Subject: [Sipping] I-D ACTION:draft-rosenberg-sipping-session-policy-req-00.txt

From: [email-masked]
Date: Tue, 07 Jan 2003 08:28:49 -0500

A New Internet-Draft is available from the on-line Internet-Drafts directories.

	Title		: Requirements for Session Policy for the Session 
                          Initiation Protocol (SIP)
	Author(s)	: J. Rosenberg
	Filename	: draft-rosenberg-sipping-session-policy-req-00.txt
	Pages		: 15
	Date		: 2003-1-6
	
The proxy server plays a central role as an intermediary in the
establishment of sessions in the Session Initiation Protocol (SIP).
In that role, they can define and impact policies on call routing,
rendezvous, and other call features.  However, there is no standard
means by which proxies can have any influence on session policies,
such as the codecs that are to be used.  As such, ad-hoc and
non-conformant techniques have been deployed to allow for such policy
mechanisms.  There is a need for a standards-based and complete
mechanism for session policies.  This document defines a set of
requirements for such a mechanism.

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---

From: Paul Kyzivat <[email-masked]>
Date: Tue, 07 Jan 2003 11:37:30 -0500

This document provides many detailed requirements. But I am still left 
with a feeling of uncertainty about the overall scope. For instance:

- I'm not sure what is in scope and out of scope for a media policy. Is 
there anything that can be negotiated via an offer/answer exchange that 
is out of scope? Certain specific features that are encoded in SDP are 
called out specifically. This might imply that it is intended that 
features not mentioned are out of scope.

An example of a specific feature not mentioned, is the a=direction 
attribute used by comedia. It is possible to imagine an intermediate 
node might want to alter an a=direction:both to be only 
a=direction:active. Does this need to be listed as a requirement before 
it can be addressed by policy?

I am somewhat concerned that this is a slippery slope. Unless we can 
clearly define what is in scope and what is out, I think we will find 
that we are simply defining a new representation of SDP, one syntactic 
element at a time. At the least, this seems to require a sort of 
meta-model for offers and answers that transcends SDP and SDPng.

- Continuing the scope question a bit, is there any intent to support 
policies that can't already be expressed via an offer/answer exchange? 
(I don't think so, but just checking.)

- There is much mention of proxies as the nodes that want to specify 
policy. But I think there are other kinds of servers that may also take 
on this role. Specifically, a B2BUA may also want to work cooperatively 
with the endpoints in same way. Of course a B2BUA already has more 
capabilities to modify the SDP directly, but it may want to be more 
subtle and friendly in its manipulations. For instance, this mechanism 
may permit a more extended pre-alert negotiation than is currently 
possible. So I would like to see these requirements phrased to permit a 
B2BUA as a policy setter if that makes sense.

	Paul

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 24 Jan 2003 22:18:45 -0500

Sorry for taking so long to respond. Comments inline.

> features not mentioned are out of scope.

Well, Section 3.2 listed a very specific set of policies. Its not any 
apsect of the O/A exchange. Anything else is out of scope. Of course, I 
am happy to discuss the scope.

> it can be addressed by policy?

Yes.

> meta-model for offers and answers that transcends SDP and SDPng.

Nowhere in this document does it describe the syntax or encoding. It may 
very well be that the right way to do this is to have the proxy include 
some kind of SDP. But, thats mechanism. For now, I am trying to focus on 
requirements, and their implications on the overall information flow.

> (I don't think so, but just checking.)

As above, the set of policies that are to be enabled are explicitly 
enumerated.

> B2BUA as a policy setter if that makes sense.

I suppose it makes sense, but I must admit the central aim was to allow 
these intermediary things to be standards compliant proxies, yet carry 
out the policy actions that people want to see.

-Jonathan R.

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