Internet Control Message Protocol

The internet is managed and monitored by routers. Any unexpected is reported by a protocol called Internet Control Message Protocol (ICMP). As it goes with its name, this protocol is used as a control and to test the internet.
It must be noted that there are a number of messages associated with this protocol, but only important ones are discussed here in brief.

DESTINATION UNREACHABLE
Used when a packet fails to reach the destination or when the router/subnet can not locate the destination.

TIME EXCEEDED
Used when the packet is dropped as its counter reaches zero. This may signify heavy traffic and/or congestion.

PARAMETER PROBLEM
Used when an illegal/incorrect parameter is inserted header field. 

SOURCE QUENCH
Used for flow control and congestion control. When a packet is discarded, this message is sent to the source informing about the congestion is the path. 

REDIRECT
Used when the router judges that the packet is wrongly routed. 

ECHO and ECHO REPLY
These messages are used to confirm the availability of the destination. ECHO message is transmitted towards destination and ECHO REPLY message from destination to source confirms presence of destination. 

Other messages along with their formats are available here.