[AVT] syncing sessions

fellow avt-ers,

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)).

appreciate any comments, pointers, etc.

thanks

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