Re: [MMUSIC] Offer/Answer PT Questions - text proposal

Colin Perkins <csp@csperkins.org> Fri, 26 February 2016 17:34 UTC

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From: Colin Perkins <csp@csperkins.org>
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Date: Fri, 26 Feb 2016 17:34:18 +0000
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To: Roman Shpount <roman@telurix.com>
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Cc: "mmusic@ietf.org" <mmusic@ietf.org>, Paul Kyzivat <pkyzivat@alum.mit.edu>
Subject: Re: [MMUSIC] Offer/Answer PT Questions - text proposal
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> On 26 Feb 2016, at 17:06, Roman Shpount <roman@telurix.com> wrote:
> 
> On Fri, Feb 26, 2016 at 4:39 AM, Colin Perkins <csp@csperkins.org <mailto:csp@csperkins.org>> wrote:
> That depends exactly what 3264 allows: if the two ends use different payload types to refer to the same codec, that’s okay; but if the two ends use the same payload type to refer to different codecs, that problematic.
> 
> Why is using the same PT to refer to different codecs is problematic? Can you point out any potential problems that this will cause from RTP perspective?

If I receive a packet with payload type X, how do I know which of the codecs it refers to?

You’d have to make the PT mapping per SSRC, but the SSRCs can change without signalling, so that doesn’t help unless you somehow label the RTP stream with a header extension that says what payload type space it’s using. 

The entire point of the PT is that it uniquely identifies a codec configuration.

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