[OpenSIPS-Devel] "Early cancel" issue in the tm module

[Maxim Sobolev]

Bogdan, thanks for looking into this for me. So OpenSIPS is somewhat better
than original code, but still not perfect. This method would work if the
INVITE has been lost or never received, but would still produce
inconsistent transaction state if provisional reply has been lost and
INVITE in fact is being processed by the far end. Then you might not hear
from the downstream UAS until much later when it follows up with either 18x
or even 200 OK.

There is no pre-cooked recipe for a stateful proxy in the RFC, but general
UAC CANCEL benaviour is defined pretty clearly, please see excerpt below.
Note the fact that INVITE and CANCEL transactions need to complete
independently, so UAC needs to continue re-transmit INVITE and hold on to
locally generated 487.

If the INVITE transaction timeouts then I think local 408 can be generated
to the UAC by the tm module.

I am pretty sure this would be RFC-correct behavior, but if you are still
in doubt, I can also raise this question on sip-implementors mailing list
and see what the community thinks.

9.1 Client Behavior

[...]
   header fields.

   Once the CANCEL is constructed, the client SHOULD check whether it
   has received any response (provisional or final) for the request
   being cancelled (herein referred to as the "original request").

   If no provisional response has been received, the CANCEL request MUST
   NOT be sent; rather, the client MUST wait for the arrival of a
   provisional response before sending the request.  If the original
   request has generated a final response, the CANCEL SHOULD NOT be
   sent, as it is an effective no-op, since CANCEL has no effect on
   requests that have already generated a final response.  When the
   client decides to send the CANCEL, it creates a client transaction
   for the CANCEL and passes it the CANCEL request along with the
   destination address, port, and transport.  The destination address,
   port, and transport for the CANCEL MUST be identical to those used to
   send the original request.

      If it was allowed to send the CANCEL before receiving a response
      for the previous request, the server could receive the CANCEL
      before the original request.

   Note that both the transaction corresponding to the original request
   and the CANCEL transaction will complete independently.  However, a
   UAC canceling a request cannot rely on receiving a 487 (Request
   Terminated) response for the original request, as an RFC 2543-
   compliant UAS will not generate such a response.  If there is no
   final response for the original request in 64*T1 seconds (T1 is




Rosenberg, et. al.          Standards Track                    [Page 54]
RFC 3261            SIP: Session Initiation Protocol           June 2002


   defined in Section 17.1.1.1), the client SHOULD then consider the
   original transaction cancelled and SHOULD destroy the client
   transaction handling the original request.


On Sat, Nov 14, 2015 at 1:06 PM, Bogdan-Andrei Iancu <bogdan at opensips.org>
wrote:

> Hi Maxim,
>
> Thank you for your detailed email on the matter. Indeed, if there is no
> reply received on the branch, OpenSIPS internally cancel the branch (stops
> retransmissions and generates a 487 reply for the INVITE). Nevertheless,
> the branch gets marked as canceled and as soon as a reply is received on it
> (provisional), a CANCEL will be fired to UAS. Of course, the reply must be
> received within the transaction lifetime (wait timer).
>
> With the approach you mentioned:
>     - could you point to the RFC section mentioning this behavior ?
>     - what happens if there is no reply at all from UAS ?
>
> Best regards,
>
> Bogdan-Andrei Iancu
> OpenSIPS Founder and Developer
> http://www.opensips-solutions.com
>
>
> On 13.11.2015 01:41, Maxim Sobolev wrote:
>
>> 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.
>>
>> 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.
>>
>> I've made a little diagram explaining the current vs. "proper" behavior.
>> You can see it at the link below:
>>
>>
>> https://docs.google.com/document/d/1mkNuqvQdw6a6j0iAmjvTssyu-VF-5i4d2Kut4Eg9qLk/pub
>>
>> 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.
>>
>> This issue tracks back to the original SER code and so that all releases
>> are affected.
>>
>>
>> -Max
>>
>
>


-- 
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