Subject: Re: [Sipping] Call Pickup Flow

From: "Deepak Mohan" <[email-masked]>
Date: 24 Jul 2001 08:32:19 -0000

Hi.

I don't think the REGISTER message really reaches the Proxy. The REGISTER message reaches the local registrar ( pickupgroup37.ss1.wcom.com ), causing a database update. This update is sensed by the proxy(ss1.wcom.com) by some mechanism (outside SIP) causing a new INVITE to be sent to B2.
 Hence, I don't think additional parameters have to be added to the SIP message ("feature=pickup"). However, changes could be made to the flow to include the registrar, and the registrar-proxy interactions (outside SIP). 

Regards,
Deepak

------------- Original Message --------------
Alan Johnston <[email-masked]> wrote:
To:SIPPING List <[email-masked]>
From:Alan Johnston <[email-masked]>
Date:Mon, 23 Jul 2001 22:21:42 -0500
Subject:[Sipping] Call Pickup Flow

All,

I think there is a problem in the directed Call Pickup flow in the
latest SIP Service Examples:

http://search.ietf.org/internet-drafts/draft-ietf-sip-service-examples-02.txt

in section 2.15 (User B2 picks up User B1's call while User B1's phone
is ringing).  The flow shows the special REGISTER being sent by User B2
which reaches the Proxy which then forks the INVITE to User B2 who then
answers.

The problem I realized is this - how does User B2 locate the correct
proxy?  For example, User B1 could receive SIP service via multiple
proxies from different providers, loadsharing proxies, or even accept
INVITEs directly bypassing a proxy.

As a result, I think the flow needs to be modified to show the special
REGISTER message reaching User B1, which then sends a redirect or acts
as a 3pcc to route the INVITE to User B2.  Of course, if the REGISTER
happens to hit the same proxy as forwarded the INVITE, the shown flow
could be used.  However, I don't think this flow will work in all cases.

This means that a SIP client which supports this feature must accept
these special REGISTER messages. As long as the REGISTER contains a
single Contact with expires=0 it can probably be identified. Another
possibility is to define a new Caller Prefs "feature=pickup" parameter
and add that to the registered URL.  That way we are being explicit as
to what this registration really means. In this case, the Request-URI
will be that of User B1, and the Contact will be that of User B2.

Comments, suggestions?

Thanks,
Alan Johnston
WorldCom
sip:[email-masked]

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---

From: Alan Johnston <[email-masked]>
Date: Tue, 24 Jul 2001 15:25:21 -0500

Deepak,

Thanks for the reply.  See my comments below.

Thanks,
Alan.

> I don't think the REGISTER message really reaches the Proxy. The REGISTER message reaches the local registrar ( pickupgroup37.ss1.wcom.com ), causing a database update. This update is sensed by the…

ABJ> This might work, although you are still left with the problem of which domain to send the REGISTER to - is the call incoming on my wcom.com line or my personal.com line?  Then there is the problem if there is no proxy in the flow at all - the call to be picked up is a point to point
call.  I still think the REGISTER (or some other message) needs to be received by the device sending the 180 Ringing response for this to work correctly.