the Internet Control Message Protocol (ICMP). Control Message is the most descriptive part of
the name--ICMP helps control and manage the work of IP and therefore is considered to be
part of TCP/IP's network layer. RFC 792 defines ICMP and includes the following excerpt,
which describes the protocol well:
example, to report an error in datagram processing. For such purposes this protocol, the
Internet Control Message Protocol (ICMP), is used. ICMP uses the basic support of IP as
if it were a higher level protocol; however, ICMP is actually an integral part of IP, and
must be implemented by every IP module.
to be familiar with several of these messages. Table 5-5 lists several ICMP messages, with the
ones most likely to be on the exam noted with an asterisk. Not surprisingly, these are the same
messages used most often. The Destination Unreachable, Time Exceeded, and Redirect
messages will be described in more detail following Table 5-5.
packet.
packet has been discarded.
message requests that the sender slow down.