On 5/4/15 10:49 AM, Sourav Dhar Chaudhuri wrote:
Hi , I have a question on To tag change during Call Forwarding [Unconditional
/ Busy/ No Answer]. As per RFC 5359, it can be noticed that To tag value is
getting changed from 181 Call is being forwarded to 180 Ringing response during
Call Forwarding.
Now my query is if a caller does not support forking then any call
originated from that caller will never be forwarded? Since To tag value is
getting changed during Call Forwarding.
Also how the caller is maintaining the state during call Forwarding? Since
initial INVITE is getting two provisional response having two different To tag.
For the initial provisional response there is no final response. Only the final
response is coming from the transferred callee. Then what will be the status
of the call leg generated for first provisional response?
This is basic SIP. A UAC that doesn't support this is *broken*.
When you see the first to-tag that establishes one early dialog. When
you see the 2nd to-tag that establishes a 2nd early dialog. You need to
maintain state for those independently.
When you get a 200 response on the 2nd early dialog you don't know with
certainty whether the first dialog is gone or not. It is *possible* that
you could still get a 200 response on that dialog until it times out,
even though it is more likely that it has been canceled downstream.
I think there are SBCs that "fix" this by hiding the existence of two
dialogs from the caller. But the caller can't really depend on that
unless it is deployed in an environment where it knows there is always
such an SBC in the path.
Thanks,
Paul
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