Scalable Video Coding (SVC) Extension for WebRTC

Terminology

The term "simulcast envelope" is defined in [[!WEBRTC]] Section 5.4.1.

This specification references objects, methods, internal slots and dictionaries defined in [[!WEBRTC]].

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 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, not MRST. Codecs with RTP payload specifications supporting SRST include VP8 [[?RFC7741]], VP9 [[?VP9-PAYLOAD]], AV1 [[?AV1-RTP]] and H.264/SVC [[?RFC6190]].

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.

The term Selective Forwarding Middlebox (SFM) is defined in Section 3.7 of [[RFC7667]].

spatialScalability is an addition to {{VideoConfiguration}} in the Media Capabilities API.

Configuration

This specification enables the configuration of encoding parameters for SVC by extending the {{RTCRtpEncodingParameters}} dictionary.

{{RTCRtpEncodingParameters}} Dictionary Extensions

partial dictionary RTCRtpEncodingParameters {
             DOMString scalabilityMode;
};

Dictionary {{RTCRtpEncodingParameters}} Members

scalabilityMode of type {{DOMString}}: A case-sensitive identifier of the scalability mode to be used for this stream. Scalability modes are defined in Section 6.

Behavior

[[!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()}}.

[[!WEBRTC]] Section 5.1 describes validation of {{RTCRtpTransceiverInit/sendEncodings}} within {{RTCPeerConnection/addTransceiver()}}. To validate {{RTCRtpEncodingParameters/scalabilityMode}}, add the following validation steps after step 3 (within step 8):

If sendEncodings contains any encoding whose {{RTCRtpEncodingParameters/scalabilityMode}} value is not supported by any codec in {{RTCRtpSender.getCapabilities(kind)}}.codecs, [= exception/throw =] an {{OperationError}}.
If {{RTCRtpEncodingParameters}} stored in sendEncodings contains more than 1 encoding with an {{RTCRtpEncodingParameters/active}} member with a value of true and sendEncodings contains any encoding whose {{RTCRtpEncodingParameters/scalabilityMode}} value represents an "S mode" and whose {{RTCRtpEncodingParameters/active}} member has a value of true, [= exception/throw =] an {{OperationError}}.

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.

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.

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.

[[!WEBRTC]] Section 5.2 describes validation of parameters within {{RTCRtpSender/setParameters()}}. Add the following to the conditions under which the operation causes a promise rejected with an {{InvalidModificationError}} (step 4 within step 6):

Before initial negotiation has concluded, encodings contains any encoding whose {{RTCRtpEncodingParameters/scalabilityMode}} value is not supported by any codec in {{RTCRtpSender.getCapabilities(kind)}}.codecs.
After initial negotiation has concluded, encodings contains an encoding whose {{RTCRtpEncodingParameters/scalabilityMode}} value is not supported by the most preferred codec.
If {{RTCRtpEncodingParameters}} stored in encodings contains more than 1 encoding with an {{RTCRtpEncodingParameters/active}} member with a value of true and encodings contains any encoding whose {{RTCRtpEncodingParameters/scalabilityMode}} value represents an "S mode" and whose {{RTCRtpEncodingParameters/active}} member has a value of true.

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()}}.

Before the initial negotiation has completed, {{RTCRtpSender/getParameters()}} returns the {{RTCRtpEncodingParameters/scalabilityMode}} value for each encoding in encodings, as last set by {{RTCPeerConnection/addTransceiver()}} or {{RTCRtpSender/setParameters()}}. If no {{RTCRtpEncodingParameters/scalabilityMode}} value was provided for an encoding in encodings, or if a value was not successfully set, then {{RTCRtpSender/getParameters()}} will not return a {{RTCRtpEncodingParameters/scalabilityMode}} value for that encoding.

After the initial negotiation has completed, {{RTCRtpSender/getParameters()}} returns the currently configured {{RTCRtpEncodingParameters/scalabilityMode}} value for each encoding in encodings 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.

If {{RTCPeerConnection/addTransceiver()}} or {{RTCRtpSender/setParameters()}} did not provide a {{RTCRtpEncodingParameters/scalabilityMode}} value for an encoding in encodings, 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 most preferred codec for that encoding. The most preferred codec and the default {{RTCRtpEncodingParameters/scalabilityMode}} for each codec are both implementation dependent. The default {{RTCRtpEncodingParameters/scalabilityMode}} SHOULD be one of the temporal scalability modes (e.g. [="L1T1"=],[="L1T2"=],[="L1T3"=], etc.).

Discovery

The [[?Media-Capabilities]] API provides information on encoder support for spatial scalability modes. {{VideoConfiguration//scalabilityMode}} is used to query whether an encoder has the ability to support this mode. The SFM can provide information on the codecs and scalability modes it can decode by providing its 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 SFM's receiver. This can be used to determine the arguments passed to the browser's {{RTCPeerConnection/addTransceiver()}} and {{RTCRtpSender/setParameters()}} methods.

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 true, the decoder can decode any {{RTCRtpEncodingParameters/scalabilityMode}} value supported by the encoder. If {{VideoConfiguration/spatialScalability}} is set to false or is absent, the decoder cannot decode spatial scalability modes, but can can decode all other {{RTCRtpEncodingParameters/scalabilityMode}} values supported by the encoder.

Negotiation

There are situations where an SFM may only support reception of a subset of codecs and scalability modes. For example, an SFM 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.

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 SFM 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 SFM can handle. For example, an SFM that can only support reception of a maximum of 2 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.

For an SFM the supported {{RTCRtpEncodingParameters/scalabilityMode}} values may depend on the negotiated RTP header extensions. For example, if the SFM 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]]) could be a prerequisite for the SFM to forward a {{RTCRtpEncodingParameters/scalabilityMode}} value. As a result, the {{RTCRtpEncodingParameters/scalabilityMode}} values supported by an SFM may not be determined until completion of the Offer/Answer negotiation.

Scalability modes

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 10.

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]] only supports temporal scalability (e.g. [="L1T2"=], [="L1T3"=]); H.264/SVC [[?RFC6190]], which supports both temporal and spatial scalability, only permits transport of simulcast on distinct SSRCs, so that it does not support "S" modes, where multiple encodings are transported on a single RTP stream.

Scalability Mode Identifier	Spatial Layers	Resolution Ratio	Temporal Layers	Inter-layer dependency	AV1 scalability_mode_idc
"L1T1"	1		1		N/A
"L1T2"	1		2		SCALABILITY_L1T2
"L1T3"	1		3		SCALABILITY_L1T3
"L2T1"	2	2:1	1	Yes	SCALABILITY_L2T1
"L2T2"	2	2:1	2	Yes	SCALABILITY_L2T2
"L2T3"	2	2:1	3	Yes	SCALABILITY_L2T3
"L3T1"	3	2:1	1	Yes	SCALABILITY_L3T1
"L3T2"	3	2:1	2	Yes	SCALABILITY_L3T2
"L3T3"	3	2:1	3	Yes	SCALABILITY_L3T3
"L2T1h"	2	1.5:1	1	Yes	SCALABILITY_L2T1h
"L2T2h"	2	1.5:1	2	Yes	SCALABILITY_L2T2h
"L2T3h"	2	1.5:1	3	Yes	SCALABILITY_L2T3h
"L3T1h"	3	1.5:1	1	Yes
"L3T2h"	3	1.5:1	2	Yes
"L3T3h"	3	1.5:1	3	Yes
"S2T1"	2	2:1	1	No	SCALABILITY_S2T1
"S2T2"	2	2:1	2	No	SCALABILITY_S2T2
"S2T3"	2	2:1	3	No	SCALABILITY_S2T3
"S2T1h"	2	1.5:1	1	No	SCALABILITY_S2T1h
"S2T2h"	2	1.5:1	2	No	SCALABILITY_S2T2h
"S2T3h"	2	1.5:1	3	No	SCALABILITY_S2T3h
"S3T1"	3	2:1	1	No	SCALABILITY_S3T1
"S3T2"	3	2:1	2	No	SCALABILITY_S3T2
"S3T3"	3	2:1	3	No	SCALABILITY_S3T3
"S3T1h"	3	1.5:1	1	No	SCALABILITY_S3T1h
"S3T2h"	3	1.5:1	2	No	SCALABILITY_S3T2h
"S3T3h"	3	1.5:1	3	No	SCALABILITY_S3T3h
"L2T2_KEY"	2	2:1	2	Yes	SCALABILITY_L3T2_KEY
"L2T2_KEY_SHIFT"	2	2:1	2	Yes	SCALABILITY_L3T2_KEY_SHIFT
"L2T3_KEY"	2	2:1	3	Yes	SCALABILITY_L3T3_KEY
"L2T3_KEY_SHIFT"	2	2:1	3	Yes	SCALABILITY_L3T3_KEY_SHIFT
"L3T1_KEY"	3	2:1	1	Yes
"L3T2_KEY"	3	2:1	2	Yes	SCALABILITY_L4T5_KEY
"L3T2_KEY_SHIFT"	3	2:1	2	Yes	SCALABILITY_L4T5_KEY_SHIFT
"L3T3_KEY"	3	2:1	3	Yes	SCALABILITY_L4T7_KEY
"L3T3_KEY_SHIFT"	3	2:1	3	Yes	SCALABILITY_L4T7_KEY_SHIFT

Guidelines for addition of {{RTCRtpEncodingParameters/scalabilityMode}} values

When proposing a {{RTCRtpEncodingParameters/scalabilityMode}} value, the following principles should be followed:

The proposed {{RTCRtpEncodingParameters/scalabilityMode}} MUST define entries to the table in Section 6, 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).
The Scalability Mode Identifier SHOULD be consistent with the existing naming scheme, which utilizes LxTy to denote a {{RTCRtpEncodingParameters/scalabilityMode}} with x spatial layers using a 2:1 resolution ratio and y temporal layers. LxTyh denotes x spatial layers with a 1.5:1 resolution ratio and y temporal layers. SxTy denotes a {{RTCRtpEncodingParameters/scalabilityMode}} with x simulcast encodings with a 2:1 resolution ratio, with each simulcast encoding containing y temporal layers. SxTyh denotes a 1.5:1 resolution ratio. LxTy_KEY denotes a {{RTCRtpEncodingParameters/scalabilityMode}} with x spatial layers using a 2:1 resolution ratio and y temporal layers in which spatial layers only depend on lower spatial layers at a key frame. LxTy_KEY_SHIFT modes denotes a {{RTCRtpEncodingParameters/scalabilityMode}} with x spatial layers using a 2:1 resolution ratio and y 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.
A dependency diagram MUST be supplied, in the format provided in Section 10.

Examples

Spatial Simulcast and Temporal Scalability

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.

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);
  }
};

This is an example with two spatial layers (with a 2:1 ratio) and three temporal layers.

let sendEncodings = [
  {scalabilityMode: 'L2T3'}
];

This is an example with three spatial simulcast layers each with three temporal layers on a single SSRC.

let sendEncodings = [
  {scalabilityMode: 'S3T3'}
]

SVC Encoder Capabilities

This is an example of {{MediaCapabilities/encodingInfo(configuration)}} returned by a browser implementing [[WEBRTC]] and [[Media-Capabilities]].

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');
}

SFM Capabilities

This is an example of receiver capabilities returned by an SFM that only supports forwarding of VP8, VP9 and AV1 temporal scalability modes.

 "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"
      ]
    }
]