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The TCP/IP model

The Advanced Research Projects Agency Network (ARPANET), which was initially funded by the US Department of Defense (DoD) was an early packet-switching network and the first network to implement the protocol suite TCP/IP. ARPANET was the test bed of the TCP/IP protocol suite which resulted in the TCP/IP model also known as the DoD model.

The TCP/IP model is a simplified model of the OSI model and has only four broad layers instead of the seven layers of the OSI model. Figure 2 shows the comparison between the two models. As can be seen from the following figure, the TCP/IP model is a much more simplified model, where the top three layers of the OSI model have been combined into a single application layer, and the physical and data link layers have been combined into a network access layer:

Figure 2: Comparing the OSI model with TCP/IP model

Some of the major differences between the two models are as follows:

The functions of the application layer in the TCP/IP model include the functions of the application, presentation and session layer of the OSI model
The OSI session layer function of graceful close/end-to-end connection setup, management, and release is taken over by the TCP/IP transport layer (Transmission Control Protocol)
The network access layer combines the functions of the OSI data link and the physical layers
The network layer in the OSI mode can be connection oriented or connectionless, while the Internet Protocol (IP) is a connectionless protocol
The transport layer in the OSI model is connection oriented, whereas, different protocols at the transport layer in the TCP/IP model provide different types of services; for example, TCP provides a connection oriented service, while UDP provides a connectionless service

Let's explore what happens when data moves from one layer to another in the TCP/IP model taking Figure 3 as an example. When data is given to the software application, for example, a web browser, the browser sends this data to the application layer, which adds a HTTP header to the data. This is known as application data. This application data is then passed on to the TCP layer, which adds a TCP header to it, thus creating a TCP segment. This segment is then passed on to the network layer (IP layer) where the IP header is added to the segment creating an IP packet or IP datagram. This IP header is then encapsulated by the data link adding a data link header and trailer, creating a Frame. This frame is then transmitted onto the transmission medium as a bit stream in the form of electrical/optical/radio signals depending upon the physical media used for communication:

Figure 3: Data flow across the TCP/IP layers

A simplified stack showing some protocols in the TCP/ IP stack is shown in the following figure:

Figure 4: Common protocols in the TCP/IP stack

Let's delve deeper into the TCP/IP model by looking at the TCP/IP headers in some more detail.