[asterisk-dev] IAX codec selection

Mihai Balea mihai at hates.ms
Thu Aug 16 10:46:49 CDT 2007

On Aug 16, 2007, at 7:37 AM, Stephen Davies wrote:

> Now I'd like to fix this.  But which is the best way:
> 1) Have the caller rearrange the codec_prefs so that the current codec
> (ilbc) is moved to the top
> 2) Have the callee note the format and use that if its anywhere in the
> allow as first option.
> I think I like (1) better for better backward compatibility, but what
> do people think?

The way I read the spec is that if you have a CODEC PREFS IE in your  
NEW message, it takes precedence over FORMAT and CAPABILITIES.  So in  
your case, the caller should arrange its codec prefs list to reflect  
its actual preferences (ilbc on top) and the callee should ignore the  
format and capabilities IEs and rely on the incoming codec prefs IE  
for its codec negotiation logic.

Generally speaking, IAX2 codec negotiation is somewhat broken.  It  
allows for only one codec to be selected as THE codec for the call.   
This is insufficient when trying to do video, since a video call  
would need two codecs, one for video and one for audio.  Also, I'm  
not sure what would happen if the caller decides to change formats in  
the middle of the call... would it have enough information about the  
callee supported codecs?

There were some discussions at Astridevcon about adopting a better  
way to manage codec negotiation.  I started writing a proposal for  
that but got caught up in other things.  I'm thinking of finishing it  
and posting it up for discussion.


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