[SIPForum-discussion] Offer-Answer SDP

Garron, James jgarron at sonusnet.com
Mon Aug 22 13:54:53 UTC 2011

This is a fairly common scenario, and the end result should end up using
G729 as both UAC and UAS list G729 as the preferred codec.  If there is
no direction attribute (e.g. a=inactive/recvonly/sendrecv) then it is
implied sendrecv.  /Jim



From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of call foreign
Sent: Friday, August 19, 2011 5:17 AM
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.


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