Subject: RE: [Sip] B2BUA

From: "Drage, Keith (Keith)" <[email-masked]>
Date: Mon, 9 Dec 2002 15:41:47 -0000

I would assume there are some instances where we want transparency of SIP contents and bodies between UAs "connected" to a focus, and therefore defining it as a B2BUA with some requirements on transparency, is actually better than saying there is no relationship between the UAs that make up the focus.

Keith

Keith Drage
Lucent Technologies
Tel: +44 1793 776249
Email: [email-masked] 

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]
> Sent: 03 December 2002 18:00
> To: [email-masked]
> Cc: [email-masked]
> Subject: RE: [Sip] B2BUA
> 
> 
> Brian,
> 
> inline.
> 
> > 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?
> 
> I agree, a conference server could assume both UA and B2BUA 
> roles, as well as proxy and redirect server roles depending 
> on the system, application, request, and so on.  But whether 
> a focus is a UA, a B2BUA, or both is somewhat besides the 
> point.  The roles need to be defined so that B2BUA behavior 
> is distinct from UA behavior.  The end-point can then use one 
> that best fits its purpose.
> 
> Cheers,
> Aki
> 
> > 
> > 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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