Subject: [SIP] Join vs. Replace vs. New

From: "Taylor, Tom-PT [NORSE:B901:EXCH]" <[email-masked]>
Date: Mon, 8 Jan 2001 13:31:10 -0500

The issue of how an invitee is expected to interpret an incoming invitation
has come up occasionally, usually in conjunction with a discussion of the
REFER method.  I would maintain that the two topics are actually
independent.

The basic problem is how the receiver of an initial INVITE can distinguish
between
 -- a new incoming call-leg, unrelated to others already in progress
 -- a call-leg intended to replace an existing call-leg
 -- a call-leg intended to join (i.e. operate simultaneously and in
interaction with) an existing call-leg.

We had some discussion of the significance of Call-Id and its possible use
for this purpose at the last meeting, but didn't come to any helpful
conclusion.  Biggs and Dean have defined the Replaces header in
draft-biggs-sip-replaces-00.txt, but this only solves part of the problem.
I suggest that we need a slight generalization of the Replaces header -- say
the Call-Intent: header, with one of its parameters indicating the intention
of the new leg.  Default in the absence of the header would be "new", to
provide equivalent to existing functionality.  The header could be present
in any initial INVITE.  We can think about whether it has uses beyond that,
say when someone wants to merge two independent calls into a single
conference.

Comments?  I'm prepared to do some writing as required to spell this out. 

Tom Taylor
Multimedia Control and Applications Standards
Ph.  +1 613 736 0961
Cell +1 613 290 0844
[email-masked]