6.1 Introduction

6.1 Introduction

ICMP is often considered part of the IP layer. It communicates error messages and other conditions that require attention. ICMP messages are usually acted on by either the IP layer or the higher layer protocol (TCP or UDP). Some ICMP messages cause errors to be returned to user processes.

ICMP messages are transmitted within IP datagrams, as shown in Figure 6.1.

Figure 6.1. ICMP messages encapsulated within an IP datagram.

RFC 792 [Postel 1981b] contains the official specification of ICMP.

Figure 6.2 shows the format of an ICMP message. The first 4 bytes have the same format for all messages, but the remainder differs from one message to the next . We'll show the exact format of each message when we describe it.

Figure 6.2. ICMP message.

There are 15 different values for the type field, which identify the particular ICMP message. Some types of ICMP messages then use different values of the code field to further specify the condition.

The checksum field covers the entire ICMP message. The algorithm used is the same as we described for the IP header checksum in Section 3.2. The ICMP checksum is required.

In this chapter we talk about ICMP messages in general and a few in detail: address mask request and reply, timestamp request and reply, and port unreachable. We discuss the echo request and reply messages in detail with the Ping program in Chapter 7, and we discuss the ICMP messages dealing with IP routing in Chapter 9.