Occasionally a gateway or destination host uses Internet
Control Message Protocol (ICMP) to communicate with a source host,
for example, to report an error in datagram processing. ICMP uses
the basic support of IP as if it were a higher level protocol, however,
ICMP is actually an integral part of IP, and implemented by every
IP module. ICMP messages are sent in several situations: for example,
when a datagram cannot reach its destination, when the gateway does
not have the buffering capacity to forward a datagram, and when
the gateway can direct the host to send traffic on a shorter route.
The Internet Protocol is not designed to be absolutely reliable.
The purpose of these control messages is to provide feedback about
problems in the communication environment, not to make IP reliable.
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