Subject: [Sipping] Call Pickup Flow

From: Alan Johnston <[email-masked]>
Date: Mon, 23 Jul 2001 22:21:42 -0500

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: "Neil Deason" <[email-masked]>
Date: Tue, 31 Jul 2001 13:04:30 +0100

Hi Alan.

The text referencing directed pickup through REGISTER has
rightfully be removed from the bis 3+ drafts. This was
because it didn't work in distributed envt's or provide
the same user experience as may be expected. (The phone
picking up rings). We also shouldn't be defining services
within the protocol spec. (At the risk of provoking those
who equate forking with a service defined in the spec).

If you really want you could show a call flow to emulate
this service in SIP using a B2BUA (yawn). But please let's
not hack REGISTER to do pickup. But really who cares there
are many better call completion features which obselete
pick up. User friendly forwarding services implemented
through CPL mean no phone need ring unanswered ;)

Cheers,
Neil.
--
Ubiquity Software Corporation, UK        http://www.ubiquity.net

> -----Original Message-----
> From: [email-masked]
> [mailto:[email-masked]]On Behalf Of
> Alan Johnston
> Sent: 24 July 2001 04:22
> To: SIPPING List
> Subject: [Sipping] Call Pickup Flow
>
>
> All,
>