[SIPForum-discussion] Codecs in 200 OK response

Mirchandani, Pushpa Pushpa.Mirchandani at charter.com
Wed Oct 9 20:53:11 UTC 2013

The question is why  is there the : m=audio 18238 RTP/AVP 0 8 18 101<tel:0%208%2018%20101> 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.



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.


On Mon, Oct 7, 2013 at 1:09 PM, Waqas Arshad <waqas.arshad000 at yahoo.com<mailto:waqas.arshad000 at yahoo.com>> wrote:

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<tel:0%208%2018%20101>
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<tel:0%208%2018%20101>

Secondly, can someone please suggest the meaning of a=X-sqn in above 200OK?

Thanks in advance.

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