ICMP Echo

ICMP echo messages are sent to a remote host and are returned in an echo-reply response. The primary use for these messages is to check the availability of the target machine.

1. Host A creates an ICMP echo datagram, using host A's address as the source, and host B's address as the destination.
2. The ICMP echo datagram is transmitted via the network to the destination (host B).
3. The destination system (host B) copies the echo information into a new echo-reply message datagram.
4. The destination system (host B) destroys the original ICMP echo message.
5. The destination machine (host B) now becomes the source of a new echo-reply datagram. Host B places it's own address in the source address field of the IP header, and host A's address in the destination field of the IP header.
6. The datagram is transmitted to the network and is routed to host A.

That's how a PING works by the way...

 

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