[Iaxclient-devel] [Asterisk-video] Re: Re: Video codec negotiation in IAX

[Steve Kann]

On Oct 25, 2006, at 10:44 AM, Mihai Balea wrote:

> The main reason for this is that RTP packets carry media type
> information in the header.
> In IAX, we'll probably have to use a full frame for this (mini frames
> have only minimal information in their 4-byte headers).
> It would be doable, but it will probably require some changes in the
> way codecs are negotiated.
> Something to think about....

That's the way IAX works now for audio -- a full frame is sent  
explicitly if the codec changes..

 From iax.c in libiax2, for example:
       if (((fts & 0xFFFF0000L) == (lastsent & 0xFFFF0000L))
                 /* High two bits are the same on timestamp, or  
sending on a trunk */ &&
             (f->frametype == AST_FRAME_VOICE)
                 /* is a voice frame */ &&
                 (f->subclass == pvt->svoiceformat)
                 /* is the same type */ ) {
                         /* Force immediate rather than delayed  
transmission */
                         now = 1;
                         /* Mark that mini-style frame is appropriate */
                         sendmini = 1;
         }

sendmini is only set on all these conditions, one of which is that  
the format is the same.


-SteveK


>
> Mihai
>
>
> On Oct 25, 2006, at 4:26 AM, Olle E Johansson wrote:
>
>> The interesting part in all of this is that Asterisk is able to
>> negotiate a SIP call with
>> multiple codecs, and change codec mid-call without re-negotation.
>> The RTP stack
>> will just notify that we changed codec and the translators seems to
>> be in place
>> somehow. I have no idea on the inner workings, but it works.
>>
>> In that case, no re-invite is needed, since it was already  
>> negotiated.
>>
>> /O
>>
>
>
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