Subject: RE: [Sip] B2BUA

From: "Rosen, Brian" <[email-masked]>
Date: Tue, 3 Dec 2002 12:29:48 -0500

Of course these things are never so simple.
On an "end system mixed" conference, an INVITE
from a new participant triggers Re-INVITES to
existing participants.  Depending on details,
a focus could hold the response from the INVITE
of the new participant until it receives responses
from all existing participants (new CODEC negotiation
for example).  That meets your definition of B2BUA.

On the other hand, a centrally mixed conference
would not normally trigger Re-INVITES.  Does that
mean that an end-system mixed focus is a B2BUA
but a centrally mixed focus is not?  Hard to
differentiate.  With both of them, a state
change, signaled by, say a NOTIFY from one to
the focus triggers NOTIFIES to (possibly a
subset of) the other participants, so that
would meet the definition.  Would you call
that a change in roles?

Brian

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]
> Sent: Tuesday, December 03, 2002 12:13 PM
> To: [email-masked]
> Cc: [email-masked]
> Subject: RE: [Sip] B2BUA
> 
> 
> 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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