ICMP Destination Unreachable

ICMP Destination Unreachable
The Destination Unreachable message is an ICMP message which is generated by the host or its inbound gateway to inform the client that the destination is unreachable for some reason. A Destination Unreachable message may be generated as a result of a TCP, UDP or another ICMP transmission. Unreachable TCP ports notably respond with TCP RST rather than a Destination Unreachable type 3 as might be expected.
http://en.wikipedia.org/wiki/ICMP_Destination_Unreachable