Subject: [Sip] comments on draft-worley-sip-many-refers-00

From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 05 Jul 2006 23:07:26 -0400

Dale,

Thanks for taking a stab at this. The 'hidden semantic' behind REFER is 
a looming potential problem that I don't think we've fully captured. 
Your draft takes a good step towards uncovering it, but I don't think it 
quite gets there.

In particular, the Summary section of the draft is where you actually 
show the differences between the various usages. None of the 'Actions' 
in the table demonstrate the UA having to do something that is 
usage-specific and not directly inferred from the request itself. That, 
to me, is the sign of trouble - that a UA has to manipulate some kind of 
other state, or take some kind of other action, as a side effect of 
REFER, and that other action or state change depends on the usage. Thus, 
at the moment, your draft doesn't seem to point out that there are any 
problems with the REFER mechanism.

One possible place this side-effect issue might show up is in the UI of 
a phone device. Without a doubt, its a better experience for a user to 
get asked, "Joe wants to transfer you to Bob, proceed?" than "You have 
received a REFER with Refer-To of sip:[email-masked], proceed?", and 
this would seem to require the device to know that the reason for the 
REFER is a transfer. You might want to look at these kinds of UI issues 
across the different usages.

You call out a transfer #1 use case in which the transferee has to send 
a BYE after the transfer. However, this is not the case AFAIK. In none 
of the transfer flows does the transferee need to send a BYE; its the 
entity doing the transfer (which obviously knows its transferring) that 
needs to do a BYE after the REFER for the INVITE succeeds.

Thanks,
Jonathan R.
-- 
Jonathan D. Rosenberg, Ph.D.                   600 Lanidex Plaza
Cisco Fellow                                   Parsippany, NJ 07054-2711
Cisco Systems
[email-masked]                              FAX:   (973) 952-5050
http://www.jdrosen.net                         PHONE: (973) 952-5000
http://www.cisco.com

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From: [email-masked]
Date: Thu, 6 Jul 2006 23:09:53 -0400

From: Jonathan Rosenberg <[email-masked]>

Thanks!  In the next revision, I'll take care of these issues.  At the
moment, my interest was just to list all the distinct meanings and
determine if it was *possible* for a UA to determine at all times
which was intended.  I don't think we've made that mistake, but until
it's pointed out that the mistake can be made, it's accidental that we
haven't.

I'm also curious if there are cases I've missed.  I'd be surprised if
I've found all of them.

Dale