Spamming attacks occur when numerous unsolicited messages are sent to a victim. Because

Computer systems and computer networks are complex entities. They combine hardware and software components to create a sys- tem that can perform operations and calculations beyond the capabilities of humans. From the integration of communication devices, storage devices, pro- cessing devices, security devices, input devices, output devices, operating systems, software, ser- vices, data, and people emerge computers and networks. The CISSP CBK states that a thorough knowledge of the hardware and software components a system comprises is an essential element of being able to implement and maintain security. The Telecommunications and Network Security domain for the CISSP certification exam deals with topics related to network components primarily network devices and protocols; specifically, how they function and how they are relevant to security. This domain is discussed in this chapter and in Chapter 4, “Communications Security and Countermeasures.” Be sure to read and study the materials in both chapters to ensure complete coverage of the essential mate- rial for the CISSP certification exam. OSI Model Communications between computers over networks is made possible by the use of protocols. A protocol is a set of rules and restrictions that define how data is transmitted over a network medium e.g., twisted-pair cable, wireless transmission, and so on. Protocols make computer-to- computer communications possible. In the early days of network development, many companies had their own proprietary protocols, which meant interaction between computers of different vendors was often difficult if not impossible. In an effort to eliminate this problem, the Interna- tional Organization for Standardization ISO developed the OSI model for protocols in the early 1980s. ISO Standard 7498 defines the OSI Reference Model also called the OSI model. History of the OSI Model The OSI model wasn’t the first or only movement to streamline networking protocols or estab- lish a common communications standard. In fact, the most widely used protocol today, the TCPIP protocol which was based upon the DARPA model, also known now as the TCPIP model, was developed in the early 1970s. The Open Systems Interconnection OSI protocol was developed to establish a common communication structure or standard for all computer systems. The actual OSI protocol was never widely adopted, but the theory behind the OSI protocol, the OSI model, was readily 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