replaceTrack(null): Allowed?

[aboba]

Currently, the argument to replaceTrack() cannot be null. Since sender.track can be null in other methods (e.g. addTransceiver), why should it not be possible to replace an existing track with null (and cause the sender to stop sending)?

[alvestrand]

See also #943

[aboba]

Some preliminary thoughts:

"Attempts to replace the track being sent with another track provided (or with a null track), without renegotiation.

To avoid track identifiers changing on the remote receiving end when a track is replaced, the sender must retain the original track identifier and stream associations and use these in subsequent negotiations.

When the replaceTrack method is invoked, the user agent must run the following steps:

Let sender be the RTCRtpSender object on which replaceTrack is invoked.
Let tranceiver be the RTCRtpTransceiver object associated with sender.
Let connection be the RTCPeerConnection object that created sender.

If transceiver.stopped is true, return a promise rejected with an InvalidStateError.

Let withTrack be the argument to this method.

If withTrack is non-null and withTrack.kind differs from the transceiver kind of transceiver, return a promise rejected with a TypeError.

If transceiver is not yet associated with a media description [JSEP] (section 3.4.1.), then set sender's track attribute to withTrack, and return a promise resolved with undefined.

Let p be a new promise.

Run the following steps in parallel:

Determine if negotiation is needed to replace the sender's existing track with withTrack. Negotiation is not needed if withTrack is null or if the sender's existing track is ended (which appears as though the track was muted). Ignore which MediaStream the track resides in and the id attribute of the track in this determination. If negotiation is needed, then reject p with InvalidModificationError and abort these steps.

If withTrack is null, have the sender stop sending, without negotiating. Otherwise, have the sender switch seamlessly to transmitting withTrack instead of the sender's existing track, without negotiating.

Queue a task that sets sender's track attribute to withTrack, and resolves p with undefined.

Return p."

[aboba]

@taylor-b Thoughts on the above?

[taylor-b]

That sounds good to me.