[SIPForum-discussion] Codecs in 200 OK response

Mirchandani, Pushpa Pushpa.Mirchandani at charter.com
Wed Oct 9 21:07:44 UTC 2013

So here is the answer per RFC 3264 (http://www.ietf.org/rfc/rfc3264.txt) link forwarded by Ayur in one of the responses.

For each "m=" line in the offer, there MUST be a corresponding "m="
   line in the answer.  The answer MUST contain exactly the same number
   of "m=" lines as the offer.  This allows for streams to be matched up
   based on their order.  This implies that if the offer contained zero
   "m=" lines, the answer MUST contain zero "m=" lines.

The actual codec negotiation  is shown in the " a " attribute in the 200OK SDP.   The " m"  attribute in 200OK  is just a response to the offer (INVITE)  per the above para.

I hope this helps.  Thanks


From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of pradeep gali
Sent: Monday, October 07, 2013 1:12 PM
To: Waqas Arshad
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Codecs in 200 OK response

If the INVITE is sending "m=audio 39838 RTP/AVP 0 8 18 101" , the 200 OK SDP is expected to be with any one Codec like "m=audio 39838 RTP/AVP 0 101.

On Mon, Oct 7, 2013 at 2:39 AM, 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
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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Pradeep Gali
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