Set ICMP (Dest Unreachable/Host) behavior for TCP packets. tcp-rst : If a TCP packet can not be delivered/forwarded to destination (e.g. no matching route exists, interface down/blocked), the TCP-Connection is terminated by sending a TCP-RST message (a TCP packet with RST-bit set in TCP-header) back to the originator of the packet. This is the default behavior of the system. The TCP RST message is send INSTEAD of an ICMP 'Destination-Unreachable/Host' message. If ipIcmpDestUnreachHost is set to disabled(2), no TCP-RST message is sent back. icmp : TCP traffic is handled like all other IP traffic. (see description of ipIcmpDestUnreachHost)