Subject: Re: [MMUSIC] Draft submission: draft-holmberg-mmusic-sdp-multiplex-negotiation

From: Paul Kyzivat <[email-masked]>
Date: Thu, 06 Oct 2011 17:03:48 -0400

On 9/16/11 4:35 AM, Miguel A. Garcia wrote:
> I think there are two major points of difference between the TOGETHER
> and the MULTIPLEX approach:
>
> 1) Usage of port numbers in the SDP offer
>
> 2) Combination of additional parameters from the SDP offer
>
> Let me analyze 1) in more detail. I believe a stream of media is
> identified by the transport addresses of both endpoints and the protocol
> (RTP/RTCP in this case). So, if we take the MULTIPLEX approach, the
> offerer will include the same port number in all its multiplexed
> streams. Now, if the answerer decides to multiplex, then we will have a
> single stream, because the transport addresses of all the RTP flows are
> equal.
>
> In this MULTIPLEX approach, if the answerer does not support the
> extension or does not want to use it, it will create an SDP answer that
> contains different port numbers. Therefore, each RTP flow will be send
> over a different 5-tuple, and each RTP flow will constitute a different
> media stream, even when all the transport addresses at one endpoint are
> equal. So I don't see this creating a problem with any existing endpoint
> that does not support the extension.

IIUC, o/a results in an exchange of addresses/ports at which media will 
be *received*. It does *not* define a 5-tuple. You learn the 5-tuple 
from the o/a only if you assume media is sent from the same addr/port at 
which it is received.

Of course its fairly common to assume this symmetric usage. In 
retrospect it should probably have been mandated. But it wasn't.

If symmetric RTP is a fundamental requirement of the multiplexing, then 
it should be called out as such.

	Thanks,
	Paul