[SIPForum-discussion] How does the SDP offer and answer works

[Andrea Puddu]

Hi all,

I'm facing a similar issue. The scenario is:
A phone (A) calls another phone (B).
In the INVITE request, A specifies "g729 annex b" as the preferred codec.
CPE forwards the request to B phone with "g729 annex b" as the preferred codec.
Phone B replies with a 200 OK specifing only "g729" as the preferred codec.
What does this mean as the phone supports both g729 a and b?
I think that phone B means that it accepts "g729 annex b", but it only specifies "g729" in its 200 OK answer. I'm afraid that media gateway expected a 200 OK with "g729 annex b" specified, if supported by phone. Otherwise media gateway does not understand the 200 OK answer, maybe because "g729" without annex specified is intended to be "g729a". So the media gateway returns the 200 OK answer to phone A with the second codec in the list (g711).
I don't know if it's mandatory that phone B specifies "g729 annex b" (if supported) instead of "g729".

A                             CPE                                      B

 --------------------->        ---------------------------->
  INVITE (g729b)                  INVITE (g729b)


<---------------------        <----------------------------
   200 OK (g711)                    200 OK (g729)
 



Andrea

On 1/17/08, Mark Holloway <mh at markholloway.com> wrote:



If A is a media gateway and B is a SIP subscriber, usually the gateway will honor the preferred codec of the subscriber.    Thus meaning A is just informing B of what he is capable of, but if B responds with rtpmap: 8 (G729) first and 0 (G711) second in the 200 OK, the call will be setup as G729a (unless B also states annexb=yes, then it will be G729b).  

 

From: discussion-bounces at sipforum.org
 [mailto:discussion-bounces at sipforum.org] On Behalf Of arun kumar
Sent: Thursday, January 17, 2008 2:32 AM

To: discussion at sipforum.org
Subject: [SIPForum-discussion] How does the SDP offer and answer works

 


Hi can any one can guide me in this


 

 

       A---------INVITE/SDP----------> B

 

      The SDP offer  has the following 

                m= audio 49170 RTP/ AVP 0 8

              a= rtpmap: 0 PCMU/8000

             a= rtpmap: 8 PCMA/8000

                   m= video 49172 RTP/AVP 32

                   a= rtpmap: 32 MPV/90000

 

    A<-----------200OK-------B

 

                B also supports all the media and codec types provided in the SDP offer of A

 

So what will be the SDP answer of B, will it send all the medai and codec supported by itself ?

 

my question is who will select the media & codec for the Media session establishment between  A & B, if more than  1 suitable codec matches between the endpoints which codec is selected and on what basis is it done. 


    
v = 0                                               


-- 

With Regards



Arun Kumar .P 

-- 
With Regards
Arun Kumar .P 

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