1.17.9 - webrtcbin sdp answer missing ssrc's?

[Nicolas Dufresne]

Le jeudi 27 août 2020 à 11:50 -0500, Trey Hutcheson a écrit :
> 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?

Consider using GST_DEBUG="webrtcbin:7,3" or similar log level. Be aware
that you need to wait for the sources to have received data before
proceeding with an offer or answer. This corner case has multiple FIXME
in the webrtcbin. That is worked reliable without special care in 1.16
was likely just luck.

Nicolas

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