Internet Control Message Protocol (ICMP)

Routers use the Internet Control Message Protocol (ICMP) to notify the source host of problems encountered in delivering a message to its destination.

ICMP Messages

Echo Request and Echo Reply – used by the Ping utility to test connectivity.

Source Quench – sent by the router to the source when the volume of data is overwhelming the router. It is a request to slow down.

Destination Unreachable – self explanatory. When a network on the route is down for example.

Time Exceeded – sent when the Time To Live (TTL) value reaches zero. Sometimes caused when a routing table indicates a route which is looped back on itself (a routing loop).

Fragmentation Needed – sent if a datagram is received with its Don’t Fragment bit set and fragmentation is required in order to forward the datagram to the next router.