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8.2 | |
TCP/IP Suite Control Messages | |
| 8.2.1 | |
Introduction to control messages |
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The Internet Control Message Protocol (ICMP)
is an integral part of the TCP/IP protocol suite. In fact, all IP
implementations must include ICMP support. The reasons for this are
simple. First, since IP does not guarantee delivery, it has no
inherent method to inform hosts when errors occur. Also, IP has no
built-in method to provide informational or control messages to hosts.
ICMP performs these functions for IP.
Unlike error messages, control messages
are not the results of lost packets or error conditions which occur
during packet transmission. Instead, they are used to inform hosts of
conditions such as network congestion or the existence of a better
gateway to a remote network. Like all ICMP messages, ICMP control
messages are encapsulated within an IP datagram.
Multiple types of control messages are
used by ICMP. Some of the most common are shown in Figure
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ICMP uses
IP datagrams in order to traverse multiple networks. 
. Many of
these are discussed in this section.