We've got Kamailio (v4.0.0) connected to some SBC, which sends SIP traffic and periodic OPTIONS pings to Kamailio's VIP. Kamailio responds to the OPTIONS pings with OK, i.e. in the main route block:
   if (is_method("OPTIONS"))
   {
      sl_send_reply("200","OK");
      exit;
   }

All works fine for a week or so, then Kamailio stopped responding to the OPTIONS pings on the VIP it listens to. But it still respond to OPTIONS pings that are sent to its real IP. The real IP is not used for receiving/sending any traffic while only the VIP is. So it seems that Kamailio is still working, but maybe having problems with the receiving buffer for the VIP?

We do see that some SIP messages sent to Kamailio's VIP are too big (sometimes over 1500 bytes). My question is, in this case, what would be expected to happen? Is it possible somehow the receiving buffer for the VIP got messed up by the big UDP messages? Any one seen similar problems? What is the suggested solution?

We're considering moving to TCP. But since this is production environment, I want to get some confidence that the problem we saw was likely to have been caused by the UDP message being too large.

Cheers,

Yufei