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[rtcweb] Reasons not to multiplex audio with video (Re: Fwd: I-D Action: draft-rosenberg-rtcweb-rtpmux-00.txt)

Harald Alvestrand <harald@alvestrand.no> Mon, 25 July 2011 12:07 UTC

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Date: Mon, 25 Jul 2011 08:06:53 -0400
From: Harald Alvestrand <harald@alvestrand.no>
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Subject: [rtcweb] Reasons not to multiplex audio with video (Re: Fwd: I-D Action: draft-rosenberg-rtcweb-rtpmux-00.txt)
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On 07/24/11 16:44, Matthew Kaufman wrote:
> In all my reading today I have not been able to find anything more concrete than the "SHOULD NOT" in section 5.2 of RFC3550. PLEASE follow up if you are aware of any other relevant specifications that would argue against using SSRC to multiplex audio and video streams over a single RTP session between a pair of compatible endpoints that agree to do so.
I have found *one* reason not mentioned in the draft:

An RTP session with both "audio" and "video" media types cannot be 
represented by an SDP description, since SDP ties RTP sessions 1-1 to 
the "m" line of the description, which contains the top-level type, and 
the codec descriptions omit the top-level type in their codec naming.

I've said elsewhere that I consider this to be a design mistake for 
entirely different reasons, but this debate has reinforced my opinion 
that it is.

                      Harald

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