Subject: Re: [Sip] B2BUA

From: William Marshall <[email-masked]>
Date: Mon, 2 Dec 2002 09:06:13 -0500 (EST)

Dean,

you wrote:
> Juan-Carlos said:
> > Back-to-Back User Agent: A SIP element being able to handle a 
> > collection of "n" User Agents (behaving each one as UAC and 
> > UAS, according to SIP rules), which are linked by some 
> > application logic that is fully independent of the SIP protocol rules.
> 
> This seems like a reasonable definition. 
> 

I dislike the use of "is" in the last line.  Many actual B2BUAs are
not "fully independent of the SIP protocol rules", and the above
definition seems to say they must be.  Perhaps "may be"?

In particular, I don't think a firewall ALG will fit in the above
definition of a B2BUA.  Neither will the 3GPP P-CSCF.  If the above
definition captures the common understanding of what a B2BUA is 
(and it certainly captures the problems of the general case), there
needs to be another type of SIP entity defined for what exists in 
practice.

The term "transparent B2BUA" has been used in the past.  I don't think
those can be considered a subcase of the general definition above.

Bill Marshall
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---

From: "Dean Willis" <[email-masked]>
Date: Tue, 3 Dec 2002 00:00:19 -0600

> needs to be another type of SIP entity defined for what exists in 

Well, "fully independent" doen't seem the same to me as "must ignore",
but perhaps "may be" would be better. Honestly, the existing text
satisfies me quite nicely, but I have noticed that it seems to confuse
people which raises the question of "how can we keep this confusion from
occurring in the mind of the reader."

> those can be considered a subcase of the general definition above.

Oh no -- I'll have to dig up my "Transparency Top 10" list again. Maybe
that should become a yearly post, sent out the week after my "B2BUA"
post . . .;-)

Really, I think that almost any of the "Top 10" transparency modes can
be violated by a B2BUA, whereas, if I recall my post, NONE are violated
by a conformant proxy. Something like a Firewall Control Proxy preserves
many of the transparency modes, while violating something fundamental
like "media transparency". It's still not really a "proxy", so it's
"some kind of B2BUA that is almost but not quite exactly a proxy".

Just because the FCP's application logic implements something that is
SIMILAR TO the SIP protocol rules, it doesn't mean that it isn't
INDEPENDENT OF thos same rules. But hey, the 3261 def works for me, so
what do I know?

--
Dean