Hello,

this sounds odd. Are you maybe using a failure route to intercept the 503 and send the INVITE to a new destination?

Cheers,

Henning

Henning Westerholt – https://skalatan.de/blog/

From: Mattis Lind <mattislind@gmail.com>
Sent: Donnerstag, 10. August 2023 15:02
To: sr-users@lists.kamailio.org
Subject: [SR-Users] Kamailio dns-failover / dns-loadbalancing with slow responding client.

Hello!

I am looking into a problem where we have Kamailio forwarding calls to two or more "recording-clients". I will try my best to describe the problem and would appreciate it if someone has an idea what to do. Please feel free to ask if you think I have forgotten to describe something that might be important or something is unclear in what I have written.

We use use_dns_failover=yes and dns_srv_lb=yes so calls get load balancing to the "recording-clients". There is also the t_set_fr(60000,1000) parameter set so that if there is no response within 1 second it would try the next recording-client. The SRV record points to two or more recording clients.

It now happens that the recording-clients sometimes have some kind of temporary problem so it will respond with a 503 after 5 seconds.

What happens is that after the 1 second timeout trying to get the INVITE through to the first recording-client Kamailio will internally generate a 408. This will cause it to failover to another recording-client which happily takes care of the INVITE and responds properly with a 200 OK.

Everything would have been just fine except for the fact that the first recording-client is just slow and finally responds with a 503. This 503 is not relayed backwards since a 200 has already been forwarded back to the caller. But when receiving the 503 Kamailio will initiate a new INVITE which is trying to set up a new call to a recording client. It looks like it is doing a new failover regardless if it already has done a failover for this failed transaction.

I don't want Kamailio to send that last INVITE when receiving the 503. How can I configure Kamailio to disregard the last 503 (except for responding with an ACK) and not initiate a new INVITE?

I have tried a lot of different changes to the configuration but failed to achieve this, unfortunately. Do I need to use the dispatcher module to achieve this?

/Mattis