Subject: [Sip] Replaces of early dialog

From: "Daniel G. Petrie" <[email-masked]>
Date: Wed, 27 Nov 2002 00:40:39 -0500

Last week in Atlanta we had a sidebar discussion of the use of
the Replaces header with early dialogs (draft-ietf-sip-replaces-02.txt).

There is a problem with using Replaces on the UAS sided of an
early dialog primarly due to forking.  In a forking situation there
are race conditions where either
   1) the To tag is explcitly specified the INVITE with Replaces
      can lose the race to UAS before the branch is canceled and
      serially forked by the proxy.
   2) if a wild card mechanism is used for the To tag you still have
     a race between INVITE with Replaces hitting the first UAS with
     no matching dialog and that of UAS where the dialog matches.

The net result is that is is very hard to get predictable results if
forking
occurs.  It seems equally difficult to determine that forking is not
used
in a particular environement.

So the choices seem to be:
   1) never allow Replaces with early dialogs
   2) allow Replaces with early dialog only on the UAC
   3) allow Replaces with early dialog on UAC and UAS with
      restricted environments.

The best choice seems to be to allow replaces with early dialogs
only on the UAC.  This still provides the ability to do "Pick up"
type services by replacing the dialog on the UAC.

This means that we loose the ability for the caller to move to
a different UA (e.g. administrator initiatates a call for an
executive or caller want to switch to a different phone down
the hall while the other end is ringing).  This feature could perhaps
be provided if the caller's UA uses 3PCC performing like a B2BUA
until the call is setup at which time it can use REFER with Replaces.

With respect to draft-ietf-sip-replaces-02.txt, section 7.1 gets removed

and replaced with a statement that early dialogs cannot be Replaced
on the UAS side.

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