[SIPForum-discussion] regarding tag parameter in From header

[Nagendrababu Maseedu]

Hi Nishant,

            I am sharing my knowledge on tags that was gathered from Jan Janak and RFC 3261.

One use case where tags are very important to have is in Forking scenarios.

When a caller sends an INVITE to a callee, the call can land on his home phone, office phone and mobile at the same time(Known as Parallel Forking).
What will happen if all the three phones of the callee are picked up? Caller would receive three 200 OK responses from three the same callee. Isn't it?
In this case, how would the caller uniquely identify the callee when the call-id , From ID and To ID remain the same?
This is the reason why a globally unique tag is added to the second half of the From and To headers.


>From tag is generated by the caller and it uniquely identifies the dialog in the caller's user agent.
To tag is generated by a callee and it uniquely identifies, just like From tag, the dialog in the callee's user agent.
This hierarchical dialog identifier is necessary because a single call invitation can create several dialogs and caller must be able to distinguish them.


               Please also refer to RC 3261 - Sec 19.3 Tags



Kind regards,

Nag.


________________________________
From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Vijay Tiwari
Sent: Thursday, May 06, 2010 1:15 PM
To: Roy, Nishant Kishore
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] regarding tag parameter in From header

Hello Nishant


1)      What is the uses of From tag in from header .
A dialog is identified by a call identifier, local tag, and a remote tag. and From tag is local tag by the definition of Dialog.

2)      Is this mandatory in the initial INVITE message ? If not in which case it will be optional ?

it is a mandatory in initial invite, if UAC  compliance of RFC 3261.

Thanks
vijay

On Wed, May 5, 2010 at 2:23 PM, Roy, Nishant Kishore <nishantkr at hp.com<mailto:nishantkr at hp.com>> wrote:
Hi All ,

Can anyone help me out with following question :


1)      What is the uses of From tag in from header .
AFAIK : it will be used in dialog creation which consist of tag from TO header and FROM header along with Call-ID , what else ?


2)      Is this mandatory in the initial INVITE message ? If not in which case it will be optional ?

Regards
Nishant


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