On 3/6/12 3:18 AM, Robert Jongbloed wrote:
> It can, but it is, IMHO, a complete pain!
>
>
>
> The specification actually says that the selection of codec is not really
> completed until the RTP packets arrive.
>
> Actually, a strict interpretation (I think) of the specification means at
> any time during the session it could send a frame of H.263, then a frame of
> H.264, then a frame of H.261! But that vanishingly unlikely to work in
> practice and no one would be insane enough to do it. J
But its not as unreasonable as you might think.
Once case - any audio codec + telephone-events as a 2nd codec. Frames of
these are then intermixed.
Another is when multiple RTP streams are multiplexed on the same RTP
session - distinguished by SSRC. (I may have the terminology wrong
there.) This hasn't been considered much in sip until recently, but is
now increasingly going to be used.
A robust implementation should be prepared to handle all of this, and do
something reasonable. (But I realize there are a lot of widely used
implementations that are not prepared for this.)
Thanks,
Paul
> I know there are some implementations that do exactly this for audio. The
> OPAL implementation unfortunately is not designed to wait till the first RTP
> packet arrives before creating codecs, starting video grabbers, blah, blah,
> blah. It does it when the 200 OK arrives. So, OPAL gets around the ambiguity
> by immediately sending a re-INVITE selecting one of the codecs the remote
> answered with. We end up with:
>
>
>
> INVITE H.264, H.263, H.261
>
> 200 OK H.264, H.263
>
> Re-INVITE H.264
>
> 200 OK H.264
>
>
>
>
>
> Robert Jongbloed
>
> OPAL/OpenH323/PTLib Architect and Co-founder.
>
>
>
> From: Vineet Menon [mailto:[email protected]]
> Sent: Tuesday, 6 March 2012 6:20 PM
> To: [email protected]; opalvoip-devel
> Subject: [Opalvoip-devel] Capability negotiation control
>
>
>
> Hi,
>
>
>
> Say UAC sends H.261, 263,264 in offer, and now UAS wants to select one among
> them. UAS supports all three.
>
>
>
> Now, can UAS send all three codec it supports or it sends only the one
> preferred by it in the response?
>
>
>
>
>
> UAC -------------------------INVITE/H.261,H.263, H.264------------------->
> UAS
>
> UAC<-----------------------200 OK/
> ???????------------------------------------ UAS
>
>
>
>
> Regards,
>
> Vineet Menon
>
>
>
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