Osi Model and Packet Filtering

OSI Model and Packet Filtering

OSI model provides easier understanding of the hierarchical approach of data communication. It contains 7 layers, each with specific functions and responsibilities. Even though the only layer that provides the actual computer to computer communication is the physical layer, each of the seven layers adds its own data overhead to the payload that is then removed on the receiving side and processed by the corresponding layer. This approach creates virtual layer to layer communication.

Physical layer

This is the lowest level. Electrical current, optical signals or radio waves are being transmitted as logical ones or zeroes at this level. This is being commonly achieved by either voltage present as logical one, or no voltage as logical zero in electrical signals, or similarly in optical signals. Devices such as NIC, hubs, repeaters, as well as wires, fiber optic cables, radio waves, etc. operate at the physical layer. From the security point of view, interception is performed at this level by the third party. This can be performed in the form of a physical connection to the media or by a reception of the radio waves. Equipment residing at this level should be kept if possible in secured location to limit points of interception. This applies mostly to devices such as hubs and repeaters.

Data link layer

Its main responsibility is to establish an error free local connection (computers on the same LAN), and to divide data into frames that can be transmitted through the physical layer. It is not totally passive as physical layer and provides error checking such as checksum (CRC). Data layer is known for its two sublayers, LLC and MAC. Media Access Control (MAC) sublayer is responsible for direct communication with NIC, defines the logical topology and sharing of the same media. Physical device address known as MAC address resides at this sublayer. CSMA/CA and CSMA/CD are examples of MAC sublayer protocols. Logical Link...