.: Advanced Linux Networking :.

One common model of network stacks is the Open System Interconnection (OSI) model. This model consists of seven layers, each of which handles a specific networking task. When a computer sends data, the information originates from a program residing at the top layer of the OSI model (known as the Application layer). The program passes data to the next layer (the Presentation layer), and so on down the stack. Each layer processes the data in some way. At the bottom of the OSI model is the Physical layer, which corresponds to the network hardware, such as cables and hubs. Data pass over the Physical layer from the sending computer to the receiving computer. (This transfer may be simple when both computers are on the same network segment, but it can involve many other systems when the two computers are at disparate points on the Internet.) On the destination system, the data pass up the network stack, ultimately reaching the recipient program at the Application layer on that system. This system may then send a reply down its network stack, and that reply passes up the stack of the first system. Figure 3.1 illustrates this process.

Each layer of the OSI model communicates directly only with the layers immediately above and below it. (In the case of the Application and Physical layers, the chain ends. Applications communicate with users or perform automated network tasks, and the Physical layer links the two computers.) On any given computer, the layers of the network stack must be written to facilitate such communication, using clearly defined interfaces. Sometimes, the components at a given layer must be interchangeable. For instance, the Application layer consists of network applications, such as Web browsers and Web servers. You should be able to swap out one Web browser or Web server for another without causing problems with the network stack. (Any given Web browser or Web server may lack certain important features, such as the ability to handle Secure Sockets Layer [SSL] encryption; however, this isn't really an issue of network stack integration.) Similarly, you should be able to swap out network cables and hubs at the Physical layer, or even replace a network card and its driver, without impacting higher-up layers.

Each layer of the network stack corresponds to its counterpart on the opposite computer. In some sense, the recipient computer's network stack undoes whatever the sender computer's network stack did. The ultimate goal is to allow Application-layer programs to communicate. Therefore, any layer should receive from the layer below exactly the data sent by its counterpart layer on the sending system. In some sense, any given layer should work as if it were talking to its counterpart on the other system, not another layer of the local protocol stack. For this reason, network stacks must be very standardized across computers, even if those computers run radically different OSs. For instance, the network stacks of such diverse OSs as Linux, Windows XP, MacOS X, and BeOS must all work in almost precisely the same ways, even if they use entirely independent code bases.