[SIPForum-discussion] Codecs in 200 OK response

[Stephen James]

Multiple codecs in the answer means that the answerer is willing to accept any of the answered codecs during the call without renegotiation.
Many (most) devices can only support a single codec and answer with a single codec. 
Larger more powerful gateways can support multiple codecs at the same time.

In our system we have a control that forces the return of a single codec in the answer.

RFC3264 is reference that indicates multiple codecs may be present in both the offer and the answer.


 
Stephen James 
sjames_1958 at yahoo.com
 
We are not princes of the earth, we are the descendants of worms, and any nobility must be earned.



________________________________
 From: "Mirchandani, Pushpa" <Pushpa.Mirchandani at charter.com>
To: Pramod Kumar <pramod.style at gmail.com>; Waqas Arshad <waqas.arshad000 at yahoo.com> 
Cc: "discussion at sipforum.org" <discussion at sipforum.org> 
Sent: Wednesday, October 9, 2013 3:53 PM
Subject: Re: [SIPForum-discussion] Codecs in 200 OK response
 


The question is why  is there the : m=audio 18238 RTP/AVP 0 8 18 101 in the 200OK? 
 
If the INVITE sends multiple codecs shouldn’t the 200OK in the SDP answer just send one that  it is accepting  for negotiation? Otherwise,  Sounds like either the sender or the answerer can receive more than one codec during the call.  So finally who makes the decision as to which codec to use for the call?  My understanding is INVITE can send multiple codecs and the answerer accepts only one.  
 
Thanks 
 
Pushpa
 
 
From:discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Pramod Kumar
Sent: Monday, October 07, 2013 11:29 PM
To: Waqas Arshad
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Codecs in 200 OK response
 
Hi Waqas,
 
Yes, 200 OK can have more than one codec in SDP answer. It is based to support the codec negotiation functionality. However it is implementation dependent how to select the final codec like select first codec from the SDP answer for that call.
 
Regards,
Pramod
 
 
On Mon, Oct 7, 2013 at 1:09 PM, Waqas Arshad <waqas.arshad000 at yahoo.com> wrote:
Hi,
 
Can someone please confirm if the SDP of 200 OK (in response to an INVITE) can contain multiple codecs in m-line or it should have only one codec at which B party has responded to establish the connection. e.g.
 
INVITE SDP:
m=audio 39838 RTP/AVP 0 8 18 101
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=sendrecv
 
200 OK SDP:
m=audio 18238 RTP/AVP 0 101
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=X-sqn: 0
a=X-cap: 1 audio RTP/AVP 100
a=X-cpar: a=rtpmap:100 X-NSE/8000
a=X-cpar: a=fmtp:100 200-202
a=X-cap: 2 image udptl t38 
 
So can 200 OK SDP contain m-line as below too or just one codec at which call is going to be established???
 
m=audio 18238 RTP/AVP 0 8 18 101
 
Secondly, can someone please suggest the meaning of a=X-sqn in above 200OK?
 
Thanks in advance.
Waqas

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