Internet Core Protocols: The Definitive Guide

ICMP Messages

ICMP is used for two different types of operations: reporting on problems that prevent delivery (such as “Destination Unreachable” errors), and probing the network through the use of request and reply messages (such as those found in the ping program’s “Echo Request” and “Echo Reply” ICMP messages).

Every ICMP message is assigned a unique " message type,” which is simply a numeric code that represents each of the predefined messages. There are a variety of predefined message types that devices can choose from when they need to send a message to another device. Many of these messages were defined in RFC 792, while others were added in RFC 950, RFC 1122, RFC 1812, and RFC 1256. In addition, some messages have been rendered obsolete (such as the Information Request and Information Reply query messages), and are no longer considered part of the standard dictionary of message types.

For a detailed listing of all of the ICMP Message Types that are currently registered, refer to the Internet Assigned Numbers Authority’s (IANA) online registry (accessible at http://www.iana.org/assignments/icmp-parameters). Note that RFC 1122 states that if a system receives an ICMP message with an unknown type, it must ignore the message. Systems that implement a specific message type are expected to implement all of the codes for that type.

Table 5.1 lists the Message Types most often used with IPv4.

Table 5.1. Message Types and Their Usage

Type	Message Description	Message Family	Defined In
0