[asterisk-users] SIP Codec negotiation

[Sam Basan]

The unwritten rule of SDP is that if possible you use the first codec of a
type listed, but you don’t have to.  If the sender says he can do
something, he had better be prepared to handle media of that type no matter
in what order it was listed.

So when you send OK with ulaw as first priority and get ACK most probably
it will be ulaw

בתאריך יום ו׳, 11 במאי 2018, 16:08, מאת Steve Edwards ‏<
asterisk.org at sedwards.com>:

> > 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
> used?
>
> 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
>              https://www.linkedin.com/in/steve-edwards-4244281
>
> --
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