Re: [rtcweb] Codec control and SDP offer/answer frequency

Christer Holmberg <> Thu, 23 August 2012 15:11 UTC

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From: Christer Holmberg <>
To: 'Harald Alvestrand' <>
Date: Thu, 23 Aug 2012 17:11:21 +0200
Thread-Topic: [rtcweb] Codec control and SDP offer/answer frequency
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Subject: Re: [rtcweb] Codec control and SDP offer/answer frequency
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>>>> Also, you would need to describe how the data channel protocol relates to negotiating being done on the signalling plane.
>>> The format of the signalling channel is defined to be outside the scope of the specification, so it's perfectly legitimate to use the data channel for that purpose - application decision!
>> Sure, applications can do whatever they want.
>> But, I think it would be good to have a standardized mechanisms which works for any application. AFAIK, that was the whole purpose of the discussion in Vancouver.
> Since we've settled on using SDP at the UA/Application interface,

Correct (eventhough someone in Seattle may have a different opinion ;)

> representing the data in SDP is a valid way of describing that interface
>- thus indeed making it available for use by any application written to that interface.


> The particular worry you raised was how fast applications can do offer/answers - which is dependent on 
> how their signalling channel is implemented, and how many intermediate nodes have to process the data.


> That's what I'm thinking will be different in the Pokerstars example than in a SIP-telephone-gateway example.

I agree that the impacts are different - but I'd like to have a solution that works in all cases.

So, if I understand you correctly: you suggest that the codec control information is transported using SDP/a=imageattr "internally" (between app and browser), but then people can send it over the wire however they want?