Internet control message protocol

When nodes fail, or become temporarily unavailable, or when certain routes become overloaded with traffic, a message mechanism called the Internet control message protocol (ICMP) reports errors and other useful information about the performance and operation of the network.

ICMP communicates between the Internet layers on two nodes and is used by routers as well as individual hosts. Although ICMP is viewed as residing within the Internet layer, its messages travel across the network encapsulated in IP datagrams in the same way as higher layer protocol (such as TCP or UDP) datagrams. The ICMP message, consisting of an ICMP header and ICMP data, is encapsulated as 'data' within an IP datagram that is, in turn, carried as 'payload' by the lower network interface layer (for example, Ethernet).

There are a variety of ICMP messages, each with a different format, yet the first three fields as contained in the first 4 bytes or 'long word' is the same for all. The various ICMP messages are shown in Figure 9.7. The three common fields are:

1. An ICMP message type (4 bits) which is a code that identifies the type of ICMP message.

2. A code (4 bits) in which interpretation depends on the type of ICMP message.

3. A checksum (16 bits) that is calculated on the entire ICMP datagram.

Table 9.2 lists the different types of ICMP messages.

ICMP Messages can be further sub-divided into two broad groups viz. ICMP error messages (destination unreachable, time exceeded, invalid parameters, source quench or redirect) and ICMP query messages (echo request and reply messages, timestamp request and reply messages, and sub-net mask request and reply messages). Here follows a few examples of ICMP error messages.

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