Subject: [Sipping] Service identification - clarifying draft-rosenberg-sipping-service-identification

From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 26 Feb 2007 10:00:18 -0500

The threads on this topic are getting long and intertwined. So, I wanted 
to send a few notes off on this topic.

First and foremost, I must apologize that my draft is far from crisp on 
the points its trying to make, and it gets obscured in what is 
admittedly an extremely contrived example.

The draft is trying to make several key points:

1. The right answer to this whole mess is to infer what the service is 
from what is actually signaled in SIP through standards-based 
extensions. If an INVITE comes with SDP that includes audio and video 
m/c-lines, then you know that this is a multimedia call. It is the usage 
of standard sip techniques, all of which are designed to work in 
heterogeneous environments, that gives you the ability to work between 
different domains, different operators with different endpoints and 
different billing plans.

2. In rare cases, you cannot accomplish #1. Or, you don't want to. Or, 
you've already designed a service which won't work without shared 
knowledge of the 'service' on each side. In these cases, SIP provides a 
mechanism for making sure that the other side supports your special 
context - Require. So, use it. But try not to, since it leads to 
non-interoperability.

3. Another issue, which is quite different IMHO, is that we have cases 
where an originating UA would like to have additional servers inserted 
into the request path for specific functions. FOr example, I, as a 
caller, want to get this recording server into this call. I want to make 
that decision on a call by call basis. My proposal for this is to use a 
Route header for a service URN. This is NOT the same problem as service 
identification since it doesn't have any of the horrible 
interoperability problems we've been discussing; the caller and called 
party don't need to agree on, or know about, this thing. Its merely a 
matter of agreement between me and my provider about what gets included.

HTH,
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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