Error Reporting (ICMP)
While IP is perfectly willing to drop data grams when the going gets tough for example. When a router does not know how to forward the data gram or when one fragment of a datagram fails to arrive at the destination it does not necessarily fail silently. IP is always configured with a companion protocol, known as Internet Control Message Protocol (ICMP) that defines a collection of error messages that are sent back to the source host whenever a router or host is unable to process an IP data gram successfully. For example, ICMP defines error message indicating that the destination host is unreachable, that the reassembly process failed, that the TTL had reached 0, that the IP header checksum failed and so on. ICMP defines a handful of control message that a router can send back to a source host. Ex., ICMP-redirect tells the source host that there is better route to the destination.