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Unit 2: Network Software

Transport Layer Notes

The transport layer of TCP/IP model corresponds to the transport layer of the OSI reference
model. It is represented by two end-to-end protocols namely, TCP (Transmission Control
Protocol) and UDP (User Datagram Protocol). TCP is a reliable connection-oriented protocol
and UDP is an unreliable connectionless protocol.

Application Layer

The TCP/IP model was the first of its kind model and therefore did not contain session or
presentation layers because of its little use to most of the applications. This layer has all the
higher-level protocols, as shown in Figure 2.5.

Host-to-network Layer

The layer below the Internet layer is not defined and varies from host and network to network.
The TCP/IP model suggests that the host has to connect to the network using some protocol so
it can send IP packets over it.

Figure 2.6: Correspondence: TCP/IP Model and the OSI Model

2.4.3 A Comparison of the OSI and TCP/IP Reference Models

In this section, you will get to know the difference between OSI and TCP/IP Reference Models.
Figure 2.6 shows the similarity between the TCP/IP and OSI reference model. Both the models
were developed based on the concept a stack of independent protocols with similar functionality
of the layers. In spite of similarity between the two models they also contrast in functionalities
provided by services, interfaces and protocols. OSI reference model clearly distinguish them
while the TCP/IP model did not explicitly distinguish them. Other differences are:

The OSI model has seven layers and the TCP/IP model has only four layers.
The OSI model was developed before the protocols were devised. The TCP/IP model was
developed after the development of the protocols.

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