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Routers and hosts use Internet Control Message Protocol (ICMP) error messages to tell a device that sent a datagram about problems that were encountered during delivery. The original ICMP version 4 (ICMPv4) defined five different error messages, which are all described in the original ICMP standard, RFC 792. These are some of the most important ICMP messages. They provide critical feedback about error conditions and may help a transmitting device take corrective action to ensure reliable and efficient datagram delivery.

In this first of four chapters on specific ICMP types, I look at the ICMPv4 ...

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