[SIPForum-discussion] Parallel forking behavior

[Dale R. Worley]

Sam Kirubakaran L <samkirubakaranl at outlook.com> writes:
> Suppose I have 3 contacts registered to the location service and while
> forking, proxy forwards the INVITE to 3 UASs.
>
> Consider the UASs are A, B and C.
>
> And A's 180 response is the first to reach the proxy which will be
> forwarded to UAC. And also the proxy had received the 180 response
> from UAS B and C.
>
> What if B sends 200 OK before A? How the proxy will process the dialog
> state ? What will be the behavior of the UAC ?

It doesn't matter which destination responds with 200 first.  All 1xx
responses are forwarded back to the UAC.  When the proxy receives a 2xx
from some target, it forwards the 2xx to the UAC and cancels the INVITEs
to all other targets.  (Any 2xx responses that arrive later are also
forwarded to the UAC; a 2xx from a target takes precedence over a
cancelation attempt.)

See RFC 3261 section 16.7 for the details.

Dale