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WebTransport using HTTP/2
draft-ietf-webtrans-http2-02

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Document Type Active Internet-Draft (webtrans WG)
Authors Alan Frindell , Eric Kinnear , Tommy Pauly , Martin Thomson , Victor Vasiliev , Guowu Xie
Last updated 2021-10-25
Replaces draft-kinnear-webtransport-http2
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draft-ietf-webtrans-http2-02
webtrans                                                     A. Frindell
Internet-Draft                                             Facebook Inc.
Intended status: Standards Track                              E. Kinnear
Expires: 28 April 2022                                          T. Pauly
                                                              Apple Inc.
                                                              M. Thomson
                                                                 Mozilla
                                                             V. Vasiliev
                                                                  Google
                                                                  G. Xie
                                                           Facebook Inc.
                                                         25 October 2021

                       WebTransport using HTTP/2
                      draft-ietf-webtrans-http2-02

Abstract

   WebTransport [OVERVIEW] is a protocol framework that enables clients
   constrained by the Web security model to communicate with a remote
   server using a secure multiplexed transport.  This document describes
   a WebTransport protocol that uses HTTP semantics to provide support
   for unidirectional streams, bidirectional streams, and datagrams.
   This protocol can be used with any HTTP version, however this
   document focuses on TCP-based HTTP versions, specifically providing
   examples of these capabilities when multiplexed within the same
   HTTP/2 [RFC7540] connection.

Note to Readers

   Discussion of this draft takes place on the WebTransport mailing list
   (webtransport@ietf.org (mailto:webtransport@ietf.org)), which is
   archived at https://mailarchive.ietf.org/arch/
   search/?email_list=webtransport.

   The repository tracking the issues for this draft can be found at[]
   (https://github.com/ietf-wg-webtrans/draft-webtransport-http2).  The
   web API draft corresponding to this document can be found at[]
   (https://w3c.github.io/webtransport/).

Status of This Memo

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

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   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."

   This Internet-Draft will expire on 28 April 2022.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Session Establishment . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Establishing a Transport-Capable HTTP/2 Connection  . . .   5
     3.2.  Extended CONNECT in HTTP/2  . . . . . . . . . . . . . . .   5
     3.3.  Creating a New Session  . . . . . . . . . . . . . . . . .   5
     3.4.  Limiting the Number of Simultaneous Sessions  . . . . . .   6
   4.  WebTransport Features . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Transport Considerations  . . . . . . . . . . . . . . . .   7
   5.  WebTransport Protocol Details . . . . . . . . . . . . . . . .   7
     5.1.  WebTransport Stream States  . . . . . . . . . . . . . . .   7
   6.  WebTransport Frames . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  WT_PADDING Frames . . . . . . . . . . . . . . . . . . . .   8
     6.2.  WT_RESET_STREAM Frames  . . . . . . . . . . . . . . . . .   8
     6.3.  WT_STOP_SENDING Frames  . . . . . . . . . . . . . . . . .   9
     6.4.  WT_STREAM Frames  . . . . . . . . . . . . . . . . . . . .  10
     6.5.  WT_MAX_DATA Frames  . . . . . . . . . . . . . . . . . . .  10
     6.6.  MAX_STREAM_DATA Frames  . . . . . . . . . . . . . . . . .  11
     6.7.  WT_MAX_STREAMS Frames . . . . . . . . . . . . . . . . . .  11

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     6.8.  WT_DATA_BLOCKED Frames  . . . . . . . . . . . . . . . . .  12
     6.9.  WT_STREAM_DATA_BLOCKED Frames . . . . . . . . . . . . . .  12
     6.10. WT_STREAMS_BLOCKED Frames . . . . . . . . . . . . . . . .  13
     6.11. WT_DATAGRAM Frames  . . . . . . . . . . . . . . . . . . .  13
   7.  Session Termination . . . . . . . . . . . . . . . . . . . . .  14
   8.  Transport Properties  . . . . . . . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  HTTP/2 SETTINGS Parameter Registration . . . . . . . . .  15
     10.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . .  16
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     11.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   This protocol satisfies the WebTransport protocol framework's
   requirements using HTTP semantics to provide support for
   unidirectional streams, bidirectional streams, and datagrams.

   By using HTTP semantics, this protocol allows deployment using any
   HTTP version, although this document in particular focuses on HTTP/2
   as the current most common TCP-based fallback to HTTP/3 running over
   QUIC [RFC9000].

   Currently, the only mechanism in HTTP/2 for server to client
   communication is server push.  That is, servers can initiate
   unidirectional push promised streams to clients, but clients cannot
   respond to them; they can only accept them or discard them.
   Additionally, intermediaries along the path may have different server
   push policies and may not forward push promised streams to the
   downstream client.  This best effort mechanism is not sufficient to
   reliably deliver messages from servers to clients, limiting server to
   client use-cases such as chat messages or notifications.

   Several techniques have been developed to workaround these
   limitations: long polling [RFC6202], WebSocket [RFC8441], and
   tunneling using the CONNECT method.  All of these approaches have
   limitations.

   This document defines a mechanism for establishing bidirectional
   communication using HTTP semantics in a manner that conforms with the
   WebTransport protocol requirements and semantics [OVERVIEW].  When
   running with HTTP/2, multiple WebTransport instances can be
   multiplexed simultaneously with regular HTTP traffic on the same
   HTTP/2 connection.

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   It is important to note that, while it is possible to have the
   WebTransport session remain entirely "self contained" within a given
   HTTP flow, certain HTTP versions may provide native features, such as
   datagram support, which can allow for a better performing
   implementation of the WebTransport client's requirements.

1.1.  Terminology

   The keywords "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.

   This document follows terminology defined in Section 1.2 of
   [OVERVIEW].  Note that this document distinguishes between a
   WebTransport server and an HTTP/2 server.  An HTTP/2 server is the
   server that terminates HTTP/2 connections; a WebTransport server is
   an application that accepts WebTransport sessions, which can be
   accessed via an HTTP/2 server.

2.  Protocol Overview

   WebTransport servers are identified by an HTTPS URI as defined in
   Section 4.2.2 of [HTTP].

   When an HTTP/2 connection is established, both the client and server
   have to send a SETTINGS_ENABLE_WEBTRANSPORT setting in order to
   indicate that they both support WebTransport over HTTP/2.

   WebTransport sessions are initiated inside a given HTTP/2 connection
   by the client, who sends an extended CONNECT request [RFC8441].  If
   the server accepts the request, an WebTransport session is
   established.  The resulting stream will be further referred to as a
   _CONNECT stream_, and its stream ID is used to uniquely identify a
   given WebTransport session within the connection.  The ID of the
   CONNECT stream that established a given WebTransport session will be
   further referred to as a _Session ID_.

   After the session is established, the peers can utilitze the
   bidirectional bytestream established by the extended CONNECT request
   to exchange WebTransport frames that multiplex flows of WebTransport
   data as _WebTransport streams_. These frames closely mirror a subset
   of QUIC frames and provide the essential WebTransport features, such
   as:

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   *  Both client and server can create a bidirectional or
      unidirectional WebTransport stream within the WebTransport session
      using a WT_STREAM frame.

   *  A datagram can be sent using a WT_DATAGRAM frame.

   Data flow on these streams is controlled via a flow control mechanism
   very similar to the one provided by QUIC using WT_MAX_DATA,
   WT_MAX_STREAM_DATA, WT_MAX_STREAMS, WT_DATA_BLOCKED,
   WT_STREAM_DATA_BLOCKED, and WT_STREAMS_BLOCKED frames.

   WebTransport streams can be closed via a WT_RESET_STREAM frame and a
   receiver can request that a sender stop sending with a
   WT_STOP_SENDING frame.

   A WebTransport session is terminated when the CONNECT stream that
   created it is closed.  This implicitly closes all WebTransport
   streams that were multiplexed over that CONNECT stream.

3.  Session Establishment

3.1.  Establishing a Transport-Capable HTTP/2 Connection

   In order to indicate support for WebTransport, both the client and
   the server MUST send a SETTINGS_ENABLE_WEBTRANSPORT value set to "1"
   in their SETTINGS frame.  Endpoints MUST NOT use any WebTransport-
   related functionality unless the parameter has been negotiated.

3.2.  Extended CONNECT in HTTP/2

   [RFC8441] defines an extended CONNECT method in Section 4, enabled by
   the SETTINGS_ENABLE_CONNECT_PROTOCOL parameter.  An endpoint does not
   need to send both SETTINGS_ENABLE_CONNECT_PROTOCOL and
   SETTINGS_ENABLE_WEBTRANSPORT; the SETTINGS_ENABLE_WEBTRANSPORT
   setting implies that an endpoint supports extended CONNECT.

3.3.  Creating a New Session

   As WebTransport sessions are established over HTTP, they are
   identified using the https URI scheme [RFC7230].

   In order to create a new WebTransport session, a client can send an
   HTTP CONNECT request.  The :protocol pseudo-header field ([RFC8441])
   MUST be set to webtransport (Section 7.1 of [WEBTRANSPORT-H3]).  The
   :scheme field MUST be https.  Both the :authority and the :path value
   MUST be set; those fields indicate the desired WebTransport server.
   An Origin header { {!RFC6454}} MUST be provided within the request.

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   Upon receiving an extended CONNECT request with a :protocol field set
   to webtransport, the HTTP server can check if it has a WebTransport
   server associated with the specified :authority and :path values.  If
   it does not, it SHOULD reply with status code 404
   (Section 6.5.4,[RFC7231]).  If it does, it MAY accept the session by
   replying with status code 200.  The WebTransport server MUST verify
   the Origin header to ensure that the specified origin is allowed to
   access the server in question.

   From the client's perspective, a WebTransport session is established
   when the client receives a 200 response.  From the server's
   perspective, a session is established once it sends a 200 response.
   Both endpoints MUST NOT send any WebTransport frames on a given
   session before that session is established.

3.4.  Limiting the Number of Simultaneous Sessions

   From a flow control perspective, WebTransport sessions count against
   HTTP/2 session flow control limits just like regular HTTP requests,
   since they are established via an HTTP CONNECT request.  This
   document does not make any effort to introduce a separate flow
   control mechanism for WebTransport sessions.  If the server needs to
   limit the rate of incoming requests, it has alternative mechanisms at
   its disposal:

   *  HTTP_STREAM_REFUSED error code defined in [RFC7540] indicates to
      the receiving HTTP/2 stack that the request was not processed in
      any way.

   *  HTTP status code 429 indicates that the request was rejected due
      to rate limiting [RFC6585].  Unlike the previous method, this
      signal is directly propagated to the application.

4.  WebTransport Features

   WebTransport over TCP-based HTTP semantics provides the following
   features described in [OVERVIEW]: unidirectional streams,
   bidirectional streams, and datagrams, initiated by either endpoint.

   WebTransport streams and datagrams that belong to different
   WebTransport sessions are identified by the CONNECT stream on which
   they are transmitted, with one WebTransport session consuming one
   CONNECT stream.

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4.1.  Transport Considerations

   Because WebTransport over TCP-based HTTP semantics relies on the
   underlying protocols to provide in order and reliable delivery, there
   are some notable differences from the way in which QUIC handles
   application data.

   Endpoints MUST send stream data in order.  As there is no ordering
   mechanism available for the receiver to reassemble incoming data,
   receivers assume that all data arriving in STREAM frames is
   contiguous and in order.

   DATAGRAM frames are delivered to the remote WebTransport endpoint
   reliably, however this does not require that the receiving
   implementation deliver that data to the application in a reliable
   manner.

5.  WebTransport Protocol Details

5.1.  WebTransport Stream States

   WebTransport streams have states that mirror the states of QUIC
   streams (Section 3 of [RFC9000]) as closely as possible to aid in
   ease of implementation.

   Because WebTransport does not provide an acknowledgement mechanism
   for WebTransport frames, it relies on the underlying transport's in
   order delivery to inform stream state transitions.  Wherever QUIC
   relies on receiving an ack for a packet to transition between stream
   states, WebTransport performs that transition immediately.

6.  WebTransport Frames

   WebTransport frames mirror their QUIC counterparts as closely as
   possible to enable maximal reuse of any applicable QUIC
   infrastructure by implementors.

   A WebTransport frame begins with a Frame Type and Frame Length which
   are followed by zero or more fields that are type-dependent.

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

                    Figure 1: WebTransport Frame Format

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   The Frame Type field indicates the type of the frame, defining what
   type-dependent fields will be present.

   The Frame Length field indicates the length of the WebTransport
   frame, including all type-dependent fields and other information.  It
   does not include the size of the Frame Type or Frame Length fields
   themselves.

   Both of these fields use a variable-length integer encoding (see
   Section 16 of [RFC9000]), with one exception.  To ensure simple and
   efficient implementations of frame parsing, the frame type and length
   MUST use the shortest possible encoding.  For example, for the frame
   types defined in this document, this means a single-byte encoding,
   even though it is possible to encode these values as a two-, four-,
   or eight-byte variable-length integer.

6.1.  WT_PADDING Frames

   A WT_PADDING frame (type=0x00) has no semantic value.  PADDING frames
   can be used to introduce additional data between other WebTransport
   frames and can also be used to provide protection against traffic
   analysis or for other reasons.

   WT_PADDING Frame {
     Type (i) = 0x00,
     Length (i),
   }

                     Figure 2: WT_PADDING Frame Format

   The padding extends to the end of the WT_PADDING frame and that many
   bytes can be discarded by the recipient.

6.2.  WT_RESET_STREAM Frames

   A WebTransport frame called WT_RESET_STREAM is introduced for either
   endpoint to abruptly terminate the sending part of a WebTransport
   stream.

   An endpoint uses a WT_RESET_STREAM frame (type=0x04) to abruptly
   terminate the sending part of a stream.

   After sending a WT_RESET_STREAM, an endpoint ceases transmission and
   retransmission of WT_STREAM frames on the identified stream.  A
   receiver of WT_RESET_STREAM can discard any data that it already
   received on that stream.

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   WT_RESET_STREAM Frame {
     Type (i) = 0x04,
     Length (i) = 0x00,
     Stream ID (i),
     Application Protocol Error Code (i),
     Final Size (i),
   }

                   Figure 3: WT_RESET_STREAM Frame Format

   The WT_RESET_STREAM frame defines the following fields:

   Stream ID: A variable-length integer encoding of the WebTransport
   stream ID of the stream being terminated.

   Application Protocol Error Code: A variable-length integer containing
   the application protocol error code that indicates why the stream is
   being closed.

   Final Size: A variable-length integer indicating the final size of
   the stream by the WT_RESET_STREAM sender, in units of bytes.  This is
   the amount of flow control credit that is consumed by a stream, see
   Section 4.5 of [RFC9000].

6.3.  WT_STOP_SENDING Frames

   A WebTransport frame called WT_STOP_SENDING is introduced to
   communicate that incoming data is being discarded on receipt per
   application request.  WT_STOP_SENDING requests that a peer cease
   transmission on a stream.

   WT_STOP_SENDING Frame {
     Type (i) = 0x05,
     Length (i) = 0x00,
     Stream ID (i),
     Application Protocol Error Code (i),
   }

                   Figure 4: WT_STOP_SENDING Frame Format

   The WT_STOP_SENDING frame defines the following fields:

   Stream ID: A variable-length integer carrying the WebTransport stream
   ID of the stream being ignored.

   Application Protocol Error Code: A variable-length integer containing
   the application-specified reason the sender is ignoring the stream.

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6.4.  WT_STREAM Frames

   WT_STREAM frames implicitly create a stream and carry stream data.
   The Type field in the WT_STREAM frame takes the form 0b00001XXX (or
   the set of values from 0x08 to 0x0f) to maximize compatibility with
   QUIC.  However, unlike QUIC, there are only one bit used to determine
   the fields that are present in the frame:

   The FIN bit (0x01) indicates that the frame marks the end of the
   stream.  The final size of the stream is the sum of the length, in
   bytes, of all data previously sent on this stream and the length of
   this frame without the length of the Stream ID.

   WT_STREAM Frame {
     Type (i) = 0x08..0x0f,
     Stream ID (i),
     Stream Data (..),
   }

                      Figure 5: WT_STREAM Frame Format

6.5.  WT_MAX_DATA Frames

   A WebTransport frame called WT_MAX_DATA is introduced to inform the
   peer of the maximum amount of data that can be sent on the
   WebTransport session as a whole.

   WT_MAX_DATA Frame {
     Type (i) = 0x10,
     Length (i) = 0x00,
     Maximum Data (i),
   }

                     Figure 6: WT_MAX_DATA Frame Format

   WT_MAX_DATA frames contain the following field:

   Maximum Data: A variable-length integer indicating the maximum amount
   of data that can be sent on the entire connection, in units of bytes.

   All data sent in WT_STREAM frames counts toward this limit.  The sum
   of the final sizes on all streams, including streams in terminal
   states, MUST NOT exceed the value advertised by a receiver.

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6.6.  MAX_STREAM_DATA Frames

   A WebTransport frame called WT_MAX_STREAM_DATA is introduced to
   inform a peer of the maximum amount of data that can be sent on a
   stream.

   WT_MAX_STREAM_DATA Frame {
     Type (i) = 0x11,
     Length (i) = 0x00,
     Stream ID (i),
     Maximum Stream Data (i),
   }

                 Figure 7: WT_MAX_STREAM_DATA Frame Format

   WT_MAX_STREAM_DATA frames contain the following fields:

   Stream ID: The stream ID of the affected WebTransport stream, encoded
   as a variable-length integer.

   Maximum Stream Data: A variable-length integer indicating the maximum
   amount of data that can be sent on the identified stream, in units of
   bytes.

   When counting data toward this limit, an endpoint accounts for the
   largest amount of data that is sent or received on the stream.  The
   data sent on a stream MUST NOT exceed the largest maximum stream data
   value advertised by the receiver.

6.7.  WT_MAX_STREAMS Frames

   A WebTransport frame called WT_MAX_STREAMS is introduced to inform
   the peer of the cumulative number of streams of a given type it is
   permitted to open.  A WT_MAX_STREAMS frame with a type of 0x12
   applies to bidirectional streams, and a WT_MAX_STREAMS frame with a
   type of 0x13 applies to unidirectional streams.

   WT_MAX_STREAMS Frame {
     Type (i) = 0x12..0x13,
     Length (i) = 0x00,
     Maximum Streams (i),
   }

                   Figure 8: WT_MAX_STREAMS Frame Format

   WT_MAX_STREAMS frames contain the following field:

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   Maximum Streams: A count of the cumulative number of streams of the
   corresponding type that can be opened over the lifetime of the
   connection.  This value cannot exceed 2^60, as it is not possible to
   encode stream IDs larger than 2^(62-1).

   An endpoint MUST NOT open more streams than permitted by the current
   stream limit set by its peer.  For instance, a server that receives a
   unidirectional stream limit of 3 is permitted to open streams 3, 7,
   and 11, but not stream 15.

   Note that this limit includes streams that have been closed as well
   as those that are open.

6.8.  WT_DATA_BLOCKED Frames

   A sender SHOULD send a WT_DATA_BLOCKED frame (type=0x14) when it
   wishes to send data but is unable to do so due to WebTransport
   session-level flow control.  WT_DATA_BLOCKED frames can be used as
   input to tuning of flow control algorithms.

   WT_DATA_BLOCKED Frame {
     Type (i) = 0x14,
     Length (i) = 0x00,
     Maximum Data (i),
   }

                   Figure 9: WT_DATA_BLOCKED Frame Format

   WT_DATA_BLOCKED frames contain the following field:

   Maximum Data: A variable-length integer indicating the session-level
   limit at which blocking occurred.

6.9.  WT_STREAM_DATA_BLOCKED Frames

   A sender SHOULD send a WT_STREAM_DATA_BLOCKED frame (type=0x15) when
   it wishes to send data but is unable to do so due to stream-level
   flow control.  This frame is analogous to WT_DATA_BLOCKED.

   WT_STREAM_DATA_BLOCKED Frame {
     Type (i) = 0x15,
     Length (i) = 0x00,
     Stream ID (i),
     Maximum Stream Data (i),
   }

               Figure 10: WT_STREAM_DATA_BLOCKED Frame Format

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   WT_STREAM_DATA_BLOCKED frames contain the following fields:

   Stream ID: A variable-length integer indicating the WebTransport
   stream that is blocked due to flow control.

   Maximum Stream Data: A variable-length integer indicating the offset
   of the stream at which the blocking occurred.

6.10.  WT_STREAMS_BLOCKED Frames

   A sender SHOULD send a WT_STREAMS_BLOCKED frame (type=0x16 or 0x17)
   when it wishes to open a stream but is unable to do so due to the
   maximum stream limit set by its peer.  A WT_STREAMS_BLOCKED frame of
   type 0x16 is used to indicate reaching the bidirectional stream
   limit, and a STREAMS_BLOCKED frame of type 0x17 is used to indicate
   reaching the unidirectional stream limit.

   A WT_STREAMS_BLOCKED frame does not open the stream, but informs the
   peer that a new stream was needed and the stream limit prevented the
   creation of the stream.

   WT_STREAMS_BLOCKED Frame {
     Type (i) = 0x16..0x17,
     Length (i) = 0x00,
     Maximum Streams (i),
   }

                 Figure 11: WT_STREAMS_BLOCKED Frame Format

   WT_STREAMS_BLOCKED frames contain the following field:

   Maximum Streams: A variable-length integer indicating the maximum
   number of streams allowed at the time the frame was sent.  This value
   cannot exceed 2^60, as it is not possible to encode stream IDs larger
   than 2^(62-1).

6.11.  WT_DATAGRAM Frames

   The WT_DATAGRAM frame type (0x31) is used to carry datagram traffic.
   Frame type 0x30 is also reserved to maintain parity with QUIC, but
   unused, as all WebTransport frames MUST contain a length field.

   WT_DATAGRAM Frame {
     Type (i) = 0x31,
     Length (i),
     Datagram Data (..),
   }

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                    Figure 12: WT_DATAGRAM Frame Format

   WT_DATAGRAM frames contain the following fields:

   Length: A variable-length integer specifying the length of the
   Datagram Data in bytes.  Note that empty (i.e., zero-length)
   datagrams are allowed.

   Datagram Data: The content of the datagram to be delivered.

   The data in WT_DATAGRAM frames is not subject to flow control.  The
   receiver MAY discard this data if it does not have sufficient space
   to buffer it.

   An intermediary could forward the data in a WT_DATAGRAM frame over
   another protocol, such as WebTransport over HTTP/3.  In QUIC, a
   datagram frame can span at most one packet.  Because of that, the
   applications have to know the maximum size of the datagram they can
   send.  However, when proxying the datagrams, the hop-by-hop MTUs can
   vary.

7.  Session Termination

   An WebTransport session over HTTP/2 is terminated when either
   endpoint closes the stream associated with the CONNECT request that
   initiated the session.  Upon learning about the session being
   terminated, the endpoint MUST stop sending new datagrams and reset
   all of the streams associated with the session.

8.  Transport Properties

   The WebTransport framework [OVERVIEW] defines a set of optional
   transport properties that clients can use to determine the presence
   of features which might allow additional optimizations beyond the
   common set of properties available via all WebTransport protocols.
   Below are details about support in Http2Transport for those
   properties.

   Stream Independence:  Http2Transport does not support stream
      independence, as HTTP/2 inherently has head of line blocking.

   Partial Reliability:  Http2Transport does not support partial
      reliability, as HTTP/2 retransmits any lost data.  This means that
      any datagrams sent via Http2Transport will be retransmitted
      regardless of the preference of the application.  The receiver is
      permitted to drop them, however, if it is unable to buffer them.

   Pooling Support:  Http2Transport supports pooling, as multiple

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      transports using Http2Transport may share the same underlying
      HTTP/2 connection and therefore share a congestion controller and
      other transport context.

   Connection Mobility:  Http2Transport does not support connection
      mobility, unless an underlying transport protocol that supports
      multipath or migration, such as MPTCP [RFC7540], is used
      underneath HTTP/2 and TLS.  Without such support, Http2Transport
      connections cannot survive network transitions.

9.  Security Considerations

   WebTransport over HTTP/2 satisfies all of the security requirements
   imposed by [OVERVIEW] on WebTransport protocols, thus providing a
   secure framework for client-server communication in cases when the
   client is potentially untrusted.

   WebTransport over HTTP/2 requires explicit opt-in through the use of
   HTTP SETTINGS; this avoids potential protocol confusion attacks by
   ensuring the HTTP/2 server explicitly supports it.  It also requires
   the use of the Origin header, providing the server with the ability
   to deny access to Web-based clients that do not originate from a
   trusted origin.

   Just like HTTP traffic going over HTTP/2, WebTransport pools traffic
   to different origins within a single connection.  Different origins
   imply different trust domains, meaning that the implementations have
   to treat each transport as potentially hostile towards others on the
   same connection.  One potential attack is a resource exhaustion
   attack: since all of the transports share both congestion control and
   flow control context, a single client aggressively using up those
   resources can cause other transports to stall.  The user agent thus
   SHOULD implement a fairness scheme that ensures that each transport
   within connection gets a reasonable share of controlled resources;
   this applies both to sending data and to opening new streams.

10.  IANA Considerations

10.1.  HTTP/2 SETTINGS Parameter Registration

   The following entry is added to the "HTTP/2 Settings" registry
   established by [RFC7540]:

   The SETTINGS_ENABLE_WEBTRANSPORT parameter indicates that the
   specified HTTP/2 connection is WebTransport-capable.

   Setting Name:  ENABLE_WEBTRANSPORT

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   Value:  0x2b603742

   Default:  0

   Specification:  This document

10.2.  Examples

   An example of negotiating a WebTransport Stream on an HTTP/2
   connection follows.  This example is intended to closely follow the
   example in Section 5.1 of [RFC8441] to help illustrate the
   differences defined in this document.

   [[ From Client ]]                   [[ From Server ]]

   SETTINGS
   SETTINGS_ENABLE_WEBTRANSPORT = 1

                                       SETTINGS
                                       SETTINGS_ENABLE_WEBTRANSPORT = 1

   HEADERS + END_HEADERS
   Stream ID = 3
   :method = CONNECT
   :protocol = webtransport
   :scheme = https
   :path = /
   :authority = server.example.com
   origin: server.example.com

                                       HEADERS + END_HEADERS
                                       Stream ID = 3
                                       :status = 200

   WT_STREAM
   Stream ID = 5
   WebTransport Data

                                       WT_STREAM + FIN
                                       Stream ID = 5
                                       WebTransport Data

   WT_STREAM + FIN
   Stream ID = 5
   WebTransport Data

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   An example of the server initiating a WebTransport Stream follows.
   The only difference here is the endpoint that sends the first
   WT_STREAM frame.

   [[ From Client ]]                   [[ From Server ]]

   SETTINGS
   SETTINGS_ENABLE_WEBTRANSPORT = 1

                                       SETTINGS
                                       SETTINGS_ENABLE_WEBTRANSPORT = 1

   HEADERS + END_HEADERS
   Stream ID = 3
   :method = CONNECT
   :protocol = webtransport
   :scheme = https
   :path = /
   :authority = server.example.com
   origin: server.example.com
                                       HEADERS + END_HEADERS
                                       Stream ID = 3
                                       :status = 200

                                       WT_STREAM
                                       Stream ID = 2
                                       WebTransport Data

   WT_STREAM + FIN
   Stream ID = 2
   WebTransport Data

                                       WT_STREAM + FIN
                                       Stream ID = 2
                                       WebTransport Data

11.  References

11.1.  Normative References

   [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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   [OVERVIEW] Vasiliev, V., "The WebTransport Protocol Framework", Work
              in Progress, Internet-Draft, draft-ietf-webtrans-overview-
              02, 28 July 2021, <https://datatracker.ietf.org/doc/html/
              draft-ietf-webtrans-overview-02>.

   [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>.

   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
              Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
              <https://www.rfc-editor.org/rfc/rfc6585>.

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

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7231>.

   [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>.

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

   [RFC9000]  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>.

   [WEBTRANSPORT-H3]
              Frindell, A., Kinnear, E., and V. Vasiliev, "WebTransport
              over HTTP/3", Work in Progress, Internet-Draft, draft-
              ietf-webtrans-http3-02, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              webtrans-http3-02>.

11.2.  Informative References

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   [RFC6202]  Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
              "Known Issues and Best Practices for the Use of Long
              Polling and Streaming in Bidirectional HTTP", RFC 6202,
              DOI 10.17487/RFC6202, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6202>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7540>.

Acknowledgments

   Thanks to Anthony Chivetta, Joshua Otto, and Valentin Pistol for
   their contributions in the design and implementation of this work.

Authors' Addresses

   Alan Frindell
   Facebook Inc.

   Email: afrind@fb.com

   Eric Kinnear
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014,
   United States of America

   Email: ekinnear@apple.com

   Tommy Pauly
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014,
   United States of America

   Email: tpauly@apple.com

   Martin Thomson
   Mozilla

   Email: mt@lowentropy.net

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   Victor Vasiliev
   Google

   Email: vasilvv@google.com

   Guowu Xie
   Facebook Inc.

   Email: woo@fb.com

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