[Sip] B2B UA definition

[Michael Thomas <mat@cisco.com>]

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

Medhavi Bhatia writes:
 > 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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