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<!DOCTYPE html>
<html lang="en" dir="ltr">
<head>
<meta charset="utf-8">
<link href="webrtc.css" rel="stylesheet">
<title>Scalable Video Coding (SVC) Extension for WebRTC</title>
<script class="remove" src="https://www.w3.org/Tools/respec/respec-w3c"></script>
<script class="remove" src="svc-respec-config.js" ></script>
</head>
<body>
<section id="abstract">
<p>
This document defines a set of ECMAScript APIs in WebIDL to extend the WebRTC
specification to enable configuration of encoding parameters for Scalable
Video Coding (SVC). Discovery of SVC encoder and decoder capabilities is
handled by the Media Capabilities specification.
</p>
</section>
<section id="sotd">
<p>
The API is based on preliminary work done in the W3C ORTC Community Group.
</p>
</section>
<section class="informative" id="intro">
<h2>Introduction</h2>
<p>
This specification extends the WebRTC specification [[WEBRTC]] to
enable configuration of encoding parameters for Scalable Video
Coding (SVC). Discovery of SVC encoder and decoder capabilities is
handled by the Media Capabilities API [[?Media-Capabilities]].
</p>
<p>
This specification does not change the behavior of WebRTC objects and
methods. Therefore, restrictions relating to Offer/Answer negotiation and
encoding parameters remain, as described in [[WEBRTC]] Section 5.2:
"{{RTCRtpSender/setParameters()}} does not cause SDP renegotiation and can
only be used to change what the media stack is sending or receiving within the
envelope negotiated by Offer/Answer."
</p>
<p>
As a result, this specification can configure encoding parameters for
codecs that do not negotiate SVC support in Offer/Answer, such as the
VP8 [[?RFC6386]], VP9 [[?VP9]] and AV1 [[?AV1]] codecs.
Configuraton of temporal scalability can also be supported
for the H.264 [[?ITU-T-REC-H.264]] and H.265 [[?ITU-T-REC-H.265]] codecs.
</p>
</section>
<section id="conformance">
<p>
This specification defines conformance criteria that apply to a single
product: the user agent that implements the interfaces that it
contains.
</p>
<p>
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.
</p>
<p>
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.
</p>
</section>
<section>
<h2>Terminology</h2>
<p>
The term "simulcast envelope" is defined in [[!WEBRTC]] Section 5.4.1.
</p>
<p>
This specification references objects, methods, internal slots
and dictionaries defined in [[!WEBRTC]].
</p>
<p>
For Scalable Video Coding (SVC), the terms "single-session transmission"
(<dfn>SST</dfn>) and "multi-session transmission" (<dfn>MST</dfn>) are
defined in [[?RFC6190]]. This specification only supports <a>SST</a> but
not <a>MST</a>.
</p>
<p>
The term "Single Real-time Transport Protocol stream Single Transport"
(<dfn>SRST</dfn>), defined in [[RFC7656]] Section 3.7, refers to SVC
implementations that transmit all layers within a single transport,
using a single Real-time Transport Protocol (RTP) stream and
synchronization source (SSRC). The term "Multiple RTP stream Single
Transport" (<dfn>MRST</dfn>), also defined in [[RFC7656]] Section 3.7,
refers to implementations that transmit all layers within a single
transport, using multiple RTP streams with a distinct SSRC for each layer.
This specification only supports <a>SRST</a>, not <a>MRST</a>. Codecs
with RTP payload specifications supporting <a>SRST</a> include VP8
[[?RFC7741]], VP9 [[?VP9-PAYLOAD]], AV1 [[?AV1-RTP-SPEC]], H.264
[[?RFC6184]] and H.265 [[?RFC7798]].
</p>
<p>
The term "S mode" refers to a scalability mode in which multiple
encodings are sent on the same SSRC. This includes the [="S2T1"=], [="S2T1h"=],
[="S2T2"=], [="S2T2h"=], [="S2T3"=], [="S2T3h"=], [="S3T1"=], [="S3T1h"=], [="S3T2"=], [="S3T2h"=],
[="S3T3"=] and [="S3T3h"=] {{RTCRtpEncodingParameters/scalabilityMode}} values.
</p>
<p>
The term <dfn data-lt="SFM" data-noexport>Selective Forwarding Middlebox</dfn>
(SFM) is defined in Section 3.7 of [[RFC7667]].
</p>
</section>
<section id="configuration">
<h2>Configuration</h2>
<p>
This specification enables the configuration of encoding parameters for SVC
by extending the {{RTCRtpEncodingParameters}} dictionary.
</p>
<section id="rtcrtpencodingparameters">
<h3>{{RTCRtpEncodingParameters}} Dictionary Extensions</h3>
<div>
<pre class="idl">partial dictionary RTCRtpEncodingParameters {
DOMString scalabilityMode;
};</pre>
<section>
<h2>Dictionary {{RTCRtpEncodingParameters}} Members</h2>
<dl data-link-for="RTCRtpEncodingParameters" data-dfn-for=
"RTCRtpEncodingParameters" class="dictionary-members">
<dt><dfn data-idl>scalabilityMode</dfn> of type {{DOMString}}</dt>
<dd>
<p>
A case-sensitive identifier of the scalability mode to be
used for this stream. Scalability modes are defined in
Section 5.
</p>
</dd>
</dl>
</section>
</div>
</section>
<section id="behavior">
<h2>Behavior</h2>
<p>
[[!WEBRTC]] describes error handling in {{RTCPeerConnection/addTransceiver()}}
(Section 5.1) and {{RTCRtpSender/setParameters()}} (Section 5.2),
including use of {{RTCError}} to indicate a
{{RTCErrorDetailType/"hardware-encoder-error"}}
due to an unsupported encoding parameter, as well as other errors.
Implementations utilize {{RTCError}} and other errors
in the prescribed manner when an invalid
{{RTCRtpEncodingParameters/scalabilityMode}} value is provided to
{{RTCRtpSender/setParameters()}} or {{RTCPeerConnection/addTransceiver()}}.
</p>
<section id="addTransceiver">
<h3>{{RTCPeerConnection/addTransceiver()}}</h3>
<p>
[[!WEBRTC]] Section 5.1 describes validation of
{{RTCRtpTransceiverInit/sendEncodings}} within
{{RTCPeerConnection/addTransceiver()}}. To validate
{{RTCRtpEncodingParameters/scalabilityMode}}, add
the following steps after step 3 of
[=RTCPeerConnection/addTransceiver sendEncodings validation steps=]:
<ol>
<li>
If <var>sendEncodings</var> contains any encoding with a {{RTCRtpEncodingParameters}}.<code>codec</code>
value <var>codec</var> [=map/exists=] and where the same encoding's {{RTCRtpEncodingParameters/scalabilityMode}}
value is not supported by <var>codec</var>, [= exception/throw =] an {{OperationError}}.
</li>
<li>
Else if <var>sendEncodings</var> contains any encoding whose
{{RTCRtpEncodingParameters/scalabilityMode}} value is
not supported by any codec in the [=RTCRtpSender/list of implemented send codecs=]
for <var>kind</var>, [= exception/throw =] an {{OperationError}}.
</li>
<li>
If {{RTCRtpEncodingParameters}} stored in
<var>sendEncodings</var> contains more than 1
encoding with an {{RTCRtpEncodingParameters/active}}
member with a value of <code>true</code> and
<var>sendEncodings</var> contains any encoding whose
{{RTCRtpEncodingParameters/scalabilityMode}} value
represents an "S mode" and whose
{{RTCRtpEncodingParameters/active}} member has a
value of <code>true</code>, [= exception/throw =] an
{{OperationError}}.
</li>
</ol>
</p>
<p>
When the {{RTCPeerConnection/addTransceiver()}} and
{{RTCRtpTransceiver/setCodecPreferences()}} methods are called
prior to conclusion of the Offer/Answer negotiation, the negotiated
codec and its capabilities may not be known. In this situation the
{{RTCRtpEncodingParameters/scalabilityMode}} values configured
in {{RTCRtpTransceiverInit/sendEncodings}} may not be supported
by the eventually negotiated codec. However, an error will only result
if the requested {{RTCRtpEncodingParameters/scalabilityMode}} value is
invalid for any supported codec, or if mixed simulcast transport is requested.
</p>
<p>
So as to ensure that the desired {{RTCRtpEncodingParameters/scalabilityMode}}
values can be applied, {{RTCRtpTransceiver/setCodecPreferences()}} can be
used to prefer or only include codecs supporting the desired configuration.
For example, if temporal scalability is desired along with spatial simulcast,
when {{RTCPeerConnection/addTransceiver()}} is called,
{{RTCRtpTransceiverInit/sendEncodings}} can be configured to send multiple
simulcast streams with different resolutions, with each stream
utilizing temporal scalability. If only the VP8, VP9 and AV1 codec implementations
support temporal scalability, {{RTCRtpTransceiver/setCodecPreferences()}}
can be used to remove the H.264/AVC codec from the Offer, improving the
chances that a codec supporting temporal scalability is negotiated.
</p>
<p>
When {{RTCRtpTransceiverInit/sendEncodings}} is used to
request the sending of multiple simulcast streams using
{{RTCPeerConnection/addTransceiver()}}, an "S mode" cannot
be requested. The browser may only be configured to send
simulcast encodings with multiple SSRCs and RIDs, or
alternatively, to send all simulcast encodings on a single
RTP stream. Simultaneously using both simulcast transport
techniques is not permitted.
</p>
</section>
<section id="setparameters">
<h3>{{RTCRtpSender/setParameters()}}</h3>
<p>
[[!WEBRTC]] Section 5.2 describes validation of <var>parameters</var>
within {{RTCRtpSender/setParameters()}}. Insert the following
conditions under which the operation causes a promise rejected
with an {{InvalidModificationError}} (step 4) of
[=RTCRtpSender/setParameters validation steps=]:
<ol>
<li>
If <var>encodings</var> contains any encoding with an existng
{{RTCRtpEncodingParameters}}.codec value <var>codec</var>, where the same encoding's
{{RTCRtpEncodingParameters/scalabilityMode}} value is not supported by <var>codec</var>.
</li>
<li>
Else if <var>sender</var>.{{RTCRtpSender/[[SendCodecs]]}} [=map/is empty=]
and <var>encodings</var> contains any encoding whose
{{RTCRtpEncodingParameters/scalabilityMode}} value is
not supported by any codec in the
[=RTCRtpSender/list of implemented send codecs=] for <var>kind</var>.
</li>
<li>
Else if <var>sender</var>.{{RTCRtpSender/[[SendCodecs]]}} [=map/is empty | is not empty=]
and <var>encodings</var> contains an encoding whose
{{RTCRtpEncodingParameters/scalabilityMode}} value is
not supported by the codec used for the encoding's RTP stream.
</li>
<li>
If {{RTCRtpEncodingParameters}} stored in
<var>encodings</var> contains more than one
encoding with an {{RTCRtpEncodingParameters/active}}
member with a value of <code>true</code> and
<var>encodings</var> contains any encoding whose
{{RTCRtpEncodingParameters/scalabilityMode}} value
represents an "S mode" and whose
{{RTCRtpEncodingParameters/active}} member has a
value of <code>true</code>.
</li>
</ol>
</p>
<p>
The [="L1T1"=] scalability mode enables SVC encoding to be turned off
using {{RTCRtpSender/setParameters()}}. If [="L1T1"=] is set using
{{RTCRtpSender/setParameters()}} then it will be returned in response
to {{RTCRtpSender/getParameters()}}.
</p>
</section>
<section id="getparameters">
<h3>{{RTCRtpSender/getParameters()}}</h3>
<p>
Before the initial negotiation has completed,
{{RTCRtpSender/getParameters()}} returns the
{{RTCRtpEncodingParameters/scalabilityMode}} value for each
encoding in <var>encodings</var>, as last set by
{{RTCPeerConnection/addTransceiver()}} or
{{RTCRtpSender/setParameters()}}. If no
{{RTCRtpEncodingParameters/scalabilityMode}} value
was provided for an encoding in <var>encodings</var>,
or if a value was not successfully set, then
{{RTCRtpSender/getParameters()}} will not return a
{{RTCRtpEncodingParameters/scalabilityMode}} value
for that encoding.
</p>
<p>
After the initial negotiation has completed, {{RTCRtpSender/getParameters()}}
returns the currently configured {{RTCRtpEncodingParameters/scalabilityMode}}
value for each encoding in <var>encodings</var> which had a value before the
initial negotiation. This MAY be different from the values requested in
{{RTCPeerConnection/addTransceiver()}} or {{RTCRtpSender/setParameters()}}.
For example, if the codecs selected during negotiation do not include an
encoder supporting the desired {{RTCRtpEncodingParameters/scalabilityMode}}
value, the user agent MAY select another value. If the configuration is not
satisfactory, {{RTCRtpSender/setParameters()}} can be used to change it.
</p>
<p>
If {{RTCPeerConnection/addTransceiver()}} or {{RTCRtpSender/setParameters()}}
did not provide a {{RTCRtpEncodingParameters/scalabilityMode}} value for
an encoding in <var>encodings</var>, then after the initial negotiation
has completed, {{RTCRtpSender/getParameters()}} will not return a
{{RTCRtpEncodingParameters/scalabilityMode}} value and the encoder will use
the default {{RTCRtpEncodingParameters/scalabilityMode}} of the codec for
that encoding's RTP stream. The default {{RTCRtpEncodingParameters/scalabilityMode}}
for each codec is implementation dependent. The default
{{RTCRtpEncodingParameters/scalabilityMode}} SHOULD be one of the
temporal scalability modes (e.g. [="L1T1"=],[="L1T2"=],[="L1T3"=], etc.).
</p>
</section>
<section id="sfmnegotiation" class="informative">
<h2>Negotiation issues</h2>
<p>
SVC is most often employed in video conferencing, where a conferencing
server such as a <a title="Selective Forwarding Middlebox">SFM</a>
selectively forwards layers to participants based on their device
characteristics and available bandwidth. In such an environment,
the application will negotiate codecs to be sent and received with
the conferencing server. However, since scalability modes are not
negotiated in Offer/Answer, the application needs to determine
which scalability modes the conference server supports by other
means.
</p>
<p>
One way to handle this is for the SFM to indicate the codecs
and scalability modes it can decode in the form of receiver
capabilities. After exchanging capabilities, the application
can compute the intersection of codecs and
{{RTCRtpEncodingParameters/scalabilityMode}} values supported
by the browser's {{RTCRtpSender}} and the
<a title="Selective Forwarding Middlebox">SFM</a>'s receiver. This
can be used to determine the arguments passed to the browser's
{{RTCPeerConnection/addTransceiver()}} and {{RTCRtpSender/setParameters()}}
methods.
</p>
<p>
There are situations where an <a title="Selective Forwarding Middlebox">SFM</a>
may only support reception of a subset of codecs and scalability modes.
For example, an <a title="Selective Forwarding Middlebox">SFM</a> that parses
codec payloads may only support the H.264/AVC codec without scalability and
the VP8 codec with temporal scalability. On the other hand, the browser
may be able to encode VP8 with temporal scalability, VP9 with temporal
and spatial scalability and or H.264/AVC with temporal scalability.
In such a situation, an application desiring to use SVC would only
be able to encode VP8 with temporal scalability.
</p>
<p>
Since sending simulcast encodings on a single stream is not negotiated within
Offer/Answer, an application using SDP signaling needs to determine whether
single stream simulcast transport is supported prior to the Offer/Answer negotiation.
This can be handled by having the <a title="Selective Forwarding Middlebox">SFM</a>
send it's receiver capabilities to the application prior to Offer/Answer.
This allows the application to determine whether single stream simulcast is
supported, and if so, what scalability modes the
<a title="Selective Forwarding Middlebox">SFM</a> can handle. For example,
an <a title="Selective Forwarding Middlebox">SFM</a> that can only support
reception of a maximum of two simulcast encodings on a single SSRC with the
AV1 codec would only indicate support for the [="S2T1"=] and [="S2T1h"=] scalability
modes in its receiver capabilities.
</p>
<p>
For an <a title="Selective Forwarding Middlebox">SFM</a> the supported
{{RTCRtpEncodingParameters/scalabilityMode}} values may depend on the negotiated RTP
header extensions. For example, if the <a title="Selective Forwarding Middlebox">SFM</a>
cannot parse codec payloads (either because it is not designed to do so, or
because the payloads are encrypted), then negotiation of an RTP header extension
(such as the AV1 Dependency Descriptor defined in Appendix A of [[?AV1-RTP-SPEC]])
could be a prerequisite for the <a title="Selective Forwarding Middlebox">SFM</a>
to forward a {{RTCRtpEncodingParameters/scalabilityMode}} value. As a result,
the {{RTCRtpEncodingParameters/scalabilityMode}} values supported by an
<a title="Selective Forwarding Middlebox">SFM</a> may not be determined until
completion of the Offer/Answer negotiation.
</p>
</section>
</section>
</section>
<section id="scalabilitymodes*">
<h3>Scalability modes</h3>
<p>
The {{RTCRtpEncodingParameters/scalabilityMode}} values supported in this specification,
as well as their associated identifiers and characteristics, are provided in the table
below. The names of the {{RTCRtpEncodingParameters/scalabilityMode}} values (which are
case sensitive) are provided, along with the scalability mode identifiers assigned in
[[?AV1]] Section 6.7.5, and links to dependency diagrams provided in Section 9.
</p>
<p>
While the [[?AV1]] and VP9 [[?VP9]] specifications support all the
{{RTCRtpEncodingParameters/scalabilityMode}} values defined in the table, other codec
specifications do not. For example, VP8 [[?RFC6386]], H.264 [[?RFC6184]] and
H.265 [[?RFC7798]] only support temporal scalability (e.g. [="L1T2"=], [="L1T3"=]).
Also VP8 [[?RFC6386]], H.264 [[?RFC6184]] and H.265 [[?RFC7798]] only permit transport
of simulcast on distinct SSRCs, so that "S" modes (where multiple encodings are
transported on a single RTP stream) are not supported.
</p>
<table class=simple>
<tbody>
<tr>
<th>Scalability Mode Identifier</th>
<th>Spatial Layers</th>
<th>Resolution Ratio</th>
<th>Temporal Layers</th>
<th>Inter-layer dependency</th>
<th>AV1 scalability_mode_idc</th>
</tr>
</tbody>
<tbody>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L1T1*">"L1T1"</a></dfn></td>
<td>1</td>
<td></td>
<td>1</td>
<td></td>
<td>N/A</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L1T2*">"L1T2"</a></dfn></td>
<td>1</td>
<td></td>
<td>2</td>
<td></td>
<td>SCALABILITY_L1T2</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L1T3*">"L1T3"</a></dfn></td>
<td>1</td>
<td></td>
<td>3</td>
<td></td>
<td>SCALABILITY_L1T3</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T1*">"L2T1"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>1</td>
<td>Yes</td>
<td>SCALABILITY_L2T1</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T2*">"L2T2"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>2</td>
<td>Yes</td>
<td>SCALABILITY_L2T2</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T3*">"L2T3"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>3</td>
<td>Yes</td>
<td>SCALABILITY_L2T3</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T1*">"L3T1"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>1</td>
<td>Yes</td>
<td>SCALABILITY_L3T1</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T2*">"L3T2"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>2</td>
<td>Yes</td>
<td>SCALABILITY_L3T2</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T3*">"L3T3"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>3</td>
<td>Yes</td>
<td>SCALABILITY_L3T3</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T1*">"L2T1h"</a></dfn></td>
<td>2</td>
<td>1.5:1</td>
<td>1</td>
<td>Yes</td>
<td>SCALABILITY_L2T1h</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T2*">"L2T2h"</a></dfn></td>
<td>2</td>
<td>1.5:1</td>
<td>2</td>
<td>Yes</td>
<td>SCALABILITY_L2T2h</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T3*">"L2T3h"</a></dfn></td>
<td>2</td>
<td>1.5:1</td>
<td>3</td>
<td>Yes</td>
<td>SCALABILITY_L2T3h</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T1*">"L3T1h"</a></dfn></td>
<td>3</td>
<td>1.5:1</td>
<td>1</td>
<td>Yes</td>
<td></td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T2*">"L3T2h"</a></dfn></td>
<td>3</td>
<td>1.5:1</td>
<td>2</td>
<td>Yes</td>
<td></td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T3*">"L3T3h"</a></dfn></td>
<td>3</td>
<td>1.5:1</td>
<td>3</td>
<td>Yes</td>
<td></td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S2T1*">"S2T1"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>1</td>
<td>No</td>
<td>SCALABILITY_S2T1</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S2T2*">"S2T2"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>2</td>
<td>No</td>
<td>SCALABILITY_S2T2</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S2T3*">"S2T3"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>3</td>
<td>No</td>
<td>SCALABILITY_S2T3</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S2T1*">"S2T1h"</a></dfn></td>
<td>2</td>
<td>1.5:1</td>
<td>1</td>
<td>No</td>
<td>SCALABILITY_S2T1h</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S2T2*">"S2T2h"</a></dfn></td>
<td>2</td>
<td>1.5:1</td>
<td>2</td>
<td>No</td>
<td>SCALABILITY_S2T2h</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S2T3*">"S2T3h"</a></dfn></td>
<td>2</td>
<td>1.5:1</td>
<td>3</td>
<td>No</td>
<td>SCALABILITY_S2T3h</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S3T1*">"S3T1"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>1</td>
<td>No</td>
<td>SCALABILITY_S3T1</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S3T2*">"S3T2"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>2</td>
<td>No</td>
<td>SCALABILITY_S3T2</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S3T3*">"S3T3"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>3</td>
<td>No</td>
<td>SCALABILITY_S3T3</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S3T1*">"S3T1h"</a></dfn></td>
<td>3</td>
<td>1.5:1</td>
<td>1</td>
<td>No</td>
<td>SCALABILITY_S3T1h</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S3T2*">"S3T2h"</a></dfn></td>
<td>3</td>
<td>1.5:1</td>
<td>2</td>
<td>No</td>
<td>SCALABILITY_S3T2h</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#S3T3*">"S3T3h"</a></dfn></td>
<td>3</td>
<td>1.5:1</td>
<td>3</td>
<td>No</td>
<td>SCALABILITY_S3T3h</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T2_KEY*">"L2T2_KEY"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>2</td>
<td>Yes</td>
<td>SCALABILITY_L3T2_KEY</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T2_KEY_SHIFT*">"L2T2_KEY_SHIFT"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>2</td>
<td>Yes</td>
<td>SCALABILITY_L3T2_KEY_SHIFT</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T3_KEY*">"L2T3_KEY"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>3</td>
<td>Yes</td>
<td>SCALABILITY_L3T3_KEY</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L2T3_KEY_SHIFT*">"L2T3_KEY_SHIFT"</a></dfn></td>
<td>2</td>
<td>2:1</td>
<td>3</td>
<td>Yes</td>
<td>SCALABILITY_L3T3_KEY_SHIFT</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T1_KEY*">"L3T1_KEY"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>1</td>
<td>Yes</td>
<td></td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T2_KEY*">"L3T2_KEY"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>2</td>
<td>Yes</td>
<td>SCALABILITY_L4T5_KEY</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T2_KEY_SHIFT*">"L3T2_KEY_SHIFT"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>2</td>
<td>Yes</td>
<td>SCALABILITY_L4T5_KEY_SHIFT</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T3_KEY*">"L3T3_KEY"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>3</td>
<td>Yes</td>
<td>SCALABILITY_L4T7_KEY</td>
</tr>
<tr>
<td><dfn data-export data-for="RTCRtpEncodingParameters/scalabilityMode"><a href="#L3T3_KEY_SHIFT*">"L3T3_KEY_SHIFT"</a></dfn></td>
<td>3</td>
<td>2:1</td>
<td>3</td>
<td>Yes</td>
<td>SCALABILITY_L4T7_KEY_SHIFT</td>
</tr>
</tbody>
</table>
<section id="addingmodes*">
<h3>Guidelines for addition of {{RTCRtpEncodingParameters/scalabilityMode}} values</h3>
<p>When proposing a {{RTCRtpEncodingParameters/scalabilityMode}} value, the following principles should be followed:</p>
<ol>
<li>
The proposed {{RTCRtpEncodingParameters/scalabilityMode}} MUST define entries to the table in
Section 5, including values for the Scalabilty Mode Identifier, spatial and
temporal layers, Resolution Ratio, Inter-layer dependency and the corresponding
AV1 scalability_mode_idc value (if assigned).
</li>
<li>
The Scalability Mode Identifier SHOULD be consistent with the existing naming scheme, which
utilizes <code>L<var>x</var>T<var>y</var></code> to denote a {{RTCRtpEncodingParameters/scalabilityMode}} with <var>x</var>
spatial layers using a 2:1 resolution ratio and <var>y</var> temporal layers.
<code>L<var>x</var>T<var>y</var>h</code> denotes <var>x</var> spatial layers with a 1.5:1 resolution ratio and
<var>y</var> temporal layers. <code>S<var>x</var>T<var>y</var></code> denotes a {{RTCRtpEncodingParameters/scalabilityMode}}
with <var>x</var> simulcast encodings with a 2:1 resolution ratio, with each
simulcast encoding containing <var>y</var> temporal layers. <code>S<var>x</var>T<var>y</var>h</code> denotes
a 1.5:1 resolution ratio. <code>L<var>x</var>T<var>y</var>_KEY</code> denotes a {{RTCRtpEncodingParameters/scalabilityMode}}
with <var>x</var> spatial layers using a 2:1 resolution ratio and <var>y</var> temporal layers in which spatial layers only
depend on lower spatial layers at a key frame. <code>L<var>x</var>T<var>y</var>_KEY_SHIFT</code> modes denotes a
{{RTCRtpEncodingParameters/scalabilityMode}} with <var>x</var> spatial layers using a 2:1 resolution ratio and
<var>y</var> temporal layers in which spatial layers only depend on lower spatial layers at a key frame and subsequent
frames have their temporal identifier shifted upward.
</li>
<li>
A dependency diagram MUST be supplied, in the format provided in Section 9.
</li>
</ol>
</section>
</section>
<section id="rtcrtpencodingparameters-example*">
<h3>Examples</h3>
<section id="simulcasttemporal-example*" class="informative">
<h4>Spatial Simulcast and Temporal Scalability</h4>
<p>
This example extends [[WEBRTC]] Section 7.1 (Example 1) to demonstrate sending three spatial
simulcast layers each with three temporal layers, using an SSRC and RID for each simulcast
layer. Only the "sendEncodings" attribute is changed from the original example.
</p>
<pre class="example highlight">
const signaling = new SignalingChannel(); // handles JSON.stringify/parse
const constraints = {audio: true, video: true};
const configuration = {'iceServers': [{'urls': 'stun:stun.example.org'}]};
let pc;
// call start() to initiate
async function start() {
pc = new RTCPeerConnection(configuration);
// let the "negotiationneeded" event trigger offer generation
pc.onnegotiationneeded = async () => {
try {
await pc.setLocalDescription();
// send the offer to the other peer
signaling.send({description: pc.localDescription});
} catch (err) {
console.error(err);
}
};
try {
// get a local stream, show it in a self-view and add it to be sent
const stream = await navigator.mediaDevices.getUserMedia(constraints);
selfView.srcObject = stream;
pc.addTransceiver(stream.getAudioTracks()[0], {direction: 'sendonly'});
pc.addTransceiver(stream.getVideoTracks()[0], {
direction: 'sendonly',
sendEncodings: [
{rid: 'q', scaleResolutionDownBy: 4.0, scalabilityMode: 'L1T3'},
{rid: 'h', scaleResolutionDownBy: 2.0, scalabilityMode: 'L1T3'},
{rid: 'f', scalabilityMode: 'L1T3'}
]
});
} catch (err) {
console.error(err);
}
}
signaling.onmessage = async ({data: {description, candidate}}) => {
try {
if (description) {
await pc.setRemoteDescription(description);
// if we got an offer, we need to reply with an answer
if (description.type == 'offer') {
await pc.setLocalDescription();
signaling.send({description: pc.localDescription});
}
} else if (candidate) {
await pc.addIceCandidate(candidate);
}
} catch (err) {
console.error(err);
}
};
</pre>
<p>
This is an example with two spatial layers (with a 2:1 ratio) and three temporal layers.
</p>
<pre class="example highlight">
let sendEncodings = [
{scalabilityMode: 'L2T3'}
];
</pre>
<p>
This is an example of mixed codec simulcast, with each simulcast layer having 3 temporal layers.
</p>
<pre class="example highlight">
let sendEncodings = [
{rid: 'q', codec: {clockRate: 90000, mimeType: 'video/AV1'}, scaleResolutionDownBy: 4.0, scalabilityMode: 'L1T3'},
{rid: 'h', codec: {clockRate: 90000, mimeType: 'video/VP8'}, scaleResolutionDownBy: 2.0, scalabilityMode: 'L1T3'},
{rid: 'f', codec: {clockRate: 90000, mimeType: 'video/VP8'}, scalabilityMode: 'L1T3'}
];
</pre>
<p>
This is an example with three spatial simulcast layers each with three temporal layers on a single SSRC.
</p>
<pre class="example highlight">
let sendEncodings = [
{scalabilityMode: 'S3T3'}
]
</pre>
</section>
<section id="media-capabilities-example*" class="informative">
<h4>SVC Encoder Capabilities</h4>
<p>
This is an example of {{MediaCapabilities/encodingInfo(configuration)}}
returned by a browser implementing [[WEBRTC]] and [[?Media-Capabilities]].
</p>
<pre class="example highlight">
const contentType = 'video/VP9';
const configuration = {
type: 'webrtc',
video: {
contentType,
width: 640,
height: 480,
bitrate: 10000,
framerate: 29.97,
scalabilityMode: 'L3T3_KEY'
}
};
try {
const info = await navigator.mediaCapabilities.encodingInfo(configuration);
if (!info.supported) {
console.log(`${contentType} is unsupported.`);
return;
}
console.log(`${contentType} is ${info.smooth || 'NOT '}smooth, and ` +
`${info.powerEfficient || 'NOT '}power efficient`);
} catch (err) {
console.error(err, ' caused encodingInfo to fail');
}
</pre>
</section>
<section id="sfm-getcapabilities-example*" class="informative" >
<h4><a title="Selective Forwarding Middlebox">SFM</a> Capabilities</h4>
<p>
This is an example of receiver capabilities returned by an
<a title="Selective Forwarding Middlebox">SFM</a> that only
supports forwarding of VP8, VP9 and AV1 temporal scalability modes.
</p>
<pre class="example highlight">
"codecs": [
{
"clockRate": 90000,
"mimeType": "video/VP8",
"scalabilityModes": [
"L1T1",
"L1T2",
"L1T3"
]
},
{
"clockRate": 90000,
"mimeType": "video/VP9",
"scalabilityModes": [
"L1T1",
"L1T2",
"L1T3"
]
},
{
"clockRate": 90000,
"mimeType": "video/AV1",
"scalabilityModes": [
"L1T1",
"L1T2",
"L1T3"
]
}
]
</pre>
</section>
</section>
<section class="informative" id="privacy">
<h2>Privacy Considerations</h2>
<p>
This section is non-normative; it specifies no new behaviour, but
instead summarizes information already present in other parts of the
specification. The privacy considerations
for the WebRTC APIs are described in [[WEBRTC]] Section 13.
</p>
<section>
<h2>Persistent information</h2>
<p>
In WebRTC, the use of scalable coding tools is not
negotiated between peers, so neither supported
{{RTCRtpEncodingParameters/scalabilityMode}} values nor
decoder support for spatial prediction is exposed in SDP.
</p>
<p>
By attempting to set
{{RTCRtpEncodingParameters/scalabilityMode}} values for
each codec using the {{RTCRtpSender/setParameters()}} API,
an application can determine the values supported by the
encoder, by noting which configuration attempts succeed
and which ones fail. However, this does not indicate
whether a {{RTCRtpEncodingParameters/scalabilityMode}}
value is supported by a hardware or software encoder
(or both). Since {{RTCRtpSender/setParameters()}}
is not supported for the {{RTCRtpReceiver}}, equivalent
experiments cannot be run to determine decoder support.
</p>
<p>
Since the {{RTCRtpEncodingParameters/scalabilityMode}}
values supported by software encoders are typically
a superset of those supported in hardware, the information
available from these experiments has a high correlation
with the browser in use, which is already available to
web pages. Once media is flowing, information on
performance characteristics or whether a
{{RTCRtpEncodingParameters/scalabilityMode}} value
is decodable for the codec in use can be obtained,
which provides more information on hardware capabilities.
</p>
<p>
The [[?Media-Capabilities]] API enables discovery of encoder and decoder
support for scalable video coding. {{VideoConfiguration//scalabilityMode}}
is used to query whether an encoder supports a
{{RTCRtpEncodingParameters/scalabilityMode}} value, indicating whether
it is "supported", "smooth" and "power efficient".
</p>
<p>
The [[?Media-Capabilities]] API also provides information on decoder support
for spatial scalablity modes. {{VideoConfiguration/spatialScalability}}
indicates whether a decoder has the ability to support spatial prediction,
which requires the ability to use frames of a resolution different than
the current resolution as a dependency. If {{VideoConfiguration/spatialScalability}}
is set to <code>true</code>, the decoder can decode any
{{RTCRtpEncodingParameters/scalabilityMode}} value supported by the encoder.
If {{VideoConfiguration/spatialScalability}} is set to <code>false</code>
or is absent, the decoder cannot decode spatial scalability modes, but can
can decode all other {{RTCRtpEncodingParameters/scalabilityMode}} values
supported by the encoder.
</p>
<p>
As noted in [[?Media-Capabilities]] Section 3.1, the Media
Capabilities API "will likely provide more accurate and
consistent information" than is available from this
specification. As noted in
[[?Media-Capabilities]] Section 3.1, "This information is
expected to have a high correlation with other information
already available to the web pages as a given class of
device is expected to have very similar decoding/encoding
capabilities."
</p>
</section>
</section>
<section class="informative" id="security">
<h2>Security Considerations</h2>
<p>
This section is non-normative; it specifies no new behaviour, but
instead summarizes information already present in other parts of the
specification. WebRTC protocol security considerations are described
in [[RFC8827]] and the security and privacy considerations
for the WebRTC APIs are described in [[WEBRTC]] Section 13.
</p>
</section>
<section id="dependencydiagrams*">
<h2>Scalability Mode Dependency Diagrams</h2>
<p>
Dependency diagrams for the scability modes defined in this specification
are provided below.
</p>
<section id="L1T1*">
<h3>L1T1</h3>
<figure>
<img alt="L1T1: a single layer" src=
"images/L1T1.svg" style="width:75%">
<figcaption>
L1T1: 1-layer encoding
</figcaption>
</figure>
</section>
<section id="L1T2*">
<h3>L1T2</h3>
<figure>
<img alt="L1T2: 2-layer temporal scalability encoding" src=
"images/L1T2.svg" style="width:75%">
<figcaption>
L1T2: 1-layer spatial and 2-layer temporal scalability encoding
</figcaption>
</figure>
</section>
<section id="L1T3*">
<h3>L1T3</h3>