HTTP Datagrams and the Capsule Protocol
draft-ietf-masque-h3-datagram-08

MASQUE                                                       D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                               L. Pardue
Expires: 29 September 2022                                    Cloudflare
                                                           28 March 2022

                HTTP Datagrams and the Capsule Protocol
                    draft-ietf-masque-h3-datagram-08

Abstract

   This document describes HTTP Datagrams, a convention for conveying
   multiplexed, potentially unreliable datagrams inside an HTTP
   connection.

   In HTTP/3, HTTP Datagrams can be conveyed natively using the QUIC
   DATAGRAM extension.  When the QUIC DATAGRAM frame is unavailable or
   undesirable, they can be sent using the Capsule Protocol, a more
   general convention for conveying data in HTTP connections.

   Both are intended for use by HTTP extensions, not applications.

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
   working documents as Internet-Drafts.  The list of current Internet-
   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."

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   This Internet-Draft will expire on 29 September 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  HTTP Datagrams  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  HTTP/3 Datagrams  . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  The H3_DATAGRAM HTTP/3 SETTINGS Parameter . . . . . .   5
     2.2.  HTTP Datagrams using Capsules . . . . . . . . . . . . . .   6
   3.  Capsules  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  HTTP Data Streams . . . . . . . . . . . . . . . . . . . .   7
     3.2.  The Capsule Protocol  . . . . . . . . . . . . . . . . . .   7
     3.3.  Error Handling  . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  The Capsule-Protocol Header Field . . . . . . . . . . . .   9
     3.5.  The DATAGRAM Capsule  . . . . . . . . . . . . . . . . . .   9
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  HTTP/3 SETTINGS Parameter . . . . . . . . . . . . . . . .  11
     5.2.  HTTP/3 Error Code . . . . . . . . . . . . . . . . . . . .  12
     5.3.  HTTP Header Field Name  . . . . . . . . . . . . . . . . .  12
     5.4.  Capsule Types . . . . . . . . . . . . . . . . . . . . . .  12
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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1.  Introduction

   HTTP extensions sometimes need to access underlying transport
   protocol features such as unreliable delivery (as offered by [DGRAM])
   to enable desirable features like an unreliable version of the
   CONNECT method, and unreliable delivery in WebSockets [RFC6455] (or
   its successors).

   In Section 2, this document describes HTTP Datagrams, a convention
   that supports the bidirectional and possibly multiplexed exchange of
   data inside an HTTP connection.  While HTTP datagrams are associated
   with HTTP requests, they are not part of message content; instead,
   they are intended for use by HTTP extensions (such as the CONNECT
   method), and are compatible with all versions of HTTP.  When the
   underlying transport protocol supports unreliable delivery (such as
   when the QUIC DATAGRAM extension is available in HTTP/3), they can
   use that capability.

   This document also describes the HTTP Capsule Protocol in Section 3,
   to allow conveyance of HTTP Datagrams when the QUIC DATAGRAM frame is
   unavailable or undesirable, such as when earlier versions of HTTP are
   in use.

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.  HTTP Datagrams

   HTTP Datagrams are a convention for conveying bidirectional and
   potentially unreliable datagrams inside an HTTP connection, with
   multiplexing when possible.  All HTTP Datagrams are associated with
   an HTTP request.

   When HTTP Datagrams are conveyed on an HTTP/3 connection, the QUIC
   DATAGRAM frame can be used to achieve these goals, including
   unreliable delivery; see Section 2.1.  Negotiation is achieved using
   a setting; see Section 2.1.1.

   When running over HTTP/2, demultiplexing is provided by the HTTP/2
   framing layer, but unreliable delivery is unavailable.  HTTP
   Datagrams are negotiated and conveyed using the Capsule Protocol; see
   Section 3.5.

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   When running over HTTP/1, requests are strictly serialized in the
   connection, and therefore demultiplexing is not available.
   Unreliable delivery is likewise not available.  HTTP Datagrams are
   negotiated and conveyed using the Capsule Protocol; see Section 3.5.

   HTTP Datagrams MUST only be sent with an association to a stream
   whose HTTP semantics explicitly supports HTTP Datagrams.  For
   example, existing HTTP methods GET and POST do not define semantics
   for associated HTTP Datagrams; therefore, HTTP Datagrams cannot be
   sent associated with GET or POST request streams.

   If an HTTP Datagram associated with a method that has no known
   semantics for HTTP Datagrams is received, the receiver MUST abort the
   corresponding stream; if HTTP/3 is in use, the stream MUST be aborted
   with H3_DATAGRAM_ERROR.  HTTP extensions can override these
   requirements by defining a negotiation mechanism and semantics for
   HTTP Datagrams.

2.1.  HTTP/3 Datagrams

   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 an HTTP/3 Datagram 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 the extension that is using HTTP Datagrams.  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.

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   HTTP/3 Datagrams MUST NOT be sent unless the corresponding stream's
   send side is open.  Once the receive side of a stream is closed,
   incoming datagrams for this stream are no longer expected so related
   state can be released.  State MAY be kept for a short time to account
   for reordering.  Once the state is released, the received associated
   datagrams MUST be silently dropped.

   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.

   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.

   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 communicating prioritization preferences.

2.1.1.  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.  If
   the H3_DATAGRAM SETTINGS parameter is received with a value that is
   neither 0 or 1, the receiver MUST terminate the connection with error
   H3_SETTINGS_ERROR.

   QUIC DATAGRAM frames MUST NOT be sent until the H3_DATAGRAM SETTINGS
   parameter has been both sent and received 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

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   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.

   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 4.

2.1.1.1.  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 sending multiple values for H3_DATAGRAM.  Once SETTINGS have been
   sent and received, an implementation that supports multiple drafts
   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 peers negotiate the
   same draft version.

2.2.  HTTP Datagrams using Capsules

   When HTTP/3 Datagrams are unavailable or undesirable, HTTP Datagrams
   can be sent using the Capsule Protocol, see Section 3.5.

3.  Capsules

   One mechanism to extend HTTP is to introduce new HTTP Upgrade Tokens
   (see Section 16.7 of [HTTP]).  In HTTP/1.x, these tokens are used via
   the Upgrade mechanism (see Section 7.8 of [HTTP]).  In HTTP/2 and
   HTTP/3, these tokens are used via the Extended CONNECT mechanism (see
   [EXT-CONNECT2] and [EXT-CONNECT3]).

   This specification introduces the Capsule Protocol.  The Capsule
   Protocol is a sequence of type-length-value tuples that definitions
   of new HTTP Upgrade Tokens 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, which is
   necessary when HTTP is running over a transport that does not support
   the QUIC DATAGRAM frame.

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3.1.  HTTP Data Streams

   This specification defines the "data stream" of an HTTP request as
   the bidirectional stream of bytes that follows the header section of
   the request message and the final, successful (i.e., 2xx) response
   message.

   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 response header section.  As a result, 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.

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

3.2.  The Capsule Protocol

   Definitions of new HTTP Upgrade Tokens can state that their
   associated request's data stream uses the Capsule Protocol.  If they
   do so, that means that the contents of the associated request's 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.

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   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.

   An intermediary can identify the use of the capsule protocol either
   through the presence of the Capsule-Protocol header field
   (Section 3.4) or by understanding the chosen HTTP Upgrade token.

   Because new protocols or extensions might define new capsule types,
   intermediaries that wish to allow for future extensibility SHOULD
   forward capsules without modification, unless the definition of the
   Capsule Type in use specifies additional intermediary processing.
   One such Capsule Type is the DATAGRAM capsule; see Section 3.5.  In
   particular, intermediaries SHOULD forward Capsules with an unknown
   Capsule Type without modification.

   Endpoints which receive a Capsule with an unknown Capsule Type MUST
   silently drop that Capsule and skip over it to parse the next
   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.  A receiver that observes a violation
   of these requirements MUST treat the HTTP message as malformed.

3.3.  Error Handling

   When an error occurs in 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].

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   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.

3.4.  The Capsule-Protocol Header Field

   The "Capsule-Protocol" header field is an Item Structured Field, see
   Section 3.3 of [STRUCT-FIELD]; its value MUST be a Boolean; any other
   value type MUST be handled as if the field were not present by
   recipients (for example, if this field is included multiple times,
   its type will become a List and the field will therefore be ignored).
   This document does not define any parameters for the Capsule-Protocol
   header field value, but future documents might define parameters.
   Receivers MUST ignore unknown parameters.

   Endpoints indicate that the Capsule Protocol is in use on a data
   stream by sending a Capsule-Protocol header field with a true value.
   A Capsule-Protocol header field with a false value 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 used on HTTP responses
   with a status code outside the 2xx range.

   When using the Capsule Protocol, HTTP endpoints SHOULD send the
   Capsule-Protocol header field to simplify intermediary processing.
   Definitions of new HTTP Upgrade Tokens that use the Capsule Protocol
   MAY alter this recommendation.

3.5.  The DATAGRAM Capsule

   This document defines the DATAGRAM capsule type (see Section 5.4 for
   the value of the capsule type).  This capsule allows HTTP Datagrams
   to be sent 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.

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   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 the extension that is using HTTP Datagrams.  Note
      that this field can be empty.

   HTTP Datagrams sent using the DATAGRAM capsule have the same
   semantics as those 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 2.1 also apply to HTTP Datagrams sent
   and received using the DATAGRAM capsule.

   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 HTTP extensions that use HTTP Datagrams will have their
   own limits on what datagram payload sizes are practical.
   Implementations SHOULD take those limits into account when parsing
   DATAGRAM capsules: if an incoming DATAGRAM capsule has a length that

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   is known to be so large as to not be usable, the implementation
   SHOULD discard the capsule without buffering its contents into
   memory.

   Note that use of the Capsule Protocol is not required to use HTTP
   Datagrams.  If an HTTP extension that uses HTTP Datagrams is only
   defined over transports that support QUIC DATAGRAM frames, it might
   not need a stream encoding.  Additionally, HTTP extensions can use
   HTTP Datagrams with their own data stream protocol.  However, new
   HTTP extensions that wish to use HTTP Datagrams SHOULD use the
   Capsule Protocol unless they have a good reason not to.

4.  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 2.1.1.

   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).

5.  IANA Considerations

5.1.  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

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5.2.  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

   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

5.3.  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

5.4.  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 3.2) is a
      62-bit 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 entry:

   Capsule Type:  DATAGRAM

   Value:  0xff37a5 (note that this will switch to a lower value before
      publication)

   Reference:  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.

6.  References

6.1.  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>.

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   [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>.

   [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>.

6.2.  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>.

   [EXT-CONNECT2]
              McManus, P., "Bootstrapping WebSockets with HTTP/2",
              RFC 8441, DOI 10.17487/RFC8441, September 2018,
              <https://www.rfc-editor.org/rfc/rfc8441>.

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   [EXT-CONNECT3]
              Hamilton, R., "Bootstrapping WebSockets with HTTP/3", Work
              in Progress, Internet-Draft, draft-ietf-httpbis-h3-
              websockets-04, 8 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              h3-websockets-04>.

   [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>.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,
              <https://www.rfc-editor.org/rfc/rfc6455>.

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.  The authors thank Mark Nottingham and Philipp Tiesel for
   their helpful comments.

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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