setCodecPreferences and RTX / RED / FEC codecs

[Orphis]

According to the spec:

codecs of type sequence<RTCRtpCodecCapability>, required
Supported media codecs as well as entries for RTX, RED and FEC mechanisms. There will only be a single entry in codecs[] for retransmission via RTX, with sdpFmtpLine not present.

Do I interpret correctly that RTX, RED and FEC could be independently enabled or disabled using setCodecPreferences() by adding those to the list?
Is this the intended way to have control over those features in WebRTC?

Also, what should happen if no proper codec is specified but only RTX, RED or FEC ones? Should we reject the setCodecPreferences() call with an error?

[aboba]

@Orphis Yes, I believe that RTX, RED and FEC can be enabled/disabled this way (preference doesn't matter for these though).

No proper codec but only RTX, RED or FEC should result in an error. In ORTC, there are checks for this kind of thing, but not yet in WebRTC.

[Orphis]

Can you clarify preference doesn't matter for these though?
Do you mean that the order doesn't matter, only the presence in the codec list?

[henbos]

How would I specify whether or not I want to use RTX? It seems to me that codec capabilities is trying to be two things at once: a codec type, AND a codec preference. But then we are using it in setCodecPreferences() as if it is a complete codec+codec preference configuration?

[aboba]

@Orphis I mean that the order of RED, RTX or FEC within the codec list is immaterial, only the presence of these codecs matters.

@henbos If you don't want to use RED, RTX (or FEC) you can remove it from the codec list.

[aboba]

Consensus from the January 22, 2019 virtual interim is to go ahead with PR #2069