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

Fri Jan 18 10:27:03 UTC 2008

Sorry,

I was wrong .... the SDP of the INVITE to B phone looks like:

......

m=audio 19386 RTP/AVP 18 8 0

a=rtpmap:18 G729/8000

a=fmtp:18 annexb=no

a=rtpmap:8 PCMA/8000

a=rtpmap:0 PCMU/8000

and the SDP of the 200 OK reply by B phone is:

..... 

m=audio 41000 RTP/AVP 18

a=rtpmap:18 G729/8000

a=sendrecv

As you can see the fmtp line is missing, even if B phone supports both
G729 a and b. It seems that CPE "suffers" the lack of the fmtp line in
the 200 OK. If the phone does not specify if it supports the annex in
the 200 OK, the CPE understand that the phone does not support that
codec! So it switches to the second codec in the list (g711).

But is it mandatory for the phone to specify the fmtp line in the 200
OK reply, if the fmtp line is specified in the related INVITE?

Thanks.

From: mh at markholloway.com
To: androjoker at hotmail.com
CC: discussion at sipforum.org
Subject: RE: [SIPForum-discussion] How does the SDP offer and answer works
Date: Fri, 18 Jan 2008 03:08:36 -0700

What is the make/model of SIP Endpoint B?    A
proper format for G729b would look like this:

Media Description, name and address (m): audio <PORT> RTP/AVP
18

Media Attribute (a): rtpmap:18 G729/8000

Media Attribute (a): fmtp:18 annexb=yes

I’ve never seen a SIP Endpoint list “g729 annex b”
in the SDP.   It looks like B is sending an invalid SDP to A for G729
which is why it falls back to G711.

From:
discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On
Behalf Of Andrea Puddu

Sent: Friday, January 18, 2008 1:47 AM

Cc: discussion at sipforum.org

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

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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