答复: Re: Question about Call Fo rwarding Busy(CFB)

Hi,

Thanks for your answer.

Using forking, Offer/Answer would be in two different dialogs. So, it has 
no such problem. But the TISPAN's standard using B2BUA mode for Call 
Forwarding.

As:

if A-C's negotiate still in the original dialog(having done A-B's 
negotiate), then it is B2BUA mode:

1. if the INVITE to C has Offer, there would be using C's Answer as Offer 
send to A, usually using UPDATE.


                 |---->Offer(1)
                 |<----Answer(2)
 Offer(3) <----|
Answer(4) ---->|

Then we would have to judge whether the Answer(4) should be intercept or 
transfer. If it should be transfered, then its the begin of next loop as 
the current one.

2. if the INVITE to C has no Offer, then C's Offer would has different 
stream as A's original desire. It is also a well known problem.


So. I want to know how you considering the problems. Thanks.

Gao





Paul Kyzivat <[email protected]> 
2009-04-09 21:32

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Re: [Sipping] Question about Call Forwarding Busy(CFB)








[email protected] wrote:
> 
> Use case: A call B(Busy), AS forward the call to C.
> 
> By TISPAN standard(current 3GPP 24.404), AS would send INVITE to C. And 
> this INVITE contain A's initial Offer.
> 
> But if considering precondition, B would has Offer/Answer with A.
> Now, should the INVITE(by AS) contain A's initial Offer or not?  Or is 
> there standard define it?

Are you assuming that the AS is acting as a proxy? Or as a B2BUA?

Assuming its a proxy:

If B is busy, then I would expect that there was no answer from B. 
Rather just an immediate 486 response. This would never reach A and so 
there is no issue.

If for some reason B did send an answer in a provisional response before 
signaling busy, then that would establish an early dialog with to-tag B.
Then later there will be a 486 response, intercepted by the AS, and not 
forwarded to A.

And then a copy of the original INVITE will be sent by the AS to C. When 
C responds with an answer, it will be with to-tag C. Hence A will see 
this as a new early (or final) dialog.

A must negotiate the preconditions independently in the two dialogs.

                 Thanks,
                 Paul




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