Subject: RE: [Sip] B2BUA

From: [email-masked]
Date: Tue, 3 Dec 2002 19:12:41 +0200

Inline.

> The key thing about a B2BUA is that it looks like a UA to whatever
> it is connected to (on both sides).  So, a conference server
> looks like a UA to each of it's ports, and it has at least two of
> them, and there is some "magic" between them.  If you look at the
> concept of the "focus" from the conferencing work, it is a
> thing that has a bunch of two ended SIP calls, one with each
> participant.  Each participant thinks it is talking to a UA.
> While some messages sent from a participant to a UA might be
> consumed by the UA (state changes), often, a message will trigger
> messages from the focus to other participants.

This just generalized the B2BUA to cover any normal UA behavior. Example, I get a MESSAGE from a friend to my UA, based on which decide to call another friend by sending an INVITE from that UA. This I think is quite normal UA operation, the "magic" being me via my UI.

> This seems, to me, to meet the definition of B2BUA in 3261,
> and more strongly meets proposed changes in the definition.
> Of course, it really doesn't matter if you consider the focus to
> be a B2BUA or not.

Hmm... Let's think of the B2BUA as 'in-UA' concatenated with 'out-UA'. By definition:

1) in-UAS receives request,
   <Magic happens>
2) out-UAC sends request'
3) out-UAC receives response'
   <Magic happens>
4) in-UAS sends response

(The same process happens with requests going from 'out' to 'in')

Within this B2BUA, 'in-UAS' shares magic with 'out-UAC', and 'in-UAC' shares magic with 'out-UAS'. However, if (as you state above), 4) follows immediately after 1), it's no longer a B2BUA, but a plain old UA, IMHO. 

Cheers,
Aki
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