[rtcweb] JSEP-bis and WebRTC-Extensions Section 5.2

[Justin Uberti <[e-mail]>]

The WebRTC W3C WG is working on an extension spec
<https://w3c.github.io/webrtc-extensions/#rtp-header-extension-control-modifications>
that suggests behavior for RTP header extension O/A that conflicts slightly
with the current version of JSEP. Rather than have this sort of ad-hoc
extensions extension to JSEP in the W3C spec, it was suggested that we
could apply a tiny patch to JSEP before JSEP-bis is finalized (it's
currently in the RFC Editor state). Murray was supportive of this but
wanted to make sure the WG was fully behind this change first.

For context, Section 5.2 in the W3C extension spec basically says that
WebRTC endpoints can control the extensions that are in use, and as a
result O/A negotiation should a) use only the extensions that are enabled
by the endpoint, and b) always reoffer all extensions, rather than the
previously negotiated set.

I've made a PR that captures this request at
https://github.com/rtcweb-wg/jsep/pull/1033/files, and it basically makes
two surgical changes:
1) replaces the use of "supported RTP header extensions" with "enabled RTP
header extensions"
2) changes the text around "extensions... present in the most recent
answer" to instead say "extensions... enabled on the associated
transceiver" (similar to how codecs are handled in JSEP).

Please let me know if this sounds reasonable to you or you have concerns.

Thanks,
Justin