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a network address entry contained in its routing table. If no match is found, the gateway
discards the datagram and sends an ICMP (see next subsection) message back to the
datagram source.
12.3.3.2
Internet Control Message Protocol (ICMP)
. ICMP is used as an adjunct
to IP when there is an error in datagram processing. 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 is implemented by every IP module.
ICMP messages are sent in several situations: for example, when a datagram cannot
reach its destination, when a gateway (router) does not have the buffering capacity to
forward a datagram, and when the gateway (router) can direct the host to send traffic on
a shorter route.
ICMP messages typically report errors in the processing of datagrams. To avoid the
possibility of infinite regress of messages about messages, and so on, no ICMP messages
are sent about ICMP messages. There are eight distinct ICMP messages:
1. Destination unreachable message
2. Time exceeded message
3. Parameter problem message
4. Source quench message
5. Redirect message
6. Echo or echo reply message
7. Timestamp or timestamp reply message
8. Information request or information reply message
12.3.3.3
IP Summary
. The IP provides connectionless service, meaning that there is no
call setup phase prior to the exchange of traffic. There are no flow control or error control
capabilities incorporated in IP. These are left to the next higher layer, the transmission
control protocol (TCP). The IP is transparent to subnetworks connecting at lower layers;
thus different types of networks can attach to an IP gateway or router. To compensate
for these deficiencies in IP, the TCP (transmission control protocol) was developed as
an upper layer to IP. It should be noted that TCP/IP can be found in both the LAN and
WAN environments.
12.3.4
The Transmission Control Protocol (TCP)
12.3.4.1
TCP Defined
. TCP (Refs. 3, 9) was designed to provide reliable commu-
nication between pairs of processes in logically distinct hosts
2
on networks and sets of
interconnected networks. TCP operates successfully in an environment where the loss,
damage, duplication or misorder of data, and network congestion can occur. This robust-
ness in spite of unreliable communications media makes TCP well-suited to support
commercial, military, and government applications. TCP appears at the transport layer
of the protocol hierarchy. Here, TCP provides connection-oriented data transfer that is
2
The IEEE (Ref. 10) defines a host as "a device to which other devices (peripherals) are connected and
that generally controls those devices" and defines a host computer (IEEE) as "a computer, attached to a
network, providing primary services such as computation, data base access or special programs or programming
languages."