3Com CoreBuilder 3500, Implementation Manual

Page: 290 / 592

290 C
HAPTER
11: I
NTERNET
P
ROTOCOL
(IP)
Internet Control
Message Protocol
(ICMP)
Because a router knows only about the next network hop, it is not aware
of problems that may be closer to the destination. Destinations may be
unreachable if:
■
Hardware is temporarily out of service.
■
You specified a nonexistent destination address.
■
The routers do not have a route to the destination network.
To help routers and hosts discover problems in packet transmission, a
mechanism called Internet Control Message Protocol (ICMP) reports
errors back to the source when routing problems occur. With ICMP, you
can determine whether a failure resulted from a local or a remote
problem.
ICMP performs these tasks:
■
Tests whether nodes can be reached (ICMP Echo Request and ICMP
Echo Reply)
A host or gateway sends an ICMP echo request to a specified
destination. If the destination receives the echo request, it sends an
ICMP echo reply to the sender. This process tests whether the
destination is reachable and responding and verifies that the network
transport hardware and software are working. The
ping
option is
frequently used to invoke this process.
■
Creates more efficient routing (ICMP Redirect)
Often the host route configuration specifies the minimum possible
routing data that is needed to communicate (for example, the address
of a single router). The host relies on routers to update its routing
table. In the process of routing packets, a router may detect that a
host is not using the best route. The router sends an ICMP Redirect to
this host, requesting that the host use a different gateway when it
sends packets to a destination. The host sends packets to that
destination using the new route if it is able to interpret ICMP Redirect
directives.