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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)                 9  ICMP Message Types and Formats

ICMP Message Types and Formats 1 ICMPv4 Destination Unreachable Messages

ICMP error messages are used by routers and hosts to tell a device that sent a datagram about problems encountered in delivering it. In the original ICMP version 4, five different error messages are defined, which are all described in the original ICMP standard, RFC 792. These are some of the most important ICMP messages, since 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 sections on specific ICMP types, I look at the ICMPv4 error messages. I begin with Destination Unreachable messages, which are sent due to datagram delivery failures, and Source Quench messages, used to tell a device to slow down the rate at which it sends datagrams. Next I describe Time Exceeded messages, sent when a datagram has been traveling the network too long or takes too long to be reassembled from fragments, and Redirect messages, which let a router provide feedback about better routes to a host. Finally, I discuss Parameter Problem messages, a generic “catch all” type used for problems not covered by other ICMP error messages.

ICMP Message Types and Formats 1 ICMPv4 Destination Unreachable Messages