Bug with iLBC mode negotiation (thanks Michael Smith)

[bennylp]

Michael Smith wrote:

> Hi,
> 
> Asterisk uses a fixed iLBC packetization of 30 ms. pjsua defaults to 20 ms.
> I can get the two to communicate if I tell pjsua to use 30 ms.
> 
>
> If I don't, pjsua encodes at 30 ms and decodes at 20 ms, even though 
> Asterisk thinks both directions are using 30 ms. This makes for some 
> interesting sounds. :)
> 
>
> According to the standard, if one side requests 30 ms, the other is 
> supposed to use it because it's the lowest bandwidth setting. (It's 
> pretty bizarre - even if the initiator offers mode=30, and the responder 
> replies with mode=20, they are both supposed to use mode=30.)
> 
>
> I found some notes about this at
> http://www.pjsip.org/trac/wiki/audio-check-codec-nego and
> http://www.pjsip.org/trac/ticket/40 but what the standard says is 
> different:
> 
>
>    It is important to emphasize the bi-directional character of the
>    "mode" parameter - both sides of a bi-directional session MUST use
>    the same "mode" value.
> 
>    The offer contains the preferred mode of the offerer.  The answerer
>    may agree to that mode by including the same mode in the answer, or
>    may include a different mode.  The resulting mode used by both
>    parties SHALL be the lower of the bandwidth modes in the offer and
>    answer.
> 
>
>    That is, an offer of "mode=20" receiving an answer of "mode=30" will
>    result in "mode=30" being used by both participants.  Similarly, an
>    offer of "mode=30" and an answer of "mode=20" will result in
>    "mode=30" being used by both participants.
> 
>
> - http://www.ietf.org/rfc/rfc3952.txt