ICMP protocol

Internet Control Message Protocol (ICMP) defines a small number of messages used for diagnostic and management purposes. ICMP depends on IP to move packets around the network on its behalf.
 
ICMP is basically IP's internal network management protocol and is not intended for use by applications. Two well known exceptions are the ping and traceroute diagnostic utilities:

• ping sends and receives ICMP "ECHO" packets, where the response packet can be taken as evidence that the target host is at least minimally active on the network, and
• traceroute sends UDP packets and infers the route taken to the target from ICMP "TIME-TO-LIVE EXCEEDED" or "PORT UNREACHABLE" packets returned by the network. (Microsoft's TRACERT sends ICMP "ECHO" packets rather than UDP packets, and so receives ICMP "TIME-TO-LIVE EXCEEDED" or "ECHO RESPONSE" packets in return.)