.1) as a connection error of an appropriate type (for
   instance, a FLOW_CONTROL_ERROR for exceeding stream data limits).

17.2.4.  Handshake Packet

   A Handshake packet uses long headers with a type value of 0x02,
   followed by the Length and Packet Number fields; see Section 17.2.
   The first byte contains the Reserved and Packet Number Length bits;
   see Section 17.2.  It is used to carry cryptographic handshake
   messages and acknowledgments from the server and client.

   Handshake Packet {
     Header Form (1) = 1,
     Fixed Bit (1) = 1,
     Long Packet Type (2) = 2,
     Reserved Bits (2),
     Packet Number Length (2),
     Version (32),
     Destination Connection ID Length (8),
     Destination Connection ID (0..160),
     Source Connection ID Length (8),
     Source Connection ID (0..160),
     Length (i),
     Packet Number (8..32),
     Packet Payload (8..),
   }

                   Figure 17: Handshake Protected Packet

   Once a client has received a Handshake packet from a server, it uses
   Handshake packets to send subsequent cryptographic handshake messages
   and acknowledgments to the server.

   The Destination Connection ID field in a Handshake packet contains a
   connection ID that is chosen by the recipient of the packet; the
   Source Connection ID includes the connection ID that the sender of
   the packet wishes to use; see Section 7.2.

   Handshake packets have their own packet number space, and thus the
   first Handshake packet sent by a server contains a packet number of
   0.

   The payload of this packet contains CRYPTO frames and could contain
   PING, PADDING, or ACK frames.  Handshake packets MAY contain
   CONNECTION_CLOSE frames of type 0x1c.  Endpoints MUST treat receipt
   of Handshake packets with other frames as a connection error of type
   PROTOCOL_VIOLATION.

   Like Initial packets (see Section 17.2.2.1), data in CRYPTO frames
   for Handshake packets is discarded -- and no longer retransmitted --
   when Handshake protection keys are discarded.

17.2.5.  Retry Packet

   As shown in Figure 18, a Retry packet uses a long packet header with
   a type value of 0x03.  It carries an address validation token created
   by the server.  It is used by a server that wishes to perform a
   retry; see Section 8.1.

   Retry Packet {
     Header Form (1) = 1,
     Fixed Bit (1) = 1,
     Long Packet Type (2) = 3,
     Unused (4),
     Version (32),
     Destination Connection ID Length (8),
     Destination Connection ID (0..160),
     Source Connection ID Length (8),
     Source Connection ID (0..160),
     Retry Token (..),
     Retry Integrity Tag (128),
   }

                          Figure 18: Retry Packet

   A Retry packet does not contain any protected fields.  The value in
   the Unused field is set to an arbitrary value by the server; a client
   MUST ignore these bits.  In addition to the fields from the long
   header, it contains these additional fields:

   Retry Token:  An opaque token that the server can use to validate the
      client's address.

   Retry Integrity Tag:  Defined in Section 5.8 ("Retry Packet
      Integrity") of [QUIC-TLS].

17.2.5.1.  Sending a Retry Packet

   The server populates the Destination Connection ID with the
   connection ID that the client included in the Source Connection ID of
   the Initial packet.

   The server includes a connection ID of its choice in the Source
   Connection ID field.  This value MUST NOT be equal to the Destination
   Connection ID field of the packet sent by the client.  A client MUST
   discard a Retry packet that contains a Source Connection ID field
   that is identical to the Destination Connection ID field of its
   Initial packet.  The client MUST use the value from the Source
   Connection ID field of the Retry packet in the Destination Connection
   ID field of subsequent packets that it sends.

   A server MAY send Retry packets in response to Initial and 0-RTT
   packets.  A server can either discard or buffer 0-RTT packets that it
   receives.  A server can send multiple Retry packets as it receives
   Initial or 0-RTT packets.  A server MUST NOT send more than one Retry
   packet in response to a single UDP datagram.

17.2.5.2.  Handling a Retry Packet

   A client MUST accept and process at most one Retry packet for each
   connection attempt.  After the client has received and processed an
   Initial or Retry packet from the server, it MUST discard any
   subsequent Retry packets that it receives.

   Clients MUST discard Retry packets that have a Retry Integrity Tag
   that cannot be validated; see Section 5.8 of [QUIC-TLS].  This
   diminishes an attacker's ability to inject a Retry packet and
   protects against accidental corruption of Retry packets.  A client
   MUST discard a Retry packet with a zero-length Retry Token field.

   The client responds to a Retry packet with an Initial packet that
   includes the provided Retry token to continue connection
   establishment.

   A client sets the Destination Connection ID field of this Initial
   packet to the value from the Source Connection ID field in the Retry
   packet.  Changing the Destination Connection ID field also results in
   a change to the keys used to protect the Initial packet.  It also
   sets the Token field to the token provided in the Retry packet.  The
   client MUST NOT change the Source Connection ID because the server
   could include the connection ID as part of its token validation
   logic; see Section 8.1.4.

   A Retry packet does not include a packet number and cannot be
   explicitly acknowledged by a client.

17.2.5.3.  Continuing a Handshake after Retry

   Subsequent Initial packets from the client include the connection ID
   and token values from the Retry packet.  The client copies the Source
   Connection ID field from the Retry packet to the Destination
   Connection ID field and uses this value until an Initial packet with
   an updated value is received; see Section 7.2.  The value of the
   Token field is copied to all subsequent Initial packets; see
   Section 8.1.2.

   Other than updating the Destination Connection ID and Token fields,
   the Initial packet sent by the client is subject to the same
   restrictions as the first Initial packet.  A client MUST use the same
   cryptographic handshake message it included in this packet.  A server
   MAY treat a packet that contains a different cryptographic handshake
   message as a connection error or discard it.  Note that including a
   Token field reduces the available space for the cryptographic
   handshake message, which might result in the client needing to send
   multiple Initial packets.

   A client MAY attempt 0-RTT after receiving a Retry packet by sending
   0-RTT packets to the connection ID provided by the server.

   A client MUST NOT reset the packet number for any packet number space
   after processing a Retry packet.  In particular, 0-RTT packets
   contain confidential information that will most likely be
   retransmitted on receiving a Retry packet.  The keys used to protect
   these new 0-RTT packets will not change as a result of responding to
   a Retry packet.  However, the data sent in these packets could be
   different than what was sent earlier.  Sending these new packets with
   the same packet number is likely to compromise the packet protection
   for those packets because the same key and nonce could be used to
   protect different content.  A server MAY abort the connection if it
   detects that the client reset the packet number.

   The connection IDs used in Initial and Retry packets exchanged
   between client and server are copied to the transport parameters and
   validated as described in Section 7.3.

17.3.  Short Header Packets

   This version of QUIC defines a single packet type that uses the short
   packet header.

17.3.1.  1-RTT Packet

   A 1-RTT packet uses a short packet header.  It is used after the
   version and 1-RTT keys are negotiated.

   1-RTT Packet {
     Header Form (1) = 0,
     Fixed Bit (1) = 1,
     Spin Bit (1),
     Reserved Bits (2),
     Key Phase (1),
     Packet Number Length (2),
     Destination Connection ID (0..160),
     Packet Number (8..32),
     Packet Payload (8..),
   }

                          Figure 19: 1-RTT Packet

   1-RTT packets contain the following fields:

   Header Form:  The most significant bit (0x80) of byte 0 is set to 0
      for the short header.

   Fixed Bit:  The next bit (0x40) of byte 0 is set to 1.  Packets
      containing a zero value for this bit are not valid packets in this
      version and MUST be discarded.  A value of 1 for this bit allows
      QUIC to coexist with other protocols; see [RFC7983].

   Spin Bit:  The third most significant bit (0x20) of byte 0 is the
      latency spin bit, set as described in Section 17.4.

   Reserved Bits:  The next two bits (those with a mask of 0x18) of byte
      0 are reserved.  These bits are protected using header protection;
      see Section 5.4 of [QUIC-TLS].  The value included prior to
      protection MUST be set to 0.  An endpoint MUST treat receipt of a
      packet that has a non-zero value for these bits, after removing
      both packet and header protection, as a connection error of type
      PROTOCOL_VIOLATION.  Discarding such a packet after only removing
      header protection can expose the endpoint to attacks; see
      Section 9.5 of [QUIC-TLS].

   Key Phase:  The next bit (0x04) of byte 0 indicates the key phase,
      which allows a recipient of a packet to identify the packet
      protection keys that are used to protect the packet.  See
      [QUIC-TLS] for details.  This bit is protected using header
      protection; see Section 5.4 of [QUIC-TLS].

   Packet Number Length:  The least significant two bits (those with a
      mask of 0x03) of byte 0 contain the length of the Packet Number
      field, encoded as an unsigned two-bit integer that is one less
      than the length of the Packet Number field in bytes.  That is, the
      length of the Packet Number field is the value of this field plus
      one.  These bits are protected using header protection; see
      Section 5.4 of [QUIC-TLS].

   Destination Connection ID:  The Destination Connection ID is a
      connection ID that is chosen by the intended recipient of the
      packet.  See Section 5.1 for more details.

   Packet Number:  The Packet Number field is 1 to 4 bytes long.  The
      packet number is protected using header protection; see
      Section 5.4 of [QUIC-TLS].  The length of the Packet Number field
      is encoded in Packet Number Length field.  See Section 17.1 for
      details.

   Packet Payload:  1-RTT packets always include a 1-RTT protected
      payload.

   The header form bit and the Destination Connection ID field of a
   short header packet are version independent.  The remaining fields
   are specific to the selected QUIC version.  See [QUIC-INVARIANTS] for
   details on how packets from different versions of QUIC are
   interpreted.

17.4.  Latency Spin Bit

   The latency spin bit, which is defined for 1-RTT packets
   (Section 17.3.1), enables passive latency monitoring from observation
   points on the network path throughout the duration of a connection.
   The server reflects the spin value received, while the client "spins"
   it after one RTT.  On-path observers can measure the time between two
   spin bit toggle events to estimate the end-to-end RTT of a
   connection.

   The spin bit is only present in 1-RTT packets, since it is possible
   to measure the initial RTT of a connection by observing the
   handshake.  Therefore, the spin bit is available after version
   negotiation and connection establishment are completed.  On-path
   measurement and use of the latency spin bit are further discussed in
   [QUIC-MANAGEABILITY].

   The spin bit is an OPTIONAL feature of this version of QUIC.  An
   endpoint that does not support this feature MUST disable it, as
   defined below.

   Each endpoint unilaterally decides if the spin bit is enabled or
   disabled for a connection.  Implementations MUST allow administrators
   of clients and servers to disable the spin bit either globally or on
   a per-connection basis.  Even when the spin bit is not disabled by
   the administrator, endpoints MUST disable their use of the spin bit
   for a random selection of at least one in every 16 network paths, or
   for one in every 16 connection IDs, in order to ensure that QUIC
   connections that disable the spin bit are commonly observed on the
   network.  As each endpoint disables the spin bit independently, this
   ensures that the spin bit signal is disabled on approximately one in
   eight network paths.

   When the spin bit is disabled, endpoints MAY set the spin bit to any
   value and MUST ignore any incoming value.  It is RECOMMENDED that
   endpoints set the spin bit to a random value either chosen
   independently for each packet or chosen independently for each
   connection ID.

   If the spin bit is enabled for the connection, the endpoint maintains
   a spin value for each network path and sets the spin bit in the
   packet header to the currently stored value when a 1-RTT packet is
   sent on that path.  The spin value is initialized to 0 in the
   endpoint for each network path.  Each endpoint also remembers the
   highest packet number seen from its peer on each path.

   When a server receives a 1-RTT packet that increases the highest
   packet number seen by the server from the client on a given network
   path, it sets the spin value for that path to be equal to the spin
   bit in the received packet.

   When a client receives a 1-RTT packet that increases the highest
   packet number seen by the client from the server on a given network
   path, it sets the spin value for that path to the inverse of the spin
   bit in the received packet.

   An endpoint resets the spin value for a network path to 0 when
   changing the connection ID being used on that network path.

18.  Transport Parameter Encoding

   The extension_data field of the quic_transport_parameters extension
   defined in [QUIC-TLS] contains the QUIC transport parameters.  They
   are encoded as a sequence of transport parameters, as shown in
   Figure 20:

   Transport Parameters {
     Transport Parameter (..) ...,
   }

                Figure 20: Sequence of Transport Parameters

   Each transport parameter is encoded as an (identifier, length, value)
   tuple, as shown in Figure 21:

   Transport Parameter {
     Transport Parameter ID (i),
     Transport Parameter Length (i),
     Transport Parameter Value (..),
   }

                  Figure 21: Transport Parameter Encoding

   The Transport Parameter Length field contains the length of the
   Transport Parameter Value field in bytes.

   QUIC encodes transport parameters into a sequence of bytes, which is
   then included in the cryptographic handshake.

18.1.  Reserved Transport Parameters

   Transport parameters with an identifier of the form "31 * N + 27" for
   integer values of N are reserved to exercise the requirement that
   unknown transport parameters be ignored.  These transport parameters
   have no semantics and can carry arbitrary values.

18.2.  Transport Parameter Definitions

   This section details the transport parameters defined in this
   document.

   Many transport parameters listed here have integer values.  Those
   transport parameters that are identified as integers use a variable-
   length integer encoding; see Section 16.  Transport parameters have a
   default value of 0 if the transport parameter is absent, unless
   otherwise stated.

   The following transport parameters are defined:

   original_destination_connection_id (0x00):  This parameter is the
      value of the Destination Connection ID field from the first
      Initial packet sent by the client; see Section 7.3.  This
      transport parameter is only sent by a server.

   max_idle_timeout (0x01):  The maximum idle timeout is a value in
      milliseconds that is encoded as an integer; see (Section 10.1).
      Idle timeout is disabled when both endpoints omit this transport
      parameter or specify a value of 0.

   stateless_reset_token (0x02):  A stateless reset token is used in
      verifying a stateless reset; see Section 10.3.  This parameter is
      a sequence of 16 bytes.  This transport parameter MUST NOT be sent
      by a client but MAY be sent by a server.  A server that does not
      send this transport parameter cannot use stateless reset
      (Section 10.3) for the connection ID negotiated during the
      handshake.

   max_udp_payload_size (0x03):  The maximum UDP payload size parameter
      is an integer value that limits the size of UDP payloads that the
      endpoint is willing to receive.  UDP datagrams with payloads
      larger than this limit are not likely to be processed by the
      receiver.

      The default for this parameter is the maximum permitted UDP
      payload of 65527.  Values below 1200 are invalid.

      This limit does act as an additional constraint on datagram size
      in the same way as the path MTU, but it is a property of the
      endpoint and not the path; see Section 14.  It is expected that
      this is the space an endpoint dedicates to holding incoming
      packets.

   initial_max_data (0x04):  The initial maximum data parameter is an
      integer value that contains the initial value for the maximum
      amount of data that can be sent on the connection.  This is
      equivalent to sending a MAX_DATA (Section 19.9) for the connection
      immediately after completing the handshake.

   initial_max_stream_data_bidi_local (0x05):  This parameter is an
      integer value specifying the initial flow control limit for
      locally initiated bidirectional streams.  This limit applies to
      newly created bidirectional streams opened by the endpoint that
      sends the transport parameter.  In client transport parameters,
      this applies to streams with an identifier with the least
      significant two bits set to 0x00; in server transport parameters,
      this applies to streams with the least significant two bits set to
      0x01.

   initial_max_stream_data_bidi_remote (0x06):  This parameter is an
      integer value specifying the initial flow control limit for peer-
      initiated bidirectional streams.  This limit applies to newly
      created bidirectional streams opened by the endpoint that receives
      the transport parameter.  In client transport parameters, this
      applies to streams with an identifier with the least significant
      two bits set to 0x01; in server transport parameters, this applies
      to streams with the least significant two bits set to 0x00.

   initial_max_stream_data_uni (0x07):  This parameter is an integer
      value specifying the initial flow control limit for unidirectional
      streams.  This limit applies to newly created unidirectional
      streams opened by the endpoint that receives the transport
      parameter.  In client transport parameters, this applies to
      streams with an identifier with the least significant two bits set
      to 0x03; in server transport parameters, this applies to streams
      with the least significant two bits set to 0x02.

   initial_max_streams_bidi (0x08):  The initial maximum bidirectional
      streams parameter is an integer value that contains the initial
      maximum number of bidirectional streams the endpoint that receives
      this transport parameter is permitted to initiate.  If this
      parameter is absent or zero, the peer cannot open bidirectional
      streams until a MAX_STREAMS frame is sent.  Setting this parameter
      is equivalent to sending a MAX_STREAMS (Section 19.11) of the
      corresponding type with the same value.

   initial_max_streams_uni (0x09):  The initial maximum unidirectional
      streams parameter is an integer value that contains the initial
      maximum number of unidirectional streams the endpoint that
      receives this transport parameter is permitted to initiate.  If
      this parameter is absent or zero, the peer cannot open
      unidirectional streams until a MAX_STREAMS frame is sent.  Setting
      this parameter is equivalent to sending a MAX_STREAMS
      (Section 19.11) of the corresponding type with the same value.

   ack_delay_exponent (0x0a):  The acknowledgment delay exponent is an
      integer value indicating an exponent used to decode the ACK Delay
      field in the ACK frame (Section 19.3).  If this value is absent, a
      default value of 3 is assumed (indicating a multiplier of 8).
      Values above 20 are invalid.

   max_ack_delay (0x0b):  The maximum acknowledgment delay is an integer
      value indicating the maximum amount of time in milliseconds by
      which the endpoint will delay sending acknowledgments.  This value
      SHOULD include the receiver's expected delays in alarms firing.
      For example, if a receiver sets a timer for 5ms and alarms
      commonly fire up to 1ms late, then it should send a max_ack_delay
      of 6ms.  If this value is absent, a default of 25 milliseconds is
      assumed.  Values of 2^14 or greater are invalid.

   disable_active_migration (0x0c):  The disable active migration
      transport parameter is included if the endpoint does not support
      active connection migration (Section 9) on the address being used
      during the handshake.  An endpoint that receives this transport
      parameter MUST NOT use a new local address when sending to the
      address that the peer used during the handshake.  This transport
      parameter does not prohibit connection migration after a client
      has acted on a preferred_address transport parameter.  This
      parameter is a zero-length value.

   preferred_address (0x0d):  The server's preferred address is used to
      effect a change in server address at the end of the handshake, as
      described in Section 9.6.  This transport parameter is only sent
      by a server.  Servers MAY choose to only send a preferred address
      of one address family by sending an all-zero address and port
      (0.0.0.0:0 or [::]:0) for the other family.  IP addresses are
      encoded in network byte order.

      The preferred_address transport parameter contains an address and
      port for both IPv4 and IPv6.  The four-byte IPv4 Address field is
      followed by the associated two-byte IPv4 Port field.  This is
      followed by a 16-byte IPv6 Address field and two-byte IPv6 Port
      field.  After address and port pairs, a Connection ID Length field
      describes the length of the following Connection ID field.
      Finally, a 16-byte Stateless Reset Token field includes the
      stateless reset token associated with the connection ID.  The
      format of this transport parameter is shown in Figure 22 below.

      The Connection ID field and the Stateless Reset Token field
      contain an alternative connection ID that has a sequence number of
      1; see Section 5.1.1.  Having these values sent alongside the
      preferred address ensures that there will be at least one unused
      active connection ID when the client initiates migration to the
      preferred address.

      The Connection ID and Stateless Reset Token fields of a preferred
      address are identical in syntax and semantics to the corresponding
      fields of a NEW_CONNECTION_ID frame (Section 19.15).  A server
      that chooses a zero-length connection ID MUST NOT provide a
      preferred address.  Similarly, a server MUST NOT include a zero-
      length connection ID in this transport parameter.  A client MUST
      treat a violation of these requirements as a connection error of
      type TRANSPORT_PARAMETER_ERROR.

   Preferred Address {
     IPv4 Address (32),
     IPv4 Port (16),
     IPv6 Address (128),
     IPv6 Port (16),
     Connection ID Length (8),
     Connection ID (..),
     Stateless Reset Token (128),
   }

                    Figure 22: Preferred Address Format

   active_connection_id_limit (0x0e):  This is an integer value
      specifying the maximum number of connection IDs from the peer that
      an endpoint is willing to store.  This value includes the
      connection ID received during the handshake, that received in the
      preferred_address transport parameter, and those received in
      NEW_CONNECTION_ID frames.  The value of the
      active_connection_id_limit parameter MUST be at least 2.  An
      endpoint that receives a value less than 2 MUST close the
      connection with an error of type TRANSPORT_PARAMETER_ERROR.  If
      this transport parameter is absent, a default of 2 is assumed.  If
      an endpoint issues a zero-length connection ID, it will never send
      a NEW_CONNECTION_ID frame and therefore ignores the
      active_connection_id_limit value received from its peer.

   initial_source_connection_id (0x0f):  This is the value that the
      endpoint included in the Source Connection ID field of the first
      Initial packet it sends for the connection; see Section 7.3.

   retry_source_connection_id (0x10):  This is the value that the server
      included in the Source Connection ID field of a Retry packet; see
      Section 7.3.  This transport parameter is only sent by a server.

   If present, transport parameters that set initial per-stream flow
   control limits (initial_max_stream_data_bidi_local,
   initial_max_stream_data_bidi_remote, and initial_max_stream_data_uni)
   are equivalent to sending a MAX_STREAM_DATA frame (Section 19.10) on
   every stream of the corresponding type immediately after opening.  If
   the transport parameter is absent, streams of that type start with a
   flow control limit of 0.

   A client MUST NOT include any server-only transport parameter:
   original_destination_connection_id, preferred_address,
   retry_source_connection_id, or stateless_reset_token.  A server MUST
   treat receipt of any of these transport parameters as a connection
   error of type TRANSPORT_PARAMETER_ERROR.

19.  Frame Types and Formats

   As described in Section 12.4, packets contain one or more frames.
   This section describes the format and semantics of the core QUIC
   frame types.

19.1.  PADDING Frames

   A PADDING frame (type=0x00) has no semantic value.  PADDING frames
   can be used to increase the size of a packet.  Padding can be used to
   increase an Initial packet to the minimum required size or to provide
   protection against traffic analysis for protected packets.

   PADDING frames are formatted as shown in Figure 23, which shows that
   PADDING frames have no content.  That is, a PADDING frame consists of
   the single byte that identifies the frame as a PADDING frame.

   PADDING Frame {
     Type (i) = 0x00,
   }

                      Figure 23: PADDING Frame Format

19.2.  PING Frames

   Endpoints can use PING frames (type=0x01) to verify that their peers
   are still alive or to check reachability to the peer.

   PING frames are formatted as shown in Figure 24, which shows that
   PING frames have no content.

   PING Frame {
     Type (i) = 0x01,
   }

                        Figure 24: PING Frame Format

   The receiver of a PING frame simply needs to acknowledge the packet
   containing this frame.

   The PING frame can be used to keep a connection alive when an
   application or application protocol wishes to prevent the connection
   from timing out; see Section 10.1.2.

19.3.  ACK Frames

   Receivers send ACK frames (types 0x02 and 0x03) to inform senders of
   packets they have received and processed.  The ACK frame contains one
   or more ACK Ranges.  ACK Ranges identify acknowledged packets.  If
   the frame type is 0x03, ACK frames also contain the cumulative count
   of QUIC packets with associated ECN marks received on the connection
   up until this point.  QUIC implementations MUST properly handle both
   types, and, if they have enabled ECN for packets they send, they
   SHOULD use the information in the ECN section to manage their
   congestion state.

   QUIC acknowledgments are irrevocable.  Once acknowledged, a packet
   remains acknowledged, even if it does not appear in a future ACK
   frame.  This is unlike reneging for TCP Selective Acknowledgments
   (SACKs) [RFC2018].

   Packets from different packet number spaces can be identified using
   the same numeric value.  An acknowledgment for a packet needs to
   indicate both a packet number and a packet number space.  This is
   accomplished by having each ACK frame only acknowledge packet numbers
   in the same space as the packet in which the ACK frame is contained.

   Version Negotiation and Retry packets cannot be acknowledged because
   they do not contain a packet number.  Rather than relying on ACK
   frames, these packets are implicitly acknowledged by the next Initial
   packet sent by the client.

   ACK frames are formatted as shown in Figure 25.

   ACK Frame {
     Type (i) = 0x02..0x03,
     Largest Acknowledged (i),
     ACK Delay (i),
     ACK Range Count (i),
     First ACK Range (i),
     ACK Range (..) ...,
     [ECN Counts (..)],
   }

                        Figure 25: ACK Frame Format

   ACK frames contain the following fields:

   Largest Acknowledged:  A variable-length integer representing the
      largest packet number the peer is acknowledging; this is usually
      the largest packet number that the peer has received prior to
      generating the ACK frame.  Unlike the packet number in the QUIC
      long or short header, the value in an ACK frame is not truncated.

   ACK Delay:  A variable-length integer encoding the acknowledgment
      delay in microseconds; see Section 13.2.5.  It is decoded by
      multiplying the value in the field by 2 to the power of the
      ack_delay_exponent transport parameter sent by the sender of the
      ACK frame; see Section 18.2.  Compared to simply expressing the
      delay as an integer, this encoding allows for a larger range of
      values within the same number of bytes, at the cost of lower
      resolution.

   ACK Range Count:  A variable-length integer specifying the number of
      ACK Range fields in the frame.

   First ACK Range:  A variable-length integer indicating the number of
      contiguous packets preceding the Largest Acknowledged that are
      being acknowledged.  That is, the smallest packet acknowledged in
      the range is determined by subtracting the First ACK Range value
      from the Largest Acknowledged field.

   ACK Ranges:  Contains additional ranges of packets that are
      alternately not acknowledged (Gap) and acknowledged (ACK Range);
      see Section 19.3.1.

   ECN Counts:  The three ECN counts; see Section 19.3.2.

19.3.1.  ACK Ranges

   Each ACK Range consists of alternating Gap and ACK Range Length
   values in descending packet number order.  ACK Ranges can be
   repeated.  The number of Gap and ACK Range Length values is
   determined by the ACK Range Count field; one of each value is present
   for each value in the ACK Range Count field.

   ACK Ranges are structured as shown in Figure 26.

   ACK Range {
     Gap (i),
     ACK Range Length (i),
   }

                           Figure 26: ACK Ranges

   The fields that form each ACK Range are:

   Gap:  A variable-length integer indicating the number of contiguous
      unacknowledged packets preceding the packet number one lower than
      the smallest in the preceding ACK Range.

   ACK Range Length:  A variable-length integer indicating the number of
      contiguous acknowledged packets preceding the largest packet
      number, as determined by the preceding Gap.

   Gap and ACK Range Length values use a relative integer encoding for
   efficiency.  Though each encoded value is positive, the values are
   subtracted, so that each ACK Range describes progressively lower-
   numbered packets.

   Each ACK Range acknowledges a contiguous range of packets by
   indicating the number of acknowledged packets that precede the
   largest packet number in that range.  A value of 0 indicates that
   only the largest packet number is acknowledged.  Larger ACK Range
   values indicate a larger range, with corresponding lower values for
   the smallest packet number in the range.  Thus, given a largest
   packet number for the range, the smallest value is determined by the
   following formula:

      smallest = largest - ack_range

   An ACK Range acknowledges all packets between the smallest packet
   number and the largest, inclusive.

   The largest value for an ACK Range is determined by cumulatively
   subtracting the size of all preceding ACK Range Lengths and Gaps.

   Each Gap indicates a range of packets that are not being
   acknowledged.  The number of packets in the gap is one higher than
   the encoded value of the Gap field.

   The value of the Gap field establishes the largest packet number
   value for the subsequent ACK Range using the following formula:

      largest = previous_smallest - gap - 2

   If any computed packet number is negative, an endpoint MUST generate
   a connection error of type FRAME_ENCODING_ERROR.

19.3.2.  ECN Counts

   The ACK frame uses the least significant bit of the type value (that
   is, type 0x03) to indicate ECN feedback and report receipt of QUIC
   packets with associated ECN codepoints of ECT(0), ECT(1), or ECN-CE
   in the packet's IP header.  ECN counts are only present when the ACK
   frame type is 0x03.

   When present, there are three ECN counts, as shown in Figure 27.

   ECN Counts {
     ECT0 Count (i),
     ECT1 Count (i),
     ECN-CE Count (i),
   }

                        Figure 27: ECN Count Format

   The ECN count fields are:

   ECT0 Count:  A variable-length integer representing the total number
      of packets received with the ECT(0) codepoint in the packet number
      space of the ACK frame.

   ECT1 Count:  A variable-length integer representing the total number
      of packets received with the ECT(1) codepoint in the packet number
      space of the ACK frame.

   ECN-CE Count:  A variable-length integer representing the total
      number of packets received with the ECN-CE codepoint in the packet
      number space of the ACK frame.

   ECN counts are maintained separately for each packet number space.

19.4.  RESET_STREAM Frames

   An endpoint uses a RESET_STREAM frame (type=0x04) to abruptly
   terminate the sending part of a stream.

   After sending a RESET_STREAM, an endpoint ceases transmission and
   retransmission of STREAM frames on the identified stream.  A receiver
   of RESET_STREAM can discard any data that it already received on that
   stream.

   An endpoint that receives a RESET_STREAM frame for a send-only stream
   MUST terminate the connection with error STREAM_STATE_ERROR.

   RESET_STREAM frames are formatted as shown in Figure 28.

   RESET_STREAM Frame {
     Type (i) = 0x04,
     Stream ID (i),
     Application Protocol Error Code (i),
     Final Size (i),
   }

                    Figure 28: RESET_STREAM Frame Format

   RESET_STREAM frames contain the following fields:

   Stream ID:  A variable-length integer encoding of the stream ID of
      the stream being terminated.

   Application Protocol Error Code:  A variable-length integer
      containing the application protocol error code (see Section 20.2)
      that indicates why the stream is being closed.

   Final Size:  A variable-length integer indicating the final size of
      the stream by the RESET_STREAM sender, in units of bytes; see
      Section 4.5.

19.5.  STOP_SENDING Frames

   An endpoint uses a STOP_SENDING frame (type=0x05) to communicate that
   incoming data is being discarded on receipt per application request.
   STOP_SENDING requests that a peer cease transmission on a stream.

   A STOP_SENDING frame can be sent for streams in the "Recv" or "Size
   Known" states; see Section 3.2.  Receiving a STOP_SENDING frame for a
   locally initiated stream that has not yet been created MUST be
   treated as a connection error of type STREAM_STATE_ERROR.  An
   endpoint that receives a STOP_SENDING frame for a receive-only stream
   MUST terminate the connection with error STREAM_STATE_ERROR.

   STOP_SENDING frames are formatted as shown in Figure 29.

   STOP_SENDING Frame {
     Type (i) = 0x05,
     Stream ID (i),
     Application Protocol Error Code (i),
   }

                    Figure 29: STOP_SENDING Frame Format

   STOP_SENDING frames contain the following fields:

   Stream ID:  A variable-length integer carrying the stream ID of the
      stream being ignored.

   Application Protocol Error Code:  A variable-length integer
      containing the application-specified reason the sender is ignoring
      the stream; see Section 20.2.

19.6.  CRYPTO Frames

   A CRYPTO frame (type=0x06) is used to transmit cryptographic
   handshake messages.  It can be sent in all packet types except 0-RTT.
   The CRYPTO frame offers the cryptographic protocol an in-order stream
   of bytes.  CRYPTO frames are functionally identical to STREAM frames,
   except that they do not bear a stream identifier; they are not flow
   controlled; and they do not carry markers for optional offset,
   optional length, and the end of the stream.

   CRYPTO frames are formatted as shown in Figure 30.

   CRYPTO Frame {
     Type (i) = 0x06,
     Offset (i),
     Length (i),
     Crypto Data (..),
   }

                       Figure 30: CRYPTO Frame Format

   CRYPTO frames contain the following fields:

   Offset:  A variable-length integer specifying the byte offset in the
      stream for the data in this CRYPTO frame.

   Length:  A variable-length integer specifying the length of the
      Crypto Data field in this CRYPTO frame.

   Crypto Data:  The cryptographic message data.

   There is a separate flow of cryptographic handshake data in each
   encryption level, each of which starts at an offset of 0.  This
   implies that each encryption level is treated as a separate CRYPTO
   stream of data.

   The largest offset delivered on a stream -- the sum of the offset and
   data length -- cannot exceed 2^62-1.  Receipt of a frame that exceeds
   this limit MUST be treated as a connection error of type
   FRAME_ENCODING_ERROR or CRYPTO_BUFFER_EXCEEDED.

   Unlike STREAM frames, which include a stream ID indicating to which
   stream the data belongs, the CRYPTO frame carries data for a single
   stream per encryption level.  The stream does not have an explicit
   end, so CRYPTO frames do not have a FIN bit.

19.7.  NEW_TOKEN Frames

   A server sends a NEW_TOKEN frame (type=0x07) to provide the client
   with a token to send in the header of an Initial packet for a future
   connection.

   NEW_TOKEN frames are formatted as shown in Figure 31.

   NEW_TOKEN Frame {
     Type (i) = 0x07,
     Token Length (i),
     Token (..),
   }

                     Figure 31: NEW_TOKEN Frame Format

   NEW_TOKEN frames contain the following fields:

   Token Length:  A variable-length integer specifying the length of the
      token in bytes.

   Token:  An opaque blob that the client can use with a future Initial
      packet.  The token MUST NOT be empty.  A client MUST treat receipt
      of a NEW_TOKEN frame with an empty Token field as a connection
      error of type FRAME_ENCODING_ERROR.

   A client might receive multiple NEW_TOKEN frames that contain the
   same token value if packets containing the frame are incorrectly
   determined to be lost.  Clients are responsible for discarding
   duplicate values, which might be used to link connection attempts;
   see Section 8.1.3.

   Clients MUST NOT send NEW_TOKEN frames.  A server MUST treat receipt
   of a NEW_TOKEN frame as a connection error of type
   PROTOCOL_VIOLATION.

19.8.  STREAM Frames

   STREAM frames implicitly create a stream and carry stream data.  The
   Type field in the STREAM frame takes the form 0b00001XXX (or the set
   of values from 0x08 to 0x0f).  The three low-order bits of the frame
   type determine the fields that are present in the frame:

   *  The OFF bit (0x04) in the frame type is set to indicate that there
      is an Offset field present.  When set to 1, the Offset field is
      present.  When set to 0, the Offset field is absent and the Stream
      Data starts at an offset of 0 (that is, the frame contains the
      first bytes of the stream, or the end of a stream that includes no
      data).

   *  The LEN bit (0x02) in the frame type is set to indicate that there
      is a Length field present.  If this bit is set to 0, the Length
      field is absent and the Stream Data field extends to the end of
      the packet.  If this bit is set to 1, the Length field is present.

   *  The FIN bit (0x01) indicates that the frame marks the end of the
      stream.  The final size of the stream is the sum of the offset and
      the length of this frame.

   An endpoint MUST terminate the connection with error
   STREAM_STATE_ERROR if it receives a STREAM frame for a locally
   initiated stream that has not yet been created, or for a send-only
   stream.

   STREAM frames are formatted as shown in Figure 32.

   STREAM Frame {
     Type (i) = 0x08..0x0f,
     Stream ID (i),
     [Offset (i)],
     [Length (i)],
     Stream Data (..),
   }

                       Figure 32: STREAM Frame Format

   STREAM frames contain the following fields:

   Stream ID:  A variable-length integer indicating the stream ID of the
      stream; see Section 2.1.

   Offset:  A variable-length integer specifying the byte offset in the
      stream for the data in this STREAM frame.  This field is present
      when the OFF bit is set to 1.  When the Offset field is absent,
      the offset is 0.

   Length:  A variable-length integer specifying the length of the
      Stream Data field in this STREAM frame.  This field is present
      when the LEN bit is set to 1.  When the LEN bit is set to 0, the
      Stream Data field consumes all the remaining bytes in the packet.

   Stream Data:  The bytes from the designated stream to be delivered.

   When a Stream Data field has a length of 0, the offset in the STREAM
   frame is the offset of the next byte that would be sent.

   The first byte in the stream has an offset of 0.  The largest offset
   delivered on a stream -- the sum of the offset and data length --
   cannot exceed 2^62-1, as it is not possible to provide flow control
   credit for that data.  Receipt of a frame that exceeds this limit
   MUST be treated as a connection error of type FRAME_ENCODING_ERROR or
   FLOW_CONTROL_ERROR.

19.9.  MAX_DATA Frames

   A MAX_DATA frame (type=0x10) is used in flow control to inform the
   peer of the maximum amount of data that can be sent on the connection
   as a whole.

   MAX_DATA frames are formatted as shown in Figure 33.

   MAX_DATA Frame {
     Type (i) = 0x10,
     Maximum Data (i),
   }

                      Figure 33: MAX_DATA Frame Format

   MAX_DATA frames contain the following field:

   Maximum Data:  A variable-length integer indicating the maximum
      amount of data that can be sent on the entire connection, in units
      of bytes.

   All data sent in STREAM frames counts toward this limit.  The sum of
   the final sizes on all streams -- including streams in terminal
   states -- MUST NOT exceed the value advertised by a receiver.  An
   endpoint MUST terminate a connection with an error of type
   FLOW_CONTROL_ERROR if it receives more data than the maximum data
   value that it has sent.  This includes violations of remembered
   limits in Early Data; see Section 7.4.1.

19.10.  MAX_STREAM_DATA Frames

   A MAX_STREAM_DATA frame (type=0x11) is used in flow control to inform
   a peer of the maximum amount of data that can be sent on a stream.

   A MAX_STREAM_DATA frame can be sent for streams in the "Recv" state;
   see Section 3.2.  Receiving a MAX_STREAM_DATA frame for a locally
   initiated stream that has not yet been created MUST be treated as a
   connection error of type STREAM_STATE_ERROR.  An endpoint that
   receives a MAX_STREAM_DATA frame for a receive-only stream MUST
   terminate the connection with error STREAM_STATE_ERROR.

   MAX_STREAM_DATA frames are formatted as shown in Figure 34.

   MAX_STREAM_DATA Frame {
     Type (i) = 0x11,
     Stream ID (i),
     Maximum Stream Data (i),
   }

                  Figure 34: MAX_STREAM_DATA Frame Format

   MAX_STREAM_DATA frames contain the following fields:

   Stream ID:  The stream ID of the affected stream, encoded as a
      variable-length integer.

   Maximum Stream Data:  A variable-length integer indicating the
      maximum amount of data that can be sent on the identified stream,
      in units of bytes.

   When counting data toward this limit, an endpoint accounts for the
   largest received offset of data that is sent or received on the
   stream.  Loss or reordering can mean that the largest received offset
   on a stream can be greater than the total size of data received on
   that stream.  Receiving STREAM frames might not increase the largest
   received offset.

   The data sent on a stream MUST NOT exceed the largest maximum stream
   data value advertised by the receiver.  An endpoint MUST terminate a
   connection with an error of type FLOW_CONTROL_ERROR if it receives
   more data than the largest maximum stream data that it has sent for
   the affected stream.  This includes violations of remembered limits
   in Early Data; see Section 7.4.1.

19.11.  MAX_STREAMS Frames

   A MAX_STREAMS frame (type=0x12 or 0x13) informs the peer of the
   cumulative number of streams of a given type it is permitted to open.
   A MAX_STREAMS frame with a type of 0x12 applies to bidirectional
   streams, and a MAX_STREAMS frame with a type of 0x13 applies to
   unidirectional streams.

   MAX_STREAMS frames are formatted as shown in Figure 35.

   MAX_STREAMS Frame {
     Type (i) = 0x12..0x13,
     Maximum Streams (i),
   }

                    Figure 35: MAX_STREAMS Frame Format

   MAX_STREAMS frames contain the following field:

   Maximum Streams:  A count of the cumulative number of streams of the
      corresponding type that can be opened over the lifetime of the
      connection.  This value cannot exceed 2^60, as it is not possible
      to encode stream IDs larger than 2^62-1.  Receipt of a frame that
      permits opening of a stream larger than this limit MUST be treated
      as a connection error of type FRAME_ENCODING_ERROR.

   Loss or reordering can cause an endpoint to receive a MAX_STREAMS
   frame with a lower stream limit than was previously received.
   MAX_STREAMS frames that do not increase the stream limit MUST be
   ignored.

   An endpoint MUST NOT open more streams than permitted by the current
   stream limit set by its peer.  For instance, a server that receives a
   unidirectional stream limit of 3 is permitted to open streams 3, 7,
   and 11, but not stream 15.  An endpoint MUST terminate a connection
   with an error of type STREAM_LIMIT_ERROR if a peer opens more streams
   than was permitted.  This includes violations of remembered limits in
   Early Data; see Section 7.4.1.

   Note that these frames (and the corresponding transport parameters)
   do not describe the number of streams that can be opened
   concurrently.  The limit includes streams that have been closed as
   well as those that are open.

19.12.  DATA_BLOCKED Frames

   A sender SHOULD send a DATA_BLOCKED frame (type=0x14) when it wishes
   to send data but is unable to do so due to connection-level flow
   control; see Section 4.  DATA_BLOCKED frames can be used as input to
   tuning of flow control algorithms; see Section 4.2.

   DATA_BLOCKED frames are formatted as shown in Figure 36.

   DATA_BLOCKED Frame {
     Type (i) = 0x14,
     Maximum Data (i),
   }

                    Figure 36: DATA_BLOCKED Frame Format

   DATA_BLOCKED frames contain the following field:

   Maximum Data:  A variable-length integer indicating the connection-
      level limit at which blocking occurred.

19.13.  STREAM_DATA_BLOCKED Frames

   A sender SHOULD send a STREAM_DATA_BLOCKED frame (type=0x15) when it
   wishes to send data but is unable to do so due to stream-level flow
   control.  This frame is analogous to DATA_BLOCKED (Section 19.12).

   An endpoint that receives a STREAM_DATA_BLOCKED frame for a send-only
   stream MUST terminate the connection with error STREAM_STATE_ERROR.

   STREAM_DATA_BLOCKED frames are formatted as shown in Figure 37.

   STREAM_DATA_BLOCKED Frame {
     Type (i) = 0x15,
     Stream ID (i),
     Maximum Stream Data (i),
   }

                Figure 37: STREAM_DATA_BLOCKED Frame Format

   STREAM_DATA_BLOCKED frames contain the following fields:

   Stream ID:  A variable-length integer indicating the stream that is
      blocked due to flow control.

   Maximum Stream Data:  A variable-length integer indicating the offset
      of the stream at which the blocking occurred.

19.14.  STREAMS_BLOCKED Frames

   A sender SHOULD send a STREAMS_BLOCKED frame (type=0x16 or 0x17) when
   it wishes to open a stream but is unable to do so due to the maximum
   stream limit set by its peer; see Section 19.11.  A STREAMS_BLOCKED
   frame of type 0x16 is used to indicate reaching the bidirectional
   stream limit, and a STREAMS_BLOCKED frame of type 0x17 is used to
   indicate reaching the unidirectional stream limit.

   A STREAMS_BLOCKED frame does not open the stream, but informs the
   peer that a new stream was needed and the stream limit prevented the
   creation of the stream.

   STREAMS_BLOCKED frames are formatted as shown in Figure 38.

   STREAMS_BLOCKED Frame {
     Type (i) = 0x16..0x17,
     Maximum Streams (i),
   }

                  Figure 38: STREAMS_BLOCKED Frame Format

   STREAMS_BLOCKED frames contain the following field:

   Maximum Streams:  A variable-length integer indicating the maximum
      number of streams allowed at the time the frame was sent.  This
      value cannot exceed 2^60, as it is not possible to encode stream
      IDs larger than 2^62-1.  Receipt of a frame that encodes a larger
      stream ID MUST be treated as a connection error of type
      STREAM_LIMIT_ERROR or FRAME_ENCODING_ERROR.

19.15.  NEW_CONNECTION_ID Frames

   An endpoint sends a NEW_CONNECTION_ID frame (type=0x18) to provide
   its peer with alternative connection IDs that can be used to break
   linkability when migrating connections; see Section 9.5.

   NEW_CONNECTION_ID frames are formatted as shown in Figure 39.

   NEW_CONNECTION_ID Frame {
     Type (i) = 0x18,
     Sequence Number (i),
     Retire Prior To (i),
     Length (8),
     Connection ID (8..160),
     Stateless Reset Token (128),
   }

                 Figure 39: NEW_CONNECTION_ID Frame Format

   NEW_CONNECTION_ID frames contain the following fields:

   Sequence Number:  The sequence number assigned to the connection ID
      by the sender, encoded as a variable-length integer; see
      Section 5.1.1.

   Retire Prior To:  A variable-length integer indicating which
      connection IDs should be retired; see Section 5.1.2.

   Length:  An 8-bit unsigned integer containing the length of the
      connection ID.  Values less than 1 and greater than 20 are invalid
      and MUST be treated as a connection error of type
      FRAME_ENCODING_ERROR.

   Connection ID:  A connection ID of the specified length.

   Stateless Reset Token:  A 128-bit value that will be used for a
      stateless reset when the associated connection ID is used; see
      Section 10.3.

   An endpoint MUST NOT send this frame if it currently requires that
   its peer send packets with a zero-length Destination Connection ID.
   Changing the length of a connection ID to or from zero length makes
   it difficult to identify when the value of the connection ID changed.
   An endpoint that is sending packets with a zero-length Destination
   Connection ID MUST treat receipt of a NEW_CONNECTION_ID frame as a
   connection error of type PROTOCOL_VIOLATION.

   Transmission errors, timeouts, and retransmissions might cause the
   same NEW_CONNECTION_ID frame to be received multiple times.  Receipt
   of the same frame multiple times MUST NOT be treated as a connection
   error.  A receiver can use the sequence number supplied in the
   NEW_CONNECTION_ID frame to handle receiving the same
   NEW_CONNECTION_ID frame multiple times.

   If an endpoint receives a NEW_CONNECTION_ID frame that repeats a
   previously issued connection ID with a different Stateless Reset
   Token field value or a different Sequence Number field value, or if a
   sequence number is used for different connection IDs, the endpoint
   MAY treat that receipt as a connection error of type
   PROTOCOL_VIOLATION.

   The Retire Prior To field applies to connection IDs established
   during connection setup and the preferred_address transport
   parameter; see Section 5.1.2.  The value in the Retire Prior To field
   MUST be less than or equal to the value in the Sequence Number field.
   Receiving a value in the Retire Prior To field that is greater than
   that in the Sequence Number field MUST be treated as a connection
   error of type FRAME_ENCODING_ERROR.

   Once a sender indicates a Retire Prior To value, smaller values sent
   in subsequent NEW_CONNECTION_ID frames have no effect.  A receiver
   MUST ignore any Retire Prior To fields that do not increase the
   largest received Retire Prior To value.

   An endpoint that receives a NEW_CONNECTION_ID frame with a sequence
   number smaller than the Retire Prior To field of a previously
   received NEW_CONNECTION_ID frame MUST send a corresponding
   RETIRE_CONNECTION_ID frame that retires the newly received connection
   ID, unless it has already done so for that sequence number.

19.16.  RETIRE_CONNECTION_ID Frames

   An endpoint sends a RETIRE_CONNECTION_ID frame (type=0x19) to
   indicate that it will no longer use a connection ID that was issued
   by its peer.  This includes the connection ID provided during the
   handshake.  Sending a RETIRE_CONNECTION_ID frame also serves as a
   request to the peer to send additional connection IDs for future use;
   see Section 5.1.  New connection IDs can be delivered to a peer using
   the NEW_CONNECTION_ID frame (Section 19.15).

   Retiring a connection ID invalidates the stateless reset token
   associated with that connection ID.

   RETIRE_CONNECTION_ID frames are formatted as shown in Figure 40.

   RETIRE_CONNECTION_ID Frame {
     Type (i) = 0x19,
     Sequence Number (i),
   }

                Figure 40: RETIRE_CONNECTION_ID Frame Format

   RETIRE_CONNECTION_ID frames contain the following field:

   Sequence Number:  The sequence number of the connection ID being
      retired; see Section 5.1.2.

   Receipt of a RETIRE_CONNECTION_ID frame containing a sequence number
   greater than any previously sent to the peer MUST be treated as a
   connection error of type PROTOCOL_VIOLATION.

   The sequence number specified in a RETIRE_CONNECTION_ID frame MUST
   NOT refer to the Destination Connection ID field of the packet in
   which the frame is contained.  The peer MAY treat this as a
   connection error of type PROTOCOL_VIOLATION.

   An endpoint cannot send this frame if it was provided with a zero-
   length connection ID by its peer.  An endpoint that provides a zero-
   length connection ID MUST treat receipt of a RETIRE_CONNECTION_ID
   frame as a connection error of type PROTOCOL_VIOLATION.

19.17.  PATH_CHALLENGE Frames

   Endpoints can use PATH_CHALLENGE frames (type=0x1a) to check
   reachability to the peer and for path validation during connection
   migration.

   PATH_CHALLENGE frames are formatted as shown in Figure 41.

   PATH_CHALLENGE Frame {
     Type (i) = 0x1a,
     Data (64),
   }

                   Figure 41: PATH_CHALLENGE Frame Format

   PATH_CHALLENGE frames contain the following field:

   Data:  This 8-byte field contains arbitrary data.

   Including 64 bits of entropy in a PATH_CHALLENGE frame ensures that
   it is easier to receive the packet than it is to guess the value
   correctly.

   The recipient of this frame MUST generate a PATH_RESPONSE frame
   (Section 19.18) containing the same Data value.

19.18.  PATH_RESPONSE Frames

   A PATH_RESPONSE frame (type=0x1b) is sent in response to a
   PATH_CHALLENGE frame.

   PATH_RESPONSE frames are formatted as shown in Figure 42.  The format
   of a PATH_RESPONSE frame is identical to that of the PATH_CHALLENGE
   frame; see Section 19.17.

   PATH_RESPONSE Frame {
     Type (i) = 0x1b,
     Data (64),
   }

                   Figure 42: PATH_RESPONSE Frame Format

   If the content of a PATH_RESPONSE frame does not match the content of
   a PATH_CHALLENGE frame previously sent by the endpoint, the endpoint
   MAY generate a connection error of type PROTOCOL_VIOLATION.

19.19.  CONNECTION_CLOSE Frames

   An endpoint sends a CONNECTION_CLOSE frame (type=0x1c or 0x1d) to
   notify its peer that the connection is being closed.  The
   CONNECTION_CLOSE frame with a type of 0x1c is used to signal errors
   at only the QUIC layer, or the absence of errors (with the NO_ERROR
   code).  The CONNECTION_CLOSE frame with a type of 0x1d is used to
   signal an error with the application that uses QUIC.

   If there are open streams that have not been explicitly closed, they
   are implicitly closed when the connection is closed.

   CONNECTION_CLOSE frames are formatted as shown in Figure 43.

   CONNECTION_CLOSE Frame {
     Type (i) = 0x1c..0x1d,
     Error Code (i),
     [Frame Type (i)],
     Reason Phrase Length (i),
     Reason Phrase (..),
   }

                  Figure 43: CONNECTION_CLOSE Frame Format

   CONNECTION_CLOSE frames contain the following fields:

   Error Code:  A variable-length integer that indicates the reason for
      closing this connection.  A CONNECTION_CLOSE frame of type 0x1c
      uses codes from the space defined in Section 20.1.  A
      CONNECTION_CLOSE frame of type 0x1d uses codes defined by the
      application protocol; see Section 20.2.

   Frame Type:  A variable-length integer encoding the type of frame
      that triggered the error.  A value of 0 (equivalent to the mention
      of the PADDING frame) is used when the frame type is unknown.  The
      application-specific variant of CONNECTION_CLOSE (type 0x1d) does
      not include this field.

   Reason Phrase Length:  A variable-length integer specifying the
      length of the reason phrase in bytes.  Because a CONNECTION_CLOSE
      frame cannot be split between packets, any limits on packet size
      will also limit the space available for a reason phrase.

   Reason Phrase:  Additional diagnostic information for the closure.
      This can be zero length if the sender chooses not to give details
      beyond the Error Code value.  This SHOULD be a UTF-8 encoded
      string [RFC3629], though the frame does not carry information,
      such as language tags, that would aid comprehension by any entity
      other than the one that created the text.

   The application-specific variant of CONNECTION_CLOSE (type 0x1d) can
   only be sent using 0-RTT or 1-RTT packets; see Section 12.5.  When an
   application wishes to abandon a connection during the handshake, an
   endpoint can send a CONNECTION_CLOSE frame (type 0x1c) with an error
   code of APPLICATION_ERROR in an Initial or Handshake packet.

19.20.  HANDSHAKE_DONE Frames

   The server uses a HANDSHAKE_DONE frame (type=0x1e) to signal
   confirmation of the handshake to the client.

   HANDSHAKE_DONE frames are formatted as shown in Figure 44, which
   shows that HANDSHAKE_DONE frames have no content.

   HANDSHAKE_DONE Frame {
     Type (i) = 0x1e,
   }

                   Figure 44: HANDSHAKE_DONE Frame Format

   A HANDSHAKE_DONE frame can only be sent by the server.  Servers MUST
   NOT send a HANDSHAKE_DONE frame before completing the handshake.  A
   server MUST treat receipt of a HANDSHAKE_DONE frame as a connection
   error of type PROTOCOL_VIOLATION.

19.21.  Extension Frames

   QUIC frames do not use a self-describing encoding.  An endpoint
   therefore needs to understand the syntax of all frames before it can
   successfully process a packet.  This allows for efficient encoding of
   frames, but it means that an endpoint cannot send a frame of a type
   that is unknown to its peer.

   An extension to QUIC that wishes to use a new type of frame MUST
   first ensure that a peer is able to understand the frame.  An
   endpoint can use a transport parameter to signal its willingness to
   receive extension frame types.  One transport parameter can indicate
   support for one or more extension frame types.

   Extensions that modify or replace core protocol functionality
   (including frame types) will be difficult to combine with other
   extensions that modify or replace the same functionality unless the
   behavior of the combination is explicitly defined.  Such extensions
   SHOULD define their interaction with previously defined extensions
   modifying the same protocol components.

   Extension frames MUST be congestion controlled and MUST cause an ACK
   frame to be sent.  The exception is extension frames that replace or
   supplement the ACK frame.  Extension frames are not included in flow
   control unless specified in the extension.

   An IANA registry is used to manage the assignment of frame types; see
   Section 22.4.

20.  Error Codes

   QUIC transport error codes and application error codes are 62-bit
   unsigned integers.

20.1.  Transport Error Codes

   This section lists the defined QUIC transport error codes that can be
   used in a CONNECTION_CLOSE frame with a type of 0x1c.  These errors
   apply to the entire connection.

   NO_ERROR (0x00):  An endpoint uses this with CONNECTION_CLOSE to
      signal that the connection is being closed abruptly in the absence
      of any error.

   INTERNAL_ERROR (0x01):  The endpoint encountered an internal error
      and cannot continue with the connection.

   CONNECTION_REFUSED (0x02):  The server refused to accept a new
      connection.

   FLOW_CONTROL_ERROR (0x03):  An endpoint received more data than it
      permitted in its advertised data limits; see Section 4.

   STREAM_LIMIT_ERROR (0x04):  An endpoint received a frame for a stream
      identifier that exceeded its advertised stream limit for the
      corresponding stream type.

   STREAM_STATE_ERROR (0x05):  An endpoint received a frame for a stream
      that was not in a state that permitted that frame; see Section 3.

   FINAL_SIZE_ERROR (0x06):  (1) An endpoint received a STREAM frame
      containing data that exceeded the previously established final
      size, (2) an endpoint received a STREAM frame or a RESET_STREAM
      frame containing a final size that was lower than the size of
      stream data that was already received, or (3) an endpoint received
      a STREAM frame or a RESET_STREAM frame containing a different
      final size to the one already established.

   FRAME_ENCODING_ERROR (0x07):  An endpoint received a frame that was
      badly formatted -- for instance, a frame of an unknown type or an
      ACK frame that has more acknowledgment ranges than the remainder
      of the packet could carry.

   TRANSPORT_PARAMETER_ERROR (0x08):  An endpoint received transport
      parameters that were badly formatted, included an invalid value,
      omitted a mandatory transport parameter, included a forbidden
      transport parameter, or were otherwise in error.

   CONNECTION_ID_LIMIT_ERROR (0x09):  The number of connection IDs
      provided by the peer exceeds the advertised
      active_connection_id_limit.

   PROTOCOL_VIOLATION (0x0a):  An endpoint detected an error with
      protocol compliance that was not covered by more specific error
      codes.

   INVALID_TOKEN (0x0b):  A server received a client Initial that
      contained an invalid Token field.

   APPLICATION_ERROR (0x0c):  The application or application protocol
      caused the connection to be closed.

   CRYPTO_BUFFER_EXCEEDED (0x0d):  An endpoint has received more data in
      CRYPTO frames than it can buffer.

   KEY_UPDATE_ERROR (0x0e):  An endpoint detected errors in performing
      key updates; see Section 6 of [QUIC-TLS].

   AEAD_LIMIT_REACHED (0x0f):  An endpoint has reached the
      confidentiality or integrity limit for the AEAD algorithm used by
      the given connection.

   NO_VIABLE_PATH (0x10):  An endpoint has determined that the network
      path is incapable of supporting QUIC.  An endpoint is unlikely to
      receive a CONNECTION_CLOSE frame carrying this code except when
      the path does not support a large enough MTU.

   CRYPTO_ERROR (0x0100-0x01ff):  The cryptographic handshake failed.  A
      range of 256 values is reserved for carrying error codes specific
      to the cryptographic handshake that is used.  Codes for errors
      occurring when TLS is used for the cryptographic handshake are
      described in Section 4.8 of [QUIC-TLS].

   See Section 22.5 for details on registering new error codes.

   In defining these error codes, several principles are applied.  Error
   conditions that might require specific action on the part of a
   recipient are given unique codes.  Errors that represent common
   conditions are given specific codes.  Absent either of these
   conditions, error codes are used to identify a general function of
   the stack, like flow control or transport parameter handling.
   Finally, generic errors are provided for conditions where
   implementations are unable or unwilling to use more specific codes.

20.2.  Application Protocol Error Codes

   The management of application error codes is left to application
   protocols.  Application protocol error codes are used for the
   RESET_STREAM frame (Section 19.4), the STOP_SENDING frame
   (Section 19.5), and the CONNECTION_CLOSE frame with a type of 0x1d
   (Section 19.19).

21.  Security Considerations

   The goal of QUIC is to provide a secure transport connection.
   Section 21.1 provides an overview of those properties; subsequent
   sections discuss constraints and caveats regarding these properties,
   including descriptions of known attacks and countermeasures.

21.1.  Overview of Security Properties

   A complete security analysis of QUIC is outside the scope of this
   document.  This section provides an informal description of the
   desired security properties as an aid to implementers and to help
   guide protocol analysis.

   QUIC assumes the threat model described in [SEC-CONS] and provides
   protections against many of the attacks that arise from that model.

   For this purpose, attacks are divided into passive and active
   attacks.  Passive attackers have the ability to read packets from the
   network, while active attackers also have the ability to write
   packets into the network.  However, a passive attack could involve an
   attacker with the ability to cause a routing change or other
   modification in the path taken by packets that comprise a connection.

   Attackers are additionally categorized as either on-path attackers or
   off-path attackers.  An on-path attacker can read, modify, or remove
   any packet it observes such that the packet no longer reaches its
   destination, while an off-path attacker observes the packets but
   cannot prevent the original packet from reaching its intended
   destination.  Both types of attackers can also transmit arbitrary
   packets.  This definition differs from that of Section 3.5 of
   [SEC-CONS] in that an off-path attacker is able to observe packets.

   Properties of the handshake, protected packets, and connection
   migration are considered separately.

21.1.1.  Handshake

   The QUIC handshake incorporates the TLS 1.3 handshake and inherits
   the cryptographic properties described in Appendix E.1 of [TLS13].
   Many of the security properties of QUIC depend on the TLS handshake
   providing these properties.  Any attack on the TLS handshake could
   affect QUIC.

   Any attack on the TLS handshake that compromises the secrecy or
   uniqueness of session keys, or the authentication of the
   participating peers, affects other security guarantees provided by
   QUIC that depend on those keys.  For instance, migration (Section 9)
   depends on the efficacy of confidentiality protections, both for the
   negotiation of keys using the TLS handshake and for QUIC packet
   protection, to avoid linkability across network paths.

   An attack on the integrity of the TLS handshake might allow an
   attacker to affect the selection of application protocol or QUIC
   version.

   In addition to the properties provided by TLS, the QUIC handshake
   provides some defense against DoS attacks on the handshake.

21.1.1.1.  Anti-Amplification

   Address validation (Section 8) is used to verify that an entity that
   claims a given address is able to receive packets at that address.
   Address validation limits amplification attack targets to addresses
   for which an attacker can observe packets.

   Prior to address validation, endpoints are limited in what they are
   able to send.  Endpoints cannot send data toward an unvalidated
   address in excess of three times the data received from that address.

      |  Note: The anti-amplification limit only applies when an
      |  endpoint responds to packets received from an unvalidated
      |  address.  The anti-amplification limit does not apply to
      |  clients when establishing a new connection or when initiating
      |  connection migration.

21.1.1.2.  Server-Side DoS

   Computing the server's first flight for a full handshake is
   potentially expensive, requiring both a signature and a key exchange
   computation.  In order to prevent computational DoS attacks, the
   Retry packet provides a cheap token exchange mechanism that allows
   servers to validate a client's IP address prior to doing any
   expensive computations at the cost of a single round trip.  After a
   successful handshake, servers can issue new tokens to a client, which
   will allow new connection establishment without incurring this cost.

21.1.1.3.  On-Path Handshake Termination

   An on-path or off-path attacker can force a handshake to fail by
   replacing or racing Initial packets.  Once valid Initial packets have
   been exchanged, subsequent Handshake packets are protected with the
   Handshake keys, and an on-path attacker cannot force handshake
   failure other than by dropping packets to cause endpoints to abandon
   the attempt.

   An on-path attacker can also replace the addresses of packets on
   either side and therefore cause the client or server to have an
   incorrect view of the remote addresses.  Such an attack is
   indistinguishable from the functions performed by a NAT.

21.1.1.4.  Parameter Negotiation

   The entire handshake is cryptographically protected, with the Initial
   packets being encrypted with per-version keys and the Handshake and
   later packets being encrypted with keys derived from the TLS key
   exchange.  Further, parameter negotiation is folded into the TLS
   transcript and thus provides the same integrity guarantees as
   ordinary TLS negotiation.  An attacker can observe the client's
   transport parameters (as long as it knows the version-specific salt)
   but cannot observe the server's transport parameters and cannot
   influence parameter negotiation.

   Connection IDs are unencrypted but integrity protected in all
   packets.

   This version of QUIC does not incorporate a version negotiation
   mechanism; implementations of incompatible versions will simply fail
   to establish a connection.

21.1.2.  Protected Packets

   Packet protection (Section 12.1) applies authenticated encryption to
   all packets except Version Negotiation packets, though Initial and
   Retry packets have limited protection due to the use of version-
   specific keying material; see [QUIC-TLS] for more details.  This
   section considers passive and active attacks against protected
   packets.

   Both on-path and off-path attackers can mount a passive attack in
   which they save observed packets for an offline attack against packet
   protection at a future time; this is true for any observer of any
   packet on any network.

   An attacker that injects packets without being able to observe valid
   packets for a connection is unlikely to be successful, since packet
   protection ensures that valid packets are only generated by endpoints
   that possess the key material established during the handshake; see
   Sections 7 and 21.1.1.  Similarly, any active attacker that observes
   packets and attempts to insert new data or modify existing data in
   those packets should not be able to generate packets deemed valid by
   the receiving endpoint, other than Initial packets.

   A spoofing attack, in which an active attacker rewrites unprotected
   parts of a packet that it forwards or injects, such as the source or
   destination address, is only effective if the attacker can forward
   packets to the original endpoint.  Packet protection ensures that the
   packet payloads can only be processed by the endpoints that completed
   the handshake, and invalid packets are ignored by those endpoints.

   An attacker can also modify the boundaries between packets and UDP
   datagrams, causing multiple packets to be coalesced into a single
   datagram or splitting coalesced packets into multiple datagrams.
   Aside from datagrams containing Initial packets, which require
   padding, modification of how packets are arranged in datagrams has no
   functional effect on a connection, although it might change some
   performance characteristics.

21.1.3.  Connection Migration

   Connection migration (Section 9) provides endpoints with the ability
   to transition between IP addresses and ports on multiple paths, using
   one path at a time for transmission and receipt of non-probing
   frames.  Path validation (Section 8.2) establishes that a peer is
   both willing and able to receive packets sent on a particular path.
   This helps reduce the effects of address spoofing by limiting the
   number of packets sent to a spoofed address.

   This section describes the intended security properties of connection
   migration under various types of DoS attacks.

21.1.3.1.  On-Path Active Attacks

   An attacker that can cause a packet it observes to no longer reach
   its intended destination is considered an on-path attacker.  When an
   attacker is present between a client and server, endpoints are
   required to send packets through the attacker to establish
   connectivity on a given path.

   An on-path attacker can:

   *  Inspect packets

   *  Modify IP and UDP packet headers

   *  Inject new packets

   *  Delay packets

   *  Reorder packets

   *  Drop packets

   *  Split and merge datagrams along packet boundaries

   An on-path attacker cannot:

   *  Modify an authenticated portion of a packet and cause the
      recipient to accept that packet

   An on-path attacker has the opportunity to modify the packets that it
   observes; however, any modifications to an authenticated portion of a
   packet will cause it to be dropped by the receiving endpoint as
   invalid, as packet payloads are both authenticated and encrypted.

   QUIC aims to constrain the capabilities of an on-path attacker as
   follows:

   1.  An on-path attacker can prevent the use of a path for a
       connection, causing the connection to fail if it cannot use a
       different path that does not contain the attacker.  This can be
       achieved by dropping all packets, modifying them so that they
       fail to decrypt, or other methods.

   2.  An on-path attacker can prevent migration to a new path for which
       the attacker is also on-path by causing path validation to fail
       on the new path.

   3.  An on-path attacker cannot prevent a client from migrating to a
       path for which the attacker is not on-path.

   4.  An on-path attacker can reduce the throughput of a connection by
       delaying packets or dropping them.

   5.  An on-path attacker cannot cause an endpoint to accept a packet
       for which it has modified an authenticated portion of that
       packet.

21.1.3.2.  Off-Path Active Attacks

   An off-path attacker is not directly on the path between a client and
   server but could be able to obtain copies of some or all packets sent
   between the client and the server.  It is also able to send copies of
   those packets to either endpoint.

   An off-path attacker can:

   *  Inspect packets

   *  Inject new packets

   *  Reorder injected packets

   An off-path attacker cannot:

   *  Modify packets sent by endpoints

   *  Delay packets

   *  Drop packets

   *  Reorder original packets

   An off-path attacker can create modified copies of packets that it
   has observed and inject those copies into the network, potentially
   with spoofed source and destination addresses.

   For the purposes of this discussion, it is assumed that an off-path
   attacker has the ability to inject a modified copy of a packet into
   the network that will reach the destination endpoint prior to the
   arrival of the original packet observed by the attacker.  In other
   words, an attacker has the ability to consistently "win" a race with
   the legitimate packets between the endpoints, potentially causing the
   original packet to be ignored by the recipient.

   It is also assumed that an attacker has the resources necessary to
   affect NAT state.  In particular, an attacker can cause an endpoint
   to lose its NAT binding and then obtain the same port for use with
   its own traffic.

   QUIC aims to constrain the capabilities of an off-path attacker as
   follows:

   1.  An off-path attacker can race packets and attempt to become a
       "limited" on-path attacker.

   2.  An off-path attacker can cause path validation to succeed for
       forwarded packets with the source address listed as the off-path
       attacker as long as it can provide improved connectivity between
       the client and the server.

   3.  An off-path attacker cannot cause a connection to close once the
       handshake has completed.

   4.  An off-path attacker cannot cause migration to a new path to fail
       if it cannot observe the new path.

   5.  An off-path attacker can become a limited on-path attacker during
       migration to a new path for which it is also an off-path
       attacker.

   6.  An off-path attacker can become a limited on-path attacker by
       affecting shared NAT state such that it sends packets to the
       server from the same IP address and port that the client
       originally used.

21.1.3.3.  Limited On-Path Active Attacks

   A limited on-path attacker is an off-path attacker that has offered
   improved routing of packets by duplicating and forwarding original
   packets between the server and the client, causing those packets to
   arrive before the original copies such that the original packets are
   dropped by the destination endpoint.

   A limited on-path attacker differs from an on-path attacker in that
   it is not on the original path between endpoints, and therefore the
   original packets sent by an endpoint are still reaching their
   destination.  This means that a future failure to route copied
   packets to the destination faster than their original path will not
   prevent the original packets from reaching the destination.

   A limited on-path attacker can:

   *  Inspect packets

   *  Inject new packets

   *  Modify unencrypted packet headers

   *  Reorder packets

   A limited on-path attacker cannot:

   *  Delay packets so that they arrive later than packets sent on the
      original path

   *  Drop packets

   *  Modify the authenticated and encrypted portion of a packet and
      cause the recipient to accept that packet

   A limited on-path attacker can only delay packets up to the point
   that the original packets arrive before the duplicate packets,
   meaning that it cannot offer routing with worse latency than the
   original path.  If a limited on-path attacker drops packets, the
   original copy will still arrive at the destination endpoint.

   QUIC aims to constrain the capabilities of a limited off-path
   attacker as follows:

   1.  A limited on-path attacker cannot cause a connection to close
       once the handshake has completed.

   2.  A limited on-path attacker cannot cause an idle connection to
       close if the client is first to resume activity.

   3.  A limited on-path attacker can cause an idle connection to be
       deemed lost if the server is the first to resume activity.

   Note that these guarantees are the same guarantees provided for any
   NAT, for the same reasons.

21.2.  Handshake Denial of Service

   As an encrypted and authenticated transport, QUIC provides a range of
   protections against denial of service.  Once the cryptographic
   handshake is complete, QUIC endpoints discard most packets that are
   not authenticated, greatly limiting the ability of an attacker to
   interfere with existing connections.

   Once a connection is established, QUIC endpoints might accept some
   unauthenticated ICMP packets (see Section 14.2.1), but the use of
   these packets is extremely limited.  The only other type of packet
   that an endpoint might accept is a stateless reset (Section 10.3),
   which relies on the token being kept secret until it is used.

   During the creation of a connection, QUIC only provides protection
   against attacks from off the network path.  All QUIC packets contain
   proof that the recipient saw a preceding packet from its peer.

   Addresses cannot change during the handshake, so endpoints can
   discard packets that are received on a different network path.

   The Source and Destination Connection ID fields are the primary means
   of protection against an off-path attack during the handshake; see
   Section 8.1.  These are required to match those set by a peer.
   Except for Initial and Stateless Resets, an endpoint only accepts
   packets that include a Destination Connection ID field that matches a
   value the endpoint previously chose.  This is the only protection
   offered for Version Negotiation packets.

   The Destination Connection ID field in an Initial packet is selected
   by a client to be unpredictable, which serves an additional purpose.
   The packets that carry the cryptographic handshake are protected with
   a key that is derived from this connection ID and a salt specific to
   the QUIC version.  This allows endpoints to use the same process for
   authenticating packets that they receive as they use after the
   cryptographic handshake completes.  Packets that cannot be
   authenticated are discarded.  Protecting packets in this fashion
   provides a strong assurance that the sender of the packet saw the
   Initial packet and understood it.

   These protections are not intended to be effective against an
   attacker that is able to receive QUIC packets prior to the connection
   being established.  Such an attacker can potentially send packets
   that will be accepted by QUIC endpoints.  This version of QUIC
   attempts to detect this sort of attack, but it expects that endpoints
   will fail to establish a connection rather than recovering.  For the
   most part, the cryptographic handshake protocol [QUIC-TLS] is
   responsible for detecting tampering during the handshake.

   Endpoints are permitted to use other methods to detect and attempt to
   recover from interference with the handshake.  Invalid packets can be
   identified and discarded using other methods, but no specific method
   is mandated in this document.

21.3.  Amplification Attack

   An attacker might be able to receive an address validation token
   (Section 8) from a server and then release the IP address it used to
   acquire that token.  At a later time, the attacker can initiate a
   0-RTT connection with a server by spoofing this same address, which
   might now address a different (victim) endpoint.  The attacker can
   thus potentially cause the server to send an initial congestion
   window's worth of data towards the victim.

   Servers SHOULD provide mitigations for this attack by limiting the
   usage and lifetime of address validation tokens; see Section 8.1.3.

21.4.  Optimistic ACK Attack

   An endpoint that acknowledges packets it has not received might cause
   a congestion controller to permit sending at rates beyond what the
   network supports.  An endpoint MAY skip packet numbers when sending
   packets to detect this behavior.  An endpoint can then immediately
   close the connection with a connection error of type
   PROTOCOL_VIOLATION; see Section 10.2.

21.5.  Request Forgery Attacks

   A request forgery attack occurs where an endpoint causes its peer to
   issue a request towards a victim, with the request controlled by the
   endpoint.  Request forgery attacks aim to provide an attacker with
   access to capabilities of its peer that might otherwise be
   unavailable to the attacker.  For a networking protocol, a request
   forgery attack is often used to exploit any implicit authorization
   conferred on the peer by the victim due to the peer's location in the
   network.

   For request forgery to be effective, an attacker needs to be able to
   influence what packets the peer sends and where these packets are
   sent.  If an attacker can target a vulnerable service with a
   controlled payload, that service might perform actions that are
   attributed to the attacker's peer but are decided by the attacker.

   For example, cross-site request forgery [CSRF] exploits on the Web
   cause a client to issue requests that include authorization cookies
   [COOKIE], allowing one site access to information and actions that
   are intended to be restricted to a different site.

   As QUIC runs over UDP, the primary attack modality of concern is one
   where an attacker can select the address to which its peer sends UDP
   datagrams and can control some of the unprotected content of those
   packets.  As much of the data sent by QUIC endpoints is protected,
   this includes control over ciphertext.  An attack is successful if an
   attacker can cause a peer to send a UDP datagram to a host that will
   perform some action based on content in the datagram.

   This section discusses ways in which QUIC might be used for request
   forgery attacks.

   This section also describes limited countermeasures that can be
   implemented by QUIC endpoints.  These mitigations can be employed
   unilaterally by a QUIC implementation or deployment, without
   potential targets for request forgery attacks taking action.
   However, these countermeasures could be insufficient if UDP-based
   services do not properly authorize requests.

   Because the migration attack described in Section 21.5.4 is quite
   powerful and does not have adequate countermeasures, QUIC server
   implementations should assume that attackers can cause them to
   generate arbitrary UDP payloads to arbitrary destinations.  QUIC
   servers SHOULD NOT be deployed in networks that do not deploy ingress
   filtering [BCP38] and also have inadequately secured UDP endpoints.

   Although it is not generally possible to ensure that clients are not
   co-located with vulnerable endpoints, this version of QUIC does not
   allow servers to migrate, thus preventing spoofed migration attacks
   on clients.  Any future extension that allows server migration MUST
   also define countermeasures for forgery attacks.

21.5.1.  Control Options for Endpoints

   QUIC offers some opportunities for an attacker to influence or
   control where its peer sends UDP datagrams:

   *  initial connection establishment (Section 7), where a server is
      able to choose where a client sends datagrams -- for example, by
      populating DNS records;

   *  preferred addresses (Section 9.6), where a server is able to
      choose where a client sends datagrams;

   *  spoofed connection migrations (Section 9.3.1), where a client is
      able to use source address spoofing to select where a server sends
      subsequent datagrams; and

   *  spoofed packets that cause a server to send a Version Negotiation
      packet (Section 21.5.5).

   In all cases, the attacker can cause its peer to send datagrams to a
   victim that might not understand QUIC.  That is, these packets are
   sent by the peer prior to address validation; see Section 8.

   Outside of the encrypted portion of packets, QUIC offers an endpoint
   several options for controlling the content of UDP datagrams that its
   peer sends.  The Destination Connection ID field offers direct
   control over bytes that appear early in packets sent by the peer; see
   Section 5.1.  The Token field in Initial packets offers a server
   control over other bytes of Initial packets; see Section 17.2.2.

   There are no measures in this version of QUIC to prevent indirect
   control over the encrypted portions of packets.  It is necessary to
   assume that endpoints are able to control the contents of frames that
   a peer sends, especially those frames that convey application data,
   such as STREAM frames.  Though this depends to some degree on details
   of the application protocol, some control is possible in many
   protocol usage contexts.  As the attacker has access to packet
   protection keys, they are likely to be capable of predicting how a
   peer will encrypt future packets.  Successful control over datagram
   content then only requires that the attacker be able to predict the
   packet number and placement of frames in packets with some amount of
   reliability.

   This section assumes that limiting control over datagram content is
   not feasible.  The focus of the mitigations in subsequent sections is
   on limiting the ways in which datagrams that are sent prior to
   address validation can be used for request forgery.

21.5.2.  Request Forgery with Client Initial Packets

   An attacker acting as a server can choose the IP address and port on
   which it advertises its availability, so Initial packets from clients
   are assumed to be available for use in this sort of attack.  The
   address validation implicit in the handshake ensures that -- for a
   new connection -- a client will not send other types of packets to a
   destination that does not understand QUIC or is not willing to accept
   a QUIC connection.

   Initial packet protection (Section 5.2 of [QUIC-TLS]) makes it
   difficult for servers to control the content of Initial packets sent
   by clients.  A client choosing an unpredictable Destination
   Connection ID ensures that servers are unable to control any of the
   encrypted portion of Initial packets from clients.

   However, the Token field is open to server control and does allow a
   server to use clients to mount request forgery attacks.  The use of
   tokens provided with the NEW_TOKEN frame (Section 8.1.3) offers the
   only option for request forgery during connection establishment.

   Clients, however, are not obligated to use the NEW_TOKEN frame.
   Request forgery attacks that rely on the Token field can be avoided
   if clients send an empty Token field when the server address has
   changed from when the NEW_TOKEN frame was received.

   Clients could avoid using NEW_TOKEN if the server address changes.
   However, not including a Token field could adversely affect
   performance.  Servers could rely on NEW_TOKEN to enable the sending
   of data in excess of the three-times limit on sending data; see
   Section 8.1.  In particular, this affects cases where clients use
   0-RTT to request data from servers.

   Sending a Retry packet (Section 17.2.5) offers a server the option to
   change the Token field.  After sending a Retry, the server can also
   control the Destination Connection ID field of subsequent Initial
   packets from the client.  This also might allow indirect control over
   the encrypted content of Initial packets.  However, the exchange of a
   Retry packet validates the server's address, thereby preventing the
   use of subsequent Initial packets for request forgery.

21.5.3.  Request Forgery with Preferred Addresses

   Servers can specify a preferred address, which clients then migrate
   to after confirming the handshake; see Section 9.6.  The Destination
   Connection ID field of packets that the client sends to a preferred
   address can be used for request forgery.

   A client MUST NOT send non-probing frames to a preferred address
   prior to validating that address; see Section 8.  This greatly
   reduces the options that a server has to control the encrypted
   portion of datagrams.

   This document does not offer any additional countermeasures that are
   specific to the use of preferred addresses and can be implemented by
   endpoints.  The generic measures described in Section 21.5.6 could be
   used as further mitigation.

21.5.4.  Request Forgery with Spoofed Migration

   Clients are able to present a spoofed source address as part of an
   apparent connection migration to cause a server to send datagrams to
   that address.

   The Destination Connection ID field in any packets that a server
   subsequently sends to this spoofed address can be used for request
   forgery.  A client might also be able to influence the ciphertext.

   A server that only sends probing packets (Section 9.1) to an address
   prior to address validation provides an attacker with only limited
   control over the encrypted portion of datagrams.  However,
   particularly for NAT rebinding, this can adversely affect
   performance.  If the server sends frames carrying application data,
   an attacker might be able to control most of the content of
   datagrams.

   This document does not offer specific countermeasures that can be
   implemented by endpoints, aside from the generic measures described
   in Section 21.5.6.  However, countermeasures for address spoofing at
   the network level -- in particular, ingress filtering [BCP38] -- are
   especially effective against attacks that use spoofing and originate
   from an external network.

21.5.5.  Request Forgery with Version Negotiation

   Clients that are able to present a spoofed source address on a packet
   can cause a server to send a Version Negotiation packet
   (Section 17.2.1) to that address.

   The absence of size restrictions on the connection ID fields for
   packets of an unknown version increases the amount of data that the
   client controls from the resulting datagram.  The first byte of this
   packet is not under client control and the next four bytes are zero,
   but the client is able to control up to 512 bytes starting from the
   fifth byte.

   No specific countermeasures are provided for this attack, though
   generic protections (Section 21.5.6) could apply.  In this case,
   ingress filtering [BCP38] is also effective.

21.5.6.  Generic Request Forgery Countermeasures

   The most effective defense against request forgery attacks is to
   modify vulnerable services to use strong authentication.  However,
   this is not always something that is within the control of a QUIC
   deployment.  This section outlines some other steps that QUIC
   endpoints could take unilaterally.  These additional steps are all
   discretionary because, depending on circumstances, they could
   interfere with or prevent legitimate uses.

   Services offered over loopback interfaces often lack proper
   authentication.  Endpoints MAY prevent connection attempts or
   migration to a loopback address.  Endpoints SHOULD NOT allow
   connections or migration to a loopback address if the same service
   was previously available at a different interface or if the address
   was provided by a service at a non-loopback address.  Endpoints that
   depend on these capabilities could offer an option to disable these
   protections.

   Similarly, endpoints could regard a change in address to a link-local
   address [RFC4291] or an address in a private-use range [RFC1918] from
   a global, unique-local [RFC4193], or non-private address as a
   potential attempt at request forgery.  Endpoints could refuse to use
   these addresses entirely, but that carries a significant risk of
   interfering with legitimate uses.  Endpoints SHOULD NOT refuse to use
   an address unless they have specific knowledge about the network
   indicating that sending datagrams to unvalidated addresses in a given
   range is not safe.

   Endpoints MAY choose to reduce the risk of request forgery by not
   including values from NEW_TOKEN frames in Initial packets or by only
   sending probing frames in packets prior to completing address
   validation.  Note that this does not prevent an attacker from using
   the Destination Connection ID field for an attack.

   Endpoints are not expected to have specific information about the
   location of servers that could be vulnerable targets of a request
   forgery attack.  However, it might be possible over time to identify
   specific UDP ports that are common targets of attacks or particular
   patterns in datagrams that are used for attacks.  Endpoints MAY
   choose to avoid sending datagrams to these ports or not send
   datagrams that match these patterns prior to validating the
   destination address.  Endpoints MAY retire connection IDs containing
   patterns known to be problematic without using them.

      |  Note: Modifying endpoints to apply these protections is more
      |  efficient than deploying network-based protections, as
      |  endpoints do not need to perform any additional processing when
      |  sending to an address that has been validated.

21.6.  Slowloris Attacks

   The attacks commonly known as Slowloris [SLOWLORIS] try to keep many
   connections to the target endpoint open and hold them open as long as
   possible.  These attacks can be executed against a QUIC endpoint by
   generating the minimum amount of activity necessary to avoid being
   closed for inactivity.  This might involve sending small amounts of
   data, gradually opening flow control windows in order to control the
   sender rate, or manufacturing ACK frames that simulate a high loss
   rate.

   QUIC deployments SHOULD provide mitigations for the Slowloris
   attacks, such as increasing the maximum number of clients the server
   will allow, limiting the number of connections a single IP address is
   allowed to make, imposing restrictions on the minimum transfer speed
   a connection is allowed to have, and restricting the length of time
   an endpoint is allowed to stay connected.

21.7.  Stream Fragmentation and Reassembly Attacks

   An adversarial sender might intentionally not send portions of the
   stream data, causing the receiver to commit resources for the unsent
   data.  This could cause a disproportionate receive buffer memory
   commitment and/or the creation of a large and inefficient data
   structure at the receiver.

   An adversarial receiver might intentionally not acknowledge packets
   containing stream data in an attempt to force the sender to store the
   unacknowledged stream data for retransmission.

   The attack on receivers is mitigated if flow control windows
   correspond to available memory.  However, some receivers will
   overcommit memory and advertise flow control offsets in the aggregate
   that exceed actual available memory.  The overcommitment strategy can
   lead to better performance when endpoints are well behaved, but
   renders endpoints vulnerable to the stream fragmentation attack.

   QUIC deployments SHOULD provide mitigations for stream fragmentation
   attacks.  Mitigations could consist of avoiding overcommitting
   memory, limiting the size of tracking data structures, delaying
   reassembly of STREAM frames, implementing heuristics based on the age
   and duration of reassembly holes, or some combination of these.

21.8.  Stream Commitment Attack

   An adversarial endpoint can open a large number of streams,
   exhausting state on an endpoint.  The adversarial endpoint could
   repeat the process on a large number of connections, in a manner
   similar to SYN flooding attacks in TCP.

   Normally, clients will open streams sequentially, as explained in
   Section 2.1.  However, when several streams are initiated at short
   intervals, loss or reordering can cause STREAM frames that open
   streams to be received out of sequence.  On receiving a higher-
   numbered stream ID, a receiver is required to open all intervening
   streams of the same type; see Section 3.2.  Thus, on a new
   connection, opening stream 4000000 opens 1 million and 1 client-
   initiated bidirectional streams.

   The number of active streams is limited by the
   initial_max_streams_bidi and initial_max_streams_uni transport
   parameters as updated by any received MAX_STREAMS frames, as
   explained in Section 4.6.  If chosen judiciously, these limits
   mitigate the effect of the stream commitment attack.  However,
   setting the limit too low could affect performance when applications
   expect to open a large number of streams.

21.9.  Peer Denial of Service

   QUIC and TLS both contain frames or messages that have legitimate
   uses in some contexts, but these frames or messages can be abused to
   cause a peer to expend processing resources without having any
   observable impact on the state of the connection.

   Messages can also be used to change and revert state in small or
   inconsequential ways, such as by sending small increments to flow
   control limits.

   If processing costs are disproportionately large in comparison to
   bandwidth consumption or effect on state, then this could allow a
   malicious peer to exhaust processing capacity.

   While there are legitimate uses for all messages, implementations
   SHOULD track cost of processing relative to progress and treat
   excessive quantities of any non-productive packets as indicative of
   an attack.  Endpoints MAY respond to this condition with a connection
   error or by dropping packets.

21.10.  Explicit Congestion Notification Attacks

   An on-path attacker could manipulate the value of ECN fields in the
   IP header to influence the sender's rate.  [RFC3168] discusses
   manipulations and their effects in more detail.

   A limited on-path attacker can duplicate and send packets with
   modified ECN fields to affect the sender's rate.  If duplicate
   packets are discarded by a receiver, an attacker will need to race
   the duplicate packet against the original to be successful in this
   attack.  Therefore, QUIC endpoints ignore the ECN field in an IP
   packet unless at least one QUIC packet in that IP packet is
   successfully processed; see Section 13.4.

21.11.  Stateless Reset Oracle

   Stateless resets create a possible denial-of-service attack analogous
   to a TCP reset injection.  This attack is possible if an attacker is
   able to cause a stateless reset token to be generated for a
   connection with a selected connection ID.  An attacker that can cause
   this token to be generated can reset an active connection with the
   same connection ID.

   If a packet can be routed to different instances that share a static
   key -- for example, by changing an IP address or port -- then an
   attacker can cause the server to send a stateless reset.  To defend
   against this style of denial of service, endpoints that share a
   static key for stateless resets (see Section 10.3.2) MUST be arranged
   so that packets with a given connection ID always arrive at an
   instance that has connection state, unless that connection is no
   longer active.

   More generally, servers MUST NOT generate a stateless reset if a
   connection with the corresponding connection ID could be active on
   any endpoint using the same static key.

   In the case of a cluster that uses dynamic load balancing, it is
   possible that a change in load-balancer configuration could occur
   while an active instance retains connection state.  Even if an
   instance retains connection state, the change in routing and
   resulting stateless reset will result in the connection being
   terminated.  If there is no chance of the packet being routed to the
   correct instance, it is better to send a stateless reset than wait
   for the connection to time out.  However, this is acceptable only if
   the routing cannot be influenced by an attacker.

21.12.  Version Downgrade

   This document defines QUIC Version Negotiation packets (Section 6),
   which can be used to negotiate the QUIC version used between two
   endpoints.  However, this document does not specify how this
   negotiation will be performed between this version and subsequent
   future versions.  In particular, Version Negotiation packets do not
   contain any mechanism to prevent version downgrade attacks.  Future
   versions of QUIC that use Version Negotiation packets MUST define a
   mechanism that is robust against version downgrade attacks.

21.13.  Targeted Attacks by Routing

   Deployments should limit the ability of an attacker to target a new
   connection to a particular server instance.  Ideally, routing
   decisions are made independently of client-selected values, including
   addresses.  Once an instance is selected, a connection ID can be
   selected so that later packets are routed to the same instance.

21.14.  Traffic Analysis

   The length of QUIC packets can reveal information about the length of
   the content of those packets.  The PADDING frame is provided so that
   endpoints have some ability to obscure the length of packet content;
   see Section 19.1.

   Defeating traffic analysis is challenging and the subject of active
   research.  Length is not the only way that information might leak.
   Endpoints might also reveal sensitive information through other side
   channels, such as the timing of packets.

22.  IANA Considerations

   This document establishes several registries for the management of
   codepoints in QUIC.  These registries operate on a common set of
   policies as defined in Section 22.1.

22.1.  Registration Policies for QUIC Registries

   All QUIC registries allow for both provisional and permanent
   registration of codepoints.  This section documents policies that are
   common to these registries.

22.1.1.  Provisional Registrations

   Provisional registrations of codepoints are intended to allow for
   private use and experimentation with extensions to QUIC.  Provisional
   registrations only require the inclusion of the codepoint value and
   contact information.  However, provisional registrations could be
   reclaimed and reassigned for another purpose.

   Provisional registrations require Expert Review, as defined in
   Section 4.5 of [RFC8126].  The designated expert or experts are
   advised that only registrations for an excessive proportion of
   remaining codepoint space or the very first unassigned value (see
   Section 22.1.2) can be rejected.

   Provisional registrations will include a Date field that indicates
   when the registration was last updated.  A request to update the date
   on any provisional registration can be made without review from the
   designated expert(s).

   All QUIC registries include the following fields to support
   provisional registration:

   Value:  The assigned codepoint.
   Status:  "permanent" or "provisional".
   Specification:  A reference to a publicly available specification for
      the value.
   Date:  The date of the last update to the registration.
   Change Controller:  The entity that is responsible for the definition
      of the registration.
   Contact:  Contact details for the registrant.
   Notes:  Supplementary notes about the registration.

   Provisional registrations MAY omit the Specification and Notes
   fields, plus any additional fields that might be required for a
   permanent registration.  The Date field is not required as part of
   requesting a registration, as it is set to the date the registration
   is created or updated.

22.1.2.  Selecting Codepoints

   New requests for codepoints from QUIC registries SHOULD use a
   randomly selected codepoint that excludes both existing allocations
   and the first unallocated codepoint in the selected space.  Requests
   for multiple codepoints MAY use a contiguous range.  This minimizes
   the risk that differing semantics are attributed to the same
   codepoint by different implementations.

   The use of the first unassigned codepoint is reserved for allocation
   using the Standards Action policy; see Section 4.9 of [RFC8126].  The
   early codepoint assignment process [EARLY-ASSIGN] can be used for
   these values.

   For codepoints that are encoded in variable-length integers
   (Section 16), such as frame types, codepoints that encode to four or
   eight bytes (that is, values 2^14 and above) SHOULD be used unless
   the usage is especially sensitive to having a longer encoding.

   Applications to register codepoints in QUIC registries MAY include a
   requested codepoint as part of the registration.  IANA MUST allocate
   the selected codepoint if the codepoint is unassigned and the
   requirements of the registration policy are met.

22.1.3.  Reclaiming Provisional Codepoints

   A request might be made to remove an unused provisional registration
   from the registry to reclaim space in a registry, or a portion of the
   registry (such as the 64-16383 range for codepoints that use
   variable-length encodings).  This SHOULD be done only for the
   codepoints with the earliest recorded date, and entries that have
   been updated less than a year prior SHOULD NOT be reclaimed.

   A request to remove a codepoint MUST be reviewed by the designated
   experts.  The experts MUST attempt to determine whether the codepoint
   is still in use.  Experts are advised to contact the listed contacts
   for the registration, plus as wide a set of protocol implementers as
   possible in order to determine whether any use of the codepoint is
   known.  The experts are also advised to allow at least four weeks for
   responses.

   If any use of the codepoints is identified by this search or a
   request to update the registration is made, the codepoint MUST NOT be
   reclaimed.  Instead, the date on the registration is updated.  A note
   might be added for the registration recording relevant information
   that was learned.

   If no use of the codepoint was identified and no request was made to
   update the registration, the codepoint MAY be removed from the
   registry.

   This review and consultation process also applies to requests to
   change a provisional registration into a permanent registration,
   except that the goal is not to determine whether there is no use of
   the codepoint but to determine that the registration is an accurate
   representation of any deployed usage.

22.1.4.  Permanent Registrations

   Permanent registrations in QUIC registries use the Specification
   Required policy (Section 4.6 of [RFC8126]), unless otherwise
   specified.  The designated expert or experts verify that a
   specification exists and is readily accessible.  Experts are
   encouraged to be biased towards approving registrations unless they
   are abusive, frivolous, or actively harmful (not merely aesthetically
   displeasing or architecturally dubious).  The creation of a registry
   MAY specify additional constraints on permanent registrations.

   The creation of a registry MAY identify a range of codepoints where
   registrations are governed by a different registration policy.  For
   instance, the "QUIC Frame Types" registry (Section 22.4) has a
   stricter policy for codepoints in the range from 0 to 63.

   Any stricter requirements for permanent registrations do not prevent
   provisional registrations for affected codepoints.  For instance, a
   provisional registration for a frame type of 61 could be requested.

   All registrations made by Standards Track publications MUST be
   permanent.

   All registrations in this document are assigned a permanent status
   and list a change controller of the IETF and a contact of the QUIC
   Working Group (quic@ietf.org).

22.2.  QUIC Versions Registry

   IANA has added a registry for "QUIC Versions" under a "QUIC" heading.

   The "QUIC Versions" registry governs a 32-bit space; see Section 15.
   This registry follows the registration policy from Section 22.1.
   Permanent registrations in this registry are assigned using the
   Specification Required policy (Section 4.6 of [RFC8126]).

   The codepoint of 0x00000001 for the protocol is assigned with
   permanent status to the protocol defined in this document.  The
   codepoint of 0x00000000 is permanently reserved; the note for this
   codepoint indicates that this version is reserved for version
   negotiation.

   All codepoints that follow the pattern 0x?a?a?a?a are reserved, MUST
   NOT be assigned by IANA, and MUST NOT appear in the listing of
   assigned values.

22.3.  QUIC Transport Parameters Registry

   IANA has added a registry for "QUIC Transport Parameters" under a
   "QUIC" heading.

   The "QUIC Transport Parameters" registry governs a 62-bit space.
   This registry follows the registration policy from Section 22.1.
   Permanent registrations in this registry are assigned using the
   Specification Required policy (Section 4.6 of [RFC8126]), except for
   values between 0x00 and 0x3f (in hexadecimal), inclusive, which are
   assigned using Standards Action or IESG Approval as defined in
   Sections 4.9 and 4.10 of [RFC8126].

   In addition to the fields listed in Section 22.1.1, permanent
   registrations in this registry MUST include the following field:

   Parameter Name:  A short mnemonic for the parameter.

   The initial contents of this registry are shown in Table 6.

      +=======+=====================================+===============+
      | Value | Parameter Name                      | Specification |
      +=======+=====================================+===============+
      | 0x00  | original_destination_connection_id  | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x01  | max_idle_timeout                    | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x02  | stateless_reset_token               | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x03  | max_udp_payload_size                | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x04  | initial_max_data                    | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x05  | initial_max_stream_data_bidi_local  | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x06  | initial_max_stream_data_bidi_remote | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x07  | initial_max_stream_data_uni         | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x08  | initial_max_streams_bidi            | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x09  | initial_max_streams_uni             | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x0a  | ack_delay_exponent                  | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x0b  | max_ack_delay                       | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x0c  | disable_active_migration            | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x0d  | preferred_address                   | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x0e  | active_connection_id_limit          | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x0f  | initial_source_connection_id        | Section 18.2  |
      +-------+-------------------------------------+---------------+
      | 0x10  | retry_source_connection_id          | Section 18.2  |
      +-------+-------------------------------------+---------------+

        Table 6: Initial QUIC Transport Parameters Registry Entries

   Each value of the form "31 * N + 27" for integer values of N (that
   is, 27, 58, 89, ...) are reserved; these values MUST NOT be assigned
   by IANA and MUST NOT appear in the listing of assigned values.

22.4.  QUIC Frame Types Registry

   IANA has added a registry for "QUIC Frame Types" under a "QUIC"
   heading.

   The "QUIC Frame Types" registry governs a 62-bit space.  This
   registry follows the registration policy from Section 22.1.
   Permanent registrations in this registry are assigned using the
   Specification Required policy (Section 4.6 of [RFC8126]), except for
   values between 0x00 and 0x3f (in hexadecimal), inclusive, which are
   assigned using Standards Action or IESG Approval as defined in
   Sections 4.9 and 4.10 of [RFC8126].

   In addition to the fields listed in Section 22.1.1, permanent
   registrations in this registry MUST include the following field:

   Frame Type Name:  A short mnemonic for the frame type.

   In addition to the advice in Section 22.1, specifications for new
   permanent registrations SHOULD describe the means by which an
   endpoint might determine that it can send the identified type of
   frame.  An accompanying transport parameter registration is expected
   for most registrations; see Section 22.3.  Specifications for
   permanent registrations also need to describe the format and assigned
   semantics of any fields in the frame.

   The initial contents of this registry are tabulated in Table 3.  Note
   that the registry does not include the "Pkts" and "Spec" columns from
   Table 3.

22.5.  QUIC Transport Error Codes Registry

   IANA has added a registry for "QUIC Transport Error Codes" under a
   "QUIC" heading.

   The "QUIC Transport Error Codes" registry governs a 62-bit space.
   This space is split into three ranges that are governed by different
   policies.  Permanent registrations in this registry are assigned
   using the Specification Required policy (Section 4.6 of [RFC8126]),
   except for values between 0x00 and 0x3f (in hexadecimal), inclusive,
   which are assigned using Standards Action or IESG Approval as defined
   in Sections 4.9 and 4.10 of [RFC8126].

   In addition to the fields listed in Section 22.1.1, permanent
   registrations in this registry MUST include the following fields:

   Code:  A short mnemonic for the parameter.

   Description:  A brief description of the error code semantics, which
      MAY be a summary if a specification reference is provided.

   The initial contents of this registry are shown in Table 7.

   +=======+===========================+================+==============+
   |Value  | Code                      |Description     |Specification |
   +=======+===========================+================+==============+
   |0x00   | NO_ERROR                  |No error        |Section 20    |
   +-------+---------------------------+----------------+--------------+
   |0x01   | INTERNAL_ERROR            |Implementation  |Section 20    |
   |       |                           |error           |              |
   +-------+---------------------------+----------------+--------------+
   |0x02   | CONNECTION_REFUSED        |Server refuses a|Section 20    |
   |       |                           |connection      |              |
   +-------+---------------------------+----------------+--------------+
   |0x03   | FLOW_CONTROL_ERROR        |Flow control    |Section 20    |
   |       |                           |error           |              |
   +-------+---------------------------+----------------+--------------+
   |0x04   | STREAM_LIMIT_ERROR        |Too many streams|Section 20    |
   |       |                           |opened          |              |
   +-------+---------------------------+----------------+--------------+
   |0x05   | STREAM_STATE_ERROR        |Frame received  |Section 20    |
   |       |                           |in invalid      |              |
   |       |                           |stream state    |              |
   +-------+---------------------------+----------------+--------------+
   |0x06   | FINAL_SIZE_ERROR          |Change to final |Section 20    |
   |       |                           |size            |              |
   +-------+---------------------------+----------------+--------------+
   |0x07   | FRAME_ENCODING_ERROR      |Frame encoding  |Section 20    |
   |       |                           |error           |              |
   +-------+---------------------------+----------------+--------------+
   |0x08   | TRANSPORT_PARAMETER_ERROR |Error in        |Section 20    |
   |       |                           |transport       |              |
   |       |                           |parameters      |              |
   +-------+---------------------------+----------------+--------------+
   |0x09   | CONNECTION_ID_LIMIT_ERROR |Too many        |Section 20    |
   |       |                           |connection IDs  |              |
   |       |                           |received        |              |
   +-------+---------------------------+----------------+--------------+
   |0x0a   | PROTOCOL_VIOLATION        |Generic protocol|Section 20    |
   |       |                           |violation       |              |
   +-------+---------------------------+----------------+--------------+
   |0x0b   | INVALID_TOKEN             |Invalid Token   |Section 20    |
   |       |                           |received        |              |
   +-------+---------------------------+----------------+--------------+
   |0x0c   | APPLICATION_ERROR         |Application     |Section 20    |
   |       |                           |error           |              |
   +-------+---------------------------+----------------+--------------+
   |0x0d   | CRYPTO_BUFFER_EXCEEDED    |CRYPTO data     |Section 20    |
   |       |                           |buffer          |              |
   |       |                           |overflowed      |              |
   +-------+---------------------------+----------------+--------------+
   |0x0e   | KEY_UPDATE_ERROR          |Invalid packet  |Section 20    |
   |       |                           |protection      |              |
   |       |                           |update          |              |
   +-------+---------------------------+----------------+--------------+
   |0x0f   | AEAD_LIMIT_REACHED        |Excessive use of|Section 20    |
   |       |                           |packet          |              |
   |       |                           |protection keys |              |
   +-------+---------------------------+----------------+--------------+
   |0x10   | NO_VIABLE_PATH            |No viable       |Section 20    |
   |       |                           |network path    |              |
   |       |                           |exists          |              |
   +-------+---------------------------+----------------+--------------+
   |0x0100-| CRYPTO_ERROR              |TLS alert code  |Section 20    |
   |0x01ff |                           |                |              |
   +-------+---------------------------+----------------+--------------+

        Table 7: Initial QUIC Transport Error Codes Registry Entries

23.  References

23.1.  Normative References

   [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

              <https://www.rfc-editor.org/info/bcp38>

   [DPLPMTUD] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/info/rfc8899>.

   [EARLY-ASSIGN]
              Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <https://www.rfc-editor.org/info/rfc7120>.

   [IPv4]     Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [QUIC-INVARIANTS]
              Thomson, M., "Version-Independent Properties of QUIC",
              RFC 8999, DOI 10.17487/RFC8999, May 2021,
              <https://www.rfc-editor.org/info/rfc8999>.

   [QUIC-RECOVERY]
              Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/info/rfc9002>.

   [QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/info/rfc9001>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [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>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,
              <https://www.rfc-editor.org/info/rfc6437>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [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>.

   [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>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,
              <https://www.rfc-editor.org/info/rfc8311>.

   [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>.

   [UDP]      Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

23.2.  Informative References

   [AEAD]     McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [ALPN]     Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

   [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>.

   [COOKIE]   Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [CSRF]     Barth, A., Jackson, C., and J. Mitchell, "Robust defenses
              for cross-site request forgery", Proceedings of the 15th
              ACM conference on Computer and communications security -
              CCS '08, DOI 10.1145/1455770.1455782, 2008,
              <https://doi.org/10.1145/1455770.1455782>.

   [EARLY-DESIGN]
              Roskind, J., "QUIC: Multiplexed Stream Transport Over
              UDP", 2 December 2013, <https://docs.google.com/document/
              d/1RNHkx_VvKWyWg6Lr8SZ-saqsQx7rFV-ev2jRFUoVD34/
              edit?usp=sharing>.

   [GATEWAY]  Hätönen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
              Sarolahti, P., and M. Kojo, "An experimental study of home
              gateway characteristics", Proceedings of the 10th ACM
              SIGCOMM conference on Internet measurement - IMC '10,
              DOI 10.1145/1879141.1879174, November 2010,
              <https://doi.org/10.1145/1879141.1879174>.

   [HTTP2]    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>.

   [IPv6]     Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [QUIC-MANAGEABILITY]
              Kuehlewind, M. and B. Trammell, "Manageability of the QUIC
              Transport Protocol", Work in Progress, Internet-Draft,
              draft-ietf-quic-manageability-11, 21 April 2021,
              <https://tools.ietf.org/html/draft-ietf-quic-
              manageability-11>.

   [RANDOM]   Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <https://www.rfc-editor.org/info/rfc1812>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,
              <https://www.rfc-editor.org/info/rfc2018>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
              Sooriyabandara, "TCP Performance Implications of Network
              Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
              December 2002, <https://www.rfc-editor.org/info/rfc3449>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <https://www.rfc-editor.org/info/rfc4787>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [RFC7983]  Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              RFC 7983, DOI 10.17487/RFC7983, September 2016,
              <https://www.rfc-editor.org/info/rfc7983>.

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,
              <https://www.rfc-editor.org/info/rfc8087>.

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,
              <https://www.rfc-editor.org/info/rfc8981>.

   [SEC-CONS] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [SLOWLORIS]
              "RSnake" Hansen, R., "Welcome to Slowloris - the low
              bandwidth, yet greedy and poisonous HTTP client!", June
              2009, <https://web.archive.org/web/20150315054838/
              http://ha.ckers.org/slowloris/>.

Appendix A.  Pseudocode

   The pseudocode in this section describes sample algorithms.  These
   algorithms are intended to be correct and clear, rather than being
   optimally performant.

   The pseudocode segments in this section are licensed as Code
   Components; see the Copyright Notice.

A.1.  Sample Variable-Length Integer Decoding

   The pseudocode in Figure 45 shows how a variable-length integer can
   be read from a stream of bytes.  The function ReadVarint takes a
   single argument -- a sequence of bytes, which can be read in network
   byte order.

   ReadVarint(data):
     // The length of variable-length integers is encoded in the
     // first two bits of the first byte.
     v = data.next_byte()
     prefix = v >> 6
     length = 1 << prefix

     // Once the length is known, remove these bits and read any
     // remaining bytes.
     v = v & 0x3f
     repeat length-1 times:
       v = (v << 8) + data.next_byte()
     return v

        Figure 45: Sample Variable-Length Integer Decoding Algorithm

   For example, the eight-byte sequence 0xc2197c5eff14e88c decodes to
   the decimal value 151,288,809,941,952,652; the four-byte sequence
   0x9d7f3e7d decodes to 494,878,333; the two-byte sequence 0x7bbd
   decodes to 15,293; and the single byte 0x25 decodes to 37 (as does
   the two-byte sequence 0x4025).

A.2.  Sample Packet Number Encoding Algorithm

   The pseudocode in Figure 46 shows how an implementation can select an
   appropriate size for packet number encodings.

   The EncodePacketNumber function takes two arguments:

   *  full_pn is the full packet number of the packet being sent.

   *  largest_acked is the largest packet number that has been
      acknowledged by the peer in the current packet number space, if
      any.

   EncodePacketNumber(full_pn, largest_acked):

     // The number of bits must be at least one more
     // than the base-2 logarithm of the number of contiguous
     // unacknowledged packet numbers, including the new packet.
     if largest_acked is None:
       num_unacked = full_pn + 1
     else:
       num_unacked = full_pn - largest_acked

     min_bits = log(num_unacked, 2) + 1
     num_bytes = ceil(min_bits / 8)

     // Encode the integer value and truncate to
     // the num_bytes least significant bytes.
     return encode(full_pn, num_bytes)

             Figure 46: Sample Packet Number Encoding Algorithm

   For example, if an endpoint has received an acknowledgment for packet
   0xabe8b3 and is sending a packet with a number of 0xac5c02, there are
   29,519 (0x734f) outstanding packet numbers.  In order to represent at
   least twice this range (59,038 packets, or 0xe69e), 16 bits are
   required.

   In the same state, sending a packet with a number of 0xace8fe uses
   the 24-bit encoding, because at least 18 bits are required to
   represent twice the range (131,222 packets, or 0x020096).

A.3.  Sample Packet Number Decoding Algorithm

   The pseudocode in Figure 47 includes an example algorithm for
   decoding packet numbers after header protection has been removed.

   The DecodePacketNumber function takes three arguments:

   *  largest_pn is the largest packet number that has been successfully
      processed in the current packet number space.

   *  truncated_pn is the value of the Packet Number field.

   *  pn_nbits is the number of bits in the Packet Number field (8, 16,
      24, or 32).

   DecodePacketNumber(largest_pn, truncated_pn, pn_nbits):
      expected_pn  = largest_pn + 1
      pn_win       = 1 << pn_nbits
      pn_hwin      = pn_win / 2
      pn_mask      = pn_win - 1
      // The incoming packet number should be greater than
      // expected_pn - pn_hwin and less than or equal to
      // expected_pn + pn_hwin
      //
      // This means we cannot just strip the trailing bits from
      // expected_pn and add the truncated_pn because that might
      // yield a value outside the window.
      //
      // The following code calculates a candidate value and
      // makes sure it's within the packet number window.
      // Note the extra checks to prevent overflow and underflow.
      candidate_pn = (expected_pn & ~pn_mask) | truncated_pn
      if candidate_pn <= expected_pn - pn_hwin and
         candidate_pn < (1 << 62) - pn_win:
         return candidate_pn + pn_win
      if candidate_pn > expected_pn + pn_hwin and
         candidate_pn >= pn_win:
         return candidate_pn - pn_win
      return candidate_pn

             Figure 47: Sample Packet Number Decoding Algorithm

   For example, if the highest successfully authenticated packet had a
   packet number of 0xa82f30ea, then a packet containing a 16-bit value
   of 0x9b32 will be decoded as 0xa82f9b32.

A.4.  Sample ECN Validation Algorithm

   Each time an endpoint commences sending on a new network path, it
   determines whether the path supports ECN; see Section 13.4.  If the
   path supports ECN, the goal is to use ECN.  Endpoints might also
   periodically reassess a path that was determined to not support ECN.

   This section describes one method for testing new paths.  This
   algorithm is intended to show how a path might be tested for ECN
   support.  Endpoints can implement different methods.

   The path is assigned an ECN state that is one of "testing",
   "unknown", "failed", or "capable".  On paths with a "testing" or
   "capable" state, the endpoint sends packets with an ECT marking --
   ECT(0) by default; otherwise, the endpoint sends unmarked packets.

   To start testing a path, the ECN state is set to "testing", and
   existing ECN counts are remembered as a baseline.

   The testing period runs for a number of packets or a limited time, as
   determined by the endpoint.  The goal is not to limit the duration of
   the testing period but to ensure that enough marked packets are sent
   for received ECN counts to provide a clear indication of how the path
   treats marked packets.  Section 13.4.2 suggests limiting this to ten
   packets or three times the PTO.

   After the testing period ends, the ECN state for the path becomes
   "unknown".  From the "unknown" state, successful validation of the
   ECN counts in an ACK frame (see Section 13.4.2.1) causes the ECN
   state for the path to become "capable", unless no marked packet has
   been acknowledged.

   If validation of ECN counts fails at any time, the ECN state for the
   affected path becomes "failed".  An endpoint can also mark the ECN
   state for a path as "failed" if marked packets are all declared lost
   or if they are all ECN-CE marked.

   Following this algorithm ensures that ECN is rarely disabled for
   paths that properly support ECN.  Any path that incorrectly modifies
   markings will cause ECN to be disabled.  For those rare cases where
   marked packets are discarded by the path, the short duration of the
   testing period limits the number of losses incurred.

Contributors

   The original design and rationale behind this protocol draw
   significantly from work by Jim Roskind [EARLY-DESIGN].

   The IETF QUIC Working Group received an enormous amount of support
   from many people.  The following people provided substantive
   contributions to this document:

   *  Alessandro Ghedini
   *  Alyssa Wilk
   *  Antoine Delignat-Lavaud
   *  Brian Trammell
   *  Christian Huitema
   *  Colin Perkins
   *  David Schinazi
   *  Dmitri Tikhonov
   *  Eric Kinnear
   *  Eric Rescorla
   *  Gorry Fairhurst
   *  Ian Swett
   *  Igor Lubashev
   *  奥 一穂 (Kazuho Oku)
   *  Lars Eggert
   *  Lucas Pardue
   *  Magnus Westerlund
   *  Marten Seemann
   *  Martin Duke
   *  Mike Bishop
   *  Mikkel Fahnøe Jørgensen
   *  Mirja Kühlewind
   *  Nick Banks
   *  Nick Harper
   *  Patrick McManus
   *  Roberto Peon
   *  Ryan Hamilton
   *  Subodh Iyengar
   *  Tatsuhiro Tsujikawa
   *  Ted Hardie
   *  Tom Jones
   *  Victor Vasiliev

Authors' Addresses

   Jana Iyengar (editor)
   Fastly

   Email: jri.ietf@gmail.com


   Martin Thomson (editor)
   Mozilla

   Email: mt@lowentropy.net