Internet Control and Message Protocol (ICMP)
The ICMP is used to provide the network layer management facilities:
- Redirection (e.g. gateway determines there is a better gateway in the same sub-network) = distributed gateways
- Destination unreachable (e.g., gateway does not have a route to the remote network or the host is down)
- Echo Request and Echo Reply for testing out a path if a higher level protocol has trouble making connection.