Copyright © 2019 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and permissive document license rules apply.
This document defines a set of ECMAScript APIs in WebIDL to extend the WebRTC 1.0 API to enable user agents to support scalable video coding (SVC).
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.
The API is based on preliminary work done in the W3C ORTC Community Group.
This document was published by the Web Real-Time Communications Working Group as a First Public Working Draft. This document is intended to become a W3C Recommendation.
Comments regarding this document are welcome. Please send them to public-webrtc@w3.org (archives).
Publication as a First Public Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
This document is governed by the 1 March 2019 W3C Process Document.
This section is non-normative.
This specification extends the WebRTC specification [WEBRTC] to enable configuration of encoding parameters for scalable video coding (SVC). Since SVC bitstreams are self-describing and SVC-capable codecs implemented in browsers require that compliant decoders be capable of decoding any legal encoding sent by an encoder, this specification does not support decoder configuration. However, it is possible for decoders that cannot decode any legal bitstream to describe the supported scalability modes.
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 word MUST in this document is to be interpreted as described in BCP 14 [RFC2119] [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-1], as this specification uses that specification and terminology.
The EventHandler
interface, representing a callback used for event handlers, and the ErrorEvent
interface are defined in [HTML51].
The concepts queue a task, fires a simple event and networking task source are defined in [HTML51].
The terms event, event handlers and event handler event types are defined in [HTML51].
When referring to exceptions, the terms throw and create are defined in [WEBIDL-1].
The term simulcast envelope refers to the maximum number of simulcast streams and the order of the encoding parameters.
The terms fulfilled, rejected, resolved, pending and settled used in the context of Promises are defined in [ECMASCRIPT-6.0].
The terms MediaStream, MediaStreamTrack, and MediaStreamConstraints are defined in [GETUSERMEDIA].
For Scalable Video Coding (SVC), the terms single-session transmission (SST) and multi-session transmission (MST) are defined in [RFC6190]. This specification only supports SST but not MST. The term Single Real-time transport protocol stream Single Transport (SRST), defined in [RFC7656] Section 3.7, refers to Scalable Video Coding (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 (MRST), 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 SRST transport. Codecs with RTP payload specifications supporting SRST transport include VP8 [RFC7741], VP9 [VP9-PAYLOAD] and H.264/SVC [RFC6190].
This specification references objects, methods, internal slots and dictionaries defined in
[WEBRTC], including the RTCPeerConnection object (defined in Section 4.4),
the RTCError object (defined in Section 11.1),
the addTrack and addTransceiver methods (defined in Section 5.1),
the setCodecPreferences method (defined in Section 5.4),
the getParameters and setParameters methods (defined in Section 5.2),
the RTCRtpParameters dictionary (defined in Section 5.2.1),
the RTCRtpSendParameters dictionary (defined in Section 5.2.2),
the RTCRtpReceiveParameters dictionary (defined in Section 5.2.3),
the RTCRtpCodingParameters dictionary (defined in Section 5.2.4),
the RTCRtpEncodingParameters dictionary (defined in Section 5.2.6),
the RTCRtpCodecCapability dictionary (defined in Section 5.2.13),
the RTCRtpTransceiver object including
its [[Sender]] and [[Receiver]] internal slots (defined
in Section 5.4), the RTCRtpSender object, including its
[[SendEncodings]] and [[LastReturnedParameters]]
internal slots (defined in Section 5.2) and the RTCRtpReceiver
object (defined in Section 5.3).
This specification extends [WEBRTC] to enable configuration of encoding
parameters for SVC, as well as discovery of the SVC capabilities of both an
encoder and decoder, by extending the RTCRtpEncodingParametersRTCRtpCodecCapability
Since this specification does not change the behavior of WebRTC
objects and methods, restrictions relating to Offer/Answer negotiation and
encoding parameters remain, as described in [WEBRTC] Section 5.2:
"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."
The configuration of SVC-capable codecs implemented in browsers fits within this restriction. Codecs such as VP8 [RFC6386], VP9 [VP9] and AV1 [AV1] mandate support for SVC and require a compliant decoder to be able to decode any compliant encoding that an encoder can send. Therefore, for these codecs there is no need to configure the decoder or to negotiate SVC support within Offer/Answer, enabling encoding parameters to be used for SVC configuration.
[WEBRTC] Section 5.2 describes error handling in
setParameters, including use of RTCError
to indicate a "hardware-encoder-error" due to an unsupported encoding
parameter, as well as OperationError for other errors.
Implementations of this specification utilize RTCError
and OperationError in the prescribed manner when an invalid
scalabilityMode is provided to
setParameters or addTranscever.
Note that since the addTransceiver and
setCodecPreferences methods can be called
before the Offer/Answer negotiation has concluded, the negotiated
codec and its capabilities may not be known, and it is
possible that the scalabilityMode values configured in
sendEncodings cannot be applied due to incompatibility with
the eventually selected codec. Since this cannot be determined at the
time addTransceiver is called, an error may only
be detected if a scalabilityMode value is invalid for any
supported codec. In order for the application to determine whether the
requested scalabilityMode values have been applied, the
RTCRtpSender.getParameters method can be called
after negotiation has completed and the sending codec has been determined.
If the configuration is not satisfactory, the application can utilize
setParameters to change it.
To influence the Offer/Answer negotiation so as to make it more likely
that the desired scalabiltyMode values can be applied,
setCodecPreferences can be used to limit the
negotiated codecs to those supporting the desired configuration.
For example, if it is desired to support temporal scalability as well as
spatial adaptation, when addTransceiver is called,
sendEncodings can be configured so as to send
multiple simulcast streams with different resolutions, with each stream
utilizing temporal scalability. Since temporal scalability is supported
by the VP8, VP9 and AV1 codecs, such a configuration could be applied
if any of these codecs were negotiated. In the event that H.264/AVC was
negotiated, temporal scalability would not be available, but
simulcast with different resolutions would be applied. If this was
unsatisfactory, a subsequent call to setParameters
could be used to adjust the parameters within the envelope negotiated
in Offer/Answer.
In situations where the decoder cannot necessarily decode anything that an
encoder can send (e.g. an H.264/SVC decoder), the getCapabilities
method can be used to retrieve the scalability modes supported by the decoder
and encoder. By exchanging capabilities, the application can compute the
intersection of the scalabilityMode values supported by the
local and remote peers, enabling it to configure scalabilityMode
values supported by both the local and remote peers using the
addTransceiver and setParameters methods.
However, in situations where SVC modes are negotiated in SDP Offer/Answer,
setParameters can only change scalabilityMode
values within the envelope negotiated by Offer/Answer, resulting in an error
if the requested scalabilityMode value is outside this envelope.
partial dictionary RTCRtpEncodingParameters {
DOMString scalabilityMode;
};
RTCRtpEncodingParameters MembersscalabilityMode of type DOMStringAn identifier of the scalability mode to be used for this stream.
The scalabilityMode selected MUST be one of the scalability modes
supported for the codec, as indicated in RTCRtpCodecCapability.
Scalability modes are defined in Section 6.
partial dictionary RTCRtpCodecCapability {
sequence<DOMString> scalabilityModes;
};
RTCRtpCodecCapability MembersscalabilityModes of type sequence<DOMString>An sequence of the scalability modes (defined in Section 6) supported by the encoder implementation.
In response to a call to RTCRtpSender.getCapabilities(kind),
conformant implementations of this specification MUST return a sequence of
scalability modes supported by each codec of that kind. If a codec does not support
encoding of any scalability modes, then the scalabilityModes member
is not provided.
In response to a call to RTCRtpReceiver.getCapabilities(kind),
decoders that do not support decoding of scalability modes (e.g. an H.264/AVC decoder) or that are required
to decode any scalability mode (such as compliant VP8, VP9 and AV1 decoders)
omit the scalabilityModes member. However, decoders that only support
decoding of a subset of scalability modes MUST return a sequence of the scalability
modes supported by that codec.
The scalabilityModes sequence represents the scalability modes supported
by a user agent. While the user agent's SVC capabilities are assumed to be static, this
may not be the case for a Selective Forwarding Unit (SFU), whose ability to forward SVC
modes may depend on the negotiated header extensions. For example, if the SFU cannot parse
codec payloads (either because it is not designed to do so, or because the payload is
confidential), then without negotiating an RTP header extension such as [FRAME-MARKING],
the SFU may not be able to forward any scalability modes. As a result, an application may
choose to exchange scalabilityModes with an SFU after the initial offer/answer
negotiation, so that it can compute the intersection of supported scalabilityModes.
Scalability mode identifiers and characteristics are indicated in the table below, which is based on the pre-defined scalability modes in [AV1] Section 6.7.5.
| Scalability Mode | Spatial Layers | Resolution Ratio | Temporal Layers | Inter-layer dependency |
|---|---|---|---|---|
| L1T2 | 1 | 2 | ||
| L1T3 | 1 | 3 | ||
| L2T1 | 2 | 2:1 | 1 | Yes |
| L2T2 | 2 | 2:1 | 2 | Yes |
| L2T3 | 2 | 2:1 | 3 | Yes |
| L3T1 | 3 | 2:1 | 1 | Yes |
| L3T2 | 3 | 2:1 | 2 | Yes |
| L3T3 | 3 | 2:1 | 3 | Yes |
| L2T1h | 2 | 1.5:1 | 1 | Yes |
| L2T2h | 2 | 1.5:1 | 2 | Yes |
| L2T3h | 2 | 1.5:1 | 3 | Yes |
| S2T1 | 2 | 2:1 | 1 | No |
| S2T2 | 2 | 2:1 | 2 | No |
| S2T3 | 2 | 2:1 | 3 | No |
| S2T1h | 2 | 1.5:1 | 1 | No |
| S2T2h | 2 | 1.5:1 | 2 | No |
| S2T3h | 2 | 1.5:1 | 3 | No |
| S3T1 | 3 | 2:1 | 1 | No |
| S3T2 | 3 | 2:1 | 2 | No |
| S3T3 | 3 | 2:1 | 3 | No |
| S3T1h | 3 | 1.5:1 | 1 | No |
| S3T2h | 3 | 1.5:1 | 2 | No |
| S3T3h | 3 | 1.5:1 | 3 | No |
| L3T2_KEY | 3 | 2:1 | 2 | Yes |
| L3T3_KEY | 3 | 2:1 | 3 | Yes |
| L4T5_KEY | 4 | 2:1 | 5 | Yes |
| L4T7_KEY | 4 | 2:1 | 7 | Yes |
| L3T2_KEY_SHIFT | 3 | 2:1 | 2 | Yes |
| L3T3_KEY_SHIFT | 3 | 2:1 | 3 | Yes |
| L4T5_KEY_SHIFT | 4 | 2:1 | 5 | Yes |
| L4T7_KEY_SHIFT | 4 | 2:1 | 7 | Yes |
This section is non-normative.
// Example of 3 spatial simulcast layers + 3 temporal layers with
// an SSRC and RID for each simulcast layer
var encodings = [
{rid: 'q', scaleResolutionDownBy: 4.0, scalabilityMode: 'L1T3'}
{rid: 'h', scaleResolutionDownBy: 2.0, scalabilityMode: 'L1T3'},
{rid: 'f', scalabilityMode: 'L1T3'},
];
// Example of 3 spatial simulcast layers + 3 temporal layers on a single SSRC
var encodings = [
{scalabilityMode: 'S3T3'}
];This section is non-normative.
// Example of 1 spatial layer + 2 temporal layers
var encodings = [
{scalabilityMode: 'L1T2'}
];
// Example of 2 spatial layers (with 2:1 ratio) + 3 temporal layers
var encodings = [
{scalabilityMode: 'L2T3'}
];This section is non-normative.
// Capabilities returned by RTCRtpSender.getCapabilities('video').codecs[]
"codecs": [
{
"clockRate": 90000,
"mimeType": "video/VP8",
"scalabilityModes": ["L1T2","L1T3"]
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=96"
},
{
"clockRate": 90000,
"mimeType": "video/VP9",
"scalabilityModes": ["L1T2","L1T3","L2T1","L2T2","L2T3","L3T1","L3T2","L3T3","L1T2h","L1T3h","L2T1h","L2T2h","L2T3h"]
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=98"
},
{
"clockRate": 90000,
"mimeType": "video/H264",
"sdpFmtpLine": "packetization-mode=1;profile-level-id=42001f;level-asymmetry-allowed=1"
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=100"
},
{
"clockRate": 90000,
"mimeType": "video/H264",
"sdpFmtpLine": "packetization-mode=0;profile-level-id=42001f;level-asymmetry-allowed=1"
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=102"
},
{
"clockRate": 90000,
"mimeType": "video/H264",
"sdpFmtpLine": "level-asymmetry-allowed=1;profile-level-id=42e01f;packetization-mode=1"
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=104"
},
{
"clockRate": 90000,
"mimeType": "video/H264",
"sdpFmtpLine": "level-asymmetry-allowed=1;profile-level-id=42e01f;packetization-mode=0"
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=106"
},
{
"clockRate": 90000,
"mimeType": "video/H264",
"sdpFmtpLine": "level-asymmetry-allowed=1;profile-level-id=4d0032;packetization-mode=1"
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=108"
},
{
"clockRate": 90000,
"mimeType": "video/H264",
"sdpFmtpLine": "level-asymmetry-allowed=1;profile-level-id=640032;packetization-mode=1"
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=110"
},
{
"clockRate": 90000,
"mimeType": "video/red"
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=112"
},
{
"clockRate": 90000,
"mimeType": "video/ulpfec"
},
{
"clockRate": 90000,
"mimeType": "video/AV1",
"scalabilityModes": ["L1T2","L1T3","L2T1","L2T2","L2T3","L3T1","L3T2","L3T3","L1T2h","L1T3h","L2T1h","L2T2h","L2T3h","S2T1","S2T2","S2T3","S3T1","S3T2","S3T3","S2T1h","S2T2h","S2T3h","S3T1h","S3T2h","S3T3h"]
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=113"
}
]This section is non-normative.
// RTCRtpReceiver.getCapabilities('video').codecs[] returned by
// SFU that can only forward VP8 and VP9 temporal scalability modes
"codecs": [
{
"clockRate": 90000,
"mimeType": "video/VP8",
"scalabilityModes": ["L1T2","L1T3"]
},
{
"clockRate": 90000,
"mimeType": "video/VP9",
"scalabilityModes": ["L1T2","L1T3","L1T2h","L1T3h"]
}
]This section is non-normative.
// RTCRtpReceiver.getCapabilities('video').codecs[] returned by a browser that can
// support all scalability modes within the VP8 and VP9 codecs.
"codecs": [
{
"clockRate": 90000,
"mimeType": "video/VP8"
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=96"
},
{
"clockRate": 90000,
"mimeType": "video/VP9"
},
{
"clockRate": 90000,
"mimeType": "video/rtx",
"sdpFmtpLine": "apt=98"
},
{
"clockRate": 90000,
"mimeType": "video/H264",
"sdpFmtpLine": "packetization-mode=1;profile-level-id=42001f;level-asymmetry-allowed=1"
},
...
]This section is non-normative; it specifies no new behaviour, but instead summarizes information already present in other parts of the specification. The overall security considerations of the APIs and protocols used in WebRTC are described in [RTCWEB-SECURITY-ARCH].
This API enables data to be communicated between browsers and other devices, including other browsers.
This means that data can be shared between applications running in different browsers, or between an application running in the same browser and something that is not a browser. This is an extension to the Web model which has had barriers against sending data between entities with different origins.
This specification provides no user prompts or chrome indicators for communication; it assumes that once the Web page has been allowed to access data, it is free to share that data with other entities as it chooses.
Since the browser is an active platform executing in a trusted network environment (inside the firewall), it is important to limit the damage that the browser can do to other elements on the local network, and it is important to protect data from interception, manipulation and modification by untrusted participants.
Mitigations include:
These measures are specified in the relevant IETF documents.
The fact that communication is taking place cannot be hidden from adversaries that can observe the network, so this has to be regarded as public information.
The WebRTC API exposes information about the
underlying media system via the RTCRtpSender.getCapabilities
and RTCRtpReceiver.getCapabilities methods, including
detailed and ordered information about the codecs that the system is able
to produce and consume. The WebRTC-SVC extension adds supported SVC modes
to that information, which is in most cases persistent across time and origins,
therefore increasing the fingerprint surface.
This section will be removed before publication.
The editors wish to thank the Working Group chairs and Team Contact, Harald Alvestrand, Stefan Håkansson and Dominique Hazaël-Massieux, for their support.
This section is non-normative.
Illustrations of the layer dependencies of various scalability modes are provided in [AV1] Section 6.7.5 and are reproduced below for convenience.
