Thanks for the tip Daniel, will give it a go! Ok, when you put it that way it actually makes more sense - thanks for a great example! I still have one question left about my current scenario - why does Kamailio retransmit final non-2xx response after receiving an ACK if the response is valid and, as far as I can tell, so is the ACK? Just to provide more context: 1) Ingress INVITE vs 504 response from Kamailio 2) 504 response from Kamailio vs Caller ACK Regards, Ivan > On 11 Dec 2020, at 14:06, Daniel-Constantin Mierla &lt;miconda(a)gmail.com&gt; wrote: > > Hello, > > actually you are wrong that you think it is a wrong to tag. The entity that generates the final reply sets the to-tag (e.g.,to be able to match ack). Even here where you talk about the serial forking, you are going to face the case of different to-tag values for each branch sent out and replied. But also think about parallel forking, that timed out with many active branches, which to-tag to use?!? > > Obviously, being open source, one can code a specific behaviour for some specific needs, but is not SIP specs compliant. If the upstream cannot deal with different to-tags, it won't really work with serial/parallel forking anyhow. > > As a side remark, seeing the comments, if route(ROUTE_ELEMENT_FINAL_FAILURE) is used only in failure_route, then transaction is always defined there, because failure_route is executed only in the transaction context. Anyhow, if you are using in many places, not knowing if transaction was created, you can use send_reply() to avoid any test in the config. > > Cheers, > Daniel > > On 11.12.20 11:04, Ivan Ribakov wrote: >> Hi all, >> >> I’m trying to implement following routing behaviour: >> >> - ingress INVITE is sequentially forked to 1 or more endpoints >> - under certain conditions sequential forking may be stopped and pre-defined error returned to the caller >> >> Here’s is part of the config responsible for handling errors: >> >> failure_route[ROUTE_ELEMENT_FINAL_FAILURE_HELPER] { >> route(ROUTE_ELEMENT_FINAL_FAILURE); >> } >> >> route[ROUTE_ELEMENT_FINAL_FAILURE] { >> # We need to check if the error happens before the first egress, because T-state is not defined yet so we can not use t_reply >> if ($var(before_first_egress) == 0) { >> t_reply("$var(sipCode)", "$var(sipMessage)"); >> } else { >> sl_send_reply("$var(sipCode)", "$var(sipMessage)"); >> } >> } >> >> >> >> Here is a simple scenario where delivering call to first endpoint times out after receiving 180 Ringing (using custom fr_inv_timer value) where: >> - .33 is caller >> - .208 is Kamailio >> - .138 is callee >> >> >> >> Notice that 504 is retransmitted by Kamailio. Here is a comparison of 180 and 504 returned by Kamailio: >> >> >> >> As can be seen, 180 was proxied from the callee and has callee’s to-tag (tag=CALLEE1), however 504 that was generated by Kamailio using t_reply() has a different to-tag (presumably generated by Kamailio). >> >> How can I tell Kamailio not to generate new to-tag but to use the last one that proxied back to the caller? >> >> Thanks in advance for any help! >> >> Regards, >> Ivan >> >> >> _______________________________________________ >> Kamailio (SER) - Users Mailing List >> sr-users(a)lists.kamailio.org <mailto:sr-users@lists.kamailio.org> >> https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users <https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users> > -- > Daniel-Constantin Mierla -- www.asipto.com <http://www.asipto.com/> > www.twitter.com/miconda <http://www.twitter.com/miconda> -- www.linkedin.com/in/miconda <http://www.linkedin.com/in/miconda> > Funding: https://www.paypal.me/dcmierla <https://www.paypal.me/dcmierla>