ICMP is a network-layer protocol that reports feedback about problems processing IP packets — its common messages (in both ICMPv4 and ICMPv6) are host reachability, destination/service unreachable, and time exceeded.

ICMP (Internet Control Message Protocol):

A network layer protocol that provides feedback about issues related to IP packet processing.

Two Versions:

Common ICMP Messages (both versions):

• Host reachability - Test if a host is available
• Destination or Service Unreachable - Report delivery failures
• Time exceeded - TTL/Hop Limit reached zero

Security Note: ICMPv4 messages are not required and are often not allowed within a network for security reasons. Firewalls frequently block ICMP to prevent network reconnaissance.

Go deeper:

• RFC 792 — Internet Control Message Protocol — the original ICMP specification defining echo, destination unreachable, and time exceeded.
• Internet Control Message Protocol — Wikipedia — overview of ICMP's role, message types, and diagnostic uses (ping, traceroute).