TCP/IP and the DoD Model
131
Internet Control Message Protocol (ICMP)
Internet Control Message Protocol (ICMP) works at the Network layer and
is used by IP for many different services. ICMP is a management protocol
and messaging service provider for IP. Its messages are carried as IP data-
grams. RFC 1256 is an annex to ICMP, which affords hosts' extended
capability in discovering routes to gateways.
Periodically, router advertisements are announced over the network,
reporting IP addresses for the router's network interfaces. Hosts listen for
these network infomercials to acquire route information. A router solicita-
tion is a request for immediate advertisements and may be sent by a host
when it starts up.
RFC 1700 references ICMP and describes how ICMP must be implemented by
all TCP/IP hosts.
The following are some common events and messages that ICMP relates to:
Destination Unreachable If a router can't send an IP datagram any fur-
ther, it uses ICMP to send a message back to the sender, advising it of the
situation. For example, if a router receives a packet destined for a network
that the router doesn't know about, it will send an ICMP Destination
Unreachable message back to the sending station.
T A B L E 3 . 2
Possible Protocols Found in the Protocol Field of an IP Header
Protocol
Protocol Number
ICMP
1
IGRP
9
IPv6
41
GRE
47
IPX in IP
111
Layer-2 tunnel
115
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