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OSI - Open Systems Interconnection Model

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  • OSI - Open Systems Interconnection Model

    The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO), maintained by the identification ISO/IEC 7498-1.
    The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology. The model partitions a communication system into seven abstraction layers.

    Layer 7, the Application Layer
    The application layer functions as a user platform in which the user and the software processes within the system can operate and access network resources. Applications and software suites that we use on a daily basis are under this layer. Common examples include protocols such as FTP and HTTP.

    Layer 6, the Presentation Layer
    The presentation layer provides a translation of data that is understandable by the next receiving layer. Traffic flow is presented in a format that can be consumed by the receiver and can optionally be encrypted with protocols such as Secure Socket Layer (SSL).

    Layer 5, the Session Layer
    The session layer identifies established system sessions between different network entities. When you access a system remotely, for example, you are creating a session between your computer and the remote system. The session layer monitor and controls such connections, allowing multiple, separate connections to different resources. Common use includes NetBIOS and RPC.

    Layer 4, the Transport Layer
    The transport layer ensures the transport or sending of data is successful. This function can include error checking operations as well as working to keep data messages in sequence. At this layer we find the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

    Layer 3, the Network Layer
    The network layer determines the path of data packets based on different factors as defined by the protocol used. At this layer we see IP addressing for routing of data packets. This layer also includes routing protocols such as the Routing Information Protocol (RIP) and the Interior Gateway Routing Protocol (IGRP). This is the know-where-to-go layer.

    Layer 2, the Data Link Layer
    The data link layer works to ensure that the data it transfers is free of errors. At this layer, data is contained in frames. Functions such as media access control and link establishment occur at this layer. This layer encompasses basic protocols such as 802.3 for Ethernet and 802.11 for WiFi.

    Layer 1, the Physical Layer
    The physical layer consists of the physical media and dumb devices that make up the infrastructure of our networks. This pertains to the cabling and connections such as category 5e and RJ45 connectors. Note that this layer also includes light and rays, which pertain to media such as fiber optics and microwave transmission equipment. Attack considerations are aligned with the physical security of site resources. Although not flashy, physical security sill bears much fruit in penetration testing and real-world scenarios.

    Data transported over a network is in a Frame at the Data Link layer, a Packet at the Network layer. In the Transport layer, TCP transport data in a Segment and UDP in a Datagram. At higher levels Protocol Data Units (PDUs) and Service Data Units (SDUs) are used..
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