ICMP Message Types

echo request/reply - used to test connectivity (ping)
time exceeded - used to report expired datagrams (TTL = 0)
redirect - used to inform hosts about better routes (gateways)
destination unreachable - used to inform a host of the impossibility of delivering traffic to a specific destination

ICMP Commands

ping - uses ICMP echo messages to determine whether a node is reachable and alive
traceroute - determines the set of routers along the path to the destination