[SIPForum-discussion] Offer-Answer SDP

Mudugere, Satish satish.mudugere at intel.com
Fri Aug 19 18:57:32 UTC 2011

As per RFC3264 (sec 5.1), absense of a= means it is sendrecv mode.

If the offerer wishes to both send and
   receive media with its peer, it MAY include an "a=sendrecv"
   attribute, or it MAY omit it, since sendrecv is the default.

You need to pick the codec in listed preferred order. Accordingly the call is using G729 codec.


From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of call foreign
Sent: Friday, August 19, 2011 3:17 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<http://host.anywhere.com>
   c=IN IP4 host.anywhere.com<http://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<http://host.example.com>
   c=IN IP4 host.example.com<http://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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