The TCP/IP protocol suite, used in the internet, was developed prior to the OSI model. Therefore the layers in the TCP/IP protocol suite do not match exactly with those in the OSI model. The TCP/IP protocol suite is made of three layers; network, transport, and application that sit on top of two lower layer protocols. The network and transport layers provide internetworking and transport functions. The three top most layers in the OSI model, however, are represented in TCP/IP by a single layer called the application layer.
TCP/IP is a hierarchical protocole made of interactive modules each of which provides a specific functionality but are not necessarily interdependent. Whereas the OSI model specifies which functions belong to each of its layers, the layers of the TCP/IP protocole suite contain relatively independent protocols that can be mixed and matched depending on the needs of the system. The term hierarchical mean that each upper level protocol is supported by one or more lower level protocols.
At the transport layer TCP/IP defines two protocols: transmission control protocol (TCP) and user datagram protocol (UDP). At the network layer, the main protocol defined by the TCP/IP is internetworking protocol (IP), although there are some other protocols that support data movement in this layer.
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