[SIPForum-discussion] Query about SDP in SIP.

JEEVANANDHAM KARTHIC KUMAR Karthic_Kumar.Jeevanandham at alcatel-lucent.com
Tue Dec 8 06:38:15 UTC 2009

Hello Sriram,
According to the RFC3261, In this case(ie initial INVITE with SDP offer
and 180 with SDP1),  200OK(INVITE) with SDP1 has to ignore by UAC. In
other words, if UAS responds 180 ringing with SDP answer. further
(provisional or final)response has to place same SDP answer.
 I can understand this from follwoing details in RFC3261 13.2.1 Creating
the Initial INVITE
      o  If the initial offer is in an INVITE, the answer MUST be in a
         reliable non-failure message from UAS back to UAC which is
         correlated to that INVITE.  For this specification, that is
         only the final 2xx response to that INVITE.  That same exact
         answer MAY also be placed in any provisional responses sent
         prior to the answer.  The UAC MUST treat the first session
         description it receives as the answer, and MUST ignore any
         session descriptions in subsequent responses to the initial
      o  Once the UAS has sent or received an answer to the initial
         offer, it MUST NOT generate subsequent offers in any responses
         to the initial INVITE.  This means that a UAS based on this
         specification alone can never generate subsequent offers until
         completion of the initial transaction.
Karthic kumar J


From: Sriram Subramanian [mailto:sriram.ngn at gmail.com] 
Sent: Tuesday, December 08, 2009 10:39 AM
Cc: lakhan patel; discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Query about SDP in SIP.

Hi Karthic ,
         I would slighlty defer from your answer which u have mentioned
below .Its basically an offer answer model and in your case explained u
had mentioned INVITE with SDP which is an offer and 180 with SDP which
is an answer then in 200 OK if u have another SDP it implies that this
is now a new offer and not an answer as there was no offer further and u
can reply to it with ACK request.Pls refer to the links provided 

On Mon, Dec 7, 2009 at 10:15 AM, JEEVANANDHAM KARTHIC KUMAR
<Karthic_Kumar.Jeevanandham at alcatel-lucent.com> wrote:

	Hi Shivlakhan Patel,
	Please ref the RFC3311.
	find my answer inline.
	Karthic kumar J 

	From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of lakhan patel
	Sent: Friday, December 04, 2009 2:37 PM
	To: discussion at sipforum.org
	Subject: [SIPForum-discussion] Query about SDP in SIP.
	Hi Guys,
	Can any one explain me the Offer-Answer in SIP.
	1) Initial Invite with Initial-SDP            (Is it Offer? I
guess YES). 
	=> YES  

	2) 180 with New-SDP and Require: 100Rel            (Is it new
Offer or Answer) 
	=> It is Answer  

	            1. In this I am sending New-SDP1 only with new
Version and Session-ID and one A-line and M-line from the Initial-SDP.
	            2. In PRACK without SDP.
	            3. In 200 Ok for invite with New-SDP1.
(Is it Answer?) 
	=>It is also Answer  

	3) Now UPDATE from A-side to B-side with New-SDP2 and again only
with new Version and Session-ID and one A-line and M-line from the
Initial-SDP.             (What should trigger at B-side). 
	=>200OK for UPDATE with SDP-2 Answer.  

	Can any one [provide me some good reference so that i can read
all these from there?
	 => Ref RFC3262 and RFC3311  

	Thanks & Regards
	Shivlakhan Patel
	Email: lakhan.p at gmail.com, lakhan.p at hotmail.com 
	IBM India Private Ltd. Bangalore
	Contact: +91-9902791177

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