Using QUIC Datagrams with HTTP/3
draft-ietf-masque-h3-datagram-01

MASQUE                                                       D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                               L. Pardue
Expires: 14 November 2021                                     Cloudflare
                                                             13 May 2021

                    Using QUIC Datagrams with HTTP/3
                    draft-ietf-masque-h3-datagram-01

Abstract

   The QUIC DATAGRAM extension provides application protocols running
   over QUIC with a mechanism to send unreliable data while leveraging
   the security and congestion-control properties of QUIC.  However,
   QUIC DATAGRAM frames do not provide a means to demultiplex
   application contexts.  This document describes how to use QUIC
   DATAGRAM frames when the application protocol running over QUIC is
   HTTP/3.  It associates datagrams with client-initiated bidirectional
   streams and defines an optional additional demultiplexing layer.

   Discussion of this work is encouraged to happen on the MASQUE IETF
   mailing list (masque@ietf.org (mailto:masque@ietf.org)) or on the
   GitHub repository which contains the draft: https://github.com/ietf-
   wg-masque/draft-ietf-masque-h3-datagram.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 14 November 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Multiplexing  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Datagram Contexts . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Context ID Allocation . . . . . . . . . . . . . . . . . .   4
   3.  HTTP/3 DATAGRAM Frame Format  . . . . . . . . . . . . . . . .   4
   4.  CAPSULE HTTP/3 Frame Definition . . . . . . . . . . . . . . .   5
     4.1.  The REGISTER_DATAGRAM_CONTEXT Capsule . . . . . . . . . .   6
     4.2.  The CLOSE_DATAGRAM_CONTEXT Capsule  . . . . . . . . . . .   7
     4.3.  The DATAGRAM Capsule  . . . . . . . . . . . . . . . . . .   8
   5.  The H3_DATAGRAM HTTP/3 SETTINGS Parameter . . . . . . . . . .   9
   6.  HTTP/1.x and HTTP/2 Support . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  HTTP/3 CAPSULE Frame  . . . . . . . . . . . . . . . . . .  10
     8.2.  HTTP SETTINGS Parameter . . . . . . . . . . . . . . . . .  10
     8.3.  Capsule Types . . . . . . . . . . . . . . . . . . . . . .  10
     8.4.  Context Extension Keys  . . . . . . . . . . . . . . . . .  11
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  12
     A.1.  CONNECT-UDP . . . . . . . . . . . . . . . . . . . . . . .  12
     A.2.  CONNECT-UDP with Timestamp Extension  . . . . . . . . . .  13
     A.3.  CONNECT-IP with IP compression  . . . . . . . . . . . . .  14
     A.4.  WebTransport  . . . . . . . . . . . . . . . . . . . . . .  15
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

   The QUIC DATAGRAM extension [DGRAM] provides application protocols
   running over QUIC [QUIC] with a mechanism to send unreliable data
   while leveraging the security and congestion-control properties of
   QUIC.  However, QUIC DATAGRAM frames do not provide a means to
   demultiplex application contexts.  This document describes how to use
   QUIC DATAGRAM frames when the application protocol running over QUIC
   is HTTP/3 [H3].  It associates datagrams with client-initiated
   bidirectional streams and defines an optional additional
   demultiplexing layer.

   Discussion of this work is encouraged to happen on the MASQUE IETF
   mailing list (masque@ietf.org (mailto:masque@ietf.org)) or on the
   GitHub repository which contains the draft: https://github.com/ietf-
   wg-masque/draft-ietf-masque-h3-datagram.

1.1.  Conventions and Definitions

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

2.  Multiplexing

   In order to allow multiple exchanges of datagrams to coexist on a
   given QUIC connection, HTTP datagrams contain two layers of
   multiplexing.  First, the QUIC DATAGRAM frame payload starts with an
   encoded stream identifier that associates the datagram with a given
   QUIC stream.  Second, datagrams carry a context identifier (see
   Section 2.1) that allows multiplexing multiple datagram contexts
   related to a given HTTP request.  Conceptually, the first layer of
   multiplexing is per-hop, while the second is end-to-end.

2.1.  Datagram Contexts

   Within the scope of a given HTTP request, contexts provide an
   additional demultiplexing layer.  Contexts determine the encoding of
   datagrams, and can be used to implicitly convey metadata.  For
   example, contexts can be used for compression to elide some parts of
   the datagram: the context identifier then maps to a compression
   context that the receiver can use to reconstruct the elided data.

   Contexts are identified within the scope of a given request by a
   numeric value, referred to as the context ID.  A context ID is a
   62-bit integer (0 to 2^62-1).

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   While stream IDs are a per-hop concept, context IDs are an end-to-end
   concept.  In other words, if a datagram travels through one or more
   intermediaries on its way from client to server, the stream ID will
   most likely change from hop to hop, but the context ID will remain
   the same.  Context IDs are opaque to intermediaries.

2.2.  Context ID Allocation

   Implementations of HTTP/3 that support the DATAGRAM extension MUST
   provide a context ID allocation service.  That service will allow
   applications co-located with HTTP/3 to request a unique context ID
   that they can subsequently use for their own purposes.  The HTTP/3
   implementation will then parse the context ID of incoming DATAGRAM
   frames and use it to deliver the frame to the appropriate application
   context.

   Even-numbered context IDs are client-initiated, while odd-numbered
   context IDs are server-initiated.  This means that an HTTP/3 client
   implementation of the context ID allocation service MUST only provide
   even-numbered IDs, while a server implementation MUST only provide
   odd-numbered IDs.  Note that, once allocated, any context ID can be
   used by both client and server - only allocation carries separate
   namespaces to avoid requiring synchronization.  Additionally, note
   that the context ID namespace is tied to a given HTTP request: it is
   possible for the same numeral context ID to be used simultaneously in
   distinct requests.

3.  HTTP/3 DATAGRAM Frame Format

   When used with HTTP/3, the Datagram Data field of QUIC DATAGRAM
   frames uses the following format (using the notation from the
   "Notational Conventions" section of [QUIC]):

   HTTP/3 Datagram {
     Quarter Stream ID (i),
     Context ID (i),
     HTTP/3 Datagram Payload (..),
   }

                   Figure 1: HTTP/3 DATAGRAM Frame Format

   Quarter Stream ID:  A variable-length integer that contains the value
      of the client-initiated bidirectional stream that this datagram is
      associated with, divided by four.  (The division by four stems
      from the fact that HTTP requests are sent on client-initiated
      bidirectional streams, and those have stream IDs that are
      divisible by four.)

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   Context ID:  A variable-length integer indicating the context ID of
      the datagram (see Section 2.1).

   HTTP/3 Datagram Payload:  The payload of the datagram, whose
      semantics are defined by individual applications.  Note that this
      field can be empty.

   Intermediaries parse the Quarter Stream ID field in order to
   associate the QUIC DATAGRAM frame with a stream.  If an intermediary
   receives a QUIC DATAGRAM frame whose payload is too short to allow
   parsing the Quarter Stream ID field, the intermediary MUST treat it
   as an HTTP/3 connection error of type H3_GENERAL_PROTOCOL_ERROR.
   Intermediaries MUST ignore any HTTP/3 Datagram fields after the
   Quarter Stream ID.

   Endpoints parse both the Quarter Stream ID field and the Context ID
   field in order to associate the QUIC DATAGRAM frame with a stream and
   context within that stream.  If an endpoint receives a QUIC DATAGRAM
   frame whose payload is too short to allow parsing the Quarter Stream
   ID field, the endpoint MUST treat it as an HTTP/3 connection error of
   type H3_GENERAL_PROTOCOL_ERROR.  If an endpoint receives a QUIC
   DATAGRAM frame whose payload is long enough to allow parsing the
   Quarter Stream ID field but too short to allow parsing the Context ID
   field, the endpoint MUST abruptly terminate the corresponding stream
   with a stream error of type H3_GENERAL_PROTOCOL_ERROR.

   If a DATAGRAM frame is received and its Quarter Stream ID maps to a
   stream that has already been closed, the receiver MUST silently drop
   that frame.  If a DATAGRAM frame is received and its Quarter Stream
   ID maps to a stream that has not yet been created, the receiver SHALL
   either drop that frame silently or buffer it temporarily while
   awaiting the creation of the corresponding stream.

4.  CAPSULE HTTP/3 Frame Definition

   CAPSULE allows reliably sending request-related information end-to-
   end, even in the presence of HTTP intermediaries.

   CAPSULE is an HTTP/3 Frame (as opposed to a QUIC frame) which SHALL
   only be sent in client-initiated bidirectional streams.
   Intermediaries MUST forward all received CAPSULE frames in their
   unmodified entirety on the same stream where it would forward DATA
   frames.  Intermediaries MUST NOT send any CAPSULE frames other than
   the ones it is forwarding.

   This specification of CAPSULE currently uses HTTP/3 frame type
   0xffcab5.  If this document is approved, a lower number will be
   requested from IANA.

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   CAPSULE HTTP/3 Frame {
     Type (i) = 0xffcab5,
     Length (i),
     Capsule Type (i),
     Capsule Data (..),
   }

                   Figure 2: CAPSULE HTTP/3 Frame Format

   The Type and Length fields follows the definition of HTTP/3 frames
   from [H3].  The payload consists of:

   Capsule Type:  The type of this capsule.

   Capsule Data:  Data whose semantics depends on the Capsule Type.

   Endpoints which receive a Capsule with an unknown Capsule Type MUST
   silently drop that Capsule.  Intermediaries MUST forward Capsules,
   even if they do not know the Capsule Type or cannot parse the Capsule
   Data.

4.1.  The REGISTER_DATAGRAM_CONTEXT Capsule

   The REGISTER_DATAGRAM_CONTEXT capsule (type=0x00) allows an endpoint
   to inform its peer of the encoding and semantics of datagrams
   associated with a given context ID.  Its Capsule Data field consists
   of:

   REGISTER_DATAGRAM_CONTEXT Capsule {
     Context ID (i),
     Extension String (..),
   }

             Figure 3: REGISTER_DATAGRAM_CONTEXT Capsule Format

   Context ID:  The context ID to register.

   Extension String:  A string of comma-separated key-value pairs to
      enable extensibility.  Keys are registered with IANA, see
      Section 8.4.

   The ABNF for the Extension String field is as follows (using syntax
   from Section 3.2.6 of [RFC7230]):

     extension-string = [ ext-member *( "," ext-member ) ]
     ext-member       = ext-member-key "=" ext-member-value
     ext-member-key   = token
     ext-member-value = token

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   Note that these registrations are unilateral and bidirectional: the
   sender of the frame unilaterally defines the semantics it will apply
   to the datagrams it sends and receives using this context ID.  Once a
   context ID is registered, it can be used in both directions.

   Endpoints MUST NOT send DATAGRAM frames using a Context ID until they
   have either sent or received a REGISTER_DATAGRAM_CONTEXT Capsule with
   the same Context ID.  However, due to reordering, an endpoint that
   receives a DATAGRAM frame with an unknown Context ID MUST NOT treat
   it as an error, it SHALL instead drop the DATAGRAM frame silently, or
   buffer it temporarily while awaiting the corresponding
   REGISTER_DATAGRAM_CONTEXT Capsule.

   Endpoints MUST NOT register the same Context ID twice on the same
   stream.  This also applies to Context IDs that have been closed using
   a CLOSE_DATAGRAM_CONTEXT capsule.  Clients MUST NOT register server-
   initiated Context IDs and servers MUST NOT register client-initiated
   Context IDs.  If an endpoint receives a REGISTER_DATAGRAM_CONTEXT
   capsule that violates one or more of these requirements, the endpoint
   MUST abruptly terminate the corresponding stream with a stream error
   of type H3_GENERAL_PROTOCOL_ERROR.

4.2.  The CLOSE_DATAGRAM_CONTEXT Capsule

   The CLOSE_DATAGRAM_CONTEXT capsule (type=0x01) allows an endpoint to
   inform its peer that it will no longer send or parse received
   datagrams associated with a given context ID.  Its Capsule Data field
   consists of:

   CLOSE_DATAGRAM_CONTEXT Capsule {
     Context ID (i),
     Extension String (..),
   }

              Figure 4: CLOSE_DATAGRAM_CONTEXT Capsule Format

   Context ID:  The context ID to close.

   Extension String:  A string of comma-separated key-value pairs to
      enable extensibility, see the definition of the same field in
      Section 4.1 for details.

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   Note that this close is unilateral and bidirectional: the sender of
   the frame unilaterally informs its peer of the closure.  Endpoints
   can use CLOSE_DATAGRAM_CONTEXT capsules to close a context that was
   initially registered by either themselves, or by their peer.
   Endpoints MAY use the CLOSE_DATAGRAM_CONTEXT capsule to immediately
   reject a context that was just registered using a
   REGISTER_DATAGRAM_CONTEXT capsule if they find its Extension String
   to be unacceptable.

   After an endpoint has either sent or received a
   CLOSE_DATAGRAM_CONTEXT frame, it MUST NOT send any DATAGRAM frames
   with that Context ID.  However, due to reordering, an endpoint that
   receives a DATAGRAM frame with a closed Context ID MUST NOT treat it
   as an error, it SHALL instead drop the DATAGRAM frame silently.

   Endpoints MUST NOT close a Context ID that was not previously
   registered.  Endpoints MUST NOT close a Context ID that has already
   been closed.  If an endpoint receives a CLOSE_DATAGRAM_CONTEXT
   capsule that violates one or more of these requirements, the endpoint
   MUST abruptly terminate the corresponding stream with a stream error
   of type H3_GENERAL_PROTOCOL_ERROR.

4.3.  The DATAGRAM Capsule

   The DATAGRAM capsule (type=0x02) allows an endpoint to send a
   datagram frame over an HTTP stream.  This is particularly useful when
   using a version of HTTP that does not support QUIC DATAGRAM frames.
   Its Capsule Data field consists of:

   DATAGRAM Capsule {
     Context ID (i),
     HTTP/3 Datagram Payload (..),
   }

                     Figure 5: DATAGRAM Capsule Format

   Context ID:  A variable-length integer indicating the context ID of
      the datagram (see Section 2.1).

   HTTP/3 Datagram Payload:  The payload of the datagram, whose
      semantics are defined by individual applications.  Note that this
      field can be empty.

   Datagrams sent using the DATAGRAM Capsule have the exact same
   semantics as datagrams sent in QUIC DATAGRAM frames.

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5.  The H3_DATAGRAM HTTP/3 SETTINGS Parameter

   Implementations of HTTP/3 that support this mechanism can indicate
   that to their peer by sending the H3_DATAGRAM SETTINGS parameter with
   a value of 1.  The value of the H3_DATAGRAM SETTINGS parameter MUST
   be either 0 or 1.  A value of 0 indicates that this mechanism is not
   supported.  An endpoint that receives the H3_DATAGRAM SETTINGS
   parameter with a value that is neither 0 or 1 MUST terminate the
   connection with error H3_SETTINGS_ERROR.

   An endpoint that sends the H3_DATAGRAM SETTINGS parameter with a
   value of 1 MUST send the max_datagram_frame_size QUIC Transport
   Parameter [DGRAM].  An endpoint that receives the H3_DATAGRAM
   SETTINGS parameter with a value of 1 on a QUIC connection that did
   not also receive the max_datagram_frame_size QUIC Transport Parameter
   MUST terminate the connection with error H3_SETTINGS_ERROR.

   When clients use 0-RTT, they MAY store the value of the server's
   H3_DATAGRAM SETTINGS parameter.  Doing so allows the client to use
   HTTP/3 datagrams in 0-RTT packets.  When servers decide to accept
   0-RTT data, they MUST send a H3_DATAGRAM SETTINGS parameter greater
   than or equal to the value they sent to the client in the connection
   where they sent them the NewSessionTicket message.  If a client
   stores the value of the H3_DATAGRAM SETTINGS parameter with their
   0-RTT state, they MUST validate that the new value of the H3_DATAGRAM
   SETTINGS parameter sent by the server in the handshake is greater
   than or equal to the stored value; if not, the client MUST terminate
   the connection with error H3_SETTINGS_ERROR.  In all cases, the
   maximum permitted value of the H3_DATAGRAM SETTINGS parameter is 1.

6.  HTTP/1.x and HTTP/2 Support

   We can provide DATAGRAM support in HTTP/2 by defining the CAPSULE
   frame in HTTP/2.

   We can provide DATAGRAM support in HTTP/1.x by defining its data
   stream format to a sequence of length-value capsules.

   TODO: Refactor this document into "HTTP Datagrams" with definitions
   for HTTP/1.x, HTTP/2, and HTTP/3.

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7.  Security Considerations

   Since this feature requires sending an HTTP/3 Settings parameter, it
   "sticks out".  In other words, probing clients can learn whether a
   server supports this feature.  Implementations that support this
   feature SHOULD always send this Settings parameter to avoid leaking
   the fact that there are applications using HTTP/3 datagrams enabled
   on this endpoint.

8.  IANA Considerations

8.1.  HTTP/3 CAPSULE Frame

   This document will request IANA to register the following entry in
   the "HTTP/3 Frames" registry:

     +------------+----------+---------------+
     | Frame Type |   Value  | Specification |
     +============+==========+===============+
     |  CAPSULE   | 0xffcab5 | This Document |
     +------------+----------+---------------+

8.2.  HTTP SETTINGS Parameter

   This document will request IANA to register the following entry in
   the "HTTP/3 Settings" registry:

     +--------------+----------+---------------+---------+
     | Setting Name |   Value  | Specification | Default |
     +==============+==========+===============+=========+
     | H3_DATAGRAM  | 0xffd276 | This Document |    0    |
     +--------------+----------+---------------+---------+

8.3.  Capsule Types

   This document establishes a registry for HTTP/3 frame type codes.
   The "HTTP Capsule Types" registry governs a 62-bit space.
   Registrations in this registry MUST include the following fields:

   Type:

   A name or label for the capsule type.

   Value:  The value of the Capsule Type field (see Section 4) is a
      62bit integer.

   Reference:  An optional reference to a specification for the type.
      This field MAY be empty.

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   Registrations follow the "First Come First Served" policy (see
   Section 4.4 of [IANA-POLICY]) where two registrations MUST NOT have
   the same Type.

   This registry initially contains the following entries:

   +---------------------------+-------+---------------+
   | Capsule Type              | Value | Specification |
   +---------------------------+-------+---------------+
   | REGISTER_DATAGRAM_CONTEXT | 0x00  | This Document |
   +---------------------------+-------+---------------+
   | CLOSE_DATAGRAM_CONTEXT    | 0x01  | This Document |
   +---------------------------+-------+---------------+
   | DATAGRAM                  | 0x02  | This Document |
   +---------------------------+-------+---------------+

8.4.  Context Extension Keys

   REGISTER_DATAGRAM_CONTEXT capsules carry key-value pairs, see
   Section 4.1.  This document will request IANA to create an "HTTP
   Datagram Context Extension Keys" registry.  Registrations in this
   registry MUST include the following fields:

   Key:  The key (see Section 4.1).  Keys MUST be valid tokens as
      defined in Section 3.2.6 of [RFC7230].

   Description:  A brief description of the key semantics, which MAY be
      a summary if a specification reference is provided.

   Reference:  An optional reference to a specification for the
      parameter.  This field MAY be empty.

   Registrations follow the "First Come First Served" policy (see
   Section 4.4 of [IANA-POLICY]) where two registrations MUST NOT have
   the same Key. This registry is initially empty.

9.  Normative References

   [DGRAM]    Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", Work in Progress, Internet-
              Draft, draft-ietf-quic-datagram-02, 16 February 2021,
              <https://tools.ietf.org/html/draft-ietf-quic-datagram-02>.

   [H3]       Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,
              <https://tools.ietf.org/html/draft-ietf-quic-http-34>.

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   [IANA-POLICY]
              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/rfc/rfc8126>.

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-34, 14 January 2021,
              <https://tools.ietf.org/html/draft-ietf-quic-transport-
              34>.

   [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/rfc/rfc2119>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7230>.

   [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/rfc/rfc8174>.

Appendix A.  Examples

A.1.  CONNECT-UDP

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   Client                                             Server

   STREAM(44): HEADERS             -------->
     :method = CONNECT-UDP
     :scheme = https
     :path = /
     :authority = target.example.org:443

   STREAM(44): CAPSULE             -------->
     Capsule Type = REGISTER_DATAGRAM_CONTEXT
     Context ID = 0
     Extension String = ""

   DATAGRAM                        -------->
     Quarter Stream ID = 11
     Context ID = 0
     Payload = Encapsulated UDP Payload

              <--------  STREAM(44): HEADERS
                           :status = 200

   /* Wait for target server to respond to UDP packet. */

              <--------  DATAGRAM
                           Quarter Stream ID = 11
                           Context ID = 0
                           Payload = Encapsulated UDP Payload

A.2.  CONNECT-UDP with Timestamp Extension

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   Client                                             Server

   STREAM(44): HEADERS            -------->
     :method = CONNECT-UDP
     :scheme = https
     :path = /
     :authority = target.example.org:443

   STREAM(44): CAPSULE            -------->
     Capsule Type = REGISTER_DATAGRAM_CONTEXT
     Context ID = 0
     Extension String = ""

   DATAGRAM                       -------->
     Quarter Stream ID = 11
     Context ID = 0
     Payload = Encapsulated UDP Payload

              <--------  STREAM(44): HEADERS
                           :status = 200

   /* Wait for target server to respond to UDP packet. */

              <--------  DATAGRAM
                           Quarter Stream ID = 11
                           Context ID = 0
                           Payload = Encapsulated UDP Payload

   STREAM(44): CAPSULE            -------->
     Capsule Type = REGISTER_DATAGRAM_CONTEXT
     Context ID = 2
     Extension String = "timestamp"

   DATAGRAM                       -------->
     Quarter Stream ID = 11
     Context ID = 2
     Payload = Encapsulated UDP Payload With Timestamp

A.3.  CONNECT-IP with IP compression

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   Client                                             Server

   STREAM(44): HEADERS            -------->
     :method = CONNECT-IP
     :scheme = https
     :path = /
     :authority = proxy.example.org:443

              <--------  STREAM(44): HEADERS
                           :status = 200

   /* Exchange CONNECT-IP configuration information. */

   STREAM(44): CAPSULE             -------->
     Capsule Type = REGISTER_DATAGRAM_CONTEXT
     Context ID = 0
     Extension String = ""

   DATAGRAM                       -------->
     Quarter Stream ID = 11
     Context ID = 0
     Payload = Encapsulated IP Packet

   /* Endpoint happily exchange encapsulated IP packets */
   /* using Quarter Stream ID 11 and Context ID 0.      */

   DATAGRAM                       -------->
     Quarter Stream ID = 11
     Context ID = 0
     Payload = Encapsulated IP Packet

   /* After performing some analysis on traffic patterns, */
   /* the client decides it wants to compress a 5-tuple.  */

   STREAM(44): CAPSULE             -------->
     Capsule Type = REGISTER_DATAGRAM_CONTEXT
     Context ID = 2
     Extension String = "ip=192.0.2.42,port=443"

   DATAGRAM                       -------->
     Quarter Stream ID = 11
     Context ID = 2
     Payload = Compressed IP Packet

A.4.  WebTransport

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   Client                                             Server

   STREAM(44): HEADERS            -------->
     :method = CONNECT
     :scheme = https
     :method = webtransport
     :path = /hello
     :authority = webtransport.example.org:443
     Origin = https://www.example.org:443

   STREAM(44): CAPSULE             -------->
     Capsule Type = REGISTER_DATAGRAM_CONTEXT
     Context ID = 0
     Extension String = ""

              <--------  STREAM(44): HEADERS
                           :status = 200

   /* Both endpoints can now send WebTransport datagrams. */

Acknowledgments

   The DATAGRAM context identifier was previously part of the DATAGRAM
   frame definition itself, the authors would like to acknowledge the
   authors of that document and the members of the IETF MASQUE working
   group for their suggestions.  Additionally, the authors would like to
   thank Martin Thomson for suggesting the use of an HTTP/3 SETTINGS
   parameter.  Furthermore, the authors would like to thank Ben Schwartz
   for writing the first proposal that used two layers of indirection.

Authors' Addresses

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043,
   United States of America

   Email: dschinazi.ietf@gmail.com

   Lucas Pardue
   Cloudflare

   Email: lucaspardue.24.7@gmail.com

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