Page 31 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes 2.4.2 TCP/IP Reference Model

The TCP/IP model is considered the oldest protocol of all computer networks like the ARPANET
and its successor Internet. TCP/IP stands for Transmission Control Protocol/Internet Protocol.
It was developed with the objective to specify a suite of protocols capable of providing transparent
communications interoperability services between computers of all sizes, regardless of the
hardware or operating system platforms supporting them. Over the years, TCP/IP has become
the most widespread of today’s protocols. One reason for TCP/IP’s popularity is the public
availability of its protocols’ specifications. In this sense, TCP/IP can justifiably be considered an
open system. Most users rely on TCP/IP for the purpose of file transfers, electronic mail (e-
mail), and remote login services.
The TCP/IP model was aimed to connect multiple networks together in a seamless way even in
case of breakdown of the subnet hardware. Not only providing seamless communication, but
also providing a flexible architecture that should support applications with divergent
requirements, ranging from transferring files to real-time speech transmission. These objectives
could be achieved because of the inclusion of the research work on packet-switching network to
the ARPAnet.

TCP corresponds to the fourth layer of OSI reference model. IP corresponds to the third layer of
the same model. TCP provides a connection type service. That is, a logical connection must be
established prior to communication to continuously transmit large amount of data with
acknowledgement. IP is a connectionless type service and prior to transmission of data, no
logical connection is needed.
TCP/IP defines a suite of communications and applications protocols in layer structure, with
each layer handling distinct communication services. TCP/IP defines a four-layer model as
shown in Figure 2.5 consisting of the internet layer, the transport layer, the application layer
and the host-to-network layer. This architecture is based on three sets of interdependent processes,
namely, application-specific processes, host-specific processes, and network-specific processes.

Figure 2.5: TCP/IP Communication Architecture

Application Layer
(Application Specific Processes)
Transport layer
(Host Specific Processes)
Internet Layer
(Routing Processes)
Host - to - Network Layer
(Network Specific Processes)

Internet Layer

The packet format and protocol at this layer is called Internet Protocol (IP). IP is a connectionless
type service that introduces IP packets into any network. The packets travel independently to
the destination. Prior to transmission of data, no logical connection is needed. The TCP/IP
Internet layer corresponds to the network layer of the OSI reference model in functionality, as
shown in Figure 2.5

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