Subject: [Sip] B2BUA

From: "Leonardo Rico" <[email-masked]>
Date: Mon, 11 Nov 2002 02:02:19 -0500

Hi,

I was looking into the issue of B2BUA and found it not clear in some 
aspects. For example some mail exchange in July 2001 says a B2BUA associates 
one or more calls meaning it can also be only one or more then 2.
All definitions I found say it is UAS on one side and UAC on the other. Can 
it also be UAC on one or more sides (meaning it initiated calls to a few 
UAs)?

What type of applications will it be used for. The drafts talk about using 
it for FW traversal, Anonymizer, 3PCC. Do you see it also functioning as a 
PBX for example?
What other type of applications do you see functioning as B2BUA?
Do you see it coupled with a SIP Proxy/Registrar/Redirect in one physical 
entity?

Thanks

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---

From: "Dean Willis" <[email-masked]>
Date: Tue, 12 Nov 2002 15:52:30 -0600

Leonardo Rico asked:
> one or more calls meaning it can also be only one or more 
> then 2. All definitions I found say it is UAS on one side and 
> UAC on the other. Can 
> What other type of applications do you see functioning as 
> B2BUA? Do you see it coupled with a SIP 
> Proxy/Registrar/Redirect in one physical 
> entity?

Ok, time for my yearly B2BUA speech.

There's a simple reason that we don't specify the B2BUA in any greater
detail in SIP. It's a UA. It can do ANYTHING A UA CAN DO, including
create and associate additional dialogs, order a beer in a bar, insult
the president, translate to a different protocol, whatever. It's a
completely unbounded problem space, NP complete, and totally
intractable. If you believe Wolfram, the B2BUA is an example of the
Universal Computer and if evolved infinitely answers Adam's Universal
Question, "you know, life, the universe, and everything."

Now, back to earth -- a B2BUA can be used in many different ways.
Sufficiently constrained, it's called a "proxy" -- a proxy being a B2BUA
that meets certain requirements for transparency.  B2BUAs that break
these transparency rules are useful for a wide variety of roles,
including the ones you mention. They might also be useful as call-center
controllers, operator-services nodes (barge-in, emergency disconnect),
lawful intercept points, conference/bridge controllers,  PBXes, etc.
Many commercial service nodes have the capacity to act as B2BUAs as well
as proxies and redirectors.

That being said, transparency is still important. The transparency rules
for proxies are there to protect functionality, including preservation
of end-node expectations and security. We will consider the transparency
requirements of proxies as we continue to define the SIP protocol. We
will NOT take into account partial-transparency issues from the infinite
diversity of B2BUAs. Therefore, reliance on a B2BUA is risky. Odds are,
somewhere, somehow, it is going to break some piece of functionality
that somebody finds critically important. You've been warned. If you use
one, and it breaks something, don't blame us.

--
Dean