Clarify behavior of getParameters()

w3c / webrtc-svc

W3C Scalable Video Coding (SVC) Extension for WebRTC

https://w3c.github.io/webrtc-svc/

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Clarify behavior of getParameters() #68

Closed aboba closed 2 years ago

aboba commented 2 years ago

There is a need for additional clarifications on the behavior of getParameters() before and after initial negotiation, as well as during a re-negotiation. Specifically:

What is returned if scalabilityMode was not set.
What is returned if an attempt was made to set scalabilityMode, but the attempt failed.
What is returned if setCodecPreferences() is used to change the preferred codec.

aboba commented 2 years ago

Closed by PR 69

aboba commented 2 years ago

Reopening for discussion at March 15 meeting.

aboba commented 2 years ago

March 15 meeting discussion about re-negotiation:

Until re-negotiation is complete, RTCRtpSender.getParameters() should return the current configuration.
Example: VP8 had been negotiated initially with 'L1T2', and then renegotiation begins to switch the preferred codec to H.264/AVC (with only 'L1T1' supported). Before re-negotiation completes, getParameters() returns 'L1T2'. After re-negotiation is completed, 'L1T1' is returned.
This appears consistent with the text on getParameters() and setParameters() in Section 5.2 RTCRtpSender Interface.

@jan-ivar Can you provide a reference to the text on [[SendEncodings]] that concerns you?

jan-ivar commented 2 years ago

False alarm, sorry. The anomaly I recalled was receiver-side in https://github.com/w3c/webrtc-pc/issues/2710:

sender getParameters gets encodings from [[SendCodecs]] which is set in the answer or pranswer.

receiver getParameters gets encodings from [[ReceiveCodecs]] which is set in SLD.

IOW the set a session description algorithm ensures that:

the receiver returns pending encodings in "have-local-offer", whereas
the sender always returns current encodings

March 15 meeting discussion about re-negotiation:

Until re-negotiation is complete, RTCRtpSender.getParameters() should return the current configuration.

Since scalabilityMode is in sender-side encodings, this decision is congruent with the current spec (and already specified).

aboba commented 2 years ago

OK. Am going to close this issue. If there are other concerns relating to setParameters() or getParameters() behavior, please reopen.