Application Layer (Layer 7)
At the very top of the OSI Reference Model stack of layers, we find layer 7, the application layer. Continuing the trend that we saw in layers 5 and 6, this one too is named very appropriately: the application layer is the one that is used by network applications. These programs are what actually implement the functions performed by users to accomplish various tasks over the network.
It's important to understand that what the OSI model calls an application is not exactly the same as what we normally think of as an application. In the OSI model, the application layer provides services for user applications to employ. For example, when you use your Web browser, that actual software is an application running on your PC. It doesn't really reside at the application layer. Rather, it makes use of the services offered by a protocol that operates at the application layer, which is called the Hypertext Transfer Protocol (HTTP). The distinction between the browser and HTTP is subtle, but important.
The reason for pointing this out is because not all user applications use the application layer of the network in the same way. Sure, your Web browser does, and so does your e-mail client and your Usenet news reader. But if you use a text editor to open a file on another machine on your network, that editor is not using the application layer. In fact, it has no clue that the file you are using is on the network: it just sees a file addressed with a name that has been mapped to a network somewhere else. The operating system takes care of redirecting what the editor does, over the network.
Similarly, not all uses of the application layer are by applications. The operating system itself can (and does) use services directly at the application layer.
That caveat aside, under normal circumstances, whenever you interact with a program on your computer that is designed specifically for use on a network, you are dealing directly with the application layer. For example, sending an e-mail, firing up a Web browser, or using an IRC chat programall of these involve protocols that reside at the application layer.
There are dozens of different application layer protocols that enable various functions at this layer. Some of the most popular ones include HTTP, FTP, SMTP, DHCP, NFS, Telnet, SNMP, POP3, NNTP and IRC. Lots of alphabet soup, sorry. J I describe all of these and more in the chapter on higher-layer protocols and applications.
As the top of the stack layer, the application layer is the only one that does not provide any services to the layer above it in the stackthere isn't one! Instead, it provides services to programs that want to use the network, and to you, the user. So the responsibilities at this layer are simply to implement the functions that are needed by users of the network. And, of course, to issue the appropriate commands to make use of the services provided by the lower layers.
As weve discussed elsewhere, the distinctions between the top layers are not very clear, and this is largely because of the decision made to not separate out session, presentation and application layer functions in the important TCP/IP protocol suite. All of the protocols mentioned above are from the TCP/IP protocol family, and some may cover all three of the top three OSI layers, two of them, or one; in the TCP/IP model, they are all applications.
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