[rtcweb] JSEPbis: Questions about send-only/recv-only codecs and Section 4.2.6

[Bernard Aboba <[e-mail]>]

The purpose of this email is to alert the RTCWEB WG to a potential issue
that has been pointed out in JSEPbis Section 4.2.6, with respect to the
operation of setCodecPreferences().

Currently, support for the H.265 codec in WebRTC is underway in multiple
browsers, including Chrome, Firefox and Safari.  Safari Tech Preview 189
has enabled by default support for both sending and receiving H.265, while
Chrome and Firefox are working on enabling reception of H.265.  This has
lead to discussion of potential interoperability issues between browsers
that can Offer H.265 on a send/recv m-line, and browsers that can only
Offer H.265 on a recv-only m-line.

At the AVTCORE WG Virtual Interim on February 13, 2024, we discussed Issue
22 relating to the handling of send-only and recv-only codecs:
https://github.com/aboba/hevc-webrtc/issues/22

draft-uberti-rtcweb-rfc8829bis Section 4.2.6 says:

"The setCodecPreferences method sets the codec preferences of a
transceiver, which in turn affect the presence and order of codecs of the
associated "m=" section on future calls to createOffer and createAnswer.  *Note
that setCodecPreferences does not directly affect which codec the
implementation decides to send. It only affects which codecs the
implementation indicates that it prefers to receive, via the offer or
answer.*  Even when a codec is excluded by setCodecPreferences, it still
may be used to send until the next offer/answer exchange discards it."

Questions arose relating to the (red) text in italics, because it does not
appear to be consistent with the definition of setCodecPreferences() within
the WebRTC-PC API specification. In particular, while the text may be
correct with respect to operation of sCP() on a send/recv or recv-only
m-line, for a send-only m-line it seems odd to insist that sCP() will only
affect receive preferences, not send preferences.

Currently within the W3C WEBRTC WG, several issues have been opened
relating to the behavior of setCodecPreferences():

   -

   w3c/webrtc-pc#2936 <https://github.com/w3c/webrtc-pc/issues/2936>
   -

   w3c/webrtc-pc#2933 <https://github.com/w3c/webrtc-pc/issues/2933>
   -

   w3c/webrtc-pc#2935 <https://github.com/w3c/webrtc-pc/pull/2935>
   -

   w3c/webrtc-pc/#2888 <https://github.com/w3c/webrtc-pc/issues/2888>