Hi Arun,
If you add To-Tag in INVITE, then It will be a re-invite. If the dialog is already established then UAC can send invite with to-tag.
UAS will generate to tag while sending response(18x/2xx) for INVITE.
1. No to-Tag in INVITE WHY?
Here your question is like. Why there is no to-tag in Initial INVITE request?
The answer will be, UAC will responsible for local tag(from tag). Then remote tag(To-Tag) will be generated by user agent server(UAS).
Note: the same answer Amit also mentioned clearly with RFC.
Thanks,
Pradeep
> Date: Tue, 5 Aug 2014 13:13:59 +0530
> From: arun.tagare5 at gmail.com
> To: Amit2.Singh at aricent.com
> CC: sip-implementors at lists.cs.columbia.edu
> Subject: Re: [Sip-implementors] : No To-Tag in INVITE why?
>
> Hi Amit,
>
> Thank you for your Response
> Thanks for explaining the dot i was having with respect to the To-Tag in
> the INVITE request.
>
> Thank you,
> With Regards
> Arun A. Tagare
>
>
> On Tue, Aug 5, 2014 at 12:25 PM, Amit Kumar Singh <Amit2.Singh at aricent.com>
> wrote:
>
> > Hi,
> >
> > Section 19.3 of RFC 3261 states the following:
> >
> > " The tag parameter is used in the To and From header fields of SIP
> > messages. It serves as a general mechanism to identify a dialog, which is
> > the combination of the Call-ID along with two tags, one from each
> > participant in the dialog. When a UA sends a request outside of a dialog,
> > it contains a From tag only, providing "half" of the dialog ID. The dialog
> > is completed from the response(s), each of which contributes the second
> > half in the To header field. The forking of SIP requests means that
> > multiple dialogs can be established from a single request. This also
> > explains the need for the two-sided dialog identifier; without a
> > contribution from the recipients, the originator could not disambiguate the
> > multiple dialogs established from a single request."
> >
> > As the RFC clearly states that the dialog is completed from the responses
> > thus it should be the UAS to generate the To Tag in the response not the
> > UAC. In the case of Forking of single SIP request, to avoid the ambiguity
> > in multiple dialogs contribution from the recipients (UAS), in our case To
> > Tag, is necessary.
> >
> > Regards,
> > Amit Kumar Singh
> > amit2.singh at aricent.com
> >
> >
> >
> > -----Original Message-----
> > From: sip-implementors-bounces at lists.cs.columbia.edu [mailto:
> > sip-implementors-bounces at lists.cs.columbia.edu] On Behalf Of Arun Tagare
> > Sent: Tuesday, August 05, 2014 11:45 AM
> > To: sip-implementors at lists.cs.columbia.edu
> > Subject: [Sip-implementors] : No To-Tag in INVITE why?
> >
> > Hi All,
> >
> > Can any one explain me
> > Why there is no To-Tag in the INVITE Request?
> >
> > And what happens if i add To-Tag in the INVITE Request?
> >
> > --
> >
> > With Regards
> >
> > Arun A. Tagare
> > +91 9449 029729
> > _______________________________________________
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>
>
> --
>
> With Regards
>
> Arun A. Tagare
> +91 9449 029729
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