Re: Failover-Failback without any Load-Balancing

Good evening,

Here is a vital fact I failed to mention previously: For the condition 
(t_branch_timeout() and !t_branch_replied()) to kick in fast when a dead 
target gives no repsonse whatsoever you need to tune the fr_timer. The 
condition will not be true before fr_timer has expired.

First I think you should read an old but still relevant (AFAIK) document 
on transaction timers - https://sip-router.org/wiki/ref_manual/timers

Then you can either change the the vaulue in the module parameter 
setting modparam("tm", "fr_timer", xxx),

or you could use t_set_fr(0, xxx); like this:

      if (is_method("INVITE|REGISTER")) {
          if (!ds_select_dst("1", "4")) {
              sl_send_reply("503", "No servers available");
              exit;
          }
          xlog*("Selected Reg Trunk $rd/$du \n");
          t_set_fr(0, xxx);
          t_on_failure("FAILOVER");
          t_relay();
          exit;
      }

To guarantee fast failover you should set xxx to e.g. 200 - meaning in 
practice that you deem the target to be down if you haven't seen a 100 
Trying in 200 ms after the INVITE was sent.

You should then use 'if (t_check_status("503|408") or 
(t_branch_timeout() and !t_branch_replied()))' as the condition in your 
failure_route.

Please have a look at t_branch_timeout() and t_branch_replied() in the 
module docs for the TM module.

Taken together as (t_branch_timeout() and !t_branch_replied()) my 
interpretation is in plain language: "No response whatsoever has been 
received and thus the branch must have timed out because the fr_timer 
has expired."

Lars

Den 2026-04-08 kl. 17:20, skrev Kama User via sr-users:
> Hi. Thanks for the answer.
>
> I divided
> "(t_branch_timeout() and !t_branch_replied())"
> into
> "t_branch_timeout() or !t_branch_replied()"
> to get more chanses to trigger ds_next_dst.
> In this case of this
> (t_check_status("503|408") or t_branch_timeout() or !t_branch_replied())
> I believe kamailio should trigger if any of these conditions hit.
>
> I haven't tried t_branch_timeout() yet.
>
> But so far only this code worked pretty much well for me
>
> request_route {
>      if (is_method("INVITE|REGISTER")){
>      	rewritehost("10.0.0.65");
>      	t_on_failure("FAILOVER");
>      	t_relay_to_tcp("10.0.0.65","5060");
>      	exit;
>      }
>
> failure_route[FAILOVER] {
>      rewritehost("10.0.0.75");
>      t_relay_to_tcp("10.0.0.75","5060");
>      exit;
> }
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