Subject: [Sip] B2B UA definition

From: "Medhavi Bhatia" <[email-masked]>
Date: Mon, 29 Jul 2002 23:50:44 -0400

Hi all,

Very many times I have seen a discussion on B2B UA's end merely in a
definition
of a B2BUA:
        "A back-to-back user agent (B2BUA) is a
         logical entity that receives a request and processes it as a
         user agent server (UAS).  In order to determine how the request
         should be answered, it acts as a user agent client (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."

However I believe this is quite incomplete. For example:

1) A B2BUA behaves similar to a proxy in that it must maintain "timer C" on
outgoing transactions - which is unlike a UAC.
2) It SHOULD maintain max forwards on request in/requests out basis.
Moreover
it SHOULD have a defined relationship between incoming and outgoing
requests.
The way the definition goes, the outgoing request can have completely new
headers,
which eventually will cause a problem in loop detection/ policy and
configuration
at a remote destination. Clearly there may be reasons for new headers, but
certain
headers should be maintained for proper SIP network operation.
3) It has a notion similar to a response context of a proxy, and thus of
forking
requests
 etc etc...

I believe that the SIP rfc describes a "server" role as a stateful/stateless
proxy and
completely specifies it. However a "server" can also be implemented as a
B2BUA,
so it should talk some more about it, or the requirements should be spelled
out
somewhere. A completely loose definition might just lead to deployment
problems
in future.

-Medhavi.

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---

From: Ben Campbell <[email-masked]>
Date: Mon, 29 Jul 2002 23:16:14 -0500

> 3) It has a notion similar to a response context of a proxy, and thus of

I think these things greatly overconstrain a b2bua. If we follow this 
path to the logical conclusion, a b2bua is just another way to implement 
a proxy. For example, is it not permissible to create a b2bua that, when 
it receives one request, generates several? Or when it receives a FOO 
request, emits a BAR request?

A b2bUA must act as a proper UAS on one side, and a proper UAC on the 
other. That should be enough. You describe some particular 
application(s) of a b2bua, where the inbound and outbound dialogs are 
linked in a way that is at least somewhat transparent to other devices. 
I think there are many b2bua applications where that is not the case at 
all. For example, can I not create a b2bua, where the downstream devices 
cannot tell that the upstream devices exist?

---

From: Michael Thomas <[email-masked]>
Date: Mon, 29 Jul 2002 21:29:54 -0700 (PDT)

Is there any reason at all to say what a B2BUA is
and/or needs to do? *Anything* can be a B2BUA, and
I don't see why its behavior should be defined any
more than defining the nature of my procmail
recipes wrt SMTP.

It was a good thing to illustrate that a B2BUA
could exist, but I just don't see that we should
go overboard...

		Mike

---

From: "Cullen Jennings" <[email-masked]>
Date: Mon, 29 Jul 2002 23:25:41 -0700

I think that a B2BUA should meet the requirements of a UAC on one side and a
UAS on the other. I am fine with it resetting max forwards and all the other
stuff you mention. If you consider a call from a UA to gateway that go
across the PSTN to another gateway to a UA, you realize the PSTN is one
large B2BUA. I don't think B2BUAs would benefit much from further definition
than it has today.

However, you could get me going about what a "transparent" B2BUA is :-)

Cullen

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of Medhavi
> Bhatia
> Sent: Monday, July 29, 2002 8:51 PM
> To: [email-masked]
> Subject: [Sip] B2B UA definition
>
>
> Hi all,
>

---

From: "David R. Oran" <[email-masked]>
Date: Tue, 30 Jul 2002 10:26:58 -0400

--On Monday, July 29, 2002 11:25 PM -0700 Cullen Jennings 
<[email-masked]> wrote:

>
> I think that a B2BUA should meet the requirements of a UAC on one side
> and a UAS on the other. I am fine with it resetting max forwards and all
> the other stuff you mention. If you consider a call from a UA to gateway
> that go across the PSTN to another gateway to a UA, you realize the PSTN
> is one large B2BUA. I don't think B2BUAs would benefit much from further
> definition than it has today.
>
That's easy. A transparent B2BUA is one whose index of refraction is close 
to zero.

	:-)

Transparent is one of the most misused terms in our entire industry, right 
up there with "compatible", and "seamless".

Banning this terminology would do everyone a big service. Oops, then 
there's most misused term in the telecom industry..."service".

Avoiding these terms would go a long way to keeping me from getting 
confused.
	:-) :-)

Dave.