[SIPForum-discussion] Offer-Answer SDP

Vivek Batra Vivek.batra at matrixcomsec.com
Sat Aug 20 04:25:28 UTC 2011



This seems simple scenario you might found in various softphones. 

According to me, both UAC and UAS agrees on multiple codecs for a session
and hence no priority applies here.

Anyone (UAC or UAS) can choose the codec randomly viz G.729 or G.723 as RTP
is bidirectional. Moreover, codec can be changed during RTP session without
further SDP negotiation. 

In case, your hardware is not capable to switch between the codec's during
runtime, best method could be to again negotiate SDP by sending single codec
(either G.729 or G.723 based on your local preference) in SDP offer.


Best Regards,

Vivek Batra  


From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of call foreign
Sent: Friday, August 19, 2011 2:47 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Offer-Answer SDP



I need ideas and explanation on this call scenario;

Origination UA sent multiple codecs;

   o=alice 2890844526 2890844526 IN IP4 host.anywhere.com
   c=IN IP4 host.anywhere.com
   t=0 0
   m=audio 62986 RTP/AVP 18 4
   a=rtpmap:18 G729/8000
   a=rtpmap:4 G723/8000

Termination UA respond with multiple codecs;

   o=bob 2890844730 2890844731 IN IP4 host.example.com
   c=IN IP4 host.example.com
   t=0 0
   m=audio 54344 RTP/AVP 18 4
    a=rtpmap:18 G729/8000
   a=rtpmap:4 G723/8000


What would be the actual media on the session considering that the
OFFER-ANSWER has been completed on the signaling with an INVITE-200OK-ACK
messages.  Please note the origination has never included attribute
"a=inactive" on the initial offer.  In addition, termination side responded
with attribute "a=sendrecv" immediately without an update offer from the
origination on which codec it choose for the final media stream.

Does this imply that the actual media stream been used on the session is
G729 if codec preference is considered?

Please share if you have encountered this scenario.


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sipforum.org/pipermail/discussion/attachments/20110820/27b9fcdf/attachment-0002.html>

More information about the discussion mailing list