Subject: [Sip] Callerprefs: Require header tag

From: Ben Campbell <[email-masked]>
Date: 21 Apr 2003 17:24:33 -0500

The use of a Require header tag does not seem to buy us much. In a
REGISTER request, the best you can expect is the registrar knows to
store the feature set parameters with the Contact. You cannot assume
that each proxy that uses the registrar's location service knows about
the "prefs" tag just because the registrar did. Also, the registrar
cannot be expected to know the capabilities of every proxy that might
possible use its location service.

I think it would be better to dispense with the Require tag completely,
and just assert that the UA cannot assume that feature parameters will
be honored in any way. This seems to fit with the concept that a proxy
can override any of this due to local policy anyway.

If we do keep the tag, then we need to explicitly document its limited
scope. 

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From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 23 Apr 2003 02:49:08 -0400

> possible use its location service.

Thats a good point. The model I had in my head was of the registrar and 
proxy being co-located, but you are right that need not be the case. 
However, practically speaking, I would expect these to be either the 
same, or made by the same vendor, in which case the assumption usually 
will hold.

> can override any of this due to local policy anyway.

There is a difference between not understanding the request, and 
understadning it, but denying it because of policy reasons.

> If we do keep the tag, then we need to explicitly document its limited

My inclination is to keep it, and document the scope. The text now reads:

The REGISTER request MAY contain a Require
header field with the value  ``pref'' if the client wants to be sure
that the registrar understands the extensions
defined in this specification. This means that the registrar will
store the caller preferences parameters, and make them available to
proxies accessing the location service within the domain. In absence of
the Require header field, a registrar that does not understand this 
extension
will simply ignore the Contact header field parameters. Note that even
if the registrar supports caller preferences, it does not mean that
all proxies in the domain which access the location service will
understand caller preferences. Therefore, the client cannot obtain a
guarantee that its preferences are taken into account.

Thanks,
Jonathan R.
-- 
Jonathan D. Rosenberg, Ph.D.                600 Lanidex Plaza
Chief Scientist                             Parsippany, NJ 07054-2711
dynamicsoft
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