Subject: RE: [SIP] transfer call flows

From: "Elwell, John" <[email-masked]>
Date: Wed, 18 Apr 2001 13:34:29 +0100

Mo,
 
I have been out of the office, which accounts for my lateness in responding.
From the responses and other documents there would seem to be a number of
proposals:
 
1. Accept-Contact (mentioned by Tom Taylor and Alan Johnstone, included in
Service-Examples by Robert Sparks).
 
2. Replaces header. Mentioned by Alan Johnstone and used in flows from Rohan
Mahy.
 
3. Refer-to with preloaded route (mentioned by Brett Tate).
 
4. Refer with all required headers (mentioned by Brett Tate). I don't know
exactly what this is, but it sounds similar to the Replaces header.
 
One observation I would make is that in addition to making sure that a new
INVITE in a Call Transfer gets to the correct UA-C, it is also necessary to
ensure that the INVITE from B to C links up with/replaces the existing call
from A to C. The Replaces header looks a good candidate for doing this, as
well as solving your original problem.

------------------------------------------------------------- 
John Elwell ([email-masked]) 
Siemens Communications Limited, 
Technology Drive, Beeston, 
Nottingham NG9 1LA, UK 
Tel: +44 115 943 4989   Fax: +44 115 943 4969 
-------------------------------------------------------------- 

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-----Original Message-----
From: Mo Zonoun [mailto:[email-masked]]
Sent: 09 April 2001 22:00
To: [email-masked]
Subject: RE: [SIP] transfer call flows

I posted the following question while back but I didn't get any response. 
How do you make sure that a new INVITE in a Call Transfer gets to the
correct UA-C when there are multiple UA candidates, including the case where
the UA-C is a gateway to another network.

Mo Zonoun 

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---

From: "Alex Hardisty" <[email-masked]>
Date: Wed, 18 Apr 2001 15:42:52 +0100

John Elwell wrote:

> ...
> INVITE in a Call Transfer gets to the correct UA-C, it is also
> necessary to ensure that the INVITE from B to C links up with/replaces the
> existing call from A to C. The Replaces header looks a good candidate for
> doing this, as well as solving your original problem.

What if UA-C is a set of geographically dispersed gateways to another
network? Consider the following scenario:

The Transferring and the Transferred Users are in the SIP network and the
Transferred-to User (represented by UA-C1) is in the other network. The
shortest route between the Transferred User and the Transferred-to User may
be via a different gateway (another instance of UA-C, UA-C2) than the
session (secondary call) between the Transferring User and the
Transferred-to User. In this case, how does UA-C1 know that the re-routed
call has been successful via UA-C2 and that the replaced original call can
now be cleared? I think this can only be achieved at the Transferred-to User
in the other network.
It would involve the use of a "correlation identity", obtained from the
Transferred-to User prior to REFER and carried in the re-routed call so that
on receipt the Transferred-To User could correlate the two calls and
initiate clearing of the original call.

Regards
--
Alex Hardisty
PQM Consultants

---

From: Joanne McMillen <[email-masked]>
Date: Wed, 18 Apr 2001 20:15:04 -0600

> network? Consider the following scenario:

	Good point and I am happy to see discussion moving to this
	level.

Joanne McMillen 	
Avaya Inc.