Internet Protocols (TCP/IP)

Internet Protocols (TCP/IP)

The Internet protocol suite (also commonly called the TCP/IP protocol suite) was originally developed by the United States Department of Defense (DoD) to provide robust service on large internetworks that incorporate a variety of computer types. In recent years, the Internet protocols constitute the most popular network protocols currently in use.

One reason for the popularity of TCP/IP is that no one vendor owns it, unlike the IPX/SPX, DNA, SNA, AppleTalk protocol suites, all of which are controlled by specific companies. TCP/IP evolved in response to input from a wide variety of industry sources. Consequently, TCP/IP is the most open of the protocol suites and is supported by the widest variety of vendors. Virtually every brand of computing equipment now supports TCP/IP.

Much of the popularity of the TCP/IP protocols comes from their early availability on Unix. The protocols were built into the Berkeley Standard Distribution (BSD) Unix implementation. Since then, TCP/IP has achieved universal acceptance in the Unix community and is a standard feature on all versions of Unix.

Figure 5.3 illustrates the relationship of the protocols in the Internet suite to the layers of the OSI reference model. Notice that the suite doesn’t include protocols for the Data Link or Physical layers. TCP/IP was designed to work over established standards such as Ethernet. Over time, TCP/IP has been interfaced to the majority of Data Link and Physical layer technologies.

The Internet protocols do not map cleanly to the OSI reference model. The DoD model was, after all, developed long before the OSI model was defined. The model for the Internet protocol suite has four layers (refer to Figure 5.3). From this model, you can see the approximate relationships of the layers. The DoD model’s layers function as follows:

The Network Access layer corresponds to the bottom two layers of the OSI model. This correspondence enables the DoD protocols to coexist with existing Data Link and Physical layer standards.
The Internet layer corresponds roughly to the OSI Network layer. Protocols at this layer move data between devices on networks.
The Host-to-Host layer can be compared to the OSI Transport layer. Host-to-Host protocols enable peer communication between hosts on the internetwork. (At the time these protocols were designed, personal computers and workstations didn’t exist, and all network computers were host computers. As a result, devices on TCP/IP networks are typically referred to as hosts. The concept of a client/server relationship didn’t exist, and all communicating hosts were assumed to be peers.)
The Process/Application layer embraces functions of the OSI Session, Presentation, and Application layers. Protocols at this layer provide network services.

One huge advantage of TCP/IP is that TCP/IP is required for communication over the Internet. One disadvantage is that the size of the protocol stack makes TCP/IP difficult to implement on some older machines. (Present-day PC models should have no problem running TCP/IP.) TCP/IP has traditionally been considered slower than other protocol stacks, but again, the power of the newer machines overcomes much of this difficulty.

A large number of protocols are associated with TCP/IP. Several of these are discussed briefly in the following sections.

Further Information