Subject: [Sip] Callerprefs: Mid-dialog requests

From: Ben Campbell <[email-masked]>
Date: 21 Apr 2003 17:30:35 -0500

The draft does not seem clear on how caller preferences affect
mid-dialog requests. I expected when reading this that caller
preferences would only apply to requests outside an existing dialog, and
that normal dialog routing would prevail for mid-dialog requests.

However, the statements about putting feature set parameters into the
Contact header when a mid-dialog request retargets the remote Contact
confuses that point for me. I am uncertain what use cases there might be
for that feature. The only thing I can think of is to convey information
to the opposite UA that it might use when re-inviting or establishing a
new dialog. But I don't think that usage really fits the scope or
purpose of this draft.

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From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 23 Apr 2003 02:38:26 -0400

> that normal dialog routing would prevail for mid-dialog requests.

Right. Section 4 says this:

    Both request handling and feature preferences can appear in any
    request, not just INVITE. However, they are only useful in requests
    where proxies need to determine a request target. If the domain in
    the request URI is not owned by any proxies along the request path,
    those proxies will never access a location service, and therefore,
    never have the opportunity to apply the caller preferences. This
    makes sense; typically, the request URI will identify a UAS for mid-
    dialog requests. In those cases, the routing decisions were already
    made on the initial request, and it makes no sense to redo them for
    subsequent requests in the dialog.

This is repeated in the section on proxy processing, which says that you 
only use caller prefs when you are the owner of the domain in the r-uri, 
and therefore, accessing a location service. That rules out mid-dialog 
requests.

> confuses that point for me. 

Thats a different application of the parameters. Thats a declaration of 
capabilities, not a request for routing preferences.

> I am uncertain what use cases there might be

One important use case is conferencing. When you end up in an ad-hoc 
conference, the UA becomes a focus, and thus needs to do a re-invite to 
update its contact to include the isfocus parameter.

-Jonathan R.

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