B. A host-based IDS watches for questionable activity on a single computer system. A network-

accepted. The OSI model serves as an abstract framework, or theoretical model, for how pro- tocols should function in an ideal world on ideal hardware. Thus, the OSI model has become a common reference point against which all protocols can be compared and contrasted. OSI Functionality The OSI model divides networking tasks into seven distinct layers. Each layer is responsible for performing specific tasks or operations toward the ultimate goal of supporting data exchange i.e., network communication between two computers. The layers are always numbered from bottom to top see Figure 3.1. They are referred to by either their name or their layer number. For example, layer 3 is also known as the Network layer. The layers are ordered specifically to indicate how information flows through the various levels of communication. Layers are said to communicate with three other layers. Each layer communicates directly with the layer above it as well as the layer below it plus the peer layer on a communication partner system. The OSI model is an open network architecture guide for network product vendors. This standard, or guide, provides a common foundation for the development of new protocols, net- working services, and even hardware devices. By working from the OSI model, vendors are able to ensure that their products will integrate with products from other companies and be sup- ported by a wide range of operating systems. If vendors developed their own networking frame- work, interoperability between products from different vendors would be next to impossible. The real benefit of the OSI model is found in its expression of how networking actually func- tions. In the most basic sense, network communications occur over a physical connection. This is true even if wireless networking devices are employed. Physical devices establish channels through which electronic signals can pass from one computer to another. These physical device channels are only one type of the seven logical channel types defined by the OSI model. Each layer of the OSI model communicates via a logical channel with its peer layer on another com- puter. This enables protocols based on the OSI model to support a type of authentication by being able to identify the remote communication entity as well as authenticate the source of the received data. F I G U R E 3 . 1 A representation of the OSI model Application 7 Presentation 6 Session 5 Transport 4 Network 3 Data Link 2 Physical 1