Subject: RE: [Sip] B2BUA

From: "Mary Barnes" <[email-masked]>
Date: Mon, 2 Dec 2002 11:46:48 -0600

Juan-Carlos,

So, I guess you're suggesting that the term "concatenation" is not accurate
or is misleading and I guess that depends upon how loosely one applies the
term concatenation here; my assumption had been that it meant that they were
"linked" (consistent with Webster's definition of concatenate) in some
manner which is unprescribed, so I think your point 2 below isn't a problem
with the current definition.  My understanding of the current definition is
that there isn't anything more for the SIP protocol to prescribe about the
operation of a B2BUA other than what is required from a protocol perspective
to function as a UAC and a UAS. From a protocol perspective, it is a logical
entity and thus whether there's one or multiple doesn't seem to apply.  I
think what you (and perhaps others) are suggesting is that there's a
definition of a B2BUA that more closely parallels the definition of a Proxy
Server since in many cases the physical rendition of the logical B2BUA
functionality would be replacing a Proxy Server in a network. And I think
this is where the problem begins, as many are opposed to this model (as it
increases the complexity, etc.) and this is why the logical definition has
remained. 

On your first point, however, I think we need to wait until the conferencing
work in SIPPING is more complete before we can suggest that a conference
server would be a B2BUA. 

Mary.

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]
Sent: Monday, December 02, 2002 11:20 AM
To: Barnes, Mary [NGC:B602:EXCH]
Cc: [email-masked]
Subject: RE: [Sip] B2BUA

Hello,
Well, if from times to times there are questions and debates on a given
subject, it is surely because the definition needs to be improved, or at
least clarified.

"Since it is a concatenation of a UAC and UAS"
1.- A B2BUA is not a "concatenation of a UAC and UAS", but a collection of
several UAs (two, three or more), which behave individually as UAS or UAC
according to SIP rules (and are seen as such from the outside of the
B2BUA). For example, a conference server where all the participants send
INVITE messages to the server could be made with UAS only (do note that
this is strictly a matter of application definition, and cannot be subject
to standardisation).
2.- UAs are not "concateneted" in the B2BUA, but they are linked by some
application logic which is by definition, a non-standard element. This is
the key element that should be precised in the RFC3261 to avoid confusion.
You cannot define the behaviour of a B2BUA, you can define only the
behaviour of each UA (and this is actually done by the SIP specification).

Hoping this answers your questions
Best regards
Juan Carlos

"Mary Barnes" <[email-masked]>@ietf.org on 02/12/2002 17:53:50

Sent by:  [email-masked]

To:   "'Phil Reynolds'" <[email-masked]>, [email-masked]
cc:
Subject:  RE: [Sip] B2BUA

Hi all,

I guess I have to ask, what's the problem with the definition in RFC 3261?

Back-to-Back user agent: A back-to-back user agent (B2BUA) is a
             logical entity that receives a request, and processes it as
             a UAS. In order to determine how the request should be
             answered, it acts as a UAC and generates requests. Unlike a
             proxy server, it maintains dialog state, and must
             participate in all requests sent on the dialogs it has
             established. Since it is a concatenation of a UAC and UAS,
             no explicit definitions are needed for its behavior.

Regards,
Mary.

---

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

Mary said:
 -----
 So, I guess you're suggesting that the term "concatenation" is not
accurate or is misleading and I guess that depends upon how loosely one
applies the term concatenation here; my assumption had been that it
meant that they were "linked" (consistent with Webster's definition of
concatenate) in some manner which is unprescribed, so I think your point
2 below isn't a problem with the current definition.  My understanding
of the current definition is that there isn't anything more for the SIP
protocol to prescribe about the operation of a B2BUA other than what is
required from a protocol perspective to function as a UAC and a UAS.
From a protocol perspective, it is a logical entity and thus whether
there's one or multiple doesn't seem to apply.  I think what you (and
perhaps others) are suggesting is that there's a definition of a B2BUA
that more closely parallels the definition of a Proxy Server since in
many cases the physical rendition of the logical B2BUA functionality
would be replacing a Proxy Server in a network. And I think this is
where the problem begins, as many are opposed to this model (as it
increases the complexity, etc.) and this is why the logical definition
has remained.  
------ 
 
Well, I think the argument is being made that this definition doesn't
(for some reason that I seem incapable of comprehending) work for some
people, and that therefore we should clarify it. At the fundamental
level, I think some people do not find the implications of this
definition obvious. There are several ways we might address this. We
might add examples. We might add "In other words, the only relationship
between the UAC and the UAS is the application logic, and that can do
anything that one can imagine an application doing", or we might just
use better wording.
 
I don't think the argument is being made that the definition in 3261 is
wrong, just that we can make it better for those who don't get it the
way it is written. Of course, if we discuss it here long enough, maybe
most of them WILL get it -- but that doesn't help the ones who aren't
"here" yet.
 
I think that Bill is suggesting that there is also something BETWEEN the
B2BUA and the conformant proxy, and that the world would be a better
place (ok, maybe not THAT much better) if our taxonomy also included
this "between" thing.
 
--
Dean