The Open Systems Interconnection (OSI) model is a theoretical way of classifying and talking about the complex process of sending data on a network. The OSI model divides the complex task of networking into various layers to facilitate the development of standards and to allow for interoperability between protocols and hardware components.
Network security devices and solutions are often described based on the OSI model. As you learn about network security, you should be familiar with the networking tasks associated with each OSI model layer. The following table summarizes key facts about each OSI model layer.
|Layer||Description and Keywords|
|Application (Layer 7)||The Application layer integrates network functionality into the host operating system, and enables network services. The Application layer does not include specific applications that provide services, but rather provides the capability for services to operate on the network.
The Application layer is associated with the data that is generated by a service or a protocol. A security device operating at the Application layer makes security decisions based on the actual data within a data stream.
|Presentation (Layer 6)||The Presentation layer formats or "presents" data into a compatible form for receipt by the Application layer or the destination system. Specifically, the Presentation layer ensures:
|Session (Layer 5)||The Session layer's primary function is managing the sessions in which data is transferred. Functions at this layer include:
|Transport (Layer 4)||The Transport layer provides a transition between the upper and lower layers of the OSI model, making the upper and lower layers transparent from each other. Two protocols associated with the Transport layer are:
|Network (Layer 3)||The Network layer describes how data is routed across networks and on to the destination.
|Data Link (Layer 2)||The Data Link layer defines the rules and procedures for hosts as they access the Physical layer.
|Physical (Layer 1)||The Physical layer sets standards for sending and receiving electrical signals between devices. Hubs operate at the physical layer because they simply forward electrical signals out all hub ports without interpreting the meaning of those signals that are present at higher layers.|