Re: [rtcweb] Signalling, SDP, and the way we think about interconnecting RTCWEB applications

Neil Stratford <> Sat, 15 October 2011 14:10 UTC

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Date: Sat, 15 Oct 2011 15:10:33 +0100
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From: Neil Stratford <>
To: Harald Alvestrand <>
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Subject: Re: [rtcweb] Signalling, SDP, and the way we think about interconnecting RTCWEB applications
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On Sat, Oct 15, 2011 at 2:44 PM, Harald Alvestrand <>wrote:

> **
> On 10/15/2011 02:08 PM, Neil Stratford wrote:
> On Sat, Oct 15, 2011 at 12:29 PM, Harald Alvestrand <>wrote:
>>>  Remember that:
>> a) codecs are (currently) part of the platform, not of the application.
>> b) one of our aims for negotiation is that if application X is deployed in
>> browsers A and B, and both browser A and B support a "best codec" W that was
>> created *after* X was last updated, then the codec W should be used.
>> Given the last point - the app *cannot* "implicitly" what codec will be
>> used. And given that it doesn't know the codec, it can't know the parameters
>> either.
>> (that said... I'm all in favour of fewer parameters. The RTP format for
>> VP8 that we're in the process of finishing has zero parameters. I hope it
>> will remain that way.
>  Can we work around this issue by delegating parameter negotiation to the
> codec itself? Each codec in the codec capability list could present (at a
> minimum) it's name, priority and a codec specific parameter negotiation
> function. We can then delegate the complicated codec specific details (when
> necessary) without needing any advance knowledge in the javascript
> application. We would need to standardise the parameter lists such that
> different codec implementations can interop, but that is the same with SDP.
> Yes, maybe we could do that. But would it make a difference?
> SDP is (among other things) exactly such a parameter list standardisation,
> each and every codec deployed with RTP tends to publish a specification on
> how to encode those parameter lists in SDP, and each and every video engine
> implementing those codecs tends to have code in it that parses that SDP and
> does the negotiation on that basis.
> Are we reinventing another encoding format for SDP again?

I guess there are a couple of points here:

1. Yes, we are re-inventing SDP, but at a lower level, so we leave it up to
the developer to use SDP, or something else if they prefer.

2. I fully support that we need to interop with SIP, and that we should make
that a simple thing to do. However, I also think that rtcweb will rapidly
become the dominant RTC platform due to the pervasiveness of browsers. Codec
developers will be targeting rtcweb as a primary platform, especially if we
make it easy to download and deploy new codecs.