Internet Control Message Protocol

The Internet Message Control Protocol (ICMP) is part of the TCP-IP

suite of protocols. ICMP messages are typically generated in response to

errors in IP datagrams (as specified in

RFC1122) or for diagnostic or routing purposes.

Many commonly used network utilities are based on ICMP messages.

The ping utility (well known on Unix) is implemented using the ICMP "Echo" and "Echo reply"

messages.

The related traceroute command is implemented by transmitting UDP datagrams

with manipulated IP Time-to-live header fields and looking for ICMP "Time to

live exceeded in transit" and "Destination unreachable" messages in response.

Each ICMP message is encapsulated directly within a single IP datagram

and is not reliably delivered.