There are five types of errors to be handled by ICMP<br>First, the destination is not reachable. When a router cannot route a datagram, or a host cannot deliver a datagram, the data will be discarded. At this time, the destination unreachable message must be sent to the sender of the datagram.<br>Second, source suppression, when routers or hosts drop packets due to network congestion. The router or host will send a source suppression message to the sender. Tells the sender that the datagram is discarded due to network congestion. Request to slow down the sending process.<br>Third, timeout. When an IP datagram with TTL value of 0 is received, the router will discard the IP datagram and send a timeout message to the sender. Or when a segment of an IP datagram does not reach the destination within a limited time, a timeout message will be generated.<br>Fourth, the parameter problem, when the router or destination host finds an IP datagram. If a parameter is ambiguous or wrong, a message about parameter problem will be generated.<br>Fifth, redirection, when the host sends an IP datagram to the default router, but the default router thinks that the metric value of reaching the destination network through another router is smaller. Then a route redirection message is generated to inform the sender that the destination network should be sent to another router.<br>
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