2. Safety and the layered model

When it comes to describing security services in terms of mechanisms, and in general when it comes to network security, it's impossible to ignore their underlying nature, which is separation into layers. In layman's terms, we speak of the TCP/IP network, but it's the "TCP/IP protocol system" that should really be referred to, as it breaks down into several modules, each performing a well-defined role in a precise order. This decomposition is necessary for reasons of abstraction, in order to separate the tasks of physical transmission on a link, switching and routing in the network, connection management on a machine and, finally, application processing. Each of these four tasks is handled independently of the others, offering simplicity and interchangeability. This layered system is known as the layered model. The data (packets of information) to be sent over the network are processed successively...