[Asterisk-video] Video codec negotiation in IAX

[Mihai Balea]

Hello all,

As you may already know, I am working on adding video support to  
iaxclient.

It appears however that IAX's support for video codec negotiation is  
lacking.  I am no IAX expert, but from what I can see, there are to  
ways of doing codec negotiation in IAX:
- use the FORMAT and CAPABILITY IEs.  This allows you to specify one  
preferred codec and a list of acceptable codecs. This is the way  
iaxclient does codec negotiation at this time.
- use the CODEC PREFS IE.  This allows you to specify a list of  
acceptable codecs, in order of preference.

The problem with video is that you need to negotiate two codecs at  
the same time (one for video and one for audio). Unfortunately, none  
of the above mentioned methods are suitable for this.  The way I see  
it, there are several ways of addressing this:
1. use a VIDEO FORMAT IE, in addition to FORMAT and CAPABILITY.  The  
CAPABILITY IE will contain acceptable video codecs as well as audio.
2. use a VIDEO CODEC PREFS IE in addition to the current CODEC PREFS  
IE.  This will more flexible since it allows for video codec  
preference order.
3. use the current FORMAT and CAPABILITY and change the semantics of  
FORMAT to allow a video codec as well as an audio codec. This might  
be easiest to implement.

As a long term solution, I'm inclined to go with #2 since it provides  
the maximum amount of flexibility.  However I would like to hear any  
suggestions you might have regarding this issue...

Thanks in advance,
Mihai