3261 Page 36
A request outside of a dialog MUST NOT contain a To tag; the tag in
   the To field of a request identifies the peer of the dialog.  Since
   no dialog is established, no tag is present.
vimal:  which implies that Cancel should have to tag in case dialog is 
already established.


3261 Page 54 last lines

Regardless of the method of the original request, as long as the
   CANCEL matched an existing transaction, the UAS answers the CANCEL
   request itself with a 200 (OK) response.  This response is
   constructed following the procedures described in Section 8.2.6
   noting that the To tag of the response to the CANCEL and the To tag
   in the response to the original request SHOULD be the same.  The
   response to CANCEL is passed to the server transaction for
   transmission.

vimal: this implies even if you dont put to tag in Cancel, it does not 
matter.

vimal: if u look at 3665, the example it has for cancel on invite request 
does not contain to tag.

cheers
v.

From: Banibrata Dutta <[EMAIL PROTECTED]>
To: "Sip-Implementors (E-mail)" <[email protected]>
Subject: [Sip-implementors] CANCEL request in the INVITE initiated Dialog
Date: Thu, 15 Dec 2005 15:02:05 +0530

hi,

a CANCEL sent for an INVITE will in the INVITE initiated Dialog. So should
the CANCEL request have both the From & To tags, or just the From tag ? I
believe that both should be sent, because the full Dialog related
information could be available from the received provisional response (s.a.
182 or 183).

thanks for any clarification.

regards,
bdutta
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