Subject: Re: [AVT] syncing sessions

From: Colin Perkins <[email-masked]>
Date: Sat, 2 Jul 2005 16:15:56 +0100

On 27 Jun 2005, at 10:23, Matthew Romaine wrote:
> I've got a question about synchronizing multiple sessions related  
> to a single payload format.
>
> I am working on a payload format which will support separation of  
> data into "required" and "enhancements" categories - aka a variant  
> on scalable streaming. In addition to sending all frame-based data  
> over one session, I would like to support sending over two sessions  
> (necessary data on one session, enhancement data on a second session).
>
> I'm sure this isn't anything new for many of you, but my Google  
> chops aren't serving me very well so I thought I would seek some  
> pointers/suggestions on payloads to use as a reference.  (Not to  
> mention using an RFC as a reference last time was a mistake :)
>
> My main question is how to specify how one session "belongs" to  
> another - i.e., is there an SDP exchange to let a client calculate  
> which two receiving-end ports need to be "associated"? Or is this  
> all strictly an application-level implementation issue? (there is  
> enough data in each payload for a client to realize which session  
> they are dealing with (required or enhancement)).

You can signal layered coding in SDP using the "slash" notation on  
either the "c=" or "m=" lines. For example, you might say:

     m=video 49170/2 RTP/AVP 31

specify that ports 49170 and 49171 form one RTP/RTCP pair and 49172  
and 49173 form the second RTP/RTCP pair. Alternatively, you might use  
the grouping framework (RFC 3388).

Colin

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