An ICMP echo-request message is sent by a host in the network to test connectivity with another host. An ICMP echo-request message is generated by the ping command. ICMP is a network-layer protocol that uses packets for reporting informational messages. When a host receives an echo-request (a ping), it responds by sending back an echo-reply message.

An ICMP redirect message is sent to the source host by the router to make the routing process more efficient.

An ICMP time-exceeded message indicates that the Time-to-Live (TTL) field of the IP packet has reached zero.

An ICMP destination-unreachable message is sent by the router to indicate that the router is unable to send the packet to its intended destination.