About this video

• Video Title: OSI Reference Model - Best Explanation
• Channel: Drunk Engineer
• Speakers: (Speaker names not explicitly mentioned in the transcript)
• Duration: 48:56

Overview

This video provides a detailed explanation of the OSI Reference Model, breaking down each of its seven layers. The presenter explains the purpose and function of each layer, starting from the Application layer down to the Physical layer, and discusses the protocols and devices associated with them. The video also touches upon the importance of open standards and how they foster competition and compatibility.

Key takeaways

• OSI Model Purpose: Developed by ISO to foster competition while maintaining compatibility, leading to open protocols that allow modification and adherence to core standards.
• Application Layer (Layer 7): Focuses on software development, programming, and acting as an interface to the network. Examples of protocols include HTTP, DNS, FTP, POP, and SMTP.
• Presentation Layer (Layer 6): Handles encoding, encryption (SSL/TLS), and compression of data to make it smaller for transmission.
• Session Layer (Layer 5): Manages dialogue and keeps network connections alive, especially when multiple applications and connections are running concurrently.
• Transport Layer (Layer 4): Responsible for delivering data segments to the correct application or service on the destination host. It uses protocols like TCP (reliable, ordered delivery) and UDP (unreliable, faster delivery) and manages ports for identifying applications/services.
• Network Layer (Layer 3): Handles routing of data packets across networks using IP addresses (IPv4 and IPv6). Routers are the key devices at this layer.
• Data Link Layer (Layer 2): Manages data frames, including adding headers and trailers. It uses MAC addresses for physical addressing and ensures data is transmitted correctly over a physical link (e.g., Ethernet).
• Physical Layer (Layer 1): Deals with the physical transmission of raw data (bits) over a medium, such as copper wires or fiber optics, handling electrical signals and timing.
• Encapsulation: Data is progressively encapsulated with headers (and a trailer at Layer 2) as it moves down the OSI layers, and de-encapsulated as it moves up.
• Open Standards vs. Proprietary Protocols: The video emphasizes the benefits of open protocols, like IP, for widespread adoption and compatibility, contrasting them with proprietary systems that can limit interoperability.

The three main responsibilities of the Presentation Layer are:

• Encoding: Translating data into a format that the network can understand, such as ASCII.
• Encryption: Securing data transmission using protocols like SSL or TLS.
• Compression: Reducing the size of data to minimize bandwidth usage.

The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a telecommunication or computing system in terms of abstraction layers. It breaks down the communication process into seven distinct layers, each with specific responsibilities.

Here's a thorough description of each layer:

1. Layer 7: Application Layer

   • Purpose: This is the layer closest to the end-user. It provides network services directly to end-user applications. It's responsible for identifying communication partners, determining resource availability, and synchronizing communication.
   • Functions: User interface, network services (email, file transfer, web browsing), application-specific protocols.
   • Examples: HTTP, FTP, SMTP, DNS, POP3.

2. Layer 6: Presentation Layer

   • Purpose: This layer ensures that data sent by the Application layer of one system can be understood by the Application layer of another system. It acts as a translator.
   • Functions: Data translation (character encoding, e.g., ASCII), data encryption/decryption, data compression/decompression.
   • Examples: SSL/TLS (for encryption), JPEG, GIF (for data formatting).

3. Layer 5: Session Layer

   • Purpose: This layer establishes, manages, and terminates connections (sessions) between applications. It controls dialogues between hosts.
   • Functions: Session establishment, maintenance, and termination; dialogue control (managing turn-taking in communication); synchronization (adding checkpoints to data streams).
   • Examples: NetBIOS, RPC (Remote Procedure Call).

4. Layer 4: Transport Layer

   • Purpose: This layer provides reliable or unreliable data transfer services to the upper layers. It segments data from the upper layers and reassembles it at the destination. It ensures complete data transfer.
   • Functions: Segmentation and reassembly, connection control (establishing and maintaining connections), flow control (managing data transmission rates), error control (detecting and correcting errors).
   • Protocols:
     • TCP (Transmission Control Protocol): Reliable, connection-oriented, ordered delivery, error checking, flow control. Used for applications where data integrity is critical (e.g., web browsing, email).
     • UDP (User Datagram Protocol): Unreliable, connectionless, faster, less overhead. Used for real-time applications where speed is more important than perfect accuracy (e.g., streaming video/audio, online gaming, DNS).
   • Key Concepts: Ports (used to identify specific applications/services on a host).

5. Layer 3: Network Layer

   • Purpose: This layer is responsible for routing packets of data from source to destination across potentially multiple networks. It handles logical addressing and path determination.
   • Functions: Logical addressing (IP addresses), routing (finding the best path across networks), packet forwarding.
   • Protocols: IP (Internet Protocol - IPv4, IPv6), ICMP (Internet Control Message Protocol).
   • Devices: Routers.

6. Layer 2: Data Link Layer

   • Purpose: This layer provides error-free transfer of data frames between two directly connected nodes. It handles physical addressing and manages access to the physical medium.
   • Functions: Framing (organizing data into frames), physical addressing (MAC addresses), error detection (often using a trailer like CRC), flow control (on a point-to-point link), media access control (determining how devices share the network medium).
   • Sublayers: Logical Link Control (LLC) and Media Access Control (MAC).
   • Protocols: Ethernet, PPP (Point-to-Point Protocol), Wi-Fi (802.11).
   • Devices: Switches, Bridges, Network Interface Cards (NICs).

7. Layer 1: Physical Layer

   • Purpose: This layer defines the physical characteristics of the network. It's responsible for the actual transmission and reception of unstructured raw data bits over a physical medium.
   • Functions: Defines physical specifications (cables, connectors, voltages, data rates), transmission mode (simplex, half-duplex, full-duplex), bit synchronization, signal encoding.
   • Media: Copper cables (e.g., Ethernet Cat5e/6), fiber optic cables, wireless radio waves.
   • Devices: Hubs, Repeaters, Modems, NICs (physical component).

Key Principles:

• Encapsulation: As data moves down the layers, each layer adds its own header (and sometimes a trailer) containing control information. This process is called encapsulation. At the receiving end, de-encapsulation reverses this process.
• Abstraction: Each layer only needs to interact with the layers directly above and below it, simplifying the overall design and maintenance of network systems.
• Standardization: The OSI model provides a standardized way to discuss and design network architectures, promoting interoperability between different vendors and technologies.

While the OSI model is a theoretical model, the TCP/IP model is the practical implementation that underlies the internet. However, the OSI model's seven-layer structure is still widely used for teaching and understanding network functions.

In the context of the OSI model, when we say SNMP operates at the Application Layer, it means:

1. User-Facing Functionality: SNMP is designed to interact with network management applications that users or network administrators use directly. These applications are built upon the services provided by the Application Layer.
2. Protocol for Network Management: SNMP is a protocol specifically created for the purpose of network management. This includes monitoring network devices, collecting information about their status and performance, and even configuring them. These are high-level tasks that directly serve the needs of network administrators.
3. Data Preparation and Formatting: At the Application Layer, protocols define how data is formatted and presented for the user or the managing application. SNMP defines specific message formats (like GET, SET, TRAP) that are used to query devices or receive notifications from them.
4. Interoperability with Lower Layers: While SNMP itself resides at the Application Layer, it relies on the services of the layers below it (Presentation, Session, Transport, Network, Data Link, Physical) to actually transmit its management messages across the network. For example, SNMP messages are typically carried over UDP (User Datagram Protocol) at the Transport Layer, which is then handled by the Network Layer and so on down to the physical medium.

In essence, saying SNMP is an Application Layer protocol means it's one of the protocols that end-users or their management software directly interact with to perform network management tasks, and it utilizes the services of the lower layers to accomplish its communication goals.