[Sip] B2B UA definition

["Medhavi Bhatia" <mbhatia@nextone.com>]

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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