Week 2 Notes: OSI Reference Model Chapter 5 - savannahc502/SavC-TechJournal-NET215 GitHub Wiki

The TCP/IP Guide by Charles M. Kozierok, Chapter 5: General OSI Reference Model Issues and Concepts

Notes on The Reading:

History of the OSI Reference Model:

  • OSI (Open Systems Interconnection) reference model provides a framework for designing networks and explaining how they work (universal language) that is widely accepted.
  • Created in 1980's by the merging of two reference models created by ISO and CCITT
  • O.G. purpose was not for education, but to create a widely adopted suite of protocols called the OSI Protocol Suite. However, TCP/IP became the standard.
  • Still very useful for education and development since it defines layers and concepts that make networks easier to understand and discuss.

The Benefits of Networking Models

  • Training and Documentation: Easier to explain in chunks
  • Specialization: Different layers provide time for specialization
  • Easier Design Modification and Enhancement: Easier to make changes
  • Modularity: Easier to interchange parts and procedures

Why Understanding the OSI Reference Model Is Important to You

  • Theoretical, but helpful in understanding networking protocols and technologies.

How to Use the OSI Reference Model

  • Use it but don't overuse it -- it's okay and normal if everything does not fit perfectly into the model.
  • Use it if it helps, don't use it if it makes things more complicated > it's a tool
  • The upper layer boundaries (session, presentation, application) get fuzzy with overlap.
  • OSI model best fits a LAN network, as there is a lot of overlap in a WAN
  • Think about the highest layer a device functions at.

OSI Reference Model Networking Layers, Sublayers, and Layer Groupings

  • Lower Layers (1-4): Physical, Data Link, Network, Transport. Deals with formatting, encoding, and transmitting data > moving it around.
  • Upper Layers (5-7): Session, Presentation, and Application. Deals with user interaction and implementing applications. The boundaries between these layers are less clear.
  • "As you go up the layer stack, you move away from concrete, hardware-specific functions to ones that are increasingly abstract, until you reach the realm of user applications at layer 7."

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"N" Notation and Other OSI Model Layer Terminology

  • Layer Names and Numbers
  • N Notation: "generically refers to a number within the computer world."
  • Protocols and Interfaces: Protocols are communications between logical or physical devices at the same layer of the model. Interfaces represent information moving between adjacent layers.
  • Network Stacks: an entire set of layers
  • Entities, Functions, Facilities, and Services

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Interfaces: Vertical (Adjacent Layer) Communication

  • Interfaces deal with transmitting data between layer N and Layer N+1 or N-1.
  • Vertical Communication

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  • This concept isn't always perfect, as some real-world implementations may cause, for example, a device in layer 7 to communicate with a layer 5 device.

Protocols: Horizontal (Corresponding Layer) Communication

  • "Each layer in the OSI Reference Model has a particular role (or roles)—a set of general tasks for which it is responsible."
  • Horizontal communication and logical communication
  • Protocol in the OSI Model "refers specifically to a set of communication rules, instructions, and procedures that describe communication between specific software or hardware elements running at the same layer on different machines within a network."

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Data Encapsulation, Protocol Data Units (PDUs), and Service Data Units (SDUs)

  • Messages sent between protocols are called protocol data units (PDUs).
  • Communications are logical, except for layer 1 which is hardware.
  • "The message used to communicate information for a particular protocol is called its protocol data unit (PDU) in OSI model terminology. That PDU is passed down to the next lower layer for transmission; since that layer is providing the service of handling that PDU, it is called the lower layer's service data unit (SDU). The SDU is encapsulated into that layer's own PDU and, in turn, sent to the next lower layer in the stack, proceeding until the physical layer is reached. The process is reversed on the recipient device. In summary, a layer N PDU is a layer N-1 SDU, which is encapsulated into a layer N-1 PDU."

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Indirect Device Connection and Message Routing

"In the OSI model, the process of routing occurs when data is sent not directly from transmitter to ultimate recipient, but indirectly through the use of an intermediate system. That device, normally called a router, connects to two or more physical networks, and thus has multiple interfaces to layer 2. When it receives data, the data passes up only to the network layer, where it is repackaged and then sent on the next leg of its journey over the appropriate layer 2 interface."

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