Clarify behavior of getParameters()

[aboba]

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]

Closed by PR 69

[aboba]

Reopening for discussion at March 15 meeting.

[aboba]

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]

False alarm, sorry. The anomaly I recalled was receiver-side in w3c/webrtc-pc#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]

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