Using Datagrams with HTTP
draft-ietf-masque-h3-datagram-06

MASQUE                                                       D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                               L. Pardue
Expires: 5 September 2022                                     Cloudflare
                                                            4 March 2022

                       Using Datagrams with HTTP
                    draft-ietf-masque-h3-datagram-06

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 with HTTP/3 by association with HTTP requests.
   Additionally, this document defines the Capsule Protocol that can
   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
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 5 September 2022.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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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 Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Multiplexing  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  HTTP/3 Datagram Format  . . . . . . . . . . . . . . . . . . .   4
     3.1.  The H3_DATAGRAM HTTP/3 SETTINGS Parameter . . . . . . . .   5
       3.1.1.  Note About Draft Versions . . . . . . . . . . . . . .   6
   4.  Capsules  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Capsule Protocol  . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Error Handling  . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  The Capsule-Protocol Header Field . . . . . . . . . . . .   8
     4.4.  The DATAGRAM Capsule  . . . . . . . . . . . . . . . . . .   9
   5.  Prioritization  . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  HTTP/3 SETTINGS Parameter . . . . . . . . . . . . . . . .  11
     7.2.  HTTP/3 Error Code . . . . . . . . . . . . . . . . . . . .  11
     7.3.  HTTP Header Field Name  . . . . . . . . . . . . . . . . .  12
     7.4.  Capsule Types . . . . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  14
     A.1.  CONNECT-UDP . . . . . . . . . . . . . . . . . . . . . . .  14
     A.2.  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 with HTTP/3 [H3] by association with HTTP
   requests.  Additionally, this document defines the Capsule Protocol
   that can convey datagrams over prior versions of HTTP.

   This document is structured as follows:

   *  Section 2 presents core concepts for multiplexing across HTTP
      versions.

   *  Section 3 defines how QUIC DATAGRAM frames are used with HTTP/3.

      -  Section 3.1 defines an HTTP/3 setting that endpoints can use to
         advertise support of the frame.

   *  Section 4 introduces the Capsule Protocol and the "data stream"
      concept.  Data streams are initiated using special-purpose HTTP
      requests, after which Capsules, an end-to-end message, can be
      sent.

      -  Section 4.4 defines Datagram Capsule types, along with guidance
         for specifying new capsule types.

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

   All HTTP Datagrams are associated with an HTTP request.

   When running over HTTP/3, multiple exchanges of datagrams need the
   ability to coexist on a given QUIC connection.  To allow this, the
   QUIC DATAGRAM frame payload starts with an encoded stream identifier
   that associates the datagram with a request stream.

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

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]):

   HTTP/3 Datagram {
     Quarter Stream 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).  The largest legal QUIC stream ID value is
      2^62-1, so the largest legal value of Quarter Stream ID is 2^60-1.
      Receipt of a frame that includes a larger value MUST be treated as
      an HTTP/3 connection error of type H3_DATAGRAM_ERROR.

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

   Receipt of a QUIC DATAGRAM frame whose payload is too short to allow
   parsing the Quarter Stream ID field MUST be treated as an HTTP/3
   connection error of type H3_DATAGRAM_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 (on the order of a
   round trip) while awaiting the creation of the corresponding stream.

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   If an HTTP/3 datagram is received and its Quarter Stream ID maps to a
   stream that cannot be created due to client-initiated bidirectional
   stream limits, it SHOULD be treated as an HTTP/3 connection error of
   type H3_ID_ERROR.  Generating an error is not mandatory in this case
   because HTTP/3 implementations might have practical barriers to
   determining the active stream concurrency limit that is applied by
   the QUIC layer.

   HTTP/3 datagrams MUST only be sent with an association to a stream
   that supports semantics for HTTP Datagrams.  For example, existing
   HTTP methods GET and POST do not define semantics for associated HTTP
   Datagrams; therefore, HTTP/3 datagrams cannot be sent associated with
   GET or POST request streams.  If an endpoint receives an HTTP/3
   datagram associated with a method that has no known semantics for
   HTTP Datagrams, it MUST abort the corresponding stream with
   H3_DATAGRAM_ERROR.  Future extensions MAY remove these requirements
   if they define semantics for such HTTP Datagrams and negotiate mutual
   support.

3.1.  The H3_DATAGRAM HTTP/3 SETTINGS Parameter

   Implementations of HTTP/3 that support HTTP Datagrams 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 HTTP Datagrams are 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.

   Endpoints MUST NOT send QUIC DATAGRAM frames until they have both
   sent and received the H3_DATAGRAM SETTINGS parameter with a value of
   1.

   When clients use 0-RTT, they MAY store the value of the server's
   H3_DATAGRAM SETTINGS parameter.  Doing so allows the client to send
   QUIC DATAGRAM frames 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.

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   It is RECOMMENDED that implementations that support receiving HTTP
   Datagrams using QUIC always send the H3_DATAGRAM SETTINGS parameter
   with a value of 1, even if the application does not intend to use
   HTTP Datagrams.  This helps to avoid "sticking out"; see Section 6.

3.1.1.  Note About Draft Versions

   [[RFC editor: please remove this section before publication.]]

   Some revisions of this draft specification use a different value (the
   Identifier field of a Setting in the HTTP/3 SETTINGS frame) for the
   H3_DATAGRAM Settings Parameter.  This allows new draft revisions to
   make incompatible changes.  Multiple draft versions MAY be supported
   by either endpoint in a connection.  Such endpoints MUST send
   multiple values for H3_DATAGRAM.  Once an endpoint has sent and
   received SETTINGS, it MUST compute the intersection of the values it
   has sent and received, and then it MUST select and use the most
   recent draft version from the intersection set.  This ensures that
   both endpoints negotiate the same draft version.

4.  Capsules

   This specification introduces the Capsule Protocol.  The Capsule
   Protocol is a sequence of type-length-value tuples that new HTTP
   Upgrade Tokens (see Section 16.7 of [HTTP]) can choose to use.  It
   allows endpoints to reliably communicate request-related information
   end-to-end on HTTP request streams, even in the presence of HTTP
   intermediaries.  The Capsule Protocol can be used to exchange HTTP
   Datagrams when HTTP is running over a transport that does not support
   the QUIC DATAGRAM frame.

   This specification defines the "data stream" of an HTTP request as
   the bidirectional stream of bytes that follow the headers in both
   directions.  In HTTP/1.x, the data stream consists of all bytes on
   the connection that follow the blank line that concludes either the
   request header section, or the 2xx (Successful) response header
   section.  (Note that only a single HTTP request starting the capsule
   protocol can be sent on HTTP/1.x connections.)  In HTTP/2 and HTTP/3,
   the data stream of a given HTTP request consists of all bytes sent in
   DATA frames with the corresponding stream ID.  The concept of a data
   stream is particularly relevant for methods such as CONNECT where
   there is no HTTP message content after the headers.

   Note that use of the Capsule Protocol is not required to use HTTP
   Datagrams.  If a new HTTP Upgrade Token is only defined over
   transports that support QUIC DATAGRAM frames, they might not need a
   stream encoding.  Additionally, definitions of new HTTP Upgrade
   Tokens can use HTTP Datagrams with their own data stream protocol.

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   However, new HTTP Upgrade Tokens that wish to use HTTP Datagrams
   SHOULD use the Capsule Protocol unless they have a good reason not
   to.

4.1.  Capsule Protocol

   Definitions of new HTTP Upgrade Tokens can state that their data
   stream uses the Capsule Protocol.  If they do so, that means that the
   contents of their data stream uses the following format (using the
   notation from the "Notational Conventions" section of [QUIC]):

   Capsule Protocol {
     Capsule (..) ...,
   }

                  Figure 2: Capsule Protocol Stream Format

   Capsule {
     Capsule Type (i),
     Capsule Length (i),
     Capsule Value (..),
   }

                          Figure 3: Capsule Format

   Capsule Type:  A variable-length integer indicating the Type of the
      capsule.  Endpoints that receive a capsule with an unknown Capsule
      Type MUST silently skip over that capsule.

   Capsule Length:  The length of the Capsule Value field following this
      field, encoded as a variable-length integer.  Note that this field
      can have a value of zero.

   Capsule Value:  The payload of this capsule.  Its semantics are
      determined by the value of the Capsule Type field.

   Because new protocols or extensions may involve defining new capsule
   types, intermediaries that wish to allow for future extensibility
   SHOULD forward capsules unmodified.  One exception to this rule is
   the DATAGRAM capsule; see Section 4.4.  An intermediary can identify
   the use of the capsule protocol either through the presence of the
   Capsule-Protocol header field (Section 4.3) or by understanding the
   chosen HTTP Upgrade token.  An intermediary that identifies the use
   of the capsule protocol MAY convert between DATAGRAM capsules and
   QUIC DATAGRAM frames when forwarding.  Definitions of new Capsule
   Types MAY specify optional custom intermediary processing.

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

   By virtue of the definition of the data stream, the Capsule Protocol
   is not in use on responses unless the response includes a 2xx
   (Successful) status code.

   The Capsule Protocol MUST NOT be used with messages that contain
   Content-Length, Content-Type, or Transfer-Encoding header fields.
   Additionally, HTTP status codes 204 (No Content), 205 (Reset
   Content), and 206 (Partial Content) MUST NOT be sent on responses
   that use the Capsule Protocol.

4.2.  Error Handling

   When an error occurs processing the capsule protocol, the receiver
   MUST treat the message as malformed or incomplete, according to the
   underlying transport protocol.  For HTTP/3, the handling of malformed
   messages is described in Section 4.1.3 of [H3].  For HTTP/2, the
   handling of malformed messages is described in Section 8.1.1 of [H2].
   For HTTP/1.1, the handling of incomplete messages is described in
   Section 8 of [H1].

   Each capsule's payload MUST contain exactly the fields identified in
   its description.  A capsule payload that contains additional bytes
   after the identified fields or a capsule payload that terminates
   before the end of the identified fields MUST be treated as a
   malformed or incomplete message.  In particular, redundant length
   encodings MUST be verified to be self-consistent.

   When a stream carrying capsules terminates cleanly, if the last
   capsule on the stream was truncated, this MUST be treated as a
   malformed or incomplete message.

4.3.  The Capsule-Protocol Header Field

   This document defines the "Capsule-Protocol" header field.  It is an
   Item Structured Field, see Section 3.3 of [STRUCT-FIELD]; its value
   MUST be a Boolean.  Its ABNF is:

   Capsule-Protocol = sf-item

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   Endpoints indicate that the Capsule Protocol is in use on the data
   stream by sending the Capsule-Protocol header field with a value of
   ?1.  A Capsule-Protocol header field with a value of ?0 has the same
   semantics as when the header is not present.  Intermediaries MAY use
   this header field to allow processing of HTTP Datagrams for unknown
   HTTP Upgrade Tokens; note that this is only possible for HTTP Upgrade
   or Extended CONNECT.

   The Capsule-Protocol header field MUST NOT be sent multiple times on
   a message.  The Capsule-Protocol header field MUST NOT be used on
   HTTP responses with a status code different from 2xx (Successful).
   This specification does not define any parameters for the Capsule-
   Protocol header field value, but future documents MAY define
   parameters.  Receivers MUST ignore unknown parameters.

   Definitions of new HTTP Upgrade Tokens that use the Capsule Protocol
   MAY use the Capsule-Protocol header field to simplify intermediary
   processing.

4.4.  The DATAGRAM Capsule

   This document defines the DATAGRAM capsule type (see Section 7.4 for
   the value of the capsule type).  This capsule allows an endpoint to
   send an HTTP Datagram on a stream using the Capsule Protocol.  This
   is particularly useful when HTTP is running over a transport that
   does not support the QUIC DATAGRAM frame.

   Datagram Capsule {
     Type (i) = DATAGRAM,
     Length (i),
     HTTP Datagram Payload (..),
   }

                     Figure 4: DATAGRAM Capsule Format

   HTTP 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 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.

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   An intermediary can 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 that are sent on a stream are
   reliably delivered in order, intermediaries can reencode DATAGRAM
   capsules into QUIC DATAGRAM frames when forwarding messages, which
   could result in loss or reordering.

   If an intermediary receives an HTTP Datagram in a QUIC DATAGRAM frame
   and is forwarding it on a connection that supports QUIC DATAGRAM
   frames, the intermediary SHOULD NOT convert that HTTP Datagram to a
   DATAGRAM capsule.  If the HTTP Datagram is too large to fit in a
   DATAGRAM frame (for example because the path MTU of that QUIC
   connection is too low or if the maximum UDP payload size advertised
   on that connection is too low), the intermediary SHOULD drop the HTTP
   Datagram instead of converting it to a DATAGRAM capsule.  This
   preserves the end-to-end unreliability characteristic that methods
   such as Datagram Packetization Layer Path MTU Discovery (DPLPMTUD)
   depend on [DPLPMTUD].  An intermediary that converts QUIC DATAGRAM
   frames to DATAGRAM capsules allows HTTP Datagrams to be arbitrarily
   large without suffering any loss; this can misrepresent the true path
   properties, defeating methods such as DPLPMTUD.

   While DATAGRAM capsules can theoretically carry a payload of length
   2^62-1, most applications will have their own limits on what datagran
   payload sizes are practical.  Implementations SHOULD take those
   limits into account when parsing DATAGRAM capsules: if an incoming
   DATAGRAM capsule has a length that is known to be so large as to not
   be usable, the implementation SHOULD discard the capsule without
   buffering its contents into memory.

5.  Prioritization

   Data streams (see Section 4.1) can be prioritized using any means
   suited to stream or request prioritization.  For example, see
   Section 11 of [PRIORITY].

   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.

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6.  Security Considerations

   Since transmitting HTTP Datagrams using QUIC DATAGRAM frames requires
   sending an HTTP/3 Settings parameter, it "sticks out".  In other
   words, probing clients can learn whether a server supports HTTP
   Datagrams over QUIC DATAGRAM frames.  As some servers might wish to
   obfuscate the fact that they offer application services that use HTTP
   datagrams, it's best for all implementations that support this
   feature to always send this Settings parameter, see Section 3.1.

   Since use of the Capsule Protocol is restricted to new HTTP Upgrade
   Tokens, it is not accessible from Web Platform APIs (such as those
   commonly accessed via JavaScript in web browsers).

7.  IANA Considerations

7.1.  HTTP/3 SETTINGS Parameter

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

   Value:  0xffd277 (note that this will switch to a lower value before
      publication)

   Setting Name:  H3_DATAGRAM

   Default:  0

   Status:  provisional (permanent if this document is approved)

   Specification:  This Document

   Change Controller:  IETF

   Contact:  HTTP_WG; HTTP working group; ietf-http-wg@w3.org

7.2.  HTTP/3 Error Code

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

   Value:  0x4A1268 (note that this will switch to a lower value before
      publication)

   Name:  H3_DATAGRAM_ERROR

   Description:  Datagram or capsule protocol parse error

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   Status:  provisional (permanent if this document is approved)

   Specification:  This Document

   Change Controller:  IETF

   Contact:  HTTP_WG; HTTP working group; ietf-http-wg@w3.org

7.3.  HTTP Header Field Name

   This document will request IANA to register the following entry in
   the "HTTP Field Name" registry:

   Field Name:  Capsule-Protocol

   Template:  None

   Status:  provisional (permanent if this document is approved)

   Reference:  This document

   Comments:  None

7.4.  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 4.1) is a
      62-bit 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 entry:

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                +==============+==========+===============+
                | Capsule Type | Value    | Specification |
                +==============+==========+===============+
                | DATAGRAM     | 0xff37a5 | This Document |
                +--------------+----------+---------------+

                  Table 1: Initial Capsule Types Registry

   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.

8.  References

8.1.  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-10, 4 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              datagram-10>.

   [H1]       Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              messaging-19>.

   [H2]       Thomson, M. and C. Benfield, "HTTP/2", Work in Progress,
              Internet-Draft, draft-ietf-httpbis-http2bis-07, 24 January
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              httpbis-http2bis-07>.

   [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>.

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/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>.

   [STRUCT-FIELD]
              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/rfc/rfc8941>.

8.2.  Informative References

   [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/rfc/rfc8899>.

   [PRIORITY] Oku, K. and L. Pardue, "Extensible Prioritization Scheme
              for HTTP", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-priority-12, 17 January 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              priority-12>.

Appendix A.  Examples

   [[RFC editor: please remove this appendix before publication.]]

A.1.  CONNECT-UDP

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   Client                                             Server

   STREAM(44): HEADERS             -------->
     :method = CONNECT
     :protocol = connect-udp
     :scheme = https
     :path = /target.example.org/443/
     :authority = proxy.example.org:443
     capsule-protocol = ?1

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

              <--------  STREAM(44): HEADERS
                           :status = 200
                           capsule-protocol = ?1

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

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

A.2.  WebTransport

   Client                                             Server

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

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

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

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Acknowledgments

   Portions of this document were previously part of the QUIC 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.
   The final design in this document came out of the HTTP Datagrams
   Design Team, whose members were Alan Frindell, Alex Chernyakhovsky,
   Ben Schwartz, Eric Rescorla, Marcus Ihlar, Martin Thomson, Mike
   Bishop, Tommy Pauly, Victor Vasiliev, and the authors of this
   document.

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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