Re: [rtcweb] SDP and ssrc-group,

["Makaraju, Maridi Raju (Raju)" <[e-mail]>]

Without commenting on this specific mechanism, in general we should NOT define solutions which assume a certain order of attributes (associated with a given m- line, or on session level) etc.

I have seen nodes, where the order of attributes has changed between the incoming- and outgoing SDP, due to the way the parser in the node is implemented.

I also don’t think we should change the semantics of the PT order in the m- line.

<Raju>

I agree with the above comments. AFAIK, SDP has session level and media level semantics, but no order (of a= lines) based semantics.

I also agree that “a=ssrc:<ssrc> fmtp:<format#>” does not give a clear answer.

How about extending http://tools.ietf.org/html/rfc5576#section-6.3 to add a new “fmtps” to associate allowed formats to an ssrc and define semantics unambiguously? Justin’s example would become like this:


     m=video 30000 RTP/AVP 100 101 108 109 110 111
     c=IN IP4 233.252.0.1
     a=rtpmap:100 VP8/90000
     a=rtpmap:101 VP9/90000
     a=rtpmap:108 rtx/90000
     a=rtpmap:109 rtx/90000
     a=fmtp:108 apt=100
     a=fmtp:109 apt=101
     a=rtpmap:110 interleaved-parityfec/90000
     a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000
     a=rtpmap:111 non-interleaved-parityfec/90000
     a=fmtp:111 L:5; D:10; ToP:2; repair-window:200000
     a=ssrc:1234 fmtps:100,101
     a=ssrc:2345 fmtps:108,109
     a=ssrc:3456 fmtps:110,111
     a=ssrc:4567 fmtps:110,111
     a=ssrc-group:FID 1234 2345
     a=ssrc-group:FEC-FR 1234 3456 4567

The fmtps mappings give the following associations:
SSRC 1234 is the primary encoding (VP8 or VP9)
SSRC 2345 is RTX (either PT 108 or 109, as needed)
SSRC 3456 is FEC (either PT 110 or 111, depending on impl)
SSRC 4567 is FEC (either PT 110 or 111, depending on impl)


</Raju>