Here is an issue that Thrupoint would like to see addressed.
At the moment derived sip sessions are created when forked responses
are received at a UAC or Proxy but there is no api method provided to
allow a UAS or B2B to create a derived session. It may need to do this
in order to send a response for a forked dialogue.
We need a way for a B2BUA to send a forked response back to the
caller, allowing a B2BUA to mimic a sequentially forking proxy.
Here is a scenario where this might be needed :-
Imagine a B2BUA app that tries, sequentially, to bridge a call from
Alice to a succession of Bob's numbers. The sequence finishes when the
first Bob number sends a 200ok. This use case needs a B2BUA so as to
allow the call to be terminated by the application – otherwise, a
proxy would be the obvious choice here, as this is essentially very
similar to sequential forking in its appearance. The problem that an
application may encounter comes from reliable early media.
Alice's initial INVITE is used to create a new linked INVITE to Bob,
creating SIP Dialog Bob-1, with an accompanying SipSession. Bob's
endpoint Bob-1 replies with a 180-rel carrying an SDP answer. As far
as Alice's phoe is concerned: the offer/answer session for this diaolg
is complete. However, let's say that Bob-1 then replies with a 4xx
response. The app intercepts this response, and sends a new derived
initial INVITE to Bob-2. If Bob-2 replies with a 200ok containing SDP,
we have a problem: how can we send the SDP answer from Bob-2? We
cannot send it down Alice's original leg, as it has already had its
final SDP answer (in the reliable 180 sent by Bob-1).
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Re: New issue - need a method to allow an application to create a derived session