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♦ Secure execution of remote applications

♦ Firewalls and intrusion detection

10.1. Networking Overview

10.1.1. The OSI Model

A protocol is, simply put, a set of rules for communication.

In order to get data over the network, for instance an E-mail from your computer to some computer at the other end of the world, lots of different hard- and software needs to work together.

All these pieces of hardware and the different software programs speak different languages. Imagine your E-mail program: it is able to talk to the computer operating system, through a specific protocol, but it is not

able to talk to the computer hardware. We need a special program in the operating system that performs this function. In turn, the computer needs to be able to communicate with the telephone line or other Internet hookup method. And behind the scenes, network connection hardware needs to be able to communicate in order to pass your E-mail from one appliance to the other, all the way to the destination computer.

All these different types of communication protocols are classified in 7 layers, which are known as the Open Systems Interconnection Reference Model, the OSI Model for short. For easy understanding, this model is reduced to a 4-layer protocol description, as described in the table below:

Table 10-1. The simplified OSI Model

Network access layer PPP, PPPoE, Ethernet

Each layer can only use the functionality of the layer below; each layer can only export functionality to the layer above. In other words: layers communicate only with adjacent layers. Let's take the example of your E-mail message again: you enter it through the application layer. In your computer, it travels down the transport and network layer. Your computer puts it on the network through the network access layer. That is also the layer that will move the message around the world. At the destination, the receiving computer will

accept the message through it's own network layer, and will display it to the recepient using the transport and application layer.

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