ipr_ttl_exc(port_nr, dest, netmask, timeout) finds a route in the output routing table whose destination is dest, ipr_ttl_exc()'s second parameter, and, if a route is found, increases the distance of the route by a multiple of 2 if the result is less than IP_MAX_TTL (#define'd as 255) and increases the distance by one if the result is greater than IP_MAX_TTL.
ipr_ttl_exc() is called by icmp_time_exceeded() upon receipt of a time-exceeded icmp message.