[SIPForum-discussion] Codecs in 200 OK response

Mirchandani, Pushpa Pushpa.Mirchandani at charter.com
Wed Oct 9 20:33:23 UTC 2013

Would you use primary and secondary choice? For example always use G.711 first ? Thanks


From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Stephen James
Sent: Monday, October 07, 2013 1:15 PM
To: Waqas Arshad; discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Codecs in 200 OK response

Yes the reponse to an INVITE can contain more than one codec. This means that the answerer can receive more than one codec type during the call.
In our system we implemented specific behavior to force our system to send only 1 codec in the answer.

Stephen James
sjames_1958 at yahoo.com<mailto: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: Waqas Arshad <waqas.arshad000 at yahoo.com<mailto:waqas.arshad000 at yahoo.com>>
To: "discussion at sipforum.org<mailto:discussion at sipforum.org>" <discussion at sipforum.org<mailto:discussion at sipforum.org>>
Sent: Monday, October 7, 2013 2:39 AM
Subject: [SIPForum-discussion] Codecs in 200 OK response


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.

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

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.

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