v=0 o=- 658899108507703479 2 IN IP4 127.0.0.1 s=- t=0 0 a=group:BUNDLE audio video a=msid-semantic: WMS FmJZgkjbR2gBS25U0hP3qoUSjvfDddQl4UjJ m=audio 1 RTP/SAVPF 111 103 104 0 8 106 105 13 126 ... a=ssrc:1400694016 cname:Yk4LvPXyWNZKkW6S a=ssrc:1400694016 msid:FmJZgkjbR2gBS25U0hP3qoUSjvfDddQl4UjJ a228df45 a=ssrc:1400694016 mslabel:FmJZgkjbR2gBS25U0hP3qoUSjvfDddQl4UjJ a=ssrc:1400694016 label:a228df45-515e-49b6-be8f-0d044b65de64 m=video 1 RTP/SAVPF 100 116 117 ... a=ssrc:1484622160 cname:Yk4LvPXyWNZKkW6S a=ssrc:1484622160 msid:FmJZgkjbR2gBS25U0hP3qoUSjvfDddQl4UjJ 00f2f3d2 a=ssrc:1484622160 mslabel:FmJZgkjbR2gBS25U0hP3qoUSjvfDddQl4UjJ a=ssrc:1484622160 label:00f2f3d2-974e-48a6-b8dc-0f70194949d5
But what is it for and why is it needed?
So in case of webrtc this line:
a=msid-semantic: WMS FmJZgkjbR2gBS25U0hP3qoUSjvfDddQl4UjJmeans that there is a WebRTC Media Stream (WMS) group identified by that stream id.
And the line:
a=ssrc:1400694016 msid:FmJZgkjbR2gBS25U0hP3qoUSjvfDddQl4UjJ a228df45means that there is a media stream with id FmJZ...jJ with an audio track with id a228df45.