[asterisk-users] SIP Codec negotiation

Steve Edwards asterisk.org at sedwards.com
Fri May 11 08:07:28 CDT 2018

> On Thu, May 10, 2018 at 11:44:14AM -0700, Steve Edwards wrote:

>> I receive an INVITE/SDP containing:
>> 	m=audio 11310 RTP/AVP 3 0 101
>> which I interpret as gsm, ulaw, rfc2833.
>> and I reply with an OK/SDP containing:
>> 	m=audio 15884 RTP/AVP 0 3 101
>> which I interpret as ulaw, gsm, rfc2833.
>> How can I tell which codec was actually used for the call?

On Fri, 11 May 2018, Daniel Tryba wrote:

> AFAIK this is undetermined. The callee can send either ulaw or gsm,
> unless the caller wants to narrow it down to 1 codec, see
> https://tools.ietf.org/html/rfc4317#section-2.2
> Most of the time the callee will pick the first (so in this case ulaw).
> But there are media gateways out there that choose g711[au] above "more
> complex" codecs regardless order in SDP. My prefer PSTN provider will
> always prefer alaw if offered since that will prevent transcoding on
> their side if the call goes to ISDN/POTS, but AMR if the call goes to
> VoLTE.

So, without examining the RTP, you cannot tell which codec was actually 

In the above example, even though the INVITE/SDP says they prefer gsm over 
ulaw and the OK/SDP says I prefer ulaw over gsm, they can choose to use 
gsm or ulaw?

Can it be asymmetrical? They send gsm and I send ulaw?

Thanks in advance,
Steve Edwards       sedwards at sedwards.com      Voice: +1-760-468-3867 PST

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