Subject: RE: [SIP] new record-route text!

From: "Christian Huitema" <[email-masked]>
Date: Tue, 9 Jan 2001 08:36:55 -0800

> Record routing is the ultimate in flexibility. A proxy that 
> doesn't need to
> be in the call path can get out of the way, and one which 
> needs to be in the
> call path to serve a useful function, can. It allows people 
> to build sip
> networks of all different flavors, and have them work with 
> the same UA.

Jonathan,

What you are describing are servers that essentially place themselves
in the middle of a call and want to keep a copy of the call state. In 
my mind, there is no difference between that and a "back-to-back UA"
implementation. The complexity of such systems derives from their
desire of being stateful "per call" (as opposed to stateful per 
transaction.) I would argue that the proposed record-route extension
only makes such servers marginally easier to write; in fact, in may make
them more complex, since it makes the whole protocol more complex.

The discussion has shown that we don't actually need this functionality
for the basic firewall case. In this case, we need to configure the UA
with
the coordinates of the proxy controlling the outgoing calls from the 
firewalled domain; the recording of a route is pretty much implicit. The

discussion has also outlined that the proxies cannot in any case rely 
entirely on the record-route feature, since UAs may always drop out
silently; any proxy that keep call state must have some way to clean
out the state on timeout, or maybe by detecting the loss of the medias.

I would not object all that much if the specification did not preclude
bypass of the recorded route, e.g. by issuing a re-invite to the 
contact address through an entirely new path. This would allow the
conversation to be redirected to a more direct path, or to a path
that is less dependent of the initial set of proxies. Come to think of 
it, there is no way to prevent UAs to do just that, which means that we
should not specify the impossible.

-- Christian Huitema

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---

From: Neil Deason <[email-masked]>
Date: Tue, 09 Jan 2001 17:43:25 +0000

> them more complex, since it makes the whole protocol more complex.

It is not just call stateful proxies that want to Record-Route. 
Plain transaction stateful ones do it for some service provision
and distributed architectures. In these cases you certainly don't 
want to be forced to use a b2b UA.

> should not specify the impossible.

As the text stands a UA is not prevented from doing this with
a fresh INVITE just not a reINVITE. Seems like reasonable
flexibility between the UAs and Proxies wants.
The text also says ...
"It is RECOMMENDED that proxies do not automatically record 
route. They should do so only if specifically required."

Cheers,
Neil.
-- 
Ubiquity Software Corporation, UK        http://www.ubiquity.net