3.2 The TCP/IP Reference Model

TCP/IP originated out of the investigative research into networking protocols that the US Department of Defense (DoD) initiated in 1969. In 1968, the DoD Advanced Research Projects Agency (ARPA) began researching the network technology that is called packet switching.

The original focus of this research was that the network be able to survive loss of subnet hardware, with existing conversations not being broken off. In other words, DoD wanted connections to remain intact as long as the source and destination nodes were functioning, even if some of the machines or transmission lines in between were suddenly put out of operation. The network that was initially constructed as a result of this research to provide a communication that could function in wartime., then called ARPANET, gradually became known as the Internet. The TCP/IP protocols played an important role in the development of the Internet. In the early 1980s, the TCP/IP protocols were developed. In 1983, they became standard protocols for ARPANET.

Because of the history of the TCP/IP protocol suite, it's often referred to as the DoD protocol suite or the Internet protocol suite.

TCP/IP model layers
Figure 3: TCP/IP model layers

Network Access Layer – The lowest layer of the TCP/IP protocol hierarchy. It defines how to use the network to transmit an IP datagram. Unlike higher-level protocols, Network Access Layer protocols must know the details of the underlying network (its packet structure, addressing, etc.) to correctly format the data being transmitted to comply with the network constraints. The TCP/IP Network Access Layer can encompass the functions of all three lower layers of the OSI reference Model (Physical, Data Link and Network layers).
As new hardware technologies appear, new Network Access protocols must be developed so that TCP/IP networks can use the new hardware. Consequently, there are many access protocols - one for each physical network standard.
Access protocol is a set of rules that defines how the hosts access the shared medium. Access protocol have to be simple, rational and fair for all the hosts.
Functions performed at this level include encapsulation of IP datagrams into the frames transmitted by the network, and mapping of IP addresses to the physical addresses used by the network. One of TCP/IP's strengths is its universal addressing scheme. The IP address must be converted into an address that is appropriate for the physical network over which the datagram is transmitted.

Internet layer – Provides services that are roughly equivalent to the OSI Network layer. The primary concern of the protocol at this layer is to manage the connections across networks as information is passed from source to destination. The Internet Protocol (IP) is the primary protocol at this layer of the TCP/IP model.

Transport layer – It is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer. Two end-to-end transport protocols have been defined here TCP and UDP Both protocols will be dicussed later.

Application Layer – includes the OSI Session, Presentation and Application layers as shown in the Figure 4. An application is any process that occurs above the Transport Layer. This includes all of the processes that involve user interaction. The application determines the presentation of the data and controls the session. There are numerous application layer protocols in TCP/IP, including Simple Mail Transfer Protocol (SMTP) and Post Office Protocol (POP) used for e-mail, Hyper Text Transfer Protocol (HTTP) used for the World-Wide-Web, and File Transfer Protocol (FTP). Most application layer protocols are associated with one or more port number. Port numbers will be dicussed later.

Figure 4: OSI and TCP/IP
OSI Reference Model Reference Models Basic Definitions in Data Networks