PSA: wildcard rtcp-fb is coming to Chromium M112+

[Philipp Hancke]

I just landed

https://chromiumdash.appspot.com/commit/815522782a92e168b80edc760b2e53e4d0e4ea0d

which you should read if you are parsing SDP yourself.

It adds

  a=rtcp-fb:* <stuff> 

lines to the SDP. This is the "wildcard" defined in

  https://www.rfc-editor.org/rfc/rfc4585.html#section-4.2

It is added for all feedback types are are either

* supported by all video codecs (i.e. rtx, ulpfec and flexfec are ignored)

* supported by Opus for audio.

There is a small semantic change here for audio:

This *may* apply TWCC feedback to PCMU

Since typically PCMU (or PCMA) is used with "telephony gateways" which don't need or support TWCC (and is a fixed bitrate codec anyway) this should not be relevant in practice.

At some undefined point in the future I hope we can switch libwebrtc (which has parsed this fine since forever) to include only the wildcard feedback and stop sending the per-codec feedback.

In unified-plan which repeats this for *every* codec and every (video) m-line this would save a lot lines in the SDP, we are talking about 40 out of 50 lines per m-line (*)

Please test that this doesn't break anything for your application.

cheers

Philipp

(*) at the expensive of increasing this from 50 to 55 for the time being.

[Alexander Abagian]

Actually, I use rtcp-fb:* for couple years in remote offer, and this somehow works. As I remember, I stole this feature it from some fippo code ("simulcast mid as rid" ?).

вторник, 31 января 2023 г. в 15:36:30 UTC+3, philipp...@googlemail.com:

[Siddharth Toshniwal]

Hi Philipp,

I am trying to figure out whether Firefox is also going to generate wildcard "rtcp-fb" attribute lines in the near future. If yes, do you know a date when that goes into production?

Regards,

Siddharth

[yiming wang]

Hi Philipp,

I saw this feature was rolled back in the updates here: https://bugs.chromium.org/p/webrtc/issues/detail?id=14802. Could you please confirm that this would not be included in Chrome v112? 

Thanks,

Yiming

[Philipp Hancke]

It is not in M112 and the earliest this will reland is M114. Still good to prepare the receive side (assuming it is not libWebRTC) for it.

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