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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:
  • Formatting and translation of data between systems.
  • Negotiation of data transfer syntax between systems, through converting character sets to the correct format.
  • Encapsulation of data into message envelopes by encryption and compression.
  • Restoration of data by decryption and decompression.
Session (Layer 5) The Session layer's primary function is managing the sessions in which data is transferred. Functions at this layer include:
  • Management of multiple sessions (each client connection is called a session). A server can concurrently maintain thousands of sessions.
  • Assignment of the session ID number to each session to keep data streams separate.
  • Set up, maintain, and tear down communication sessions.
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:
  • The Transmission Control Protocol (TCP) provides services that ensure accurate and timely delivery of network communications between two hosts. TCP provides the following services to ensure message delivery:
    • Sequencing of data packets
    • Flow control
    • Error checking
    TCP is referred to as a connection-orientedprotocol because it includes these delivery guarantees.
  • The User Datagram Protocol (UDP) is similar to TCP, but does not include mechanisms for ensuring timely and accurate delivery. Because it has less overhead, it offers fast communications, but at the expense of possible errors or data loss. UDP is referred to as a connectionless protocol because it lacks these delivery guarantee mechanisms.
Network (Layer 3) The Network layer describes how data is routed across networks and on to the destination.
  • Protocols associated with the Network layer include IP and IPX.
  • The logical host address, in the form of the IP address, is defined at the Network layer.
  • Routers operate at the Network layer by reading the IP address in the packet to make forwarding decisions.
Data Link (Layer 2) The Data Link layer defines the rules and procedures for hosts as they access the Physical layer.
  • The physical device address, in the form of the MAC address used with Ethernet, is defined at the Data Link layer.
  • Network interface cards (NICs) contain the MAC address and perform functions at the Data Link layer.
  • Switches operate at the Data Link layer by reading the MAC address in a frame to make forwarding decisions.
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.