Using Datagrams with HTTP
draft-ietf-masque-h3-datagram-03

MASQUE                                                       D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                               L. Pardue
Expires: 13 January 2022                                      Cloudflare
                                                            12 July 2021

                       Using Datagrams with HTTP
                    draft-ietf-masque-h3-datagram-03

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.
   Additionally, this document defines how to convey datagrams over
   prior versions of HTTP.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the MASQUE WG mailing list
   (masque@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/masque/.

   Source for this draft and an issue tracker can be found at
   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.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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Copyright Notice

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

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   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 . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Context ID Allocation . . . . . . . . . . . . . . . . . .   4
   3.  HTTP/3 DATAGRAM Format  . . . . . . . . . . . . . . . . . . .   4
   4.  CAPSULE HTTP/3 Frame Definition . . . . . . . . . . . . . . .   6
     4.1.  The REGISTER_DATAGRAM_CONTEXT Capsule . . . . . . . . . .   7
     4.2.  The REGISTER_DATAGRAM_NO_CONTEXT Capsule  . . . . . . . .   9
     4.3.  The CLOSE_DATAGRAM_CONTEXT Capsule  . . . . . . . . . . .  10
     4.4.  The DATAGRAM Capsule  . . . . . . . . . . . . . . . . . .  11
   5.  Context Extensibility . . . . . . . . . . . . . . . . . . . .  12
     5.1.  The CLOSE_CODE Context Extension Type . . . . . . . . . .  13
     5.2.  The DETAILS Context Extension Type  . . . . . . . . . . .  13
   6.  The H3_DATAGRAM HTTP/3 SETTINGS Parameter . . . . . . . . . .  13
   7.  Prioritization  . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  HTTP/1.x and HTTP/2 Support . . . . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  HTTP/3 CAPSULE Frame . . . . . . . . . . . . . . . . . .  14
     10.2.  HTTP/3 SETTINGS Parameter  . . . . . . . . . . . . . . .  15
     10.3.  Capsule Types  . . . . . . . . . . . . . . . . . . . . .  15
     10.4.  Context Extension Types  . . . . . . . . . . . . . . . .  16
     10.5.  Context Close Codes  . . . . . . . . . . . . . . . . . .  17
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  17
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  18
     A.1.  CONNECT-UDP . . . . . . . . . . . . . . . . . . . . . . .  18
     A.2.  CONNECT-UDP with Timestamp Extension  . . . . . . . . . .  19
     A.3.  CONNECT-IP with IP compression  . . . . . . . . . . . . .  20
     A.4.  WebTransport  . . . . . . . . . . . . . . . . . . . . . .  21

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   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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.  Additionally, this document defines how to
   convey datagrams over prior versions of HTTP.

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

   When running over HTTP/3, multiple exchanges of datagrams need the
   ability to coexist on a given QUIC connection.  To allow this, 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
   optionally 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.

   When running over HTTP/2, the first level of demultiplexing is
   provided by the HTTP/2 framing layer.  When running over HTTP/1,
   requests are strictly serialized in the connection, therefore the
   first layer of demultiplexing is not needed.

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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 optional, their use is negotiated on each request stream
   using registration capsules, see Section 4.1 and Section 4.2.  When
   contexts are used, they 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).

   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 Datagrams MUST provide a context ID
   allocation service.  That service will allow applications co-located
   with HTTP to request a unique context ID that they can subsequently
   use for their own purposes.  The HTTP implementation will then parse
   the context ID of incoming HTTP Datagrams 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 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 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]):

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

                      Figure 1: HTTP/3 DATAGRAM 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.)

   Context ID:  A variable-length integer indicating the context ID of
      the datagram (see Section 2.1).  Whether or not this field is
      present depends on which registration capsules were exchanged on
      the associated stream: if a REGISTER_DATAGRAM_CONTEXT capsule (see
      Section 4.1) has been sent or received on this stream, then the
      field is present; if a REGISTER_DATAGRAM_NO_CONTEXT capsule (see
      Section 4.2) has been sent or received, then this field is absent;
      if neither has been sent or received, then it is not yet possible
      to parse this datagram and the receiver MUST either drop that
      datagram silently or buffer it temporarily while awaiting the
      registration capsule.

   HTTP 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.  The
   Context ID field is optional and its use is negotiated end-to-end,
   see Section 4.2.  Therefore intermediaries cannot know whether the
   Context ID field is present or absent and they 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

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   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.

   Endpoints MUST NOT send HTTP/3 datagrams unless the corresponding
   stream's send side is open.  On a given endpoint, once the receive
   side of a stream is closed, incoming datagrams for this stream are no
   longer expected so the endpoint can release related state.  Endpoints
   MAY keep state for a short time to account for reordering.  Once the
   state is released, the endpoint MUST silently drop received
   associated datagrams.

   If an HTTP/3 datagram is received and its Quarter Stream ID maps to a
   stream that has not yet been created, the receiver SHALL either drop
   that datagram 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 forward received CAPSULE frames on the same stream
   where it would forward DATA frames.  Each Capsule Type determines
   whether it is opaque or transparent to intermediaries: opaque
   capsules are forwarded unmodified while transparent ones can be
   parsed, added, or removed by intermediaries.

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

   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.

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

   Unless otherwise specified, all Capsule Types are defined as opaque
   to intermediaries.  Intermediaries MUST forward all received opaque
   CAPSULE frames in their unmodified entirety.  Intermediaries MUST NOT
   send any opaque CAPSULE frames other than the ones it is forwarding.
   All Capsule Types defined in this document are opaque, with the
   exception of the DATAGRAM Capsule, see Section 4.4.  Definitions of
   new Capsule Types MAY specify that the newly introduced type is
   transparent.  Intermediaries MUST treat unknown Capsule Types as
   opaque.

   Intermediaries respect the order of opaque CAPSULE frames: if an
   intermediary receives two opaque CAPSULE frames in a given order, it
   MUST forward them in the same order.

   Endpoints which receive a Capsule with an unknown Capsule Type MUST
   silently drop that Capsule.

   Receipt of a CAPSULE HTTP/3 Frame on a stream that is not a client-
   initiated bidirectional stream MUST be treated as a connection error
   of type H3_FRAME_UNEXPECTED.

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),
     Context Extensions (..),
   }

             Figure 3: REGISTER_DATAGRAM_CONTEXT Capsule Format

   Context ID:  The context ID to register.

   Context Extensions:  See Section 5.

   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.

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   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.

   Endpoints MUST NOT send a REGISTER_DATAGRAM_CONTEXT capsule on a
   stream before they have sent at least one HEADERS frame on that
   stream.  This removes the need to buffer REGISTER_DATAGRAM_CONTEXT
   capsules when the endpoint needs information from headers to
   determine how to react to the capsule.  If an endpoint receives a
   REGISTER_DATAGRAM_CONTEXT capsule on a stream that hasn't yet
   received a HEADERS frame, the endpoint MUST abruptly terminate the
   corresponding stream with a stream error of type
   H3_GENERAL_PROTOCOL_ERROR.

   Servers MUST NOT send a REGISTER_DATAGRAM_CONTEXT capsule on a stream
   before they have received at least one REGISTER_DATAGRAM_CONTEXT
   capsule or one REGISTER_DATAGRAM_NO_CONTEXT capsule from the client
   on that stream.  This ensures that clients control whether datagrams
   are allowed for a given request.  If a client receives a
   REGISTER_DATAGRAM_CONTEXT capsule on a stream where the client has
   not yet sent a REGISTER_DATAGRAM_CONTEXT capsule, the client MUST
   abruptly terminate the corresponding stream with a stream error of
   type H3_GENERAL_PROTOCOL_ERROR.

   Servers MUST NOT send a REGISTER_DATAGRAM_CONTEXT capsule on a stream
   where it has received a REGISTER_DATAGRAM_NO_CONTEXT capsule.  If a
   client receives a REGISTER_DATAGRAM_CONTEXT capsule on a stream where
   the client has sent a REGISTER_DATAGRAM_NO_CONTEXT capsule, the
   client MUST abruptly terminate the corresponding stream with a stream
   error of type H3_GENERAL_PROTOCOL_ERROR.

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4.2.  The REGISTER_DATAGRAM_NO_CONTEXT Capsule

   The REGISTER_DATAGRAM_NO_CONTEXT capsule (type=0x03) allows a client
   to inform the server that datagram contexts will not be used with
   this stream.  It also informs the server of the encoding and
   semantics of datagrams associated with this stream.  Its Capsule Data
   field consists of:

   REGISTER_DATAGRAM_NO_CONTEXT Capsule {
     Context Extensions (..),
   }

           Figure 4: REGISTER_DATAGRAM_NO_CONTEXT Capsule Format

   Context Extensions:  See Section 5.

   Note that this registration is unilateral and bidirectional: the
   client unilaterally defines the semantics it will apply to the
   datagrams it sends and receives with this stream.

   Endpoints MUST NOT send DATAGRAM frames without a Context ID until
   they have either sent or received a REGISTER_DATAGRAM_NO_CONTEXT
   Capsule.  However, due to reordering, an endpoint that receives a
   DATAGRAM frame before receiving either a REGISTER_DATAGRAM_CONTEXT
   capsule or a REGISTER_DATAGRAM_NO_CONTEXT capsule MUST NOT treat it
   as an error, it SHALL instead drop the DATAGRAM frame silently, or
   buffer it temporarily while awaiting a REGISTER_DATAGRAM_NO_CONTEXT
   capsule or the corresponding REGISTER_DATAGRAM_CONTEXT capsule.

   Servers MUST NOT send the REGISTER_DATAGRAM_NO_CONTEXT capsule.  If a
   client receives a REGISTER_DATAGRAM_NO_CONTEXT capsule, the client
   MUST abruptly terminate the corresponding stream with a stream error
   of type H3_GENERAL_PROTOCOL_ERROR.

   Clients MUST NOT send more than one REGISTER_DATAGRAM_NO_CONTEXT
   capsule on a stream.  If a server receives a second
   REGISTER_DATAGRAM_NO_CONTEXT capsule on the same stream, the server
   MUST abruptly terminate the corresponding stream with a stream error
   of type H3_GENERAL_PROTOCOL_ERROR.

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   Clients MUST NOT send a REGISTER_DATAGRAM_NO_CONTEXT capsule on a
   stream before they have sent at least one HEADERS frame on that
   stream.  This removes the need to buffer REGISTER_DATAGRAM_CONTEXT
   capsules when the server needs information from headers to determine
   how to react to the capsule.  If a server receives a
   REGISTER_DATAGRAM_NO_CONTEXT capsule on a stream that hasn't yet
   received a HEADERS frame, the server MUST abruptly terminate the
   corresponding stream with a stream error of type
   H3_GENERAL_PROTOCOL_ERROR.

   Clients MUST NOT send both REGISTER_DATAGRAM_CONTEXT capsules and
   REGISTER_DATAGRAM_NO_CONTEXT capsules on the same stream.  If a
   server receives both a REGISTER_DATAGRAM_CONTEXT capsule and a
   REGISTER_DATAGRAM_NO_CONTEXT capsule on the same stream, the server
   MUST abruptly terminate the corresponding stream with a stream error
   of type H3_GENERAL_PROTOCOL_ERROR.

   Extensions MAY define a different mechanism to negotiate the presence
   of contexts, and they MAY do so in a way which is opaque to
   intermediaries.

4.3.  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),
     Context Extensions (..),
   }

              Figure 5: CLOSE_DATAGRAM_CONTEXT Capsule Format

   Context ID:  The context ID to close.

   Context Extensions:  See Section 5.

   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 Context Extensions
   field to be unacceptable.

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   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.

   All CLOSE_DATAGRAM_CONTEXT capsules MUST contain a CLOSE_CODE context
   extension, see Section 5.1.  If an endpoint receives a
   CLOSE_DATAGRAM_CONTEXT capsule without a CLOSE_CODE context
   extension, the endpoint MUST abruptly terminate the corresponding
   stream with a stream error of type H3_GENERAL_PROTOCOL_ERROR.

4.4.  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 Datagram Payload (..),
   }

                     Figure 6: DATAGRAM Capsule Format

   Context ID:  A variable-length integer indicating the context ID of
      the datagram (see Section 2.1).  Whether or not this field is
      present depends on which registration capsules were exchanged on
      the associated stream: if a REGISTER_DATAGRAM_CONTEXT capsule (see
      Section 4.1) has been sent or received on this stream, then the
      field is present; if a REGISTER_DATAGRAM_NO_CONTEXT capsule (see
      Section 4.2) has been sent or received, then this field is absent;
      if neither has been sent or received, then it is not yet possible
      to parse this datagram and the receiver MUST either drop that
      datagram silently or buffer it temporarily while awaiting the
      registration capsule.

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

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   Datagrams sent using the DATAGRAM Capsule have the exact same
   semantics as datagrams sent in QUIC DATAGRAM frames.  In particular,
   the restrictions on when it is allowed to send an HTTP Datagram and
   how to process them from Section 3 also apply to HTTP Datagrams sent
   and received using the DATAGRAM capsule.

   The DATAGRAM Capsule is transparent to intermediaries, meaning that
   intermediaries MAY parse it and send DATAGRAM Capsules that they did
   not receive.  This allows an intermediary to reencode HTTP Datagrams
   as it forwards them: in other words, an intermediary MAY send a
   DATAGRAM Capsule to forward an HTTP Datagram which was received in a
   QUIC DATAGRAM frame, and vice versa.

   Note that while DATAGRAM capsules are sent on a stream,
   intermediaries can reencode HTTP Datagrams into QUIC DATAGRAM frames
   over the next hop, and those could be dropped.  Because of this,
   applications have to always consider HTTP Datagrams to be unreliable,
   even if they were initially sent in a capsule.

5.  Context Extensibility

   In order to facilitate extensibility of contexts, the
   REGISTER_DATAGRAM_CONTEXT, REGISTER_DATAGRAM_NO_CONTEXT, and the
   CLOSE_DATAGRAM_CONTEXT capsules carry a Context Extensions field.
   That field contains a sequence of context extensions:

     Context Extensions {
       Context Extension (..) ...,
     }

   Each context extension is encoded as a (type, length, value) tuple:

     Context Extension {
       Context Extension Type (i),
       Context Extension Length (i),
       Context Extension Value (..),
     }

   Context Extension Types are registered with IANA, see Section 10.4.
   The Context Extension Length field contains the length of the Context
   Extension Value field in bytes.  The semantics of the Context
   Extension Value field are defined by the corresponding Context
   Extension Type.

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5.1.  The CLOSE_CODE Context Extension Type

   The CLOSE_CODE context extension type (type=0x00) allows an endpoint
   to provide additional information as to why a datagram context was
   closed.  This type SHALL only be sent in CLOSE_DATAGRAM_CONTEXT
   capsules.  Its Context Extension Value field consists of a single
   variable-length integer which contains the close code.  The following
   codes are defined:

   NO_ERROR (code=0x00):  This indicates that the registration was
      closed without any additional information.

   DENIED (code=0x01):  This indicates that the sender has rejected the
      context registration based on its local policy.  The endpoint that
      had originally registered this context MUST NOT try to register
      another context with the same context extensions on this stream.

   RESOURCE_LIMIT (code=0x02):  This indicates that the context was
      closed to save resources.  The recipient SHOULD limit its future
      registration of resource-incentive contexts.

   Receipt of an unknown close code MUST be treated as if the NO_ERROR
   code was present.  Close codes are registered with IANA, see
   Section 10.5.

5.2.  The DETAILS Context Extension Type

   The DETAILS context extension type (type=0x01) allows an endpoint to
   provide additional details to context capsules.  It is meant for
   debugging purposes.  Its Context Extension Value field consists of a
   human-readable string encoded in UTF-8.

6.  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.

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   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.

7.  Prioritization

   Prioritization of HTTP/3 datagrams is not defined in this document.
   Future extensions MAY define how to prioritize datagrams, and MAY
   define signaling to allow endpoints to communicate their
   prioritization preferences.

8.  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 and add definitions for HTTP/1.x and
   HTTP/2.

9.  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.

10.  IANA Considerations

10.1.  HTTP/3 CAPSULE Frame

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

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

10.2.  HTTP/3 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    |
     +--------------+----------+---------------+---------+

10.3.  Capsule Types

   This document establishes a registry for HTTP capsule 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.

   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:

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   +------------------------------+-------+---------------+
   | Capsule Type                 | Value | Specification |
   +------------------------------+-------+---------------+
   | REGISTER_DATAGRAM_CONTEXT    | 0x00  | This Document |
   +------------------------------+-------+---------------+
   | CLOSE_DATAGRAM_CONTEXT       | 0x01  | This Document |
   +------------------------------+-------+---------------+
   | DATAGRAM                     | 0x02  | This Document |
   +------------------------------+-------+---------------+
   | REGISTER_DATAGRAM_NO_CONTEXT | 0x03  | This Document |
   +------------------------------+-------+---------------+

   Capsule types with a value of the form 41 * N + 23 for integer values
   of N are reserved to exercise the requirement that unknown capsule
   types be ignored.  These capsules have no semantics and can carry
   arbitrary values.  These values MUST NOT be assigned by IANA and MUST
   NOT appear in the listing of assigned values.

10.4.  Context Extension Types

   This document establishes a registry for HTTP datagram context
   extension type codes.  The "HTTP Context Extension Types" registry
   governs a 62-bit space.  Registrations in this registry MUST include
   the following fields:

   Type:

   A name or label for the context extension type.

   Value:  The value of the Context Extension Type field (see Section 5)
      is a 62bit integer.

   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 Type nor Value.

   This registry initially contains the following entries:

   +------------------------------+-------+---------------+
   | Context Extension Type       | Value | Specification |
   +------------------------------+-------+---------------+
   | CLOSE_CODE                   | 0x00  | This Document |
   +------------------------------+-------+---------------+
   | DETAILS                      | 0x01  | This Document |
   +------------------------------+-------+---------------+

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   Context extension types with a value of the form 41 * N + 17 for
   integer values of N are reserved to exercise the requirement that
   unknown context extension types be ignored.  These extensions have no
   semantics and can carry arbitrary values.  These values MUST NOT be
   assigned by IANA and MUST NOT appear in the listing of assigned
   values.

10.5.  Context Close Codes

   This document establishes a registry for HTTP context extension type
   codes.  The "HTTP Context Close Codes" registry governs a 62-bit
   space.  Registrations in this registry MUST include the following
   fields:

   Type:

   A name or label for the close code.

   Value:  The value of the CLOSE_CODE Context Extension Value field
      (see Section 5.1) is a 62bit integer.

   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 Type nor Value.

   This registry initially contains the following entries:

   +------------------------------+-------+---------------+
   | Context Close Code           | Value | Specification |
   +------------------------------+-------+---------------+
   | NO_ERROR                     | 0x00  | This Document |
   +------------------------------+-------+---------------+
   | DENIED                       | 0x01  | This Document |
   +------------------------------+-------+---------------+
   | RESOURCE_LIMIT               | 0x02  | This Document |
   +------------------------------+-------+---------------+

   Context close codes with a value of the form 41 * N + 19 for integer
   values of N are reserved to exercise the requirement that unknown
   context close codes be treated as NO_ERROR.  These values MUST NOT be
   assigned by IANA and MUST NOT appear in the listing of assigned
   values.

11.  Normative References

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   [DGRAM]    Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", Work in Progress, Internet-
              Draft, draft-ietf-quic-datagram-03, 12 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              datagram-03>.

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

   [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://datatracker.ietf.org/doc/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>.

   [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
     Context Extension = {}

   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
     Context Extension = {}

   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
     Context Extension = {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
   Context Extension = {}

 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
   Context Extension = {IP_COMPRESSION=tcp,192.0.2.6:9876,192.0.2.7:443}

 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_NO_CONTEXT
     Context Extension = {}

              <--------  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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