Match header extensions to all media sections in offer

[ellenfkh]

Description

We noticed that once pion has negotiated headers and codecs for a given Kind, when a subsequent offer SDP adds a media section for that same Kind with different header extensions, the generated answer uses the original headers for all media sections.

This doesn't happen in Chrome, which never seems to send mixed headers like this. Firefox does, though.

We ran into this when sending an offer in which the first video media section had simulcast disabled, and thus didn't set this attribute

a=extmap:8/sendonly urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id

but the second video media section had simulcast enabled, so included that header.

The resulting answer didn't include that header in either media section, but the second media section did correctly have these attributes

a=rid:f recv
a=rid:h recv
a=rid:q recv
a=simulcast:recv f;h;q

Resulting in this error

Incoming unhandled RTP ssrc(3870936274), OnTrack will not be fired. stream id RTP Extensions required for Simulcast

This patch fixes our manual test case, but I suspect there are more places in the code where this assumption is being made. I'm also not sure whether Plan B vs Unified expects different behavior in this case.

Reference issue

Fixes #...

[adriancable]

This looks good to me. If one other person can quickly eyeball this (@Sean-Der / @davidzhao ?) then I am happy to merge it. @ellenfkh - can you kindly squash the commits and make sure the new commit message passes the lint rules (begins with a capital letter, no more than 50 characters). Thanks!

[codecov]

Codecov Report

All modified lines are covered by tests ✅

see 50 files with indirect coverage changes

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[davidzhao]

Thank you for the PR. It looks good!

Sorry for the delays getting to this one.