



WEBTRANS                                                     A. Frindell
Internet-Draft                                                  Facebook
Intended status: Standards Track                              E. Kinnear
Expires: 23 April 2026                                        Apple Inc.
                                                             V. Vasiliev
                                                                  Google
                                                         20 October 2025


                        WebTransport over HTTP/3
                      draft-ietf-webtrans-http3-14

Abstract

   WebTransport [OVERVIEW] is a protocol framework that enables
   application clients constrained by the Web security model to
   communicate with a remote application server using a secure
   multiplexed transport.  This document describes a WebTransport
   protocol that is based on HTTP/3 [HTTP3] and provides support for
   unidirectional streams, bidirectional streams, and datagrams, all
   multiplexed within the same HTTP/3 connection.

About This Document

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

   The latest revision of this draft can be found at https://ietf-wg-
   webtrans.github.io/draft-ietf-webtrans-http3/#go.draft-ietf-webtrans-
   http3.html.  Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-webtrans-http3/.

   Discussion of this document takes place on the WebTransport Working
   Group mailing list (mailto:webtransport@ietf.org), which is archived
   at https://mailarchive.ietf.org/arch/browse/webtransport/.  Subscribe
   at https://www.ietf.org/mailman/listinfo/webtransport/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-webtrans/draft-ietf-webtrans-http3.

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/.



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   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
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   This Internet-Draft will expire on 23 April 2026.

Copyright Notice

   Copyright (c) 2025 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  QUIC, WebTransport, and HTTP/3  . . . . . . . . . . . . .   4
       2.1.1.  Minimizing Implementation Complexity  . . . . . . . .   5
     2.2.  Protocol Overview . . . . . . . . . . . . . . . . . . . .   5
   3.  Session Establishment . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Establishing a WebTransport-Capable HTTP/3 Connection . .   6
     3.2.  Creating a New Session  . . . . . . . . . . . . . . . . .   8
     3.3.  Application Protocol Negotiation  . . . . . . . . . . . .   9
     3.4.  Prioritization  . . . . . . . . . . . . . . . . . . . . .  10
   4.  WebTransport Features . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Transport Properties  . . . . . . . . . . . . . . . . . .  11
     4.2.  Unidirectional streams  . . . . . . . . . . . . . . . . .  12
     4.3.  Bidirectional Streams . . . . . . . . . . . . . . . . . .  12
     4.4.  Resetting Data Streams  . . . . . . . . . . . . . . . . .  13
     4.5.  Datagrams . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.6.  Buffering Incoming Streams and Datagrams  . . . . . . . .  14
     4.7.  Interaction with the HTTP/3 GOAWAY frame  . . . . . . . .  15
     4.8.  Use of Keying Material Exporters  . . . . . . . . . . . .  16
   5.  Flow Control  . . . . . . . . . . . . . . . . . . . . . . . .  16
     5.1.  Negotiating the Use of Flow Control . . . . . . . . . . .  17
     5.2.  Limiting the Number of Simultaneous Sessions  . . . . . .  18
     5.3.  Limiting the Number of Streams Within a Session . . . . .  18
     5.4.  Data Limits . . . . . . . . . . . . . . . . . . . . . . .  19



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     5.5.  Flow Control SETTINGS . . . . . . . . . . . . . . . . . .  19
     5.6.  Flow Control Capsules . . . . . . . . . . . . . . . . . .  20
       5.6.1.  Flow Control and Intermediaries . . . . . . . . . . .  20
       5.6.2.  WT_MAX_STREAMS Capsule  . . . . . . . . . . . . . . .  20
       5.6.3.  WT_STREAMS_BLOCKED Capsule  . . . . . . . . . . . . .  21
       5.6.4.  WT_MAX_DATA Capsule . . . . . . . . . . . . . . . . .  22
       5.6.5.  WT_DATA_BLOCKED Capsule . . . . . . . . . . . . . . .  23
   6.  Session Termination . . . . . . . . . . . . . . . . . . . . .  24
   7.  Considerations for Future Versions  . . . . . . . . . . . . .  25
     7.1.  Negotiating the Draft Version . . . . . . . . . . . . . .  26
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     9.1.  Upgrade Token Registration  . . . . . . . . . . . . . . .  27
     9.2.  HTTP/3 SETTINGS Parameter Registration  . . . . . . . . .  27
     9.3.  Frame Type Registration . . . . . . . . . . . . . . . . .  28
     9.4.  Stream Type Registration  . . . . . . . . . . . . . . . .  29
     9.5.  HTTP/3 Error Code Registration  . . . . . . . . . . . . .  29
     9.6.  Capsule Types . . . . . . . . . . . . . . . . . . . . . .  30
     9.7.  Protocol Negotiation HTTP Header Fields . . . . . . . . .  32
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     10.2.  Informative References . . . . . . . . . . . . . . . . .  34
   Appendix A.  Changelog  . . . . . . . . . . . . . . . . . . . . .  34
     A.1.  Changes between draft versions 02 and 07  . . . . . . . .  34
   Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   HTTP/3 [HTTP3] is a protocol defined on top of QUIC [RFC9000] that
   can multiplex HTTP requests over a QUIC connection.  This document
   defines a mechanism for multiplexing non-HTTP data with HTTP/3 in a
   manner that conforms with the WebTransport protocol requirements and
   semantics [OVERVIEW].  Using the mechanism described here, multiple
   WebTransport instances, or sessions, can be multiplexed
   simultaneously with regular HTTP traffic on the same HTTP/3
   connection.

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.






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   This document follows terminology defined in Section 1.2 of
   [OVERVIEW].  Note that this document distinguishes between a
   WebTransport server and an HTTP/3 server.  An HTTP/3 server is the
   server that terminates HTTP/3 connections; a WebTransport server is
   an application that accepts WebTransport sessions, which can be
   accessed via an HTTP/3 server.  An application client is user or
   developer-provided code, often untrusted, that utilizes the interface
   offered by the WebTransport client to communicate with an application
   server.  The application server uses the interface offered by the
   WebTransport server to accept incoming WebTransport sessions.

2.  Overview

2.1.  QUIC, WebTransport, and HTTP/3

   QUIC version 1 [RFC9000] is a secure transport protocol with flow
   control and congestion control.  QUIC supports application data
   exchange via streams; reliable and ordered byte streams that can be
   multiplexed.  Stream independence can mitigate head-of-line blocking.
   While QUIC provides streams as a transport service, it is
   unopinionated about their usage.  The applicability of streams is
   described by section 4 of [RFC9308].

   HTTP is an application-layer protocol, defined by "HTTP Semantics"
   [RFC9110].  HTTP/3 is the application mapping for QUIC, defined in
   [RFC9114].  It describes how QUIC streams are used to carry control
   data or HTTP request and response message sequences in the form of
   frames and describes details of stream and connection lifecycle
   management.  HTTP/3 offers two features in addition to HTTP
   Semantics: QPACK header compression [RFC9208] and Server Push
   Section 4.6 of [RFC9114].

   WebTransport session establishment involves interacting at the HTTP
   layer with a resource.  For Web user agents and other WebTransport
   clients, this interaction is important for security reasons,
   especially to ensure that the resource is willing to use
   WebTransport.

   Although WebTransport requires HTTP for its handshake, when HTTP/3 is
   in use, HTTP is not used for anything else related to an established
   session.  Instead, QUIC streams begin with a sequence of header bytes
   that links them to the established session.  The remainder of the
   stream is the body, which carries the payload supplied by the
   application using WebTransport.  This process is similar to
   WebSockets over HTTP/1.1 [ORIGIN], where access to the underlying
   byte stream is enabled after both sides have completed an initial
   handshake.




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   The layering of QUIC, HTTP/3, and WebTransport is shown in Figure 1.
   Once a WebTransport session is established, applications have nearly
   direct access to QUIC.

   ,--------------------------------,
   |            WebTransport        |
   ,----------------,---------------,
   | HTTP Semantics |               |
   |      and       |               |
   | Session Setup  | Nearly direct |
   ,----------------,               |
   |     HTTP/3     |               |
   ,----------------`---------------,
   |               QUIC             |
   `--------------------------------'

                      Figure 1: WebTransport Layering

2.1.1.  Minimizing Implementation Complexity

   WebTransport has minimal interaction with HTTP and HTTP/3.  Clients
   and servers can constrain their use of features to only those
   required to complete a WebTransport handshake:

   *  Generating/parsing the request method, host, path, protocol,
      optional Origin header field, and perhaps some extra header
      fields.

   *  Generating/parsing the response status code and possibly some
      extra header fields.

   A WebTransport endpoint, whether a client or a server, can likely
   perform several of its HTTP-level requirements using bytestring
   comparisons.

   While HTTP/3 encodes HTTP messages using QPACK, this complexity can
   be minimized.  When receiving, a WebTransport endpoint can disable
   dynamic decompression entirely but must always support static
   decompression and Huffman decoding.  When sending, endpoints can opt
   to never use dynamic compression, static compression, or Huffman
   encoding.

2.2.  Protocol Overview

   WebTransport servers in general are identified by a pair of authority
   value and path value (defined in [RFC3986] Sections 3.2 and 3.3
   correspondingly).




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   When an HTTP/3 connection is established, both the client and the
   server send a SETTINGS_WT_MAX_SESSIONS setting to indicate support
   for WebTransport over HTTP/3.  This process also negotiates the use
   of additional HTTP/3 extensions to enable both endpoints to open
   WebTransport streams.

   WebTransport sessions are initiated inside a given HTTP/3 connection
   by the client, who sends an extended CONNECT request [RFC9220].  If
   the server accepts the request, a 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 endpoints can exchange data
   using the following mechanisms:

   *  A client can create a bidirectional stream and transfer its
      ownership to WebTransport by providing a special signal in the
      first bytes.

   *  A server can create a bidirectional stream and transfer its
      ownership to WebTransport by providing a special signal in the
      first bytes.

   *  Both client and server can create a unidirectional stream using a
      special stream type.

   *  Both client and server can send datagrams using HTTP Datagrams
      [HTTP-DATAGRAM].

   A WebTransport session is terminated when the CONNECT stream that
   created it is closed.

3.  Session Establishment

3.1.  Establishing a WebTransport-Capable HTTP/3 Connection

   A WebTransport-Capable HTTP/3 connection requires the client and
   server to both signal support for WebTransport over HTTP/3 using a
   setting.

   This document defines a SETTINGS_WT_MAX_SESSIONS setting for
   indicating the number of WebTransport sessions a connection supports.
   The default value for the SETTINGS_WT_MAX_SESSIONS setting is "0",
   meaning that the endpoint is not willing to receive any WebTransport
   sessions.  Both clients and servers supporting WebTransport over



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   HTTP/3 MUST send the SETTINGS_WT_MAX_SESSIONS setting with a value
   greater than "0".  Clients MUST NOT attempt to establish WebTransport
   sessions until they have received the setting indicating WebTransport
   support from the server.

   WebTransport over HTTP/3 uses extended CONNECT in HTTP/3 as described
   in [RFC9220], which defines the SETTINGS_ENABLE_CONNECT_PROTOCOL
   setting.  Clients also signal support for WebTransport by using the
   "webtransport" upgrade token in extended CONNECT requests when
   establishing sessions (see Section 9.1).

   WebTransport over HTTP/3 requires support for HTTP/3 datagrams and
   the Capsule Protocol, and both the client and the server indicate
   support for HTTP/3 datagrams by sending a SETTINGS_H3_DATAGRAM
   setting value set to 1 in their SETTINGS frame (see Section 2.1.1 of
   [HTTP-DATAGRAM]).

   WebTransport over HTTP/3 also requires support for QUIC datagrams.
   To indicate support, both the client and the server send a
   max_datagram_frame_size transport parameter with a value greater than
   0 (see Section 3 of [QUIC-DATAGRAM]).

   WebTransport over HTTP/3 relies on the RESET_STREAM_AT frame defined
   in [RESET-STREAM-AT].  To indicate support, both the client and the
   server enable the extension by sending an empty reset_stream_at
   transport parameter as described in Section 3 of [RESET-STREAM-AT].

   In summary, servers supporting WebTransport over HTTP/3 send:

   *  A SETTINGS_WT_MAX_SESSIONS setting with a value greater than "0"

   *  A SETTINGS_ENABLE_CONNECT_PROTOCOL setting with a value of "1"

   *  A SETTINGS_H3_DATAGRAM setting with a value of 1

   *  A max_datagram_frame_size transport parameter with a value greater
      than 0

   *  An empty reset_stream_at transport parameter

   Clients supporting WebTransport over HTTP/3 send:

   *  A SETTINGS_WT_MAX_SESSIONS setting with a value greater than "0"

   *  A SETTINGS_H3_DATAGRAM setting with a value of 1

   *  A max_datagram_frame_size transport parameter with a value greater
      than 0



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   *  An empty reset_stream_at transport parameter

   Servers should note that CONNECT requests to establish new
   WebTransport sessions, in addition to other messages, can arrive
   before the client's SETTINGS are received (see Section 4.6).  If the
   server receives SETTINGS that do not have correct values for every
   required setting, or transport parameters that do not have correct
   values for every required transport parameter, the server MUST treat
   all established and newly incoming WebTransport sessions as
   malformed, as described in Section 4.1.2 of [HTTP3].

   A client MUST NOT establish WebTransport sessions if the server's
   SETTINGS do not have correct values for every required setting or if
   the server's transport parameters do not have correct values for
   every required transport parameter.

   [[RFC editor: please remove the following paragraph before
   publication.]]

   For draft versions of WebTransport only, the server MUST NOT process
   any incoming WebTransport requests until the client settings have
   been received, as the client might be using a version of the
   WebTransport extension that is different from the one used by the
   server.

3.2.  Creating a New Session

   As WebTransport sessions are established over HTTP/3, they are
   identified using the https URI scheme (Section 4.2.2 of [HTTP]).

   In order to create a new WebTransport session, a WebTransport client
   sends an HTTP extended CONNECT request.  In this request:

   *  The :protocol pseudo-header field([RFC8441]) MUST be set to
      webtransport.

   *  The :scheme field MUST be https.

   *  Both the :authority and the :path value MUST be set; these fields
      identify the desired WebTransport server resource.

   *  If the WebTransport session is coming from a browser client, an
      Origin header [RFC6454] MUST be provided within the request.
      Otherwise, the header is OPTIONAL.

   Upon receiving an extended CONNECT request with a :protocol field set
   to webtransport, the HTTP/3 server can check if it has a WebTransport
   server associated with the specified :authority and :path values.  If



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   it does not, it SHOULD reply with status code 404 (Section 15.5.5 of
   [HTTP]).  When the request contains the Origin header, the
   WebTransport server MUST verify the Origin header to ensure that the
   specified origin is allowed to access the server in question.  If the
   verification fails, the WebTransport server SHOULD reply with status
   code 403 (Section 15.5.4 of [HTTP]).  If all checks pass, the
   WebTransport server MAY accept the session by replying with a 2xx
   series status code, as defined in Section 15.3 of [HTTP].

   From the client's perspective, a WebTransport session is established
   when the client receives a 2xx response.  From the server's
   perspective, a session is established once it sends a 2xx response.

   The server may reply with a 3xx response, indicating a redirection
   (Section 15.4 of [HTTP]).  The WebTransport client MUST NOT
   automatically follow such redirects, as it potentially could have
   already sent data for the WebTransport session in question; it MAY
   notify the application client about the redirect.

   Clients cannot initiate WebTransport in 0-RTT packets, as the CONNECT
   method is not considered safe (see Section 10.9 of [HTTP3]).
   However, WebTransport-related SETTINGS parameters may be retained
   from the previous session as described in Section 7.2.4.2 of [HTTP3].
   If the server accepts 0-RTT, the server MUST NOT reduce the limit of
   maximum open WebTransport sessions, or other initial flow control
   values, from the values negotiated during the previous session; such
   change would be deemed incompatible, and MUST result in a
   H3_SETTINGS_ERROR connection error.

   The webtransport HTTP Upgrade Token uses the Capsule Protocol as
   defined in [HTTP-DATAGRAM].  The Capsule Protocol is negotiated when
   the server sends a 2xx response.  The capsule-protocol header field
   Section 3.4 of [HTTP-DATAGRAM] is not required by WebTransport and
   can safely be ignored by WebTransport endpoints.

3.3.  Application Protocol Negotiation

   WebTransport over HTTP/3 offers a protocol negotiation mechanism,
   similar to TLS Application-Layer Protocol Negotiation Extension
   (ALPN) [RFC7301]; the intent is to simplify porting existing
   protocols that use QUIC and rely on this functionality.










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   The client MAY include a WT-Available-Protocols header field in the
   CONNECT request.  The WT-Available-Protocols field enumerates the
   possible protocols in preference order, with the most preferred
   protocol listed first.  If the server receives such a header, it MAY
   include a WT-Protocol field in a successful (2xx) response.  If it
   does, the server MUST include a single choice from the client's list
   in that field.  Servers MAY reject the request if the client did not
   include a suitable protocol.

   Both WT-Available-Protocols and WT-Protocol are Structured Fields
   [FIELDS].  WT-Available-Protocols is a List.  WT-Protocol is defined
   as an Item.  In both cases, the only valid value type is a String.
   Any value type other than String MUST be treated as an error that
   causes the entire field to be ignored.  No semantics are defined for
   parameters on either field; parameters MUST be ignored.

   The value in the WT-Protocol response header field MUST be one of the
   values listed in WT-Available-Protocols of the request.  Otherwise,
   the WT-Protocol field MUST be ignored.

   The semantics of individual values used in WT-Available-Protocols and
   WT-Protocol are determined by the WebTransport resource in question
   and are not required to be registered in IANA's "ALPN Protocol IDs"
   registry.

3.4.  Prioritization

   WebTransport sessions are initiated using extended CONNECT.  While
   Section 11 of [RFC9218] describes how extensible priorities can be
   applied to data sent on a CONNECT stream, WebTransport extends the
   types of data that are exchanged in relation to the request and
   response, which requires additional considerations.

   WebTransport CONNECT requests and responses MAY contain the Priority
   header field (Section 5 of [RFC9218]); clients MAY reprioritize by
   sending PRIORITY_UPDATE frames (Section 7 of [RFC9218]).  In
   extension to [RFC9218], it is RECOMMENDED that clients and servers
   apply the scheduling guidance in both Section 9 of [RFC9218] and
   Section 10 of [RFC9218] for all data that they send in the enclosing
   WebTransport session, including Capsules, WebTransport streams and
   datagrams.  WebTransport does not provide any priority signaling
   mechanism for streams and datagrams within a WebTransport session;
   such mechanisms can be defined by application protocols using
   WebTransport.  It is RECOMMENDED that such mechanisms only affect
   scheduling within a session and not scheduling of other data on the
   same HTTP/3 connection.





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   The client/server priority merging guidance in Section 8 of [RFC9218]
   also applies to WebTransport sessions.  For example, a client that
   receives a response Priority header field could alter its view of a
   WebTransport session priority and alter the scheduling of outgoing
   data as a result.

   Endpoints that prioritize WebTransport sessions need to consider how
   they interact with other sessions or requests on the same HTTP/3
   connection.

4.  WebTransport Features

   WebTransport over HTTP/3 provides the following features described in
   [OVERVIEW]: unidirectional streams, bidirectional streams, and
   datagrams, all of which can be initiated by either endpoint.
   Protocols designed for use with WebTransport over HTTP/3 are
   constrained to these features.  The Capsule Protocol is an
   implementation detail of WebTransport over HTTP/3 and is not a
   WebTransport feature.

   Session IDs are used to demultiplex streams and datagrams belonging
   to different WebTransport sessions.  On the wire, session IDs are
   encoded using the QUIC variable length integer scheme described in
   [RFC9000].

   The client MAY optimistically open unidirectional and bidirectional
   streams, as well as send datagrams, on a session for which it has
   sent the CONNECT request, even if it has not yet received the
   server's response to the request.  On the server side, opening
   streams and sending datagrams is possible as soon as the CONNECT
   request has been received.

   If at any point a session ID is received that cannot be a valid ID
   for a client-initiated bidirectional stream, the recipient MUST close
   the connection with an H3_ID_ERROR error code.

4.1.  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 WebTransport over HTTP/3 for the
   properties defined by the WebTransport framework.

   Unreliable Delivery:  WebTransport over HTTP/3 supports unreliable




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      delivery.  Resetting a stream results in lost stream data no
      longer being retransmitted.  WebTransport over HTTP/3 also
      supports datagrams, which are not retransmitted.

   Pooling:  WebTransport over HTTP/3 provides optional support for
      pooling.  Endpoints can use the SETTINGS_WT_MAX_SESSIONS setting
      to indicate if pooling is supported on a particular HTTP/3
      connection (Section 3.1).

4.2.  Unidirectional streams

   WebTransport endpoints can initiate unidirectional streams.  The
   HTTP/3 unidirectional stream type SHALL be 0x54.  The body of the
   stream SHALL be the stream type, followed by the session ID, encoded
   as a variable-length integer, followed by the user-specified stream
   data (Figure 2).

   Unidirectional Stream {
       Stream Type (i) = 0x54,
       Session ID (i),
       User-Specified Stream Data (..)
   }

            Figure 2: Unidirectional WebTransport stream format

4.3.  Bidirectional Streams

   All client-initiated bidirectional streams are reserved by HTTP/3 as
   request streams, which are a sequence of HTTP/3 frames with a variety
   of rules (see Sections 4.1 and 6.1 of [HTTP3]).

   WebTransport extends HTTP/3 to allow clients to declare and to use
   alternative request stream rules.  Once a client receives settings
   indicating WebTransport support (Section 3.1), it MUST send a special
   signal value, encoded as a variable-length integer, as the first
   bytes of each bidirectional WebTransport stream it initiates to
   indicate how the remaining bytes on the stream are used.

   WebTransport extends HTTP/3 by defining rules for all server-
   initiated bidirectional streams.  Once a server receives an incoming
   CONNECT request establishing a WebTransport session (Section 3.1), it
   can open a bidirectional stream for use with that session and MUST
   send a special signal value, encoded as a variable-length integer, as
   the first bytes of the stream in order to indicate how the remaining
   bytes on the stream are used.






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   Clients and servers use the signal value 0x41 to open a bidirectional
   WebTransport stream.  Following this is the associated session ID,
   encoded as a variable-length integer; the rest of the stream is the
   application payload of the WebTransport stream (Figure 3).

   Bidirectional Stream {
       Signal Value (i) = 0x41,
       Session ID (i),
       Stream Body (..)
   }

             Figure 3: Bidirectional WebTransport stream format

   This document reserves the special signal value 0x41 as a WT_STREAM
   frame type.  While it is registered as an HTTP/3 frame type to avoid
   collisions, WT_STREAM lacks length and is not a proper HTTP/3 frame;
   it is an extension of HTTP/3 frame syntax that MUST be supported by
   any peer negotiating WebTransport.  Endpoints that implement this
   extension are also subject to additional frame handling requirements.
   Endpoints MUST NOT send WT_STREAM as a frame type on HTTP/3 streams
   other than the very first bytes of a request stream.  Receiving this
   frame type in any other circumstances MUST be treated as a connection
   error of type H3_FRAME_ERROR.

4.4.  Resetting Data Streams

   A WebTransport endpoint may send a RESET_STREAM or a STOP_SENDING
   frame for a WebTransport data stream.  Those signals are propagated
   by the WebTransport implementation to the application.

   A WebTransport application MUST provide an error code for those
   operations.  Since WebTransport shares the error code space with
   HTTP/3, WebTransport application errors for streams are limited to an
   unsigned 32-bit integer, assuming values between 0x00000000 and
   0xffffffff.  WebTransport implementations MUST remap those error
   codes into the error range reserved for WT_APPLICATION_ERROR, where
   0x00000000 corresponds to 0x52e4a40fa8db, and 0xffffffff corresponds
   to 0x52e5ac983162.  Note that there are codepoints inside that range
   of form "0x1f * N + 0x21" that are reserved by Section 8.1 of
   [HTTP3]; those have to be skipped when mapping the error codes (i.e.,
   the two HTTP/3 error codepoints adjacent to a reserved codepoint
   would map to two adjacent WebTransport application error codepoints).
   An example pseudocode can be seen in Figure 4.








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       first = 0x52e4a40fa8db
       last = 0x52e5ac983162

       def webtransport_code_to_http_code(n):
           return first + n + floor(n / 0x1e)

       def http_code_to_webtransport_code(h):
           assert(first <= h <= last)
           assert((h - 0x21) % 0x1f != 0)
           shifted = h - first
           return shifted - floor(shifted / 0x1f)

          Figure 4: Pseudocode for converting between WebTransport
                 application errors and HTTP/3 error codes

   WebTransport data streams are associated with sessions through a
   header at the beginning of the stream; resetting a stream might
   result in that data being discarded when using a RESET_STREAM frame.
   To prevent this, WebTransport implementations MUST use the
   RESET_STREAM_AT frame [RESET-STREAM-AT] with a Reliable Size set to
   at least the size of the WebTransport header when resetting a
   WebTransport data stream.  This ensures reliable delivery of the ID
   field associating the data stream with a WebTransport session.

   WebTransport endpoints MUST forward the error code for a stream
   associated with a known session to the application that owns that
   session; similarly, intermediaries MUST reset such streams with a
   corresponding error code when receiving a reset from their peer.

4.5.  Datagrams

   Datagrams can be sent using HTTP Datagrams.  The WebTransport
   datagram payload is sent unmodified in the "HTTP Datagram Payload"
   field of an HTTP Datagram (Section 2.1 of [HTTP-DATAGRAM]).  Note
   that the payload field directly follows the Quarter Stream ID field,
   which is at the start of the QUIC DATAGRAM frame payload and refers
   to the CONNECT stream that established the WebTransport session.

4.6.  Buffering Incoming Streams and Datagrams

   In WebTransport over HTTP/3, the client MUST wait for receipt of the
   server's SETTINGS frame before establishing any WebTransport sessions
   by sending CONNECT requests using the WebTransport upgrade token (see
   Section 3.1).  This ensures that the client will always know what
   versions of WebTransport can be used on a given HTTP/3 connection.






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   Clients can, however, send a SETTINGS frame, multiple WebTransport
   CONNECT requests, WebTransport data streams, and WebTransport
   datagrams all within a single flight.  As those can arrive out of
   order, a WebTransport server could be put into a situation where it
   receives a stream or a datagram without a corresponding session.
   Similarly, a client may receive a server-initiated stream or a
   datagram before receiving the CONNECT response headers from the
   server.

   To handle this case, WebTransport endpoints SHOULD buffer streams and
   datagrams until they can be associated with an established session.
   To avoid resource exhaustion, endpoints MUST limit the number of
   buffered streams and datagrams.  When the number of buffered streams
   is exceeded, a stream SHALL be closed by sending a RESET_STREAM and/
   or STOP_SENDING with the WT_BUFFERED_STREAM_REJECTED error code.
   When the number of buffered datagrams is exceeded, a datagram SHALL
   be dropped.  It is up to an implementation to choose what stream or
   datagram to discard.

4.7.  Interaction with the HTTP/3 GOAWAY frame

   HTTP/3 defines a graceful shutdown mechanism (Section 5.2 of [HTTP3])
   that allows a peer to send a GOAWAY frame indicating that it will no
   longer accept any new incoming requests or pushes.

   A client receiving GOAWAY cannot initiate CONNECT requests for new
   WebTransport sessions on that HTTP/3 connection; it must open a new
   HTTP/3 connection to initiate new WebTransport sessions with the same
   peer.

   An HTTP/3 GOAWAY frame is also a signal to applications to initiate
   shutdown for all WebTransport sessions.  To shut down a single
   WebTransport session, either endpoint can send a WT_DRAIN_SESSION
   (0x78ae) capsule.

   WT_DRAIN_SESSION Capsule {
     Type (i) = WT_DRAIN_SESSION,
     Length (i) = 0
   }

                 Figure 5: WT_DRAIN_SESSION Capsule Format

   After sending or receiving either a WT_DRAIN_SESSION capsule or a
   HTTP/3 GOAWAY frame, an endpoint MAY continue using the session and
   MAY open new WebTransport streams.  The signal is intended for the
   application using WebTransport, which is expected to attempt to
   gracefully terminate the session as soon as possible.




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   The WT_DRAIN_SESSION capsule is useful when an end-to-end
   WebTransport session passes through an intermediary.  For example,
   when the backend shuts down, it sends a GOAWAY to the intermediary.
   The intermediary can convert this signal to a WT_DRAIN_SESSION
   capsule on the client-facing session, without impacting other
   requests or sessions carried on that connection.

4.8.  Use of Keying Material Exporters

   WebTransport over HTTP/3 supports the use of TLS keying material
   exporters Section 7.5 of [RFC8446].  Since the underlying QUIC
   connection may be shared by multiple WebTransport sessions,
   WebTransport defines a mechanism for deriving a TLS exporter that
   separates keying material for different sessions.  If the application
   requests an exporter for a given WebTransport session with a
   specified label and context, the resulting exporter SHALL be a TLS
   exporter as defined in Section 7.5 of [RFC8446] with the label set to
   "EXPORTER-WebTransport" and the context set to the serialization of
   the "WebTransport Exporter Context" struct as defined below.

   WebTransport Exporter Context {
     WebTransport Session ID (64),
     WebTransport Application-Supplied Exporter Label Length (8),
     WebTransport Application-Supplied Exporter Label (8..),
     WebTransport Application-Supplied Exporter Context Length (8),
     WebTransport Application-Supplied Exporter Context (..)
   }

               Figure 6: WebTransport Exporter Context struct

   A TLS exporter API might permit the context field to be omitted.  In
   this case, as with TLS 1.3, the WebTransport Application-Supplied
   Exporter Context becomes zero-length if omitted.

5.  Flow Control

   Flow control governs the amount of resources that can be consumed or
   data that can be sent.  When using WebTransport over HTTP/3,
   endpoints can limit the number of sessions that a peer can create on
   a single HTTP/3 connection and the number of streams that a peer can
   create within a session.  Endpoints can also limit the amount of data
   that can be consumed by each session and by each stream within a
   session.








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   WebTransport over HTTP/3 provides a connection-level limit that
   governs the number of sessions that can be created on an HTTP/3
   connection (see Section 5.2).  It also provides session-level limits
   that govern the number of streams that can be created in a session
   and limit the amount of data that can be exchanged across all streams
   in each session (see Section 5.3).

   The underlying QUIC connection provides connection and stream level
   flow control.  The QUIC connection data limit defines the total
   amount of data that can be sent across all WebTransport sessions and
   other non-WebTransport streams.  A QUIC stream's data limit controls
   the amount of data that can be sent on that stream, WebTransport or
   otherwise (see Section 4 of [RFC9000]).

5.1.  Negotiating the Use of Flow Control

   A WebTransport endpoint that allows a WebTransport session to share
   an underlying transport connection with other WebTransport sessions
   MUST enable flow control.  This prevents an application from
   consuming excessive resources on a single session and starving
   traffic for other sessions (see Section 8).

   Flow control is enabled when both endpoints declare their intent to
   use flow control, even if SETTINGS_WT_MAX_SESSIONS is sent with a
   value of "1".  Endpoints declare their intent to use flow control by
   taking any of the following actions:

   *  Sending SETTINGS_WT_MAX_SESSIONS with a value greater than "1".

   *  Sending SETTINGS_WT_INITIAL_MAX_STREAMS_UNI with any value other
      than "0".

   *  Sending SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI with any value other
      than "0".

   *  Sending SETTINGS_WT_INITIAL_MAX_DATA with any value other than
      "0".

   If both endpoints take at least one of these actions, flow control is
   enabled, and the limits described in the entirety of Section 5 apply.

   The inclusion of the flow control SETTINGS in these criteria allows
   endpoints to agree to explicitly enable flow control, even if only a
   single WebTransport session is supported.







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   If flow control is not enabled, clients MUST NOT attempt to establish
   more than one simultaneous WebTransport session.  A server that
   receives more than one session on an underlying transport connection
   when flow control is not enabled MUST reset the excessive CONNECT
   streams with a H3_REQUEST_REJECTED status (see Section 5.2).

   Also, if flow control is not enabled, an endpoint MUST ignore receipt
   of any flow control capsules (see Section 5.6), since the peer might
   not have received SETTINGS at the time they were sent or packets
   might have been reordered.

5.2.  Limiting the Number of Simultaneous Sessions

   This document defines a SETTINGS_WT_MAX_SESSIONS setting that allows
   the server to limit the maximum number of concurrent WebTransport
   sessions on a single HTTP/3 connection.  The client MUST NOT open
   more simultaneous sessions than indicated in the server SETTINGS
   parameter.  The server MUST NOT close the connection if the client
   opens sessions exceeding this limit, as the client and the server do
   not have a consistent view of how many sessions are open due to the
   asynchronous nature of the protocol; instead, it MUST reset all of
   the CONNECT streams it is not willing to process with the
   H3_REQUEST_REJECTED status defined in Section 8.1 of [HTTP3].

5.3.  Limiting the Number of Streams Within a Session

   The WT_MAX_STREAMS capsule (Section 5.6.2) establishes a limit on the
   number of streams within a WebTransport session.  Like the QUIC
   MAX_STREAMS frame (Section 19.11 of [RFC9000]), this capsule has two
   types that provide separate limits for unidirectional and
   bidirectional streams that a peer initiates.

   Note that the CONNECT stream for the session is not included in
   either the bidirectional or the unidirectional stream limits; the
   number of CONNECT streams a client can open is limited by the
   SETTINGS_WT_MAX_SESSIONS setting and QUIC flow control's stream
   limits.

   The session-level stream limit applies in addition to the QUIC
   MAX_STREAMS frame, which provides a connection-level stream limit.
   New streams can only be created within the session if both the
   stream- and the connection-level limit permit, see Section 4.6 of
   [RFC9000] for details on how QUIC stream limits are applied.

   Unlike the the QUIC MAX_STREAMS frame, there is no simple
   relationship between the value in this frame and stream IDs in QUIC
   STREAM frames.  This especially applies if there are other users of
   streams on the connection.



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   The WT_STREAMS_BLOCKED capsule (Section 5.6.3) can be sent to
   indicate that an endpoint was unable to create a stream due to the
   session-level stream limit.

   Note that enforcing this limit requires reliable resets for stream
   headers so that both endpoints can agree on the number of streams
   that are open.

5.4.  Data Limits

   The WT_MAX_DATA capsule (Section 5.6.4) establishes a limit on the
   amount of data that can be sent within a WebTransport session.  This
   limit counts all data that is sent on streams of the corresponding
   type, excluding the stream header (see Section 4.2 and Section 4.3).
   The stream header is excluded from this limit so that this limit does
   not prevent the sending of information that is essential in linking
   new streams to a specific WebTransport session.

   For streams that were reset, implementing WT_MAX_DATA requires that
   the QUIC stack provide the WebTransport implementation with
   information about the final size of streams; see Section 4.5 of
   [RFC9000].  This guarantees that both endpoints agree on how much
   WebTransport session flow control credit was consumed by the sender
   on that stream.

   The WT_DATA_BLOCKED capsule (Section 5.6.5) can be sent to indicate
   that an endpoint was unable to send data due to a limit set by the
   WT_MAX_DATA capsule.

   Because WebTransport over HTTP/3 uses a native QUIC stream for each
   WebTransport stream, per-stream data limits are provided by QUIC
   natively (see Section 4.1 of [RFC9000]).  The WT_MAX_STREAM_DATA and
   WT_STREAM_DATA_BLOCKED capsules (Part XX of
   [I-D.ietf-webtrans-http2]) are not used and so are prohibited.
   Endpoints MUST treat receipt of a WT_MAX_STREAM_DATA or a
   WT_STREAM_DATA_BLOCKED capsule as a session error.

5.5.  Flow Control SETTINGS

   Initial flow control limits can be exchanged via HTTP/3 SETTINGS
   (Section 9.2) by providing non-zero values for

   *  WT_MAX_STREAMS via SETTINGS_WT_INITIAL_MAX_STREAMS_UNI and
      SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI

   *  WT_MAX_DATA via SETTINGS_WT_INITIAL_MAX_DATA





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5.6.  Flow Control Capsules

   WebTransport over HTTP/3 uses several capsules for flow control, and
   all of these capsules define special intermediary handling as
   described in Section 3.2 of [HTTP-DATAGRAM].  These capsules,
   referred to as the "flow control capsules", are WT_MAX_DATA,
   WT_MAX_STREAMS, WT_DATA_BLOCKED, and WT_STREAMS_BLOCKED.

5.6.1.  Flow Control and Intermediaries

   Because flow control in WebTransport is hop-by-hop and does not
   provide an end-to-end signal, intermediaries MUST consume flow
   control signals and express their own flow control limits to the next
   hop.  The intermediary can send these signals via HTTP/3 flow control
   messages, HTTP/2 flow control messages, or as WebTransport flow
   control capsules, where appropriate.  Intermediaries are responsible
   for storing any data for which they advertise flow control credit if
   that data cannot be immediately forwarded to the next hop.

   In practice, an intermediary that translates flow control signals
   between similar WebTransport protocols, such as between two HTTP/3
   connections, can often simply reexpress the same limits received on
   one connection directly on the other connection.

   An intermediary that does not want to be responsible for storing data
   that cannot be immediately sent on its translated connection can
   ensure that it does not advertise a higher flow control limit on one
   connection than the corresponding limit on the translated connection.

5.6.2.  WT_MAX_STREAMS Capsule

   An HTTP capsule [HTTP-DATAGRAM] 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 capsule with a type
   of 0x190B4D3F applies to bidirectional streams, and a WT_MAX_STREAMS
   capsule with a type of 0x190B4D40 applies to unidirectional streams.

   Note that, because Maximum Streams is a cumulative value representing
   the total allowed number of streams, including previously closed
   streams, endpoints repeatedly send new WT_MAX_STREAMS capsules with
   increasing Maximum Streams values as streams are opened.

   WT_MAX_STREAMS Capsule {
     Type (i) = 0x190B4D3F..0x190B4D40,
     Length (i),
     Maximum Streams (i),
   }




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                  Figure 7: WT_MAX_STREAMS Capsule Format

   WT_MAX_STREAMS capsules contain the following field:

   Maximum Streams:  A count of the cumulative number of streams of the
      corresponding type that can be opened over the lifetime of the
      session.  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.

   Unlike in QUIC, where MAX_STREAMS frames can be delivered in any
   order, WT_MAX_STREAMS capsules are sent on the WebTransport session's
   connect stream and are delivered in order.  If an endpoint receives a
   WT_MAX_STREAMS capsule with a Maximum Streams value less than a
   previously received value, it MUST close the WebTransport session by
   resetting the connect stream with the WT_FLOW_CONTROL_ERROR error
   code.

   The WT_MAX_STREAMS capsule defines special intermediary handling, as
   described in Section 3.2 of [HTTP-DATAGRAM].  Intermediaries MUST
   consume WT_MAX_STREAMS capsules for flow control purposes and MUST
   generate and send appropriate flow control signals for their limits.

   Initial values for these limits MAY be communicated by sending non-
   zero values for SETTINGS_WT_INITIAL_MAX_STREAMS_UNI and
   SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI.

5.6.3.  WT_STREAMS_BLOCKED Capsule

   A sender SHOULD send a WT_STREAMS_BLOCKED capsule (type=0x190B4D43 or
   0x190B4D44) 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 capsule of type 0x190B4D43 is used to indicate
   reaching the bidirectional stream limit, and a STREAMS_BLOCKED
   capsule of type 0x190B4D44 is used to indicate reaching the
   unidirectional stream limit.

   A WT_STREAMS_BLOCKED capsule 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.




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   WT_STREAMS_BLOCKED Capsule {
     Type (i) = 0x190B4D43..0x190B4D44,
     Length (i),
     Maximum Streams (i),
   }

                Figure 8: WT_STREAMS_BLOCKED Capsule Format

   WT_STREAMS_BLOCKED capsules contain the following field:

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

   The WT_STREAMS_BLOCKED capsule defines special intermediary handling,
   as described in Section 3.2 of [HTTP-DATAGRAM].  Intermediaries MUST
   consume WT_STREAMS_BLOCKED capsules for flow control purposes and
   MUST generate and send appropriate flow control signals for their
   limits.

5.6.4.  WT_MAX_DATA Capsule

   An HTTP capsule [HTTP-DATAGRAM] called WT_MAX_DATA (type=0x190B4D3D)
   is introduced to inform the peer of the maximum amount of data that
   can be sent on the WebTransport session as a whole.

   This limit counts all data that is sent on streams of the
   corresponding type, excluding the stream header (see Section 4.2 and
   Section 4.3).  For streams that were reset, implementing WT_MAX_DATA
   requires that the QUIC stack provide the WebTransport implementation
   with information about the final size of streams (see Section 4.5 of
   [RFC9000]).

   WT_MAX_DATA Capsule {
     Type (i) = 0x190B4D3D,
     Length (i),
     Maximum Data (i),
   }

                    Figure 9: WT_MAX_DATA Capsule Format

   WT_MAX_DATA capsules contain the following field:

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




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   All data sent in WT_STREAM capsules counts toward this limit.  The
   sum of the lengths of Stream Data fields in WT_STREAM capsules MUST
   NOT exceed the value advertised by a receiver.

   Unlike in QUIC, where MAX_DATA frames can be delivered in any order,
   WT_MAX_DATA capsules are sent on the WebTransport session's connect
   stream and are delivered in order.  If an endpoint receives a
   WT_MAX_DATA capsule with a Maximum Data value less than a previously
   received value, it MUST close the WebTransport session by resetting
   the connect stream with the WT_FLOW_CONTROL_ERROR error code.

   The WT_MAX_DATA capsule defines special intermediary handling, as
   described in Section 3.2 of [HTTP-DATAGRAM].  Intermediaries MUST
   consume WT_MAX_DATA capsules for flow control purposes and MUST
   generate and send appropriate flow control signals for their limits
   (see Section 5.6.1).

   The initial value for this limit MAY be communicated by sending a
   non-zero value for SETTINGS_WT_INITIAL_MAX_DATA.

5.6.5.  WT_DATA_BLOCKED Capsule

   A sender SHOULD send a WT_DATA_BLOCKED capsule (type=0x190B4D41) when
   it wishes to send data but is unable to do so due to WebTransport
   session-level flow control.  WT_DATA_BLOCKED capsules can be used as
   input to tuning of flow control algorithms.

   WT_DATA_BLOCKED Capsule {
     Type (i) = 0x190B4D41,
     Length (i),
     Maximum Data (i),
   }

                 Figure 10: WT_DATA_BLOCKED Capsule Format

   WT_DATA_BLOCKED capsules contain the following field:

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

   The WT_DATA_BLOCKED capsule defines special intermediary handling, as
   described in Section 3.2 of [HTTP-DATAGRAM].  Intermediaries MUST
   consume WT_DATA_BLOCKED capsules for flow control purposes and MUST
   generate and send appropriate flow control signals for their limits
   (see Section 5.6.1).






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6.  Session Termination

   A WebTransport session over HTTP/3 is considered terminated when
   either of the following conditions is met:

   *  the CONNECT stream is closed, either cleanly or abruptly, on
      either side; or

   *  a WT_CLOSE_SESSION capsule is either sent or received.

   Upon learning that the session has been terminated, the endpoint MUST
   reset the send side and abort reading on the receive side of all
   unidirectional and bidirectional streams associated with the session
   (see Section 2.4 of [RFC9000]) using the WT_SESSION_GONE error code;
   it MUST NOT send any new datagrams or open any new streams.

   To terminate a session with a detailed error message, an application
   MAY provide such a message for the WebTransport endpoint to send in
   an HTTP capsule [HTTP-DATAGRAM] of type WT_CLOSE_SESSION (0x2843).
   The format of the capsule SHALL be as follows:

   WT_CLOSE_SESSION Capsule {
     Type (i) = WT_CLOSE_SESSION,
     Length (i),
     Application Error Code (32),
     Application Error Message (..8192),
   }

                 Figure 11: WT_CLOSE_SESSION Capsule Format

   WT_CLOSE_SESSION has the following fields:

   Application Error Code:  A 32-bit error code provided by the
      application closing the session.

   Application Error Message:  A UTF-8 encoded error message string
      provided by the application closing the session.  The message
      takes up the remainder of the capsule, and its length MUST NOT
      exceed 1024 bytes.

   Note that the Application Error Code field does not mirror the Error
   Code field in QUIC's CONNECTION_CLOSE frame (Section 19.19 of
   [RFC9000]) because WebTransport application errors use a subset of
   the HTTP/3 Error Code space and need to fit within those bounds, see
   Section 4.4.






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   An endpoint that sends a WT_CLOSE_SESSION capsule MUST immediately
   send a FIN on the CONNECT Stream.  The endpoint MAY also send a
   STOP_SENDING with error code WT_SESSION_GONE to indicate it is no
   longer reading from the CONNECT stream.  The recipient MUST either
   close or reset the stream in response.  If any additional stream data
   is received on the CONNECT stream after receiving a WT_CLOSE_SESSION
   capsule, the stream MUST be reset with code H3_MESSAGE_ERROR.

   Cleanly terminating a CONNECT stream without a WT_CLOSE_SESSION
   capsule SHALL be semantically equivalent to terminating it with a
   WT_CLOSE_SESSION capsule that has an error code of 0 and an empty
   error string.

   In some scenarios, an endpoint might want to send a WT_CLOSE_SESSION
   with detailed close information and then immediately close the
   underlying QUIC connection.  If the endpoint were to do both of those
   simultaneously, the peer could potentially receive the
   CONNECTION_CLOSE before receiving the WT_CLOSE_SESSION, thus never
   receiving the application error data contained in the latter.  To
   avoid this, the endpoint SHOULD wait until all CONNECT streams have
   been closed by the peer before sending the CONNECTION_CLOSE; this
   gives WT_CLOSE_SESSION properties similar to that of the QUIC
   CONNECTION_CLOSE mechanism as a best-effort mechanism of delivering
   application close metadata.

7.  Considerations for Future Versions

   Future versions of WebTransport that change the syntax of the CONNECT
   requests used to establish WebTransport sessions will need to modify
   the upgrade token used to identify WebTransport, allowing servers to
   offer multiple versions simultaneously (see Section 9.1).

   Servers that support future incompatible versions of WebTransport
   signal that support by changing the codepoint used for the
   SETTINGS_WT_MAX_SESSIONS setting (see Section 9.2).  Clients can
   select the associated upgrade token, if applicable, to use when
   establishing a new session, ensuring that servers will always know
   the syntax in use for every incoming request.

   Changes to future stream formats require changes to the
   Unidirectional Stream type (see Section 4.2) and Bidirectional Stream
   signal value (see Section 4.3) to allow recipients of incoming frames
   to determine the WebTransport version, and corresponding wire format,
   used for the session associated with that stream.







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7.1.  Negotiating the Draft Version

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

   The wire format aspects of the protocol are negotiated by changing
   the codepoint used for the SETTINGS_WT_MAX_SESSIONS setting.  Because
   of that, any WebTransport endpoint MUST wait for the peer's SETTINGS
   frame before sending or processing any WebTransport traffic.  When
   multiple versions are supported by both of the peers, the most recent
   version supported by both is selected.

8.  Security Considerations

   WebTransport over HTTP/3 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
   application is potentially untrusted.

   WebTransport over HTTP/3 requires explicit opt-in through the use of
   an HTTP/3 setting; this avoids potential protocol confusion attacks
   by ensuring the HTTP/3 server explicitly supports it.  It also
   requires the use of the Origin header for browser traffic, providing
   the server with the ability to deny access to Web-based applications
   that do not originate from a trusted origin.

   Just like HTTP traffic going over HTTP/3, 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 WebTransport sessions share both congestion
   control and flow control context, a single application aggressively
   using up those resources can cause other sessions to stall.  A
   WebTransport endpoint MUST implement flow control mechanisms if it
   allows a WebTransport session to share the transport connection with
   other WebTransport sessions.  WebTransport endpoints SHOULD implement
   a fairness scheme that ensures that each session that shares a
   transport connection gets a reasonable share of controlled resources;
   this applies both to sending data and to opening new streams.

   An application could attempt to exhaust resources by opening too many
   WebTransport sessions at once.  In cases when the application is
   untrusted, the WebTransport client SHOULD limit the number of
   outgoing sessions it will open.







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9.  IANA Considerations

9.1.  Upgrade Token Registration

   The following entry is added to the "Hypertext Transfer Protocol
   (HTTP) Upgrade Token Registry" registry established by Section 16.7
   of [HTTP].

   The "webtransport" label identifies HTTP/3 used as a protocol for
   WebTransport:

   Value:  webtransport

   Description:  WebTransport over HTTP/3

   Reference:  This document and [I-D.ietf-webtrans-http2]

9.2.  HTTP/3 SETTINGS Parameter Registration

   The following entry is added to the "HTTP/3 Settings" registry
   established by [HTTP3]:

   The SETTINGS_WT_MAX_SESSIONS setting indicates that the specified
   HTTP/3 endpoint is WebTransport-capable and the number of concurrent
   sessions it is willing to receive.  The default value for the
   SETTINGS_WT_MAX_SESSIONS setting is "0", meaning that the endpoint is
   not willing to receive any WebTransport sessions.

   Setting Name:  WT_MAX_SESSIONS

   Value:  0x14e9cd29

   Default:  0

   Specification:  This document

   The SETTINGS_WT_INITIAL_MAX_STREAMS_UNI setting indicates the initial
   value for the unidirectional max stream limit, otherwise communicated
   by the WT_MAX_STREAMS capsule (see Section 5.6.2).  The default value
   for the SETTINGS_WT_INITIAL_MAX_STREAMS_UNI setting is "0",
   indicating that the endpoint needs to send WT_MAX_STREAMS capsules on
   each individual WebTransport session before its peer is allowed to
   create any unidirectional streams within that session.

   Note that this limit applies to all WebTransport sessions that use
   the HTTP/3 connection on which this SETTING is sent.

   Setting Name:  SETTINGS_WT_INITIAL_MAX_STREAMS_UNI



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   Value:  0x2b64

   Default:  0

   Specification:  This document

   The SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI setting indicates the
   initial value for the bidirectional max stream limit, otherwise
   communicated by the WT_MAX_STREAMS capsule (see Section 5.6.2).  The
   default value for the SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI setting is
   "0", indicating that the endpoint needs to send WT_MAX_STREAMS
   capsules on each individual WebTransport session before its peer is
   allowed to create any bidirectional streams within that session.

   Note that this limit applies to all WebTransport sessions that use
   the HTTP/3 connection on which this SETTING is sent.

   Setting Name:  SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI

   Value:  0x2b65

   Default:  0

   Specification:  This document

   The SETTINGS_WT_INITIAL_MAX_DATA setting indicates the initial value
   for the session data limit, otherwise communicated by the WT_MAX_DATA
   capsule (see Section 5.6.4).  The default value for the
   SETTINGS_WT_INITIAL_MAX_DATA setting is "0", indicating that the
   endpoint needs to send a WT_MAX_DATA capsule within each session
   before its peer is allowed to send any stream data within that
   session.

   Note that this limit applies to all WebTransport sessions that use
   the HTTP/3 connection on which this SETTING is sent.

   Setting Name:  SETTINGS_WT_INITIAL_MAX_DATA

   Value:  0x2b61

   Default:  0

   Specification:  This document

9.3.  Frame Type Registration

   The following entry is added to the "HTTP/3 Frame Type" registry
   established by [HTTP3]:



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   The WT_STREAM frame is reserved for the purpose of avoiding collision
   with WebTransport HTTP/3 extensions:

   Code:  0x41

   Frame Type:  WT_STREAM

   Specification:  This document

9.4.  Stream Type Registration

   The following entry is added to the "HTTP/3 Stream Type" registry
   established by [HTTP3]:

   The "WebTransport stream" type allows unidirectional streams to be
   used by WebTransport:

   Code:  0x54

   Stream Type:  WebTransport stream

   Specification:  This document

   Sender:  Both

9.5.  HTTP/3 Error Code Registration

   The following entries are added to the "HTTP/3 Error Code" registry
   established by [HTTP3]:

   Name:  WT_BUFFERED_STREAM_REJECTED

   Value:  0x3994bd84

   Description:  WebTransport data stream rejected due to lack of
      associated session.

   Specification:  This document.

   Name:  WT_SESSION_GONE

   Value:  0x170d7b68

   Description:  WebTransport data stream aborted because the associated
      WebTransport session has been closed.  Also used to indicate that
      the endpoint is no longer reading from the CONNECT stream.

   Specification:  This document.



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   Name:  WT_FLOW_CONTROL_ERROR

   Value:  0x045d4487

   Description:  WebTransport session aborted because a flow control
      error was encountered.

   Specification:  This document.

   In addition, the following range of entries is registered:

   Name:  WT_APPLICATION_ERROR

   Value:  0x52e4a40fa8db to 0x52e5ac983162 inclusive, with the
      exception of the codepoints of form 0x1f * N + 0x21.

   Description:  WebTransport application error codes.

   Specification:  This document.

9.6.  Capsule Types

   The following entries are added to the "HTTP Capsule Types" registry
   established by [HTTP-DATAGRAM]:

   The WT_CLOSE_SESSION capsule.

   Value:  0x2843
   Capsule Type:  WT_CLOSE_SESSION
   Status:  permanent
   Specification:  This document
   Change Controller:  IETF
   Contact:  WebTransport Working Group webtransport@ietf.org
      (mailto:webtransport@ietf.org)
   Notes:  None

   The WT_DRAIN_SESSION capsule.

   Value:  0x78ae
   Capsule Type:  WT_DRAIN_SESSION
   Status:  provisional (when this document is approved this will become
      permanent)
   Specification:  This document
   Change Controller:  IETF
   Contact:  WebTransport Working Group webtransport@ietf.org
      (mailto:webtransport@ietf.org)
   Notes:  None




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   The WT_MAX_STREAMS capsule:

   Value:  0x190B4D3F..0x190B4D40
   Capsule Type:  WT_MAX_STREAMS
   Status:  permanent
   Specification:  This document
   Change Controller:  IETF
   Contact:  WebTransport Working Group webtransport@ietf.org
      (mailto:webtransport@ietf.org)
   Notes:  None

   The WT_STREAMS_BLOCKED capsule:

   Value:  0x190B4D43..0x190B4D44
   Capsule Type:  WT_STREAMS_BLOCKED
   Status:  permanent
   Specification:  This document
   Change Controller:  IETF
   Contact:  WebTransport Working Group webtransport@ietf.org
      (mailto:webtransport@ietf.org)
   Notes:  None

   The WT_MAX_DATA capsule:

   Value:  0x190B4D3D
   Capsule Type:  WT_MAX_DATA
   Status:  permanent
   Specification:  This document
   Change Controller:  IETF
   Contact:  WebTransport Working Group webtransport@ietf.org
      (mailto:webtransport@ietf.org)
   Notes:  None

   The WT_DATA_BLOCKED capsule:

   Value:  0x190B4D41
   Capsule Type:  WT_DATA_BLOCKED
   Status:  permanent
   Specification:  This document
   Change Controller:  IETF
   Contact:  WebTransport Working Group webtransport@ietf.org
      (mailto:webtransport@ietf.org)
   Notes:  None








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9.7.  Protocol Negotiation HTTP Header Fields

   The following HTTP header fields are used for negotiating a protocol
   (Section 3.3.  These are added to the "HTTP Field Name" registry
   established in Section 18.4 of [HTTP]:

   The WT-Available-Protocols field:

   Field Name:  WT-Available-Protocols
   Status:  permanent
   Structured Type:  List
   Reference:  Section 3.3
   Comments:  None

   The WT-Protocol field:

   Field Name:  WT-Protocol
   Status:  permanent
   Structured Type:  Item
   Reference:  Section 3.3
   Comments:  None

10.  References

10.1.  Normative References

   [FIELDS]   Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,
              <https://www.rfc-editor.org/rfc/rfc9651>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

   [HTTP-DATAGRAM]
              Schinazi, D. and L. Pardue, "HTTP Datagrams and the
              Capsule Protocol", RFC 9297, DOI 10.17487/RFC9297, August
              2022, <https://www.rfc-editor.org/rfc/rfc9297>.

   [HTTP3]    Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.

   [OVERVIEW] Kinnear, E. and V. Vasiliev, "The WebTransport Protocol
              Framework", Work in Progress, Internet-Draft, draft-ietf-
              webtrans-overview-11, 20 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              webtrans-overview-11>.



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   [QUIC-DATAGRAM]
              Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", RFC 9221,
              DOI 10.17487/RFC9221, March 2022,
              <https://www.rfc-editor.org/rfc/rfc9221>.

   [RESET-STREAM-AT]
              Seemann, M. and K. Oku, "QUIC Stream Resets with Partial
              Delivery", Work in Progress, Internet-Draft, draft-ietf-
              quic-reliable-stream-reset-07, 14 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              reliable-stream-reset-07>.

   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/rfc/rfc6454>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [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>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.



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   [RFC9114]  Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.

   [RFC9218]  Oku, K. and L. Pardue, "Extensible Prioritization Scheme
              for HTTP", RFC 9218, DOI 10.17487/RFC9218, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9218>.

   [RFC9220]  Hamilton, R., "Bootstrapping WebSockets with HTTP/3",
              RFC 9220, DOI 10.17487/RFC9220, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9220>.

10.2.  Informative References

   [I-D.ietf-webtrans-http2]
              Frindell, A., Kinnear, E., Pauly, T., Thomson, M.,
              Vasiliev, V., and G. Xie, "WebTransport over HTTP/2", Work
              in Progress, Internet-Draft, draft-ietf-webtrans-http2-12,
              7 July 2025, <https://datatracker.ietf.org/doc/html/draft-
              ietf-webtrans-http2-12>.

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

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.

   [RFC9208]  Melnikov, A., "IMAP QUOTA Extension", RFC 9208,
              DOI 10.17487/RFC9208, March 2022,
              <https://www.rfc-editor.org/rfc/rfc9208>.

   [RFC9308]  Kühlewind, M. and B. Trammell, "Applicability of the QUIC
              Transport Protocol", RFC 9308, DOI 10.17487/RFC9308,
              September 2022, <https://www.rfc-editor.org/rfc/rfc9308>.

Appendix A.  Changelog

A.1.  Changes between draft versions 02 and 07

   The following changes make the draft-02 and draft-07 versions of this
   protocol incompatible:

   *  draft-07 requires SETTINGS_WEBTRANSPORT_MAX_SESSIONS (#86) and
      uses it for version negotiation (#129)





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   *  draft-07 explicitly requires SETTINGS_ENABLE_CONNECT_PROTOCOL to
      be enabled (#93)

   *  draft-07 explicitly requires SETTINGS_H3_DATAGRAM to be enabled
      (#106)

   *  draft-07 only allows WEBTRANSPORT_STREAM at the beginning of the
      stream

   The following changes that are present in draft-07 can be also
   implemented by a draft-02 implementation safely:

   *  Expanding stream reset error code space from 8 to 32 bits (#115)

   *  WEBTRANSPORT_SESSION_GONE error code (#75)

   *  Handling for HTTP GOAWAY (#76)

   *  DRAIN_WEBTRANSPORT_SESSION capsule (#79)

   *  Disallowing following redirects automatically (#113)

Index

   S W

      S

         SETTINGS_WT_INITIAL_MAX_DATA  Section 5.1, Paragraph 3.4.1;
            Section 5.5, Paragraph 2.2.1; Section 5.6.4, Paragraph 9;
            Section 9.2, Paragraph 10; Section 9.2, Paragraph 12.2.1
         SETTINGS_WT_INITIAL_MAX_STREAMS_BIDI  Section 5.1, Paragraph
            3.3.1; Section 5.5, Paragraph 2.1.1; Section 5.6.2,
            Paragraph 10; Section 9.2, Paragraph 7; Section 9.2,
            Paragraph 9.2.1
         SETTINGS_WT_INITIAL_MAX_STREAMS_UNI  Section 5.1, Paragraph
            3.2.1; Section 5.5, Paragraph 2.1.1; Section 5.6.2,
            Paragraph 10; Section 9.2, Paragraph 4; Section 9.2,
            Paragraph 6.2.1

      W

         WT_DATA_BLOCKED  Section 5.4, Paragraph 3; Section 5.6,
            Paragraph 1; Section 5.6.5, Paragraph 1; Section 5.6.5,
            Paragraph 3; Section 5.6.5, Paragraph 5; Section 9.6,
            Paragraph 13.4.1
         WT_MAX_DATA  Section 5.4, Paragraph 1; Section 5.4, Paragraph




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            2; Section 5.4, Paragraph 3; Section 5.5, Paragraph 2.2.1;
            Section 5.6, Paragraph 1; Section 5.6.4, Paragraph 1;
            Section 5.6.4, Paragraph 2; Section 5.6.4, Paragraph 4;
            Section 5.6.4, Paragraph 7; Section 5.6.4, Paragraph 8;
            Section 9.2, Paragraph 10; Section 9.6, Paragraph 11.4.1
         WT_MAX_STREAMS  Section 5.3, Paragraph 1; Section 5.5,
            Paragraph 2.1.1; Section 5.6, Paragraph 1; Section 5.6.2,
            Paragraph 1; Section 5.6.2, Paragraph 2; Section 5.6.2,
            Paragraph 4; Section 5.6.2, Paragraph 8; Section 5.6.2,
            Paragraph 9; Section 9.2, Paragraph 4; Section 9.2,
            Paragraph 7; Section 9.6, Paragraph 7.4.1
         WT_STREAMS_BLOCKED  Section 5.3, Paragraph 5; Section 5.6,
            Paragraph 1; Section 5.6.3, Paragraph 1; Section 5.6.3,
            Paragraph 2; Section 5.6.3, Paragraph 4; Section 5.6.3,
            Paragraph 6; Section 9.6, Paragraph 9.4.1

Authors' Addresses

   Alan Frindell
   Facebook
   Email: afrind@fb.com


   Eric Kinnear
   Apple Inc.
   Email: ekinnear@apple.com


   Victor Vasiliev
   Google
   Email: vasilvv@google.com




















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