ICMP

As has been mentioned, IP provides a service for datagram transmission without
guaranteeing the arrival at the destination. Furthermore, as it is not a
connection-oriented protocol, there is no coordination among the origin, destination,
and all of the intermediate gateways intervening in the communication.
Therefore, it is helpful to have a mechanism for informing the source of
the message about the cause of a loss or any other event associated with a datagram.
The ICMP permits this utilization of the data part of a standard IP
datagram [7]. It is important to note that including information in the data
field of a datagram should be considered not a higher level protocol but an
integral part of IP, and should be included in all IP implementations.
The errors detected by the ICMP are varied. One is host unreachable,
which informs about the impossibility of reaching the destination because it
cannot be found either because it is disconnected or because there is an error
in the address. Others are time exceeded, which reports that the TTL field of
the datagram has been decreased down to zero without reaching its destination
and source quench, which indicates a state of congestion in the gateway.
These messages can be used, for example, so that the local host stops the
transmission when it detects congestion at a gateway in the network. It is important to note that ICMP is a protocol permitting the gateways
to report an error, but it does not provide mechanisms for its correction. In
fact, ICMP does not specify what to do when an error is produced. It is even
possible that the source of the message can do nothing about it (typically due
to congestion at an intermediate gateway). Only the source of the message is
informed, and not the intermediate gateways, is because usually an IP datagram
includes only origin and destination addresses and not the complete
route followed. This makes it impossible for a gateway to know the path followed
by a specific datagram, especially if the routing mechanism used
updates the tables dynamically.