Re: [SR-Users] Handling of CANCEL in case of no matching INVITE

Hi Bill, plain-kamailio cannot send BYEs (not sure if some module can). The point is the proxy is sort of "passive element" and doesn't initiate transactions on its own. Why isn't it enough to have the BYEs sent by UAC? I mean sometimes there can be some confusing situations (say forking downstream) and it is eventually the UAC that's in the best position to know what to BYE or CANCEL. The point is this all seems a UAS business to me, for which a proxy can only do some approximiations risking it will be wrong. Like in the case 2, you may not even notice the ACK is already on its way (because you didn't record-route)... Getting control of this would really mean using the dialog module and running in B2BUA mode rather than proxy. -jiri On 5/3/13 11:59 AM, Vassilis Radis wrote: > Thank you jiri, > > I totally agree, but I have a question that occured to me now and I cant > find answer: > > If Kamailio receives a CANCEL from a UAC after kamailio has responded > with a 200 to the corresponding INVITE, what does t_relay_cancel() do in > the following 2 > cases: > > 1. CANCEL received before the ACK from UAC. This is legal from the side > of the UAC, since the UAC may have sent the CANCEL before it receives the > 200 to the > INVITE that kamailio sent. > 2. CANCEL received after the ACK . This is illegal, but anyway, kamailio > must do something. I can't find in the RFC if it should respond 200 to the > CANCEL and > drop it, or 481(Call/Transaction Does Not Exist). Although 481 seems more > resonable, 16.10 section of RFC says: "... If a matching response context > is found, > the element MUST immediately return a 200 (OK) response to the CANCEL > request.". It does not discriminate if the corresponding INVITE has been > 200ed or not. Any > insights? > > In the spirit of what you wrote in your reply, could I configure it > somehow, to take initiative and translate the CANCEL to a BYE downstream, > no matter whether > CANCEL is received before or after the ACK? > > Bill > > > On Fri, May 3, 2013 at 12:26 PM, Jiri Kuthan &lt;jiri(a)iptel.org <mailto: > jiri(a)iptel.org&gt;&gt; wrote: > > On 5/3/13 11:04 AM, Vassilis Radis wrote: > > Hello all, > > In the documentation of the t_relay_cancel() (TM module) there is > an example that reads: > > if (method == CANCEL) { > if (!t_relay_cancel()) { # implicit drop if relaying was > successful, > # nothing to do > > # corresponding INVITE transaction found but error occurred > sl_reply("500", "Internal Server Error"); > drop; > } > # bad luck, corresponding INVITE transaction is missing, > # do the same as for INVITEs > } > > What bothers me is the phrase#do the same as or INVITEs, because > in RFC(http://tools.ietf.org/__**html/rfc3261#section-16.10<http://tools.ie… > > <http://tools.ietf.org/html/**rfc3261#section-16.10<http://tools.ietf.org/html/rfc3261#section-16.10>> > ) it says: > > If a response context is not found, the element does not have any > knowledge of the request to apply the CANCEL to. It > MUST*statelessly* > > forward the CANCEL request (it may have statelessly forwarded the > associated request previously). > > > So aren't we supposed to immediately statelessly forward the > CANCEL if t_relay_cancel() did not find the INVITE transaction, instead of > doing the same > as INVITEs, which could be t_relay() (not stateless)?. Like below: > > > if (method == CANCEL) { > if (!t_relay_cancel()) { # implicit drop if relaying was > successful, > > # nothing to do > > # corresponding INVITE transaction found but error occurred > sl_reply("500", "Internal Server Error"); > drop; > > } > # bad luck, corresponding INVITE transaction is missing, > > forward(); > } > > Am I correct or am i missing something? > > > > It could be even more standardized this way indeed. Aparts > from standards, measured by common sense I cannot realize > that either way would break something. The CANCEL should > not be appearing there at all. Perhaps it would make a difference > when SER is restarted and one after it went alive again, > UAC sent a CANCEL. Then what you suggest would perform better. > If you want to make sure you have captured this case, also make > sure that branch ids are configured to be produced statelessly. > Otherwise SER-proxied CANCEL won't match the previous INVITE anyhow. > > other than that, SER is much better than RFC3261. Its > registrar is stateless (in departure from the RFC, otherwise > it would be more vulnerable to DoS attacks), INVITE transaction > is hold after a 200 passes through (otherwise it would not > absorb retransmissions, create another transaction and > cause interop issues), and probably more. > > I just wanted to say it is really important to look first > at what interop issues one may have or not, as SER the > way it is is likely to produce better interop than > consequent standard compliance. In most cases you > can achieve it by configuration changes, I just think > you don't want to :) > > -jiri > > > > Thank you in advance, > Bill > > > > ______________________________**___________________ > > SIP Express Router (SER) and Kamailio (OpenSER) - sr-users > mailing list > sr-users(a)lists.sip-router.org <mailto:sr-users@lists.sip-** > router.org &lt;sr-users(a)lists.sip-router.org&gt;&gt; > http://lists.sip-router.org/__**cgi-bin/mailman/listinfo/sr-__** > users <http://lists.sip-router.org/__cgi-bin/mailman/listinfo/sr-__users>< > http://lists.sip-router.org/**cgi-bin/mailman/listinfo/sr-**users<http:/… > > > > >