Tim, unfortunately ICMP UNREACHABLE can be sent some intermediate
router during routing flip process. For this reason some customer
prefer cut off this sort of ICMP - it would break running TCP connection.
Understood, however the conditions to terminate the connection
is not just as simple as UNREACHABLE. A few possible conditions:
(1) UNREACHABLE && TCP_SYN_STATE
(2) UNREACHABLE && TCP_SYN_STATE && sk->time_in_state