Re: [MMUSIC] Scope of RTP payload types in BUNDLE?

Colin Perkins <csp@csperkins.org> Mon, 27 May 2013 19:09 UTC

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From: Colin Perkins <csp@csperkins.org>
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Subject: Re: [MMUSIC] Scope of RTP payload types in BUNDLE?
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On 27 May 2013, at 19:49, Paul Kyzivat wrote:
> On 5/27/13 2:29 PM, Colin Perkins wrote:
>> What would that other characteristic be? The only thing you have is the SSRC, and SDP doesn't scope "a=rtpmap:" lines to be per "a=ssrc:" line, and RTP certainly doesn't treat payload types as per SSRC.
> 
> This is all part of defining the precise semantics of bundling!
> 
> In the example I gave below, what I am suggesting is that:
> 
> - SSRC 1111 is associated with m-line X, and SSRC 22222 is associated
>  with m-line Y.
> 
> - When a packet is received, it must first be associated with
>  an m-line. If it has SSRC=1111 then it can be associated with
>  m-line X. Then, m-line X gives the mapping of PT 96 to audio
>  and AMR. If it has SSRC=2222 then it can be associated with
>  m-line Y. Then, m-line Y gives the mapping of PT 96 to audio
>  and G.7291. (If a packet with some other SSRC is received
>  then the mapping to an m-line is unknown, unless we introduce
>  some other rule.)
> 
> - in some other example, if some PT is unique to a single m-line,
>  then a packet with that PT can be associated to that m-line.
> 
> Have I made myself clear yet?


Yes, but I think that's the wrong approach. 

When an RTP or RTCP packet arrives it first needs to be associated with an RTP source (step B in the diagram in http://www.ietf.org/mail-archive/web/mmusic/current/msg11251.html). That RTP source then, if you care about such things, can be associated with an m= line (step C in the diagram). 

You can use some combination of the payload type and a=ssrc: lines to perform the mapping at step C. That's fine. I'm arguing, however, that step B is important for RTP to work right, and that we can't just map to sources based on the payload type.

-- 
Colin Perkins
http://csperkins.org/