[Asterisk-Dev] IAX2 protocol question

[Steve Kann]

Tim Robbins wrote:

> Matt Riddell wrote:
>
>> Tim Robbins wrote:
>>
>>>  From reading the Asterisk sources, it looks as if the FORMAT 
>>> information element, when sent as part of an ACCEPT frame, lets the 
>>> calling IAX endpoint know which formats are supported by both 
>>> parties. However, libiax2 (and therefore the majority of IAX 
>>> softphones) does not send a FORMAT IE as part of the ACCEPT. In this 
>>> situation, what is the "right" way to determine which format voice 
>>> frames should be sent in?
>>>
>>> Thanks for any suggestions.
>>
>>
>>
>> The iaxclient libraries have just recently had multiple codecs added. 
>> You might want to check that you have the latest version.
>
>
> I was actually more interested in how codec negotiation is supposed to 
> work in the IAX2 protocol itself (and specifically, what an endpoint 
> should do if the ACCEPT frame is missing a FORMAT IE), not any 
> particular implementation.

That's a good question.  Here's what I just wrote to iaxclient-devel:


1) The caller sends both format and capabilities when it makes a new call.
2) The remote side chooses a format, based on it's own capabilities, 
preferences, and the caller's capabilities and preferences.
3) The remote side ACCEPTs the call, and sends back the format to use.

So, this is what I implemented in iaxclient and libiax2.

In step2, asterisk seems to do this:

1) Use the caller's preferred format, if it supports it,
2) Choose amongst all capabilities, using a hard-coded order of preference.

In step2, in iaxclient, I implemented it as:
1) Use the caller's preferred format, if we support it,
2) Use _our_ preferred format, if they support it,
3) Finally, look at the union of capabilities, and (the same) hard-coded 
order.