[rtcweb] JSEP question: How to set up simulcast for server-originated calls?

[Harald Alvestrand <[e-mail]>]

In JSEP, we have the following text:


   In addition, JSEP does not provide a mechanism to handle an incoming
   offer requesting simulcast from the JSEP endpoint.  This means that
   setting up simulcast in the case where the JSEP endpoint receives the
   initial offer requires out-of-band signaling or SDP inspection.

In WEBRTC, we have been grappling with the problem of simulcast setup,
and it seems that we have no obvious API surface that can be used for
this purpose - creating transceivers (as we do for outgoing) doesn't fit
the bill, because incoming SDP causes transceivers to be created wihout
interacting with the API surface.

I tried to drum up a test that would create such a setup based on SDP,
but found that in conformance with JSEP, this doesn't work.

Can someone tell me why we decided to not allow an incoming offer with
a=simulcast and a=rid ... recv in it to generate outgoing simulcast
capability?

The test in question:

https://github.com/web-platform-tests/wpt/pull/13902


Harald



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