[SIPForum-discussion] Need clarification on Re-Invite ..

Tue May 25 04:42:25 UTC 2010

Hi,

I am sharing my views on this topic. I hope this explanation helps. I would appreciate if anyone could correct me if my examples don't fit in this scenario:).

For a moment, let us forget that we have a method in SIP called UPDATE.

1.       Suppose 'A' has called 'B'.
2.       The UA core of B will process the INVITE that it received from A. Let us assume that the 200 OK final response is not yet generated.
3.       In the meantime, A decides to update his codec list (say, he thinks of opening a video chat/stream).
4.       If he sends another INVITE, the UAS of B would be confused. B is still processing the earlier INVITE and it cannot take another INVITE before establishing the initial dialog.
5.       Following references provide the error responses that would be generated when re-invite is sent within a dialog.

                  Reference: RFC 3261, 14.1, UAC behavior for target refresh.

                  Reference: RFC 3261, 14.2, UAS behavior for target refresh.

So, how can one ever update his media capabilities before a dialog is established? Any system that is compliant to RFC 3261 will not allow target refresh within a dialog using re-invite.

Well, we can use UPDATE method rightly for this use case.
Following lines are taken from RFC 3311 - Abstract.

UPDATE allows a client to update parameters of a session (such as the set of media streams and their codecs) but has no impact on the state of a dialog.  In that sense, it is like a re-INVITE, but unlike re-INVITE, it can be sent before the initial INVITE has been completed.  This makes it very useful for   updating session parameters within early dialogs.

If you refer to early media related RFCs (3960, 3959, 3262), you will find that there is a dialog started even before a session is established (i.e, even before a final 2xx response is sent).
Such a dialog which is started even before receiving/sending 200 OK is called early dialog. One cannot change the media parameters of the early dialog with re-invite. It can be done only with UPDATE method for obvious reasons mentioned in my earlier paragraphs.

Kind regards,
Nag.

________________________________
From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of shiba shankar rout
Sent: Monday, May 24, 2010 11:28 PM
To: Ravi Ranjan Sinha
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Need clarification on Re-Invite ..

No Ravi,

Its not like that. UPDATE can also be used after session establishment.   After Session establishment UAC uses UPDATE as Session Refresher.

Best Regards,
S S Rout

On Wed, Apr 28, 2010 at 2:28 PM, Ravi Ranjan Sinha <ravi.sinha at aricent.com<mailto:ravi.sinha at aricent.com>> wrote:
Difference between Update & Re-INVITE:

Update could be used before the session is established (before final response & ACK), while Re-INVIE can be used only after session is established.

Regards,
Ravi SINHA
Société Aricent - Prestataire du Groupe SFR
DGRE / Direction Intégration Validation et Bout-en-Bout/CoreMobile
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ravi-ranjan.sinha.prestataire at sfr.com<mailto:ravi-ranjan.sinha.prestataire at sfr.com>
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________________________________
From: discussion-bounces at sipforum.org<mailto:discussion-bounces at sipforum.org> [discussion-bounces at sipforum.org<mailto:discussion-bounces at sipforum.org>] On Behalf Of Murali Vuppandla [mvuppand at cisco.com<mailto:mvuppand at cisco.com>]

Sent: Monday, April 26, 2010 12:20 PM
To: discussion at sipforum.org<mailto:discussion at sipforum.org>
Subject: [SIPForum-discussion] Need clarification on Re-Invite ..

There are two SIP  subscribers A and B configured with G729 codec.

A calls B. B is ringing. Now, I want to simulate a scenario wherein  A sends Re-Invite to B and B responds accordingly. What actually makes A send Re-invite to B before the session is established(do we need any extra configurations to simulate this scenario). Any real time scenarios in this regard is much helpful.

And what's the exact difference between UPDATE and RE-INVITE. I had read somewhere UPDATE is sent before the session is established whereas Re-Invite is sent after the session is established.

Please clarify me on this.

Regards,
Murali.

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