The DoD model

In reality, the TCP/IP standard does not adhere 100 percent to the OSI model. As we said, the model is only a reference guide, and protocols do not have to follow it exactly. The TCP/IP model fits more closely to the DoD (Department of Defense) model of a network protocol shown in Figure 6-2. TCP/IP is not as abstracted as the OSI model, and many of the components fit into the DoD model. For example, the TCP/IP application usually takes care of the format of the data that is sent and also the creation of a TCP/IP session.

Note We spent so much time on the OSI model because everyone refers to it as the standard rep resentation of how a network protocol can be implemented. You will see people refer to the OSI model more often than the DoD model.

Application

Transport

Network

Link

Figure 6-2: The DoD model

The DoD model is so named because it was a TCP/IP four-layer protocol originally developed by the United States Department of Defense when defining TCP/IP. The seven layers of the OSI network model have a many-to-one mapping to the four layers used in the DoD model.

Tip For additional information about the OSI and DoD networking models and the relationships between the various layers that they define, see Internet sites such as www. comptechdoc ■ ■ '} . org/i ndependent/networki ng/gui de/netstandards.html and www .novel 1 . com/ i nfo/primer/prim05.html.

So there you have it, a TCP/IP conceptual overview. The information will become clearer as we progress through the chapter.

Note A wealth of good books about TCP/IP are available, as well as a plethora of Internet resources.

This chapter provides an overview of networking theory to make it easier to understand how Linux uses networks and what aspects of networking you may need to configure. This is not a networking book, so we've provided only as much detail as necessary for basic understanding.

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