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<pre class="metadata">
Title: WebTransport
Shortname: webtransport
Level: none
Status: w3c/ED
Group: webtransport
ED: https://w3c.github.io/webtransport/
TR: https://www.w3.org/TR/webtransport/
Editor: Nidhi Jaju, w3cid 136840, Google
Editor: Victor Vasiliev, w3cid 113328, Google
Editor: Jan-Ivar Bruaroey, w3cid 79152, Mozilla
Former Editor: Bernard Aboba, Microsoft Corporation
Former Editor: Peter Thatcher, Google
Former Editor: Robin Raymond, Optical Tone Ltd.
Former Editor: Yutaka Hirano, Google
Abstract:
This document defines a set of ECMAScript APIs in WebIDL to allow data to be
sent and received between a browser and server, utilizing [[!WEB-TRANSPORT-HTTP3]]
and [[!WEB-TRANSPORT-HTTP2]]. This specification is being developed in conjunction
with protocol specifications developed by the IETF WEBTRANS Working Group.
Repository: w3c/webtransport
Indent: 2
Markup Shorthands: markdown yes
Boilerplate: omit conformance
</pre>
<pre class="biblio">
{
"quic": {
"authors": ["Jana Iyengar", "Martin Thomson"],
"href": "https://www.rfc-editor.org/rfc/rfc9000",
"title": "QUIC: A UDP-Based Multiplexed and Secure Transport",
"status": "Proposed Standard",
"publisher": "IETF"
},
"quic-datagram": {
"authors": ["Tommy Pauly", "Eric Kinnear", "David Schinazi"],
"href": "https://www.rfc-editor.org/rfc/rfc9221",
"title": "An Unreliable Datagram Extension to QUIC",
"status": "Proposed Standard",
"publisher": "IETF"
},
"web-transport-overview": {
"authors": ["Victor Vasiliev"],
"href": "https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview",
"title": "WebTransport Protocol Framework",
"status": "Internet-Draft",
"publisher": "IETF"
},
"web-transport-http3": {
"authors": ["Alan Frindell", "Eric Kinnear", "Victor Vasiliev"],
"href": "https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3/",
"title": "WebTransport over HTTP/3",
"status": "Internet-Draft",
"publisher": "IETF"
},
"web-transport-http2": {
"authors": ["Alan Frindell", "Eric Kinnear", "Tommy Pauly", "Martin Thomson", "Victor Vasiliev", "Guowu Xie"],
"href": "https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http2/",
"title": "WebTransport over HTTP/2",
"status": "Internet-Draft",
"publisher": "IETF"
}
}
</pre>
<pre class="link-defaults">
spec:infra; type:dfn; for:set; text:for each
spec:infra; type:dfn; for:/; text:set
spec:streams; type:interface; text:ReadableStream
spec:fetch; type:dfn; for:/; text:fetch
spec:fetch; type:dfn; for:/; text:credentials
spec:url; type:dfn; text:scheme
spec:url; type:dfn; text:fragment
spec:infra; type:dfn; for:/; text:ASCII case-insensitive
spec:infra; type:dfn; text:list
</pre>
<pre class="anchors">
url: https://html.spec.whatwg.org/multipage/origin.html#concept-origin; type: dfn; text: origin; for:/
urlPrefix: http://www.ecma-international.org/ecma-262/6.0/index.html; spec: ECMASCRIPT-6.0
type: dfn
text: fulfilled; url: sec-promise-objects
text: rejected; url: sec-promise-objects
text: pending; url: sec-promise-objects
text: resolved; url: sec-promise-objects
text: settled; url: sec-promise-objects
text: the typed array constructors table; url: #table-49
urlPrefix: https://heycam.github.io/webidl/; spec: WEBIDL
type: dfn
text: created; for:DOMException; url: dfn-create-exception
text: name; for:DOMException; url: domexception-name
text: message; for:DOMException; url: domexception-message
</pre>
# Introduction # {#introduction}
*This section is non-normative.*
This specification uses [[!WEB-TRANSPORT-HTTP3]] and [[!WEB-TRANSPORT-HTTP2]] to
send data to and receive data from servers. It can be used like WebSockets but
with support for multiple streams, unidirectional streams, out-of-order delivery,
and reliable as well as unreliable transport.
Note: The API presented in this specification represents a preliminary proposal
based on work-in-progress within the IETF WEBTRANS WG. Since the [[!WEB-TRANSPORT-HTTP3]]
and [[!WEB-TRANSPORT-HTTP2]] specifications are a work-in-progress, both the protocol
and API are likely to change significantly going forward.
# Conformance # {#conformance}
As well as sections marked as non-normative, all authoring guidelines,
diagrams, examples, and notes in this specification are non-normative.
Everything else in this specification is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in
[[!RFC2119]] and [[!RFC8174]] when, and only when, they appear in all capitals, as shown here.
This specification defines conformance criteria that apply to a single product:
the user agent that implements the interfaces that it contains.
Conformance requirements phrased as algorithms or specific steps may be
implemented in any manner, so long as the end result is equivalent. (In
particular, the algorithms defined in this specification are intended to be
easy to follow, and not intended to be performant.)
Implementations that use ECMAScript to implement the APIs defined in this
specification MUST implement them in a manner consistent with the ECMAScript
Bindings defined in the Web IDL specification [[!WEBIDL]], as this
specification uses that specification and terminology.
# Protocol concepts # {#protocol-concepts}
There are two main protocol concepts for WebTransport: sessions and streams. Each [=WebTransport
session=] can contain multiple [=WebTransport streams=].
These should not be confused with [=protocol names=] which is an application-level API construct.
## WebTransport session ## {#webtransport-session}
A <dfn for="protocol">WebTransport session</dfn> is a session of WebTransport over an HTTP/3
or HTTP/2 <dfn>underlying [=connection=]</dfn>.
There may be multiple [=WebTransport sessions=] on one [=connection=], when pooling is enabled.
A [=WebTransport session=] has the following capabilities defined in [[!WEB-TRANSPORT-OVERVIEW]]:
<table class="data" dfn-for="session">
<thead>
<tr>
<th>capability
<th>definition
</tr>
</thead>
<tbody>
<tr>
<td><dfn>send a [datagram](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#name-datagrams)</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.2](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.2-6.2.1)
</tr>
<tr>
<td><dfn>receive a [datagram](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#name-datagrams)</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.2](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.2-6.4.1)
</tr>
<tr>
<td><dfn>create an [=stream/outgoing unidirectional=] stream</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-7.2.1)
</tr>
<tr>
<td><dfn>create a [=stream/bidirectional=] stream</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-7.4.1)
</tr>
<tr>
<td><dfn>receive an [=stream/incoming unidirectional=] stream</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-7.6.1)
</tr>
<tr>
<td><dfn>receive a [=stream/bidirectional=] stream</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-7.8.1)
</tr>
</tbody>
</table>
To <dfn for=session>establish</dfn> a [=WebTransport session=] with an [=/origin=] |origin| and a
|protocols| array, follow [[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.1](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.1-2.2.1),
with using |origin|, [=ASCII serialization of an origin|serialized=] and [=isomorphic encoded=],
as the [:Origin:] header of the request.
When establishing a session, the client MUST NOT provide any [=credentials=].
The resulting underlying transport stream is referred to as the session's <dfn>CONNECT stream</dfn>.
Additionally, if the |protocols| array is non-empty,
add a `WT-Available-Protocols` header field to the CONNECT request, containing
[=isomorphic encoded=] protocols from |protocols| in the order given, following [[!WEB-TRANSPORT-HTTP3]]
[Section 3.4](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3/#section-3.4).
<div class="note">
This should reference [[!WEB-TRANSPORT-OVERVIEW]] instead pending
[issue 15](https://github.com/ietf-wg-webtrans/draft-ietf-webtrans-overview/issues/15).
</div>
A [=WebTransport session=] |session| is <dfn for=session>draining</dfn> when the
[=CONNECT stream=] receives an [=session-signal/DRAIN_WEBTRANSPORT_SESSION=] capsule, or when a
[=session-signal/GOAWAY=] frame is received, as described in [[!WEB-TRANSPORT-HTTP3]]
[Section 4.6](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3/#section-4.6).
To <dfn for=session>terminate</dfn> a [=WebTransport session=] |session| with an optional integer
|code| and an optional [=byte sequence=] |reason|, follow [[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.1](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.1-2.2.1).
A [=WebTransport session=] |session| is <dfn for=session>terminated</dfn>, with optionally
an integer |code| and a [=byte sequence=] |reason|, when the [=CONNECT stream=] is closed by the server,
as described at [[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.1](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.1-4.2.1).
A [=WebTransport session=] has the following signals:
<table class="data" dfn-for="session-signal">
<thead>
<tr>
<th>event
<th>definition
</tr>
</thead>
<tbody>
<tr>
<td><dfn>DRAIN_WEBTRANSPORT_SESSION</dfn>
<td>[[!WEB-TRANSPORT-HTTP3]]
[Section 4.6](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3/#section-4.6)
</tr>
<tr>
<td><dfn>GOAWAY</dfn>
<td>[[!WEB-TRANSPORT-HTTP3]]
[Section 4.6](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3/#section-4.6)
</tr>
</tbody>
</table>
## WebTransport stream ## {#webtransport-stream}
A <dfn for="protocol">WebTransport stream</dfn> is a concept for a reliable in-order stream of
bytes on a [=WebTransport session=], as described in [[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3).
A [=WebTransport stream=] is one of <dfn for=stream>incoming unidirectional</dfn>,
<dfn for=stream>outgoing unidirectional</dfn> or <dfn for=stream>bidirectional</dfn>.
A [=WebTransport stream=] has the following capabilities:
<table class="data" dfn-for="stream">
<thead>
<tr>
<th>capability
<th>definition
<th>[=stream/incoming unidirectional=]
<th>[=stream/outgoing unidirectional=]
<th>[=stream/bidirectional=]
</tr>
</thead>
<tbody>
<tr>
<td><dfn>send</dfn> bytes (potentially with FIN)
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-9.2.1)
<td>No
<td>Yes
<td>Yes
</tr>
<tr>
<td><dfn>receive</dfn> bytes (potentially with FIN)
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-9.4.1)
<td>Yes
<td>No
<td>Yes
</tr>
<tr>
<td><dfn>send STOP_SENDING</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-9.8.1)
<td>Yes
<td>No
<td>Yes
</tr>
<tr>
<td><dfn>reset</dfn> a [=WebTransport stream=]
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-9.6.1)
<td>No
<td>Yes
<td>Yes
</tr>
</tbody>
</table>
A [=WebTransport stream=] has the following signals:
<table class="data" dfn-for="stream-signal">
<thead>
<tr>
<th>event
<th>definition
<th>[=stream/incoming unidirectional=]
<th>[=stream/outgoing unidirectional=]
<th>[=stream/bidirectional=]
</tr>
</thead>
<tbody>
<tr>
<td><dfn>STOP_SENDING</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-9.8.1)
<td>No
<td>Yes
<td>Yes
</tr>
<tr>
<td><dfn>RESET_STREAM</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-9.6.1)
<td>Yes
<td>No
<td>Yes
</tr>
<tr>
<td><dfn>flow control</dfn>
<td>[[!WEB-TRANSPORT-OVERVIEW]]
[Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-overview-06#section-4.3-5)
<td>No
<td>Yes
<td>Yes
</tr>
</tbody>
</table>
# `WebTransportDatagramDuplexStream` Interface # {#duplex-stream}
A <dfn interface>WebTransportDatagramDuplexStream</dfn> is a generic duplex stream.
<pre class="idl">
[Exposed=(Window,Worker), SecureContext]
interface WebTransportDatagramDuplexStream {
readonly attribute ReadableStream readable;
readonly attribute WritableStream writable;
readonly attribute unsigned long maxDatagramSize;
attribute unrestricted double? incomingMaxAge;
attribute unrestricted double? outgoingMaxAge;
attribute unrestricted double incomingHighWaterMark;
attribute unrestricted double outgoingHighWaterMark;
};
</pre>
## Internal slots ## {#datagramduplexstream-internal-slots}
A {{WebTransportDatagramDuplexStream}} object has the following internal slots.
<table class="data" dfn-for="WebTransportDatagramDuplexStream" dfn-type="attribute">
<thead>
<tr>
<th>Internal Slot
<th>Description (<em>non-normative</em>)
</tr>
</thead>
<tbody>
<tr>
<td><dfn>`[[Readable]]`</dfn>
<td class="non-normative">A {{ReadableStream}} for incoming datagrams.
</tr>
<tr>
<td><dfn>`[[Writable]]`</dfn>
<td class="non-normative">A {{WritableStream}} for outgoing datagrams.
</tr>
<tr>
<td><dfn>`[[IncomingDatagramsQueue]]`</dfn>
<td class="non-normative">A queue of pairs of an incoming datagram and a timestamp.
</tr>
<tr>
<td><dfn>`[[IncomingDatagramsPullPromise]]`</dfn>
<td class="non-normative">A promise set by [=pullDatagrams=], to wait for an incoming datagram.
</tr>
<tr>
<td><dfn>`[[IncomingDatagramsHighWaterMark]]`</dfn>
<td class="non-normative">An {{unrestricted double}} representing the
[=high water mark=] of the incoming datagrams.
</tr>
<tr>
<td><dfn>`[[IncomingDatagramsExpirationDuration]]`</dfn>
<td class="non-normative">An {{unrestricted double}} representing the
expiration duration for incoming datagrams (in milliseconds), or null.
</tr>
<tr>
<td><dfn>`[[OutgoingDatagramsQueue]]`</dfn>
<td class="non-normative">A queue of tuples of an outgoing datagram, a timestamp and a promise
which is resolved when the datagram is sent or discarded.
</tr>
<tr>
<td><dfn>`[[OutgoingDatagramsHighWaterMark]]`</dfn>
<td class="non-normative">An {{unrestricted double}} representing the
[=high water mark=] of the outgoing datagrams.
</tr>
<tr>
<td><dfn>`[[OutgoingDatagramsExpirationDuration]]`</dfn>
<td class="non-normative">An {{unrestricted double}} value representing the
expiration duration for outgoing datagrams (in milliseconds), or null.
</tr>
<tr>
<td><dfn>`[[OutgoingMaxDatagramSize]]`</dfn>
<td class="non-normative">An integer representing the maximum size for an outgoing datagram.
</tr>
</tbody>
</table>
The user agent MAY update {{[[OutgoingMaxDatagramSize]]}} for any {{WebTransport}} object whose
{{[[State]]}} is either `"connecting"` or `"connected"`.
To <dfn export for="WebTransportDatagramDuplexStream" lt="create|creating">create</dfn> a
{{WebTransportDatagramDuplexStream}} given a
<dfn export for="WebTransportDatagramDuplexStream/create"><var>readable</var></dfn>, and
a <dfn export for="WebTransportDatagramDuplexStream/create"><var>writable</var></dfn>,
perform the following steps.
1. Let |stream| be a [=new=] {{WebTransportDatagramDuplexStream}}, with:
: {{WebTransportDatagramDuplexStream/[[Readable]]}}
:: |readable|
: {{WebTransportDatagramDuplexStream/[[Writable]]}}
:: |writable|
: {{[[IncomingDatagramsQueue]]}}
:: an empty queue
: {{[[IncomingDatagramsPullPromise]]}}
:: null
: {{[[IncomingDatagramsHighWaterMark]]}}
:: an [=implementation-defined=] value
: {{[[IncomingDatagramsExpirationDuration]]}}
:: null
: {{[[OutgoingDatagramsQueue]]}}
:: an empty queue
: {{[[OutgoingDatagramsHighWaterMark]]}}
:: an [=implementation-defined=] value
<div class="note">
<p>This implementation-defined value should be tuned to ensure decent throughput, without
jeopardizing the timeliness of transmitted data.</p>
</div>
: {{[[OutgoingDatagramsExpirationDuration]]}}
:: null
: {{[[OutgoingMaxDatagramSize]]}}
:: an [=implementation-defined=] integer.
1. Return |stream|.
## Attributes ## {#datagram-duplex-stream-attributes}
: <dfn for="WebTransportDatagramDuplexStream" attribute>readable</dfn>
:: The getter steps are:
1. Return [=this=].{{WebTransportDatagramDuplexStream/[[Readable]]}}.
: <dfn for="WebTransportDatagramDuplexStream" attribute>writable</dfn>
:: The getter steps are:
1. Return [=this=].{{WebTransportDatagramDuplexStream/[[Writable]]}}.
: <dfn for="WebTransportDatagramDuplexStream" attribute>incomingMaxAge</dfn>
:: The getter steps are:
1. Return [=this=].{{[[IncomingDatagramsExpirationDuration]]}}.
:: The setter steps, given |value|, are:
1. If |value| is negative or NaN, [=throw=] a {{RangeError}}.
1. If |value| is `0`, set |value| to null.
1. Set [=this=].{{[[IncomingDatagramsExpirationDuration]]}} to |value|.
: <dfn for="WebTransportDatagramDuplexStream" attribute>maxDatagramSize</dfn>
:: The maximum size data that may be passed to {{WebTransportDatagramDuplexStream/writable}}.
The getter steps are to return [=this=].{{[[OutgoingMaxDatagramSize]]}}.
: <dfn for="WebTransportDatagramDuplexStream" attribute>outgoingMaxAge</dfn>
:: The getter steps are:
1. Return [=this=]'s {{[[OutgoingDatagramsExpirationDuration]]}}.
:: The setter steps, given |value|, are:
1. If |value| is negative or NaN, [=throw=] a {{RangeError}}.
1. If |value| is `0`, set |value| to null.
1. Set [=this=].{{[[OutgoingDatagramsExpirationDuration]]}} to |value|.
: <dfn for="WebTransportDatagramDuplexStream" attribute>incomingHighWaterMark</dfn>
:: The getter steps are:
1. Return [=this=].{{[[IncomingDatagramsHighWaterMark]]}}.
:: The setter steps, given |value|, are:
1. If |value| is negative or NaN, [=throw=] a {{RangeError}}.
1. If |value| is < `1`, set |value| to `1`.
1. Set [=this=].{{[[IncomingDatagramsHighWaterMark]]}} to |value|.
: <dfn for="WebTransportDatagramDuplexStream" attribute>outgoingHighWaterMark</dfn>
:: The getter steps are:
1. Return [=this=].{{[[OutgoingDatagramsHighWaterMark]]}}.
:: The setter steps, given |value|, are:
1. If |value| is negative or NaN, [=throw=] a {{RangeError}}.
1. If |value| is < `1`, set |value| to `1`.
1. Set [=this=].{{[[OutgoingDatagramsHighWaterMark]]}} to |value|.
## Procedures ## {#datagram-duplex-stream-procedures}
To <dfn>pullDatagrams</dfn>, given a {{WebTransport}} object |transport|, run these steps:
1. Let |datagrams| be |transport|.{{[[Datagrams]]}}.
1. Assert: |datagrams|.{{[[IncomingDatagramsPullPromise]]}} is null.
1. Let |queue| be |datagrams|.{{[[IncomingDatagramsQueue]]}}.
1. If |queue| is empty, then:
1. Set |datagrams|.{{[[IncomingDatagramsPullPromise]]}} to a new promise.
1. Return |datagrams|.{{[[IncomingDatagramsPullPromise]]}}.
1. Let |datagram| and |timestamp| be the result of [=dequeuing=] |queue|.
1. If |datagrams|.{{WebTransportDatagramDuplexStream/[[Readable]]}}'s
[=ReadableStream/current BYOB request view=] is not null, then:
1. Let |view| be |datagrams|.{{WebTransportDatagramDuplexStream/[[Readable]]}}'s
[=ReadableStream/current BYOB request view=].
1. If |view|'s [=BufferSource/byte length=] is less than the size of |datagram|, return
[=a promise rejected with=] a {{RangeError}}.
1. Let |elementSize| be the element size specified in [=the typed array constructors table=] for
|view|.\[[TypedArrayName]]. If |view| does not have a \[[TypedArrayName]] internal slot
(i.e. it is a {{DataView}}), let |elementSize| be 0.
1. If |elementSize| is not 1, return [=a promise rejected with=] a {{TypeError}}.
1. [=ReadableStream/Pull from bytes=] |datagram| into |datagrams|.{{WebTransportDatagramDuplexStream/[[Readable]]}}.
1. Return [=a promise resolved with=] undefined.
To <dfn>receiveDatagrams</dfn>, given a {{WebTransport}} object |transport|, run these steps:
1. Let |timestamp| be a timestamp representing now.
1. Let |queue| be |datagrams|.{{[[IncomingDatagramsQueue]]}}.
1. Let |duration| be |datagrams|.{{[[IncomingDatagramsExpirationDuration]]}}.
1. If |duration| is null, then set |duration| to an [=implementation-defined=] value.
1. Let |session| be |transport|.{{[[Session]]}}.
1. While there are [=session/receive a datagram|available incoming datagrams=] on |session|:
1. Let |datagram| be the result of [=session/receiving a datagram=] with |session|.
1. Let |timestamp| be a timestamp representing now.
1. Let |chunk| be a pair of |datagram| and |timestamp|.
1. [=queue/Enqueue=] |chunk| to |queue|.
1. Let |toBeRemoved| be the length of |queue| minus |datagrams|.{{[[IncomingDatagramsHighWaterMark]]}}.
1. If |toBeRemoved| is positive, repeat [=queue/dequeuing=] |queue| |toBeRemoved|
([rounded down](https://tc39.github.io/ecma262/#eqn-floor)) times.
1. While |queue| is not empty:
1. Let |bytes| and |timestamp| be |queue|'s first element.
1. If more than |duration| milliseconds have passed since |timestamp|, then [=queue/dequeue=] |queue|.
1. Otherwise, [=iteration/break=] this loop.
1. If |queue| is not empty and |datagrams|.{{[[IncomingDatagramsPullPromise]]}} is non-null, then:
1. Let |bytes| and |timestamp| be the result of [=queue/dequeuing=] |queue|.
1. Let |promise| be |datagrams|.{{[[IncomingDatagramsPullPromise]]}}.
1. Set |datagrams|.{{[[IncomingDatagramsPullPromise]]}} to null.
1. [=WebTransport/Queue a network task=] with |transport| to run the following steps:
1. Let |chunk| be a new {{Uint8Array}} object representing |bytes|.
1. [=ReadableStream/Enqueue=] |chunk| to |datagrams|.{{WebTransportDatagramDuplexStream/[[Readable]]}}.
1. [=Resolve=] |promise| with undefined.
The user agent SHOULD run [=receiveDatagrams=] for any {{WebTransport}} object whose
{{[[State]]}} is `"connected"` as soon as reasonably possible whenever the algorithm can make
progress.
<div algorithm>
The <dfn>writeDatagrams</dfn> algorithm is given a |transport| as parameter and
|data| as input. It is defined by running the following steps:
1. Let |timestamp| be a timestamp representing now.
1. If |data| is not a {{BufferSource}} object, then return [=a promise rejected with=] a {{TypeError}}.
1. Let |datagrams| be |transport|.{{[[Datagrams]]}}.
1. If |datagrams|.{{[[OutgoingMaxDatagramSize]]}} is less than |data|'s \[[ByteLength]], return
[=a promise resolved with=] undefined.
1. Let |promise| be a new promise.
1. Let |bytes| be a copy of bytes which |data| represents.
1. Let |chunk| be a tuple of |bytes|, |timestamp| and |promise|.
1. Enqueue |chunk| to |datagrams|.{{[[OutgoingDatagramsQueue]]}}.
1. If the length of |datagrams|.{{[[OutgoingDatagramsQueue]]}} is less than
|datagrams|.{{[[OutgoingDatagramsHighWaterMark]]}}, then [=resolve=] |promise| with undefined.
1. Return |promise|.
Note: The associated {{WritableStream}} calls [=writeDatagrams=] only when all the promises that
have been returned by [=writeDatagrams=] have been resolved. Hence the timestamp and the expiration
duration work well only when the web developer pays attention to
{{WritableStreamDefaultWriter/ready|WritableStreamDefaultWriter.ready}}.
</div>
<div algorithm>
To <dfn>sendDatagrams</dfn>, given a {{WebTransport}} object |transport|, run these steps:
1. Let |queue| be |datagrams|.{{[[OutgoingDatagramsQueue]]}}.
1. Let |duration| be |datagrams|.{{[[OutgoingDatagramsExpirationDuration]]}}.
1. If |duration| is null, then set |duration| to an [=implementation-defined=] value.
1. While |queue| is not empty:
1. Let |bytes|, |timestamp| and |promise| be |queue|'s first element.
1. If more than |duration| milliseconds have passed since |timestamp|, then:
1. Remove the first element from |queue|.
1. [=Queue a network task=] with |transport| to [=resolve=] |promise| with undefined.
1. Otherwise, break this loop.
1. If |transport|.{{[[State]]}} is not `"connected"`, then return.
1. Let |maxSize| be |datagrams|.{{[[OutgoingMaxDatagramSize]]}}.
1. While |queue| is not empty:
1. Let |bytes|, |timestamp| and |promise| be |queue|'s first element.
1. If |bytes|'s length ≤ |maxSize|:
1. If it is not possible to send |bytes| to the network immediately, then break this loop.
1. [=session/Send a datagram=], with |transport|.{{[[Session]]}} and |bytes|.
1. Remove the first element from |queue|.
1. [=Queue a network task=] with |transport| to [=resolve=] |promise| with undefined.
</div>
The user agent SHOULD run [=sendDatagrams=] for any {{WebTransport}} object whose
{{[[State]]}} is `"connecting"` or `"connected"` as soon as reasonably possible whenever the
algorithm can make progress.
Note: Writing datagrams while the transport's {{[[State]]}} is `"connecting"` is allowed. The
datagrams are stored in {{[[OutgoingDatagramsQueue]]}}, and they can be discarded
in the same manner as when in the `"connected"` state. Once the transport's {{[[State]]}} becomes
`"connected"`, it will start sending the queued datagrams.
# `WebTransport` Interface # {#web-transport}
`WebTransport` provides an API to the underlying transport functionality
defined in [[!WEB-TRANSPORT-OVERVIEW]].
<pre class="idl">
[Exposed=(Window,Worker), SecureContext]
interface WebTransport {
constructor(USVString url, optional WebTransportOptions options = {});
Promise<WebTransportConnectionStats> getStats();
readonly attribute Promise<undefined> ready;
readonly attribute WebTransportReliabilityMode reliability;
readonly attribute WebTransportCongestionControl congestionControl;
[EnforceRange] attribute unsigned short? anticipatedConcurrentIncomingUnidirectionalStreams;
[EnforceRange] attribute unsigned short? anticipatedConcurrentIncomingBidirectionalStreams;
readonly attribute DOMString protocol;
readonly attribute Promise<WebTransportCloseInfo> closed;
readonly attribute Promise<undefined> draining;
undefined close(optional WebTransportCloseInfo closeInfo = {});
readonly attribute WebTransportDatagramDuplexStream datagrams;
Promise<WebTransportBidirectionalStream> createBidirectionalStream(
optional WebTransportSendStreamOptions options = {});
/* a ReadableStream of WebTransportBidirectionalStream objects */
readonly attribute ReadableStream incomingBidirectionalStreams;
Promise<WebTransportSendStream> createUnidirectionalStream(
optional WebTransportSendStreamOptions options = {});
/* a ReadableStream of WebTransportReceiveStream objects */
readonly attribute ReadableStream incomingUnidirectionalStreams;
WebTransportSendGroup createSendGroup();
static readonly attribute boolean supportsReliableOnly;
};
enum WebTransportReliabilityMode {
"pending",
"reliable-only",
"supports-unreliable",
};
</pre>
## Internal slots ## {#webtransport-internal-slots}
A {{WebTransport}} object has the following internal slots.
<table class="data" dfn-for="WebTransport" dfn-type="attribute">
<thead>
<tr>
<th>Internal Slot
<th>Description (<em>non-normative</em>)
</tr>
</thead>
<tbody>
<tr>
<td><dfn>`[[SendStreams]]`</dfn>
<td class="non-normative">An [=ordered set=] of {{WebTransportSendStream}}s owned by this {{WebTransport}}.
</tr>
<tr>
<td><dfn>`[[ReceiveStreams]]`</dfn>
<td class="non-normative">An [=ordered set=] of {{WebTransportReceiveStream}}s owned by this
{{WebTransport}}.
</tr>
<tr>
<td><dfn>`[[IncomingBidirectionalStreams]]`</dfn>
<td class="non-normative">A {{ReadableStream}} consisting of {{WebTransportBidirectionalStream}}
objects.
</tr>
<tr>
<td><dfn>`[[IncomingUnidirectionalStreams]]`</dfn>
<td class="non-normative">A {{ReadableStream}} consisting of {{WebTransportReceiveStream}}s.
</tr>
<tr>
<td><dfn>`[[State]]`</dfn>
<td class="non-normative">An enum indicating the state of the transport. One of `"connecting"`,
`"connected"`, `"draining"`, `"closed"`, and `"failed"`.
</tr>
<tr>
<td><dfn>`[[Ready]]`</dfn>
<td class="non-normative">A promise fulfilled when the associated [=WebTransport session=]
gets [=session/established=], or rejected if the [=session/establish|establishment process=]
failed.
</tr>
<tr>
<td><dfn>`[[Reliability]]`</dfn>
<td class="non-normative">A {{WebTransportReliabilityMode}} indicating whether
the first hop supports unreliable (UDP) transport or whether only reliable
(TCP fallback) transport is available. Returns `"pending"` until a connection
has been established.
</tr>
<tr>
<td><dfn>`[[CongestionControl]]`</dfn>
<td class="non-normative">A {{WebTransportCongestionControl}} indicating
whether a preference for a congestion control algorithm optimized for
throughput or low latency was requested by the application and satisfied
by the user agent, or `"default"`.
</tr>
<tr>
<td><dfn>`[[AnticipatedConcurrentIncomingUnidirectionalStreams]]`</dfn>
<td class="non-normative">The number of concurrently open
[=incoming unidirectional=] streams the application anticipates the server creating, or null.
</tr>
<tr>
<td><dfn>`[[AnticipatedConcurrentIncomingBidirectionalStreams]]`</dfn>
<td class="non-normative">The number of concurrently open
[=bidirectional=] streams the application anticipates the server creating, or null.
</tr>
<tr>
<td><dfn>`[[Protocol]]`</dfn>
<td class="non-normative">A string indicating the application-level protocol selected by the server,
if any. Initially an empty string.
</tr>
<tr>
<td><dfn>`[[Closed]]`</dfn>
<td class="non-normative">A promise fulfilled when the associated {{WebTransport}} object is
closed gracefully, or rejected when it is closed abruptly or failed on initialization.
</tr>
<tr>
<td><dfn>`[[Draining]]`</dfn>
<td class="non-normative">A promise fulfilled when the associated [=WebTransport session=]
receives a [=session-signal/DRAIN_WEBTRANSPORT_SESSION=] capsule or a
[=session-signal/GOAWAY=] frame.
</tr>
<tr>
<td><dfn>`[[Datagrams]]`</dfn>
<td class="non-normative">A {{WebTransportDatagramDuplexStream}}.
</tr>
<tr>
<td><dfn>`[[Session]]`</dfn>
<td class="non-normative">A [=WebTransport session=] for this {{WebTransport}} object, or null.
</tr>
</table>
## Constructor ## {#webtransport-constructor}
<div algorithm="webtransport-contructor">
When the {{WebTransport/constructor()}} constructor is invoked, the user
agent MUST run the following steps:
1. Let |baseURL| be [=this=]'s [=relevant settings object=]'s [=API base URL=].
1. Let |parsedURL| be the [=URL record=] resulting from [=URL parser|parsing=]
{{WebTransport/constructor(url, options)/url}} with |baseURL|.
1. If |parsedURL| is a failure, [=throw=] a {{SyntaxError}} exception.
1. If |parsedURL| [=scheme=] is not `https`, [=throw=] a {{SyntaxError}} exception.
1. If |parsedURL| [=fragment=] is not null, [=throw=] a {{SyntaxError}} exception.
1. Let |allowPooling| be {{WebTransport/constructor(url, options)/options}}'s
{{WebTransportOptions/allowPooling}}.
1. Let |dedicated| be the negation of |allowPooling|.
1. Let |serverCertificateHashes| be {{WebTransport/constructor(url, options)/options}}'s
{{WebTransportOptions/serverCertificateHashes}} if it exists, and null otherwise.
1. If |dedicated| is false and |serverCertificateHashes| is non-null, then [=throw=] a
{{NotSupportedError}} exception.
1. Let |requireUnreliable| be {{WebTransport/constructor(url, options)/options}}'s
{{WebTransportOptions/requireUnreliable}}.
1. Let |congestionControl| be {{WebTransport/constructor(url, options)/options}}'s
{{WebTransportOptions/congestionControl}}.
1. If |congestionControl| is not `"default"`, and the user agent does not support any
congestion control algorithms that optimize for |congestionControl|, as allowed by
[[!RFC9002]] [Section 7](https://www.rfc-editor.org/rfc/rfc9002#section-7),
then set |congestionControl| to `"default"`.
1. Let |protocols| be {{WebTransport/constructor(url, options)/options}}'s
{{WebTransportOptions/protocols}}
1. If any of the values in |protocols| occur more than once, fail to match
the requirements for elements that comprise the value of `WT-Protocol`
fields as defined by the WebTransport protocol, or have an [=isomorphic encoded=]
length of 0 or exceeding 512, [=throw=] a {{SyntaxError}} exception.
[[!WEB-TRANSPORT-HTTP3]]
[Section 3.4](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3/#section-3.4).
1. Let |anticipatedConcurrentIncomingUnidirectionalStreams| be {{WebTransport/constructor(url, options)/options}}'s
{{WebTransportOptions/anticipatedConcurrentIncomingUnidirectionalStreams}}.
1. Let |anticipatedConcurrentIncomingBidirectionalStreams| be {{WebTransport/constructor(url, options)/options}}'s
{{WebTransportOptions/anticipatedConcurrentIncomingBidirectionalStreams}}.
1. Let |incomingDatagrams| be a [=new=] {{ReadableStream}}.
1. Let |outgoingDatagrams| be a [=new=] {{WritableStream}}.
1. Let |datagrams| be the result of [=WebTransportDatagramDuplexStream/creating=] a
{{WebTransportDatagramDuplexStream}}, its [=WebTransportDatagramDuplexStream/create/readable=] set to
|incomingDatagrams| and its [=WebTransportDatagramDuplexStream/create/writable=] set to |outgoingDatagrams|.
1. Let |transport| be a newly constructed {{WebTransport}} object, with:
: {{[[SendStreams]]}}
:: an empty [=ordered set=]
: {{[[ReceiveStreams]]}}
:: an empty [=ordered set=]
: {{[[IncomingBidirectionalStreams]]}}
:: a new {{ReadableStream}}
: {{[[IncomingUnidirectionalStreams]]}}
:: a new {{ReadableStream}}
: {{[[State]]}}
:: `"connecting"`
: {{[[Ready]]}}
:: a new promise
: {{[[Reliability]]}}
:: "pending"
: {{[[CongestionControl]]}}
:: |congestionControl|
: {{[[AnticipatedConcurrentIncomingUnidirectionalStreams]]}}
:: |anticipatedConcurrentIncomingUnidirectionalStreams|
: {{[[AnticipatedConcurrentIncomingBidirectionalStreams]]}}
:: |anticipatedConcurrentIncomingBidirectionalStreams|
: {{[[Protocol]]}}
:: an empty string
: {{[[Closed]]}}
:: a new promise
: {{[[Draining]]}}
:: a new promise
: {{[[Datagrams]]}}
:: |datagrams|
: {{[[Session]]}}
:: null
1. Let |pullDatagramsAlgorithm| be an action that runs [=pullDatagrams=] with |transport|.
1. Let |writeDatagramsAlgorithm| be an action that runs [=writeDatagrams=] with |transport|.
Note: Using 64kB buffers with datagrams is recommended because the effective
maximum WebTransport datagram frame size has an upper bound of the QUIC maximum datagram frame size
which is recommended to be 64kB (See [[!QUIC-DATAGRAM]] [Section 3](https://datatracker.ietf.org/doc/html/rfc9221#section-3)).
This will ensure the stream is not errored due to a datagram being larger than the buffer.
1. [=ReadableStream/Set up with byte reading support=] |incomingDatagrams| with
[=ReadableStream/set up with byte reading support/pullAlgorithm=] set to
|pullDatagramsAlgorithm|, and [=ReadableStream/set up/highWaterMark=] set to 0.
1. [=WritableStream/Set up=] |outgoingDatagrams| with [=WritableStream/set up/writeAlgorithm=]
set to |writeDatagramsAlgorithm|.
1. Let |pullBidirectionalStreamAlgorithm| be an action that runs [=pullBidirectionalStream=]
with |transport|.
1. [=ReadableStream/Set up=] |transport|.{{[[IncomingBidirectionalStreams]]}} with
[=ReadableStream/set up/pullAlgorithm=] set to |pullBidirectionalStreamAlgorithm|, and
[=ReadableStream/set up/highWaterMark=] set to 0.
1. Let |pullUnidirectionalStreamAlgorithm| be an action that runs [=pullUnidirectionalStream=]
with |transport|.
1. [=ReadableStream/Set up=] |transport|.{{[[IncomingUnidirectionalStreams]]}} with
[=ReadableStream/set up/pullAlgorithm=] set to |pullUnidirectionalStreamAlgorithm|, and
[=ReadableStream/set up/highWaterMark=] set to 0.
1. [=Initialize WebTransport over HTTP=] with |transport|, |parsedURL|, |dedicated|,
|requireUnreliable|, |congestionControl|, |protocols|, and |serverCertificateHashes|.
1. Return |transport|.
</div>
<div algorithm>
To <dfn>initialize WebTransport over HTTP</dfn>, given a {{WebTransport}} object
<var>transport</var>, a [=URL record=] |url|, a boolean |dedicated|, a boolean
|requireUnreliable|, a {{WebTransportCongestionControl}} |congestionControl|,
a |protocols| array, and a
sequence<{{WebTransportHash}}> |serverCertificateHashes|, run these steps.
1. Let |client| be |transport|'s [=relevant settings object=].
1. Let |origin| be |client|'s [=environment settings object/origin=].
1. Let |request| be a new [=/request=] whose [=request/URL=] is |url|, [=request/client=] is
|client|, [=request/policy container=] is |client|'s
[=environment settings object/policy container=], [=request/destination=] is an empty string,
[=request/origin=] is |origin| and [=request/redirect mode=] is "error".
1. Run <a>report Content Security Policy violations for |request|</a>.
1. If [=should request be blocked by Content Security Policy?=] with |request| returns
<b>"Blocked"</b>, or if |request| [=block bad port|should be blocked due to a bad port=]
returns <b>blocked</b>, then abort the remaining steps and [=queue a network task=] with |transport|
to run these steps:
1. If |transport|.{{[[State]]}} is `"closed"` or `"failed"`, then abort these steps.
1. Let |error| be a newly [=DOMException/created=] {{WebTransportError}} whose
{{WebTransportErrorOptions/source}} is `"session"`.
1. [=Cleanup=] |transport| with |error|.
1. Let |networkPartitionKey| be the result of [=determining the network partition key=] with
|transport|'s [=relevant settings object=].
1. Run the following steps [=in parallel=], but [=abort when=] |transport|.{{[[State]]}} becomes `"closed"` or `"failed"`:
1. Let |newConnection| be "`no`" if |dedicated| is false; otherwise "`yes-and-dedicated`".
1. Let |connection| be the result of [=obtain a connection|obtaining a connection=] with
|networkPartitionKey|, |url|, false, |newConnection|, and |requireUnreliable|. If the user agent
supports more than one congestion control algorithm, choose one appropriate for
|congestionControl| for sending of data on this |connection|. When obtaining a connection, if
|serverCertificateHashes| is specified, instead of using the default certificate verification
algorithm, consider the certificate valid if it meets the [=custom certificate
requirements=] and if [=verify a certificate hash|verifying the certificate hash=] against
|serverCertificateHashes| returns true. If either condition is not met, let |connection| be
failure.
1. If |connection| is failure, then abort the remaining steps and [=queue a network task=] with
|transport| to run these steps:
1. If |transport|.{{[[State]]}} is `"closed"` or `"failed"`, then abort these steps.
1. Let |error| be a newly [=DOMException/created=] {{WebTransportError}} whose
{{WebTransportErrorOptions/source}} is `"session"`.
1. [=Cleanup=] |transport| with |error|.
Note: Redirects are not followed. Network errors caused by redirection are intentionally
indistinguishable from other network errors. In cross-origin contexts, this would reveal
information that would normally be blocked by CORS. In same-origin contexts, it might
encourage applications to abuse the handshake as a vector for passing information.
1. Wait for |connection| to receive the first SETTINGS frame, and let |settings| be a dictionary that
represents the SETTINGS frame.
1. If |settings| doesn't contain SETTINGS_ENABLE_WEBTRANPORT with a value of 1, or it doesn't
contain H3_DATAGRAM with a value of 1, then abort the remaining steps and [=queue a network
task=] with |transport| to run these steps:
1. If |transport|.{{[[State]]}} is `"closed"` or `"failed"`, then abort these steps.
1. Let |error| be a newly [=DOMException/created=] {{WebTransportError}} whose
{{WebTransportErrorOptions/source}} is `"session"`.
1. [=Cleanup=] |transport| with |error|.
1. [=session/Establish=] a [=WebTransport session=] with |origin| and |protocols| on |connection|.
Note: This step also contains the transport parameter exchange specified in [[!QUIC-DATAGRAM]].
1. If the previous step fails, abort the remaining steps and [=queue a network task=] with
|transport| to run these steps:
1. If |transport|.{{[[State]]}} is `"closed"` or `"failed"`, then abort these steps.
1. Let |error| be a newly [=DOMException/created=] {{WebTransportError}} whose
{{WebTransportErrorOptions/source}} is `"session"`.
1. [=Cleanup=] |transport| with |error|.
1. Let |session| be the established [=WebTransport session=].
1. Assert: |maxDatagramSize| is an integer.
1. [=Queue a network task=] with |transport| to run these steps:
1. If |transport|.{{[[State]]}} is not `"connecting"`:
1. [=In parallel=], [=session/terminate=] |session|.
1. Abort these steps.
1. Set |transport|.{{[[State]]}} to `"connected"`.
1. Set |transport|.{{[[Session]]}} to |session|.
1. Set |transport|.{{[[Protocol]]}} to either the string value of the `WT-Protocol` header field
in the 2xx response to the CONNECT request if present, following [[!WEB-TRANSPORT-HTTP3]]
[Section 3.4](https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3/#section-3.4),
or `""` if not present.
<div class="note">
This should reference [[!WEB-TRANSPORT-OVERVIEW]] instead pending
[issue 15](https://github.com/ietf-wg-webtrans/draft-ietf-webtrans-overview/issues/15).
</div>
1. If the connection is an HTTP/3 connection, set |transport|.{{[[Reliability]]}} to `"supports-unreliable"`.
1. If the connection is an HTTP/2 connection [[!WEB-TRANSPORT-HTTP2]], set |transport|'s {{[[Reliability]]}} to `"reliable-only"`.
1. [=Resolve=] |transport|.{{[[Ready]]}} with undefined.
</div>
<div algorithm="pullBidirectionalStream">
To <dfn>pullBidirectionalStream</dfn>, given a {{WebTransport}} object <var>transport</var>, run
these steps.
1. If |transport|.{{[[State]]}} is `"connecting"`, then return the result of performing the
following steps [=upon fulfillment=] of |transport|.{{[[Ready]]}}:
1. Return the result of [=pullBidirectionalStream=] with |transport|.
1. If |transport|.{{[[State]]}} is not `"connected"`, then return a new [=rejected=] promise with
an {{InvalidStateError}}.
1. Let |session| be |transport|.{{[[Session]]}}.
1. Let |p| be a new promise.
1. Run the following steps [=in parallel=]:
1. Wait until there is an [=session/receive a bidirectional stream|available incoming bidirectional
stream=].
1. Let |internalStream| be the result of [=session/receiving a bidirectional stream=].
1. [=Queue a network task=] with |transport| to run these steps:
1. Let |stream| be the result of [=BidirectionalStream/creating=] a
{{WebTransportBidirectionalStream}} with |internalStream| and |transport|.
1. [=ReadableStream/Enqueue=] |stream| to |transport|.{{[[IncomingBidirectionalStreams]]}}.
1. [=Resolve=] |p| with undefined.
1. Return |p|.
</div>
<div algorithm>
To <dfn>pullUnidirectionalStream</dfn>, given a {{WebTransport}} object <var>transport</var>, run
these steps.
1. If |transport|.{{[[State]]}} is `"connecting"`, then return the result of performing the
following steps [=upon fulfillment=] of |transport|.{{[[Ready]]}}:
1. Return the result of [=pullUnidirectionalStream=] with |transport|.
1. If |transport|.{{[[State]]}} is not `"connected"`, then return a new [=rejected=] promise with
an {{InvalidStateError}}.
1. Let |session| be |transport|.{{[[Session]]}}.
1. Let |p| be a new promise.
1. Run the following steps [=in parallel=]:
1. Wait until there is an
[=session/receive an incoming unidirectional stream|available incoming unidirectional stream=].
1. Let |internalStream| be the result of [=session/receiving an incoming unidirectional stream=].
1. [=Queue a network task=] with |transport| to run these steps:
1. Let |stream| be the result of [=WebTransportReceiveStream/creating=] a {{WebTransportReceiveStream}} with
|internalStream| and |transport|.
1. [=ReadableStream/Enqueue=] |stream| to |transport|.{{[[IncomingUnidirectionalStreams]]}}.
1. [=Resolve=] |p| with undefined.
1. Return |p|.
</div>
## Attributes ## {#webtransport-attributes}
: <dfn for="WebTransport" attribute>ready</dfn>
:: On getting, it MUST return [=this=]'s {{[[Ready]]}}.
: <dfn for="WebTransport" attribute>closed</dfn>
:: On getting, it MUST return [=this=]'s {{[[Closed]]}}.
: <dfn for="WebTransport" attribute>draining</dfn>
:: On getting, it MUST return [=this=]'s {{[[Draining]]}}.
: <dfn for="WebTransport" attribute>datagrams</dfn>
:: A single duplex stream for sending and receiving datagrams over this session.
The getter steps for the `datagrams` attribute SHALL be:
1. Return [=this=]'s {{[[Datagrams]]}}.
: <dfn for="WebTransport" attribute>incomingBidirectionalStreams</dfn>
:: Returns a {{ReadableStream}} of {{WebTransportBidirectionalStream}}s that have been
received from the server.
Note: Whether the incoming streams already have data on them will depend on server behavior.