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ICMP is an error reporting protocol for
IP. When datagram delivery errors occur, ICMP is used to report these
errors back to the source of the datagram. For example, if Workstation
1 in Figure
 is sending
a datagram to Workstation 6, but interface Fa0/0 on Router C goes
down, Router C then utilizes ICMP to send a message back to
Workstation 1 indicating that the datagram could not be delivered.
ICMP does not correct the encountered network problem; it merely
reports the problem.
When Router C receives the datagram
from Workstation 1, it knows only the source and destination IP
addresses of the datagram. It does not know about the exact path the
datagram took on the way to Router C. Therefore, Router C can only
notify Workstation 1 of the failure, and no ICMP messages are sent to
Router A and Router B. ICMP reports on the status of the delivered
packet only to the source device. It does not propagate information
about network changes to routers.
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