|  | 9.2 | Layer 3 Concepts
| 9.2.3 | Internet Control Message Protocol (ICMP)
All TCP/IP hosts implement ICMP. ICMP messages are carried in IP datagrams and are used to send error and control messages. ICMP uses the following types of defined messages. Others exist that are not included on this list: Destination Unreachable Time to Live Exceeded Parameter Problem Source Quench Redirect Echo Echo Reply Timestamp Timestamp Reply Information Request Information Reply Address Request Address Reply How ICMP testing works If a router receives a packet that it is unable to deliver to its final destination, the router sends an ICMP unreachable message to the source as shown in Figure . The message might be undeliverable because there is no known route to the destination. In Figure an echo reply is a successful reply to a ping command. However, results could include other ICMP messages, such as unreachables and timeouts. -