OSI Model

Network - Open Systems Interconnection Model

Why do we need a communication model?

  • Agnostic applications
    • Without a standard model, your application must have knowledge of the underlying network medium
    • Imagine if you have to author different versions of your apps so that it works on wifi vs ethernet vs LTE vs fiber
  • Network Equipment Management
    • Without a standard model, upgrading network equipment becomes difficult
  • Decoupled Innovation
    • Innovations can be done in each layer separately without affecting the rest of the models

What is the OSI Model?

  • 7 Layers each describe a specific networking component
  • Layer 7 - Application - HTTP/FTP/gRPC
  • Layer 6 - Presentation - Encoding, Serialization
  • Layer 5 - Session - Connection establishment, TLS
  • Layer 4 - Transport - UDP/TCP
  • Layer 3 - Network - IP
  • Layer 2 - Data link - Frames, Mac address Ethernet
  • Layer 1 - Physical - Electric signals, fiber or radio waves

The OSI Layers - an Example (Sender)

Example sending a POST request to an HTTPS webpage:

  • Layer 7 - Application
    • POST request with JSON data to HTTPS server
  • Layer 6 - Presentation
    • Serialize JSON to flat byte strings
  • Layer 5 - Session
    • Request to establish TCP connection/TLS
  • Layer 4 - Transport
    • Sends SYN request target port 443
  • Layer 3 - Network
    • SYN is placed in IP packet(s) and adds the source/dest IPs
  • Layer 2 - Data link
    • Each packet goes into a single frame and adds the source/dest MAC addresses
  • Layer 1 - Physical
    • Each frame becomes string of bits which converted into either a radio signal (wifi), electric signal (ethernet), or light (fiber)

Take it with a grain of salt, it's not always cut and dry

The OSI Layers - an Example (Receiver)

Receiver computer receives the POST request the other way around:

  • Layer 1 - Physical
    • Radio, electric or light is received and converted into digital bits
  • Layer 2 - Data link
    • The bits from Layer 1 are assembled into frames
  • Layer 3 - Network
    • The frames from layer 2 are assembled into IP packet
  • Layer 4 - Transport
    • The IP packets from layer 3 are assembled into TCP segments
    • Deals with Congestion control/flow control/retransmission in case of TCP
    • If Segment is SYN we don't need to go further into more layers as we are still processing the connection request
  • Layer 5 - Session
    • The connection session is established or identified
    • We only arrive at this layer when necessary (three way handshake is done)
  • Layer 6 - Presentation
    • Deserialize flat byte strings back to JSON for the app to consume
  • Layer 7 - Application
    • Application understands the JSON POST request and your express json or apache request receive event is triggered