Subject: PINT Proxies and Gateway behaviour

From: [email-masked] (Lawrence Conroy)
Date: Sun, 29 Nov 1998 23:33:10 +0000

This is really a re-iteration of the discussions we've had so far on the
list and at the meetings (and in developing the PINT draft :) concerning the
behaviour of PINT Proxies, Registration, and Gateways, in particular where
there is more than one Gateway registered as handling a given PINT service.
I'm sending this as it still causes some confusion, hence this "heads up":

Section 1.4.5 (page 15) of SIP (draft 10 of November 12th 1998) states that
"A location server MAY return several locations because the user is logged
in at several hosts or because the location server has (temporarily)
inaccurate information. The SIP server combines the results to yield a list
of zero or more locations".

It goes on to suggest that "A SIP Proxy can sequentially or in parallel try
the addresses until the call is successful (2xx response), or until the
callee has declined the call (6xx response)".

As mentioned in the PINT draft, the behaviour of a PINT proxy MIGHT differ
slightly from this, as the "user" in the PINT case is the PINT service,
and it is quite possible that a number of Gateways could register as being
capable of providing this service.

If a SIP proxy were to try a number of PINT servers in parallel, then it
could well get a number of 2xx OK responses back; after all, they HAVE
registered as being able and (one assumes) willing to provide the requested
service.

The impact of this is that the design of a PINT Gateway should be careful to
treat an ACK to it's 200 response as being a commitment from the client, and
NOT to action the request on receiving the initial INVITE.

This is probably obvious, but can cause quite subtle problems in practice,
as the parties to a PINT service are not directly involved in the SIP/PINT
IP network transactions.

All the best, Lawrence
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| Lawrence Conroy,    | "These Opinions must be mine, 'cos if they    |
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From: Jonathan Rosenberg <[email-masked]>
Date: Sun, 29 Nov 1998 23:17:59 -0500

> NOT to action the request on receiving the initial INVITE.

In fact, I don't see this behavior as different at all from regular SIP.
I believe it says somewhere in the SIP spec that for a multimedia
conference, the callee should not send audio to the caller until the ACK
is received. In general, for an arbitary invitation to a session, the
UAC may get 200 OK's from users the UAC does not want to "connect" to.
So, the UAC can either send an ACK or send a BYE to effectively "hang
up" before the call starts. 

-Jonathan R.
-- 
Jonathan D. Rosenberg                       Lucent Technologies
Member of Technical Staff                   101 Crawfords Corner Rd.
High Speed Networks Research                Holmdel, NJ 07733
FAX: (732) 834-5379                         Rm. 4C-526
EMAIL: [email-masked]
URL: http://www.cs.columbia.edu/~jdrosen
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