1.17.9 - webrtcbin sdp answer missing ssrc's?

[Trey Hutcheson]

I have a media server that currently runs against gstreamer 1.16.2. When I
run that same code on top of the current 1.17.9 master, I sometimes get no
media to my remote peers.

The workflow is simple. The browser provides the offer, and webrtcbin
provides the answer (signaling is our own custom app). Looking at the sdp,
I see that in the cases where media never starts flowing that the sdp
answer does not include ssrc's.

Example fragment from good sdp answer:
m=video 9 UDP/TLS/RTP/SAVPF 96
c=IN IP4 0.0.0.0
a=ice-ufrag:fENRu4J+ohkX+c21z/3vzhvQ0a7LMqrX
a=ice-pwd:64ese6hPqiLwjB8QytOBokIXJGqf37Az
a=mid:0
a=rtcp-mux
a=setup:active
a=rtpmap:96 VP8/90000
a=rtcp-fb:96 nack pli
a=rtcp-fb:96 ccm fir
a=framerate:30
a=ssrc:1009469877 msid:user286747991 at host-a7789f8b webrtctransceiver5
a=ssrc:1009469877 cname:user286747991 at host-a7789f8b

Example fragment from bad sdp answer:
m=video 9 UDP/TLS/RTP/SAVPF 96
c=IN IP4 0.0.0.0
a=ice-ufrag:2t2Wy+2pPcDPVPmVyixyMH2O/s63DFiX
a=ice-pwd:gPMprJzvik/ulzX3S92hcsGsdBMcYfgd
a=mid:0
a=rtcp-mux
a=setup:active
a=rtpmap:96 VP8/90000
a=rtcp-fb:96 nack pli
a=rtcp-fb:96 ccm fir
a=sendonly

Notice that the second fragment does not include a=ssrc lines (nor does it
include the framerate attribute).

This is troubling because the issue is intermittent. It happens with great
frequency. If, however, I set GST_DEBUG to 4, then it happens far less
often, though still occasionally. That would indicate to me it is some
internal race condition?
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