Re: [rtcweb] SDP and ssrc-group,

"Makaraju, Maridi Raju (Raju)" <> Wed, 22 October 2014 20:05 UTC

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From: "Makaraju, Maridi Raju (Raju)" <>
To: Suhas Nandakumar <>, Bernard Aboba <>
Thread-Topic: [rtcweb] SDP and ssrc-group,
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Date: Wed, 22 Oct 2014 20:05:33 +0000
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Subject: Re: [rtcweb] SDP and ssrc-group,
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Just thinking loud
 As long as there is enough information to unabigously map the incoming streams to the SDP, i dont see we need any more information to be added.

I feel the combination of SSRC and Payload Type in the RTP packet is good enough to find the mapping to the appropriate media line (if the PTs are not repeated) when needed.

So in the initial examples above, ssrc-group communicated in SDP along with semantics (FEC,FID) when applied to incoming SSRCs and the associated payload type has all the needed info for the receiver to demux at the rtp level as well as map at the SDP level. Thus i dont see a need for specifying PT association explicitly in SDP for the ssrc-groups.
For a simple SDP offer/answer case it is probably not needed. But think of a case where SDP answerer does not support a=ssrc-group:FID and also associated retransmission ‘apt’ payload then it will not include them in SDP answer. But it needs to know which ssrc is associated to the original payload so that it allows only that ssrc.