<div dir="ltr">Hi Volks, there seems to be an issue in the way how the tm handles early CANCEL, i.e. when a CANCEL arriving before the downstream UAS gets chance to generate a 100 Trying reply, or that reply is still in flight (or maybe 100 Trying is lost). In that case the OpenSIPS stops outbound INVITE re-transmits and generates both 200 OK for CANCEL and 487 Transaction Terminated for the INVITE. This only works if initial INVITE has not reached the target UAS, otherwise inconsistent state of session is produced, with UAC thinking that the transaction is over with, while the UAS is still proceeding with call setup. Needless to say this can produce all kind of weird things ranging from irritated users to billing mismatches.<div><br></div><div>This behavior comes from the RFC requirement that UAC cannot generate CANCEL until at least one provisional reply has arrived, but implementation is completely wrong in my view. Instead, it should be only generating 200 Cancelling for cancel immediately (to stop any CANCEL retransmits) and continue with re-transmitting INVITEs in due course until either transaction timeout occurs in 32 or so seconds or 100 Trying finally comes and then outbound CANCEL transaction can be fired immediately and the rest of the logic can proceed as happens now on regular CANCEL.<div><br></div><div>I've made a little diagram explaining the current vs. "proper" behavior. You can see it at the link below:</div><div><br></div><div><a href="https://docs.google.com/document/d/1mkNuqvQdw6a6j0iAmjvTssyu-VF-5i4d2Kut4Eg9qLk/pub">https://docs.google.com/document/d/1mkNuqvQdw6a6j0iAmjvTssyu-VF-5i4d2Kut4Eg9qLk/pub</a><br></div><div><br></div><div>In general this is very non-intuitive, but for INVITE transactions in no circumstances retransmits should be terminated. Once the first INVITE has left the port, there is no way for the SIP proxy to know if missing provisional response is due to that invite never being received or due to response being lost or some propagation/processing delay in between.<br></div><div><br></div><div>This issue tracks back to the original SER code and so that all releases are affected.</div><div><br></div><div><br></div><div>-Max</div>
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