TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
The OSI Reference Model is used to describe the architecture of networking protocols and technologies and to show how they relate to one another. In the chapter describing the OSI model, I mentioned that its seven layers could be organized into two layer groupings: the lower layers (1 through 4) and the upper layers (5 through 7). While there are certainly other ways to divide the OSI layers, I feel this split best reflects the different roles that the layers play in a network.
The lower layers are concerned primarily with the mechanics of formatting, encoding and sending data over a network; they involve software elements but are often closely associated with networking hardware devices. In contrast, the upper layers are concerned mainly with user interaction and the implementation of software applications, protocols and services that let us actually make use of the network. These elements generally don't need to worry about details, relying on the lower layers to ensure that data gets to where it needs to go reliably.
In this chapter I describe the details of the many protocols and applications that run on the upper layers in modern networks and internetworks. The organization of this chapter is quite different than the previous one. I felt that there was benefit to explaining the technologies in each of the lower layers separately. This is possible because with a few exceptions, the dividing lines between the lower layers are fairly well-established, and this helped show how the layers differ.
The upper layers are much more difficult to separate from each other, because there are many technologies and applications that implement more than one of layers 5 through 7. Furthermore, even differentiating between these layers becomes less important near the top of the networking stack. In fact, the TCP/IP protocol suite uses an architecture that lumps all the higher layers together anyway.
For these reasons, this chapter is divided functionally and not by layer. It contains four different sections that cover distinct higher-layer protocol and application areas. The first discusses naming system, especially the TCP/IP Domain Name System. The second overviews file and resource sharing protocols, with a focus on the Network File System. The third covers network configuration and management protocols, which includes the host configuration protocols BOOTP and DHCP. The last and largest section covers end-user applications and application protocols, including general file transfer, electronic mail, Usenet, the World Wide Web, interactive protocols (such as Telnet) and administration utilities.
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