1. Introduction
The [WEBRTC-NV-USE-CASES] document describes the use-case of
-
Untrusted JavaScript Cloud Conferencing
This specification provides access to encoded media, which is the output of the encoder part of a codec and the input to the decoder part of a codec which allows the user agent to apply encryption locally.
The interface is inspired by [WEBCODECS] to provide access to such functionality while retaining the setup flow of RTCPeerConnection
2. Specification
The Streams definition doesn’t use WebIDL much, but the WebRTC spec does. This specification shows the IDL extensions for WebRTC.
It uses an additional API on RTCRtpSender
and RTCRtpReceiver
to
insert the processing into the pipeline.
typedef (SFrameTransform or RTCRtpScriptTransform ); // New methods for RTCRtpSender and RTCRtpReceiver
RTCRtpTransform partial interface RTCRtpSender {attribute RTCRtpTransform ?transform ; };partial interface RTCRtpReceiver {attribute RTCRtpTransform ?transform ; };
2.1. Extension operation
At the time when a codec is initialized as part of the encoder, and the
corresponding flag is set in the RTCPeerConnection
's RTCConfiguration
argument, ensure that the codec is disabled and produces no output.
2.1.1. Stream creation
At construction of each RTCRtpSender
or RTCRtpReceiver
, run the following steps:
-
Initialize this.
[[transform]]
to null. -
Initialize this.
[[readable]]
to a newReadableStream
. -
Set up this.
[[readable]]
. this.[[readable]]
is provided frames using the readEncodedData algorithm given this as parameter. -
Initialize this.
[[writable]]
to a newWritableStream
. -
Set up this.
[[writable]]
with its writeAlgorithm set to writeEncodedData given this as parameter and its highWaterMark set toInfinity
.highWaterMark is set to Infinity to explicitly disable backpressure.
-
Initialize this.
[[pipeToController]]
to null. -
Initialize this.
[[lastReceivedFrameCounter]]
to0
. -
Initialize this.
[[lastEnqueuedFrameCounter]]
to0
. -
Queue a task to run the following steps:
Streams backpressure can optimize throughput while limiting processing and memory consumption by pausing data production as early as possible in a data pipeline. This proves useful in contexts where reliability is essential and latency is less of a concern. On the other hand, WebRTC media pipelines favour low latency over reliability, for instance by allowing to drop frames at various places and by using recovery mechanisms. Buffering within a transform would add latency without allowing web applications to adapt much. The User Agent is responsible for doing these adaptations, especially since it controls both ends of the transform. For those reasons, streams backpressure is disabled in WebRTC encoded transforms.
2.1.2. Stream processing
The readEncodedData algorithm is given a rtcObject as parameter. It is defined by running the following steps:
-
Wait for a frame to be produced by rtcObject’s encoder if it is a
RTCRtpSender
or rtcObject’s packetizer if it is aRTCRtpReceiver
. -
Increment rtcObject.
[[lastEnqueuedFrameCounter]]
by1
. -
Let frame be the newly produced frame.
-
Set frame.
[[owner]]
to rtcObject. -
Set frame.
[[counter]]
to rtcObject.[[lastEnqueuedFrameCounter]]
. -
Enqueue frame in rtcObject.
[[readable]]
.
The writeEncodedData algorithm is given a rtcObject as parameter and a frame as input. It is defined by running the following steps:
-
If frame.
[[owner]]
is not equal to rtcObject, abort these steps and return a promise resolved with undefined. A processor cannot create frames, or move frames between streams. -
If frame.
[[counter]]
is equal or smaller than rtcObject.[[lastReceivedFrameCounter]]
, abort these steps and return a promise resolved with undefined. A processor cannot reorder frames, although it may delay them or drop them. -
Set rtcObject.
[[lastReceivedFrameCounter]]
to frame[[counter]]
. -
Let data be frame.
[[data]]
. -
Let serializedFrame be StructuredSerializeWithTransfer(frame, « data »).
-
Let frameCopy be StructuredDeserialize(serializedFrame, frame’s relevant realm).
-
Enqueue frameCopy for processing as if it came directly from the encoded data source, by running one of the following steps:
-
If rtcObject is a
RTCRtpSender
, enqueue frameCopy to rtcObject’s packetizer, to be processed in parallel. -
If rtcObject is a
RTCRtpReceiver
, enqueue frameCopy it to rtcObject’s decoder, to be processed in parallel.
-
-
Return a promise resolved with undefined.
On sender side, as part of readEncodedData, frames produced by rtcObject’s encoder MUST be enqueued in rtcObject.[[readable]]
in the encoder’s output order.
As writeEncodedData ensures that the transform cannot reorder frames, the encoder’s output order is also the order followed by packetizers to generate RTP packets and assign RTP packet sequence numbers.
The packetizer may expect the transformed data to still conform to the original format, e.g. a series of NAL units separated by Annex B start codes.
On receiver side, as part of readEncodedData, frames produced by rtcObject’s packetizer MUST be enqueued in rtcObject.[[readable]]
in the same encoder’s output order.
To ensure the order is respected, the depacketizer will typically use RTP packet sequence numbers to reorder RTP packets as needed before enqueuing frames in rtcObject.[[readable]]
.
As writeEncodedData ensures that the transform cannot reorder frames, this will be the order expected by rtcObject’s decoder.
2.2. Extension attribute
A RTCRtpTransform has two private slots called [[readable]]
and [[writable]]
.
Each RTCRtpTransform has an association steps set, which is empty by default.
The transform
getter steps are:
-
Return this.
[[transform]]
.
The transform
setter steps are:
-
Let transform be the argument to the setter.
-
Let checkedTransform set to transform if it is not null or to an identity transform stream otherwise.
-
Let reader be the result of getting a reader for checkedTransform.
[[readable]]
. -
Let writer be the result of getting a writer for checkedTransform.
[[writable]]
. -
Initialize newPipeToController to a new
AbortController
. -
If this.
[[pipeToController]]
is not null, run the following steps:-
Add the chain transform algorithm to this.
[[pipeToController]]
.signal. -
signal abort on this.
[[pipeToController]]
.
-
-
Else, run the chain transform algorithm steps.
-
Set this.
[[pipeToController]]
to newPipeToController. -
Set this.
[[transform]]
to transform. -
Run the steps in the set of association steps of transform with this.
The chain transform algorithm steps are defined as:
-
If newPipeToController.signal is aborted, abort these steps.
-
Release reader.
-
Release writer.
-
Assert that newPipeToController is the same object as rtcObject.
[[pipeToController]]
. -
Call pipeTo with rtcObject.
[[readable]]
, checkedTransform.[[writable]]
, preventClose equal to false, preventAbort equal to false, preventCancel equal to true and newPipeToController.signal. -
Call pipeTo with checkedTransform.
[[readable]]
, rtcObject.[[writable]]
, preventClose equal to true, preventAbort equal to true, preventCancel equal to false and newPipeToController.signal.
This algorithm is defined so that transforms can be updated dynamically. There is no guarantee on which frame will happen the switch from the previous transform to the new transform.
If a web application sets the transform synchronously at creation of the RTCRtpSender
(for instance when calling addTrack), the transform will receive the first frame generated by the RTCRtpSender
's encoder.
Similarly, if a web application sets the transform synchronously at creation of the RTCRtpReceiver
(for instance when calling addTrack, or at track event handler), the transform will receive the first full frame generated by the RTCRtpReceiver
's packetizer.
3. SFrameTransform
The API presented in this section allows applications to process SFrame data as defined in [SFrame].
enum {
SFrameTransformRole ,
"encrypt" };
"decrypt" dictionary {
SFrameTransformOptions SFrameTransformRole = "encrypt"; };
role typedef [EnforceRange ]unsigned long long ;
SmallCryptoKeyID typedef (SmallCryptoKeyID or bigint ); [
CryptoKeyID Exposed =(Window ,DedicatedWorker )]interface :
SFrameTransform EventTarget {constructor (optional SFrameTransformOptions = {});
options Promise <undefined >setEncryptionKey (CryptoKey ,
key optional CryptoKeyID );
keyID attribute EventHandler ; };
onerror SFrameTransform includes GenericTransformStream ;enum {
SFrameTransformErrorEventType ,
"authentication" ,
"keyID" }; [
"syntax" Exposed =(Window ,DedicatedWorker )]interface :
SFrameTransformErrorEvent Event {(
constructor DOMString ,
type SFrameTransformErrorEventInit );
eventInitDict readonly attribute SFrameTransformErrorEventType ;
errorType readonly attribute CryptoKeyID ?;
keyID readonly attribute any ; };
frame dictionary :
SFrameTransformErrorEventInit EventInit {required SFrameTransformErrorEventType ;
errorType required any ;
frame CryptoKeyID ?; };
keyID
The new SFrameTransform(options)
constructor steps are:
-
Let transformAlgorithm be an algorithm which takes a frame as input and runs the SFrame transform algorithm with this and frame.
-
Set this.
[[transform]]
to a newTransformStream
. -
Set up this.
[[transform]]
with transformAlgorithm set to transformAlgorithm. -
Let options be the method’s first argument.
-
Set this.
[[role]]
to options["role
"]. -
Set this.
[[readable]]
to this.[[transform]]
.[[readable]]
. -
Set this.
[[writable]]
to this.[[transform]]
.[[writable]]
.
3.1. Algorithm
The SFrame transform algorithm, given sframe as a SFrameTransform object and frame, runs these steps:
-
Let role be sframe.
[[role]]
. -
If frame.
[[owner]]
is aRTCRtpSender
, set role to 'encrypt'. -
If frame.
[[owner]]
is aRTCRtpReceiver
, set role to 'decrypt'. -
Let data be undefined.
-
If frame is a
BufferSource
, set data to frame. -
If frame is a
RTCEncodedAudioFrame
, set data to frame.data
-
If frame is a
RTCEncodedVideoFrame
, set data to frame.data
-
If data is undefined, abort these steps.
-
Let buffer be the result of running the SFrame algorithm with data and role as parameters. This algorithm is defined by the SFrame specification and returns an
ArrayBuffer
. -
If the SFrame algorithm exits abruptly with an error, queue a task to run the following sub steps:
-
If the processing fails on decryption side due to data not following the SFrame format, fire an event named
error
at sframe, using theSFrameTransformErrorEvent
interface with itserrorType
attribute set tosyntax
and itsframe
attribute set to frame. -
If the processing fails on decryption side due to the key identifier parsed in data being unknown, fire an event named
error
at sframe, using theSFrameTransformErrorEvent
interface with itserrorType
attribute set tokeyID
, itsframe
attribute set to frame and itskeyID
attribute set to the keyID value parsed in the SFrame header. -
If the processing fails on decryption side due to validation of the authentication tag, fire an event named
error
at sframe, using theSFrameTransformErrorEvent
interface with itserrorType
attribute set toauthentication
and itsframe
attribute set to frame. -
Abort these steps.
-
-
If frame is a
BufferSource
, set frame to buffer. -
If frame is a
RTCEncodedAudioFrame
, set frame.data
to buffer. -
If frame is a
RTCEncodedVideoFrame
, set frame.data
to buffer. -
Enqueue frame in sframe.
[[transform]]
.
3.2. Methods
ThesetEncryptionKey(key, keyID)
method steps are:
-
Let promise be a new promise.
-
If keyID is a
bigint
which cannot be represented as a integer between 0 and 264-1 inclusive, reject promise with aRangeError
exception. -
Otherwise, in parallel, run the following steps:
-
Set key with its optional keyID as key material to use for the SFrame transform algorithm, as defined by the SFrame specification.
-
If setting the key material fails, reject promise with an
InvalidModificationError
exception and abort these steps. -
Resolve promise with undefined.
-
-
Return promise.
4. RTCRtpScriptTransform
4.1. RTCEncodedVideoFrameType
dictionary
// New enum for video frame types. Will eventually re-use the equivalent defined // by WebCodecs.enum RTCEncodedVideoFrameType {"empty" ,"key" ,"delta" , };
Enum value | Description |
---|---|
empty
|
This frame contains no data. |
key
|
This frame can be decoded without reference to any other frames. |
delta
|
This frame references another frame and can not be decoded without that frame. |
4.2. RTCEncodedVideoFrameMetadata
dictionary
dictionary RTCEncodedVideoFrameMetadata {unsigned long long ;
frameId sequence <unsigned long long >;
dependencies unsigned short ;
width unsigned short ;
height unsigned long ;
spatialIndex unsigned long ;
temporalIndex unsigned long synchronizationSource ;octet payloadType ;sequence <unsigned long >contributingSources ;long long timestamp ; // microsecondsunsigned long rtpTimestamp ;DOMString mimeType ; };
4.2.1. Members
-
synchronizationSource
, of type unsigned longunsigned long -
The synchronization source (ssrc) identifier is an unsigned integer value per [RFC3550] used to identify the stream of RTP packets that the encoded frame object is describing.
-
payloadType
, of type octetoctet -
The payload type is an unsigned integer value in the range from 0 to 127 per [RFC3550] that is used to describe the format of the RTP payload.
-
contributingSources
, of typesequence<unsigned long>
sequence<unsigned long> -
The list of contribution sources (csrc list) as defined in [RFC3550].
-
timestamp
, of type long longlong long -
The media presentation timestamp (PTS) in microseconds of raw frame, matching the
timestamp
for raw frames which correspond to this frame. -
rtpTimestamp
, of type unsigned longunsigned long -
The RTP timestamp identifier is an unsigned integer value per [RFC3550] that reflects the sampling instant of the first octet in the RTP data packet.
-
mimeType
, of type DOMStringDOMString -
The codec MIME media type/subtype defined in the IANA media types registry [IANA-MEDIA-TYPES], e.g. video/VP8.
4.3. RTCEncodedVideoFrame
interface
// New interfaces to define encoded video and audio frames. Will eventually // re-use or extend the equivalent defined in WebCodecs. [Exposed =(Window ,DedicatedWorker ),Serializable ]interface RTCEncodedVideoFrame {readonly attribute RTCEncodedVideoFrameType type ;attribute ArrayBuffer data ;RTCEncodedVideoFrameMetadata getMetadata (); };
4.3.1. Members
-
type
, of type RTCEncodedVideoFrameType, readonlyRTCEncodedVideoFrameType -
The type attribute allows the application to determine when a key frame is being sent or received.
-
data
, of type ArrayBufferArrayBuffer -
The encoded frame data. The format of the data depends on the video codec that is used to encode/decode the frame which can be determined by looking at the
mimeType
. For SVC, each spatial layer is transformed separately.Since packetizers may drop certain elements, e.g. AV1 temporal delimiter OBUs, the input to an receive-side transform may be different from the output of a send-side transform.
The following table gives a number of examples:mimeType Data format video/VP8 The data starts with the "uncompressed data chunk" defined in section 9.1 of [RFC6386] and is followed by the rest of the frame data. The VP8 payload descriptor is not accessible. video/VP9 The data is a frame as described in Section 6 of [VP9]. The VP9 payload descriptor is not accessible. video/H264 The data is a series of NAL units in Annex B format, as defined in [ITU-T-REC-H.264] Annex B. video/AV1 The data is a series of OBUs compliant to the low-overhead bitstream format as described in Section 5 of [AV1]. The AV1 aggregation header is not accessible.
4.3.2. Methods
-
getMetadata()
-
Returns the metadata associated with the frame.
4.3.3. Serialization
RTCEncodedVideoFrame
objects are serializable objects [HTML].
Their serialization steps, given value, serialized, and forStorage, are:
-
If forStorage is true, then throw a
DataCloneError
. -
Set serialized.
[[type]]
to the value of value.type
-
Set serialized.
[[metadata]]
to an internal representation of value’s metadata. -
Set serialized.
[[data]]
to value.[[data]]
Their deserialization steps, given serialized, value and realm, are:
-
Set value.
type
to serialized.[[type]]
-
Set value’s metadata to the platform object representation of serialized.
[[metadata]]
-
Set value.
[[data]]
to serialized.[[data]]
.
The internal form of a serialized RTCEncodedVideoFrame is not observable;
it is defined chiefly so that it can be used with frame cloning in the writeEncodedData algorithm and in the structuredClone()
operation.
An implementation is therefore free to choose whatever method works best.
4.4. RTCEncodedAudioFrameMetadata
dictionary
dictionary RTCEncodedAudioFrameMetadata {unsigned long synchronizationSource ;octet payloadType ;sequence <unsigned long >contributingSources ;short sequenceNumber ;unsigned long rtpTimestamp ;DOMString mimeType ; };
4.4.1. Members
-
synchronizationSource
, of type unsigned longunsigned long -
The synchronization source (ssrc) identifier is an unsigned integer value per [RFC3550] used to identify the stream of RTP packets that the encoded frame object is describing.
-
payloadType
, of type octetoctet -
The payload type is an unsigned integer value in the range from 0 to 127 per [RFC3550] that is used to describe the format of the RTP payload.
-
contributingSources
, of typesequence<unsigned long>
sequence<unsigned long> -
The list of contribution sources (csrc list) as defined in [RFC3550].
-
sequenceNumber
, of type shortshort -
The RTP sequence number as defined in [RFC3550]. Only exists for incoming audio frames.
Comparing two sequence numbers requires serial number arithmetic described in [RFC1982].
-
rtpTimestamp
, of type unsigned longunsigned long -
The RTP timestamp identifier is an unsigned integer value per [RFC3550] that reflects the sampling instant of the first octet in the RTP data packet.
-
mimeType
, of type DOMStringDOMString -
The codec MIME media type/subtype defined in the IANA media types registry [IANA-MEDIA-TYPES], e.g. audio/opus.
4.5. RTCEncodedAudioFrame
interface
[Exposed =(Window ,DedicatedWorker ),Serializable ]interface RTCEncodedAudioFrame {attribute ArrayBuffer data ;RTCEncodedAudioFrameMetadata getMetadata (); };
4.5.1. Members
-
data
, of type ArrayBufferArrayBuffer -
The encoded frame data. The format of the data depends on the audio codec that is used to encode/decode the frame which can be determined by looking at the
mimeType
. The following table gives a number of examples:mimeType Data format audio/opus The data is Opus packets, as described in section 3 of [RFC6716]. audio/PCMU The data is a sequence of bytes of arbitrary length, where each byte is a u-law encoded PCM sample as defined by Table 2a and 2b in [ITU-G.711]. audio/PCMA The data is a sequence of bytes of arbitrary length, where each byte is an A-law encoded PCM sample as defined by Tables 1a and 1b in [ITU-G.711]. audio/G722 The data is G.722 audio as described in [ITU-G.722]. audio/RED The data is Redundant Audio Data as described in section 3 of [RFC2198]. audio/CN The data is Comfort Noise as described in section 3 of [RFC3389].
4.5.2. Methods
-
getMetadata()
-
Returns the metadata associated with the frame.
4.5.3. Serialization
RTCEncodedAudioFrame
objects are serializable objects [HTML].
Their serialization steps, given value, serialized, and forStorage, are:
-
If forStorage is true, then throw a
DataCloneError
. -
Set serialized.
[[metadata]]
to an internal representation of value’s metadata. -
Set serialized.
[[data]]
to value.[[data]]
Their deserialization steps, given serialized, value and realm, are:
-
Set value’s metadata to the platform object representation of serialized.
[[metadata]]
-
Set value.
[[data]]
to serialized.[[data]]
.
4.6. Interfaces
[Exposed =DedicatedWorker ]interface :
RTCTransformEvent Event {readonly attribute RTCRtpScriptTransformer ; };
transformer partial interface DedicatedWorkerGlobalScope {attribute EventHandler ; }; [
onrtctransform Exposed =DedicatedWorker ]interface :
RTCRtpScriptTransformer EventTarget { // Attributes and methods related to the transformer sourcereadonly attribute ReadableStream ;
readable Promise <unsigned long long >(
generateKeyFrame optional DOMString );
rid Promise <undefined >(); // Attributes and methods related to the transformer sink
sendKeyFrameRequest readonly attribute WritableStream ;
writable attribute EventHandler ; // Attributes for configuring the Javascript code
onkeyframerequest readonly attribute any options ; }; [Exposed =Window ]interface {
RTCRtpScriptTransform constructor (Worker ,
worker optional any ,
options optional sequence <object >); }; [
transfer Exposed =DedicatedWorker ]interface :
KeyFrameRequestEvent Event {(
constructor DOMString ,
type optional DOMString );
rid readonly attribute DOMString ?; };
rid
4.7. Operations
The new RTCRtpScriptTransform(worker, options, transfer)
constructor steps are:
-
Set t1 to an identity transform stream.
-
Set t2 to an identity transform stream.
-
Set this.
[[writable]]
to t1.[[writable]]
. -
Set this.
[[readable]]
to t2.[[readable]]
. -
Let serializedOptions be the result of StructuredSerializeWithTransfer(options, transfer).
-
Let serializedReadable be the result of StructuredSerializeWithTransfer(t1.
[[readable]]
, « t1.[[readable]]
»). -
Let serializedWritable be the result of StructuredSerializeWithTransfer(t2.
[[writable]]
, « t2.[[writable]]
»). -
Queue a task on the DOM manipulation task source worker’s global scope to run the following steps:
-
Let transformerOptions be the result of StructuredDeserialize(serializedOptions, the current Realm).
-
Let readable be the result of StructuredDeserialize(serializedReadable, the current Realm).
-
Let writable be the result of StructuredDeserialize(serializedWritable, the current Realm).
-
Let transformer be a new
RTCRtpScriptTransformer
. -
Set transformer.
[[options]]
to transformerOptions. -
Set transformer.
[[readable]]
to readable. -
Set transformer.
[[writable]]
to writable. -
Fire an event named
rtctransform
usingRTCTransformEvent
withtransformer
set to transformer on worker’s global scope.
-
// FIXME: Describe error handling (worker closing flag true at RTCRtpScriptTransform creation time. And worker being terminated while transform is processing data).
Each RTCRtpScriptTransform has the following set of association steps, given rtcObject:
-
Let transform be the
RTCRtpScriptTransform
object that owns the association steps. -
Let encoder be rtcObject’s encoder if rtcObject is a
RTCRtpSender
or undefined otherwise. -
Let depacketizer be rtcObject’s depacketizer if rtcObject is a
RTCRtpReceiver
or undefined otherwise. -
Queue a task on the DOM manipulation task source worker’s global scope to run the following steps:
-
Let transformer be the
RTCRtpScriptTransformer
object associated to transform. -
Set transformer.
[[encoder]]
to encoder. -
Set transformer.
[[depacketizer]]
to depacketizer.
-
The generateKeyFrame(rid)
method steps are:
-
Let promise be a new promise.
-
Run the generate key frame algorithm with promise, this.
[[encoder]]
and rid. -
Return promise.
The sendKeyFrameRequest()
method steps are:
-
Let promise be a new promise.
-
Run the send request key frame algorithm with promise and this.
[[depacketizer]]
. -
Return promise.
4.8. Attributes
A RTCRtpScriptTransformer
has the following private slots called [[depacketizer]]
, [[encoder]]
, [[options]]
, [[readable]]
and [[writable]]
.
In addition, a RTCRtpScriptTransformer
is always associated with its parent RTCRtpScriptTransform
transform.
This allows algorithms to go from an RTCRtpScriptTransformer
object to its RTCRtpScriptTransform
parent and vice versa.
The options
getter steps are:
-
Return this.
[[options]]
.
The readable
getter steps are:
-
Return this.
[[readable]]
.
The writable
getter steps are:
-
Return this.
[[writable]]
.
The onbandwidthestimate
EventHandler has type bandwidthestimate.
The onkeyframerequest
EventHandler has type keyframerequest.
4.9. Events
The following event fires on an RTCRtpScriptTransformer
:
-
keyframerequest of type
KeyFrameRequestEvent
- fired when the sink determines that a key frame has been requested.
The steps that generate an event of type KeyFrameRequestEvent
are as follows:
Given a RTCRtpScriptTransformer
transform:
When transform’s [[encoder]]
receives a keyframe request, for instance from an incoming RTCP Picture Loss Indication (PLI)
or Full Intra Refresh (FIR), queue a task to perform the following steps:
-
Set rid to the RID of the appropriate layer, or undefined if the request is not for a specific layer.
-
Fire an event named
keyframerequest
at transform usingKeyFrameRequestEvent
with itscancelable
attribute initialized to "true", and withrid
set to rid. -
If the event’s canceled flag is true, abort these steps.
-
Run the generate key frame algorithm with a new promise, transform.
[[encoder]]
and rid.
4.10. KeyFrame Algorithms
The generate key frame algorithm, given promise, encoder and rid, is defined by running these steps:
-
If encoder is undefined, reject promise with
InvalidStateError
, abort these steps. -
If encoder is not processing video frames, reject promise with
InvalidStateError
, abort these steps. -
If rid is defined, but does not conform to the grammar requirements specified in Section 10 of [RFC8851], then reject promise with
TypeError
and abort these steps. -
In parallel, run the following steps:
-
Gather a list of video encoders, named videoEncoders from encoder, ordered according negotiated RIDs if any.
-
If rid is defined, remove from videoEncoders any video encoder that does not match rid.
-
If rid is undefined, remove from videoEncoders all video encoders except the first one.
-
If videoEncoders is empty, reject promise with
NotFoundError
and abort these steps. videoEncoders is expected to be empty if the correspondingRTCRtpSender
is not active, or the correspondingRTCRtpSender
track is ended. -
Let videoEncoder be the first encoder in videoEncoders.
-
If rid is undefined, set rid to the RID value corresponding to videoEncoder.
-
Create a pending key frame task called task with task.
[[rid]]
set to rid and task.[[promise]]
| set to promise. -
If encoder.
[[pendingKeyFrameTasks]]
is undefined, initialize encoder.[[pendingKeyFrameTasks]]
to an empty set. -
Let shouldTriggerKeyFrame be
false
if encoder.[[pendingKeyFrameTasks]]
contains a task whose[[rid]]
value is equal to rid, andtrue
otherwise. -
Add task to encoder.
[[pendingKeyFrameTasks]]
. -
If shouldTriggerKeyFrame is
true
, instruct videoEncoder to generate a key frame for the next provided video frame.
-
For any RTCRtpScriptTransformer
named transformer, the following steps are run just before any frame is enqueued in transformer.[[readable]]
:
-
Let encoder be transformer.
[[encoder]]
. -
If encoder or encoder.
[[pendingKeyFrameTasks]]
is undefined, abort these steps. -
If frame is not a video
"key"
frame, abort these steps. -
For each task in encoder.
[[pendingKeyFrameTasks]]
, run the following steps:-
If frame was generated by a video encoder identified by task.
[[rid]]
, run the following steps:-
Remove task from encoder.
[[pendingKeyFrameTasks]]
. -
Resolve task.
[[promise]]
with frame’s timestamp.
-
-
By resolving the promises just before enqueuing the corresponding key frame in a RTCRtpScriptTransformer
's readable,
the resolution callbacks of the promises are always executed just before the corresponding key frame is exposed.
If the promise is associated to several rid values, it will be resolved when the first key frame corresponding to one the rid value is enqueued.
The send request key frame algorithm, given promise and depacketizer, is defined by running these steps:
-
If depacketizer is undefined, reject promise with
InvalidStateError
, abort these steps. -
If depacketizer is not processing video packets, reject promise with
InvalidStateError
, abort these steps. -
In parallel, run the following steps:
-
If sending a Full Intra Request (FIR) by depacketizer’s receiver is not deemed appropriate, resolve promise with undefined and abort these steps. Section 4.3.1 of [RFC5104] provides guidelines of how and when it is appropriate to sending a Full Intra Request.
-
Generate a Full Intra Request (FIR) packet as defined in section 4.3.1 of [RFC5104] and send it through depacketizer’s receiver.
-
Resolve promise with undefined.
-
5. RTCRtpSender extension
An additional API on RTCRtpSender
is added to complement the generation of key frame added to RTCRtpScriptTransformer
.
partial interface RTCRtpSender {Promise <undefined >generateKeyFrame (optional sequence <DOMString >); };
rids
5.1. Extension operation
The generateKeyFrame(rids)
method steps are:
-
Let promise be a new promise.
-
In parallel, run the generate key frame algorithm with promise, this’s encoder and rids.
-
Return promise.
6. Privacy and security considerations
This API gives Javascript access to the content of media streams. This is also available from other sources, such as Canvas and WebAudio.
However, streams that are isolated (as specified in [WEBRTC-IDENTITY]) or tainted with another origin, cannot be accessed using this API, since that would break the isolation rule.
The API will allow access to some aspects of timing information that are otherwise unavailable, which allows some fingerprinting surface.
The API will give access to encoded media, which means that the JS application will have full control over what’s delivered to internal components like the packetizer or the decoder. This may require additional care with auditing how data is handled inside these components.
For instance, packetizers may expect to see data only from trusted encoders, and may not be audited for reception of data from untrusted sources.
7. Examples
See the explainer document.