Re: [rtcweb] JSEP: Issues with a=ssrc and RTP payload type switching

Christer Holmberg <> Fri, 12 June 2015 08:33 UTC

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From: Christer Holmberg <>
To: Magnus Westerlund <>, Harald Alvestrand <>, Simon Perreault <>, Roman Shpount <>
Thread-Topic: [rtcweb] JSEP: Issues with a=ssrc and RTP payload type switching
Date: Fri, 12 Jun 2015 08:33:09 +0000
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Subject: Re: [rtcweb] JSEP: Issues with a=ssrc and RTP payload type switching
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Keep in mind that whatever changes we do in JSEP must be "compatible" with BUNDLE :)

However, BUNDLE currently doesn't mandate the usage of the SDP 'ssrc' attribute, but it does have the following rule on SSRC usage:

	o  A given SSRC MUST NOT transmit RTP packets using payload types
      	that originate from different bundled "m=" lines.

However, that doesn't seem to be related to the ongoing discussion.



-----Original Message-----
From: rtcweb [] On Behalf Of Magnus Westerlund
Sent: 9. kesäkuuta 2015 11:20
To: Harald Alvestrand; Simon Perreault; Roman Shpount
Subject: Re: [rtcweb] JSEP: Issues with a=ssrc and RTP payload type switching

Harald Alvestrand skrev den 2015-06-08 17:17:
> Den 08. juni 2015 17:15, skrev Simon Perreault:
>> Le 2015-06-08 09:09, Harald Alvestrand a écrit :
>>> This has another implication:
>>> When doing PT switching for comfort noise, you MUST have a CN with 
>>> the same clock rate as your normal audio codec.
>>> CN is exactly the type of PT-switching that, if it occurs at all, 
>>> occurs very frequently.
>> I'm not following. We're discussing generating a new SSRC on clock 
>> rate-switching, not on PT-switching. For the CN example, you would 
>> typically include in your SDP one CN payload type per possible clock 
>> type so that you can switch payload types without switching the clock rate.
> Yep. It's only because I've seen SDP trying to match CN/8000 with
> OPUS/48000 that I'm mentioning it - it's obvious, but only after it's 
> bitten you.

Yes, this is likely one of these things that the ones that considered the issues see as obvious, but is in fact not until you actually tried it and therefore could have benefited from a note about the need for having one PT configuration per timestamp rate used by the codecs that may use the CN.

As the RTP usage is in IESG review, I am hesitant to add such discussion into the RTP usage document. As it is really a configuration issue, I think it can be discussed in JSEP, but the audio draft might be an even better fit adding to the CN text.


Magnus Westerlund

Services, Media and Network features, Ericsson Research EAB/TXM
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