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.
Echo Request and Echo Reply used by thePing 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 Dont Fragment bit set and fragmentation is required in order to forward the datagram to the next router.