OSI Model

Network communication happens in seven layers. Each layer has a specific job and only needs to care about the layers directly above and below it.

Layer 7: APPLICATION
|- What: Web browsers, email, chat, databases
|- Protocol: HTTP, HTTPS, SSH, DNS, FTP
`- Example: OpenWebUI (web interface)
 
Layer 6: PRESENTATION
|- What: Encryption, compression, formatting
`- Example: TLS (HTTPS uses TLS)
 
Layer 5: SESSION
|- What: Manages conversation flow
`- Example: WebSocket connections
 
Layer 4: TRANSPORT
|- What: Ensures data arrives correctly
|- Protocol: TCP (reliable), UDP (fast)
`- Example: TCP port 80 (HTTP), 443 (HTTPS)
 
Layer 3: NETWORK
|- What: Routes data across networks
|- Protocol: IP (v4, v6)
`- Example: 192.168.1.20 (Proxmox IP)
 
Layer 2: DATA LINK
|- What: Moves data between devices on same network
|- Protocol: Ethernet, WiFi
`- Example: MAC address (physical hardware address)
 
Layer 1: PHYSICAL
|- What: Cables, radio waves, electricity
`- Example: Cat6 ethernet cable, WiFi antenna

Layers Working Together

Here is what the stack looks like when a browser request leaves your machine and is unpacked on the server.

You open browser: http://192.168.1.20:8080
 
-> Sending
Layer 7: Browser sends GET /api/chat
Layer 6: Data compressed/formatted
Layer 5: WebSocket session established
Layer 4: TCP stream created
Layer 3: IP header added - from=192.168.1.100, to=192.168.1.20
Layer 2: Ethernet frame built, MAC addresses added
Layer 1: Electrical signals sent through cable
 
-> Receiving (server side, reversed)
Layer 1: Signals -> bits
Layer 2: Frame unpacked
Layer 3: IP packet routed to Proxmox
Layer 4: TCP stream assembled
Layer 7: nginx processes the request

The important practical point is that failures usually map to a layer. A DNS issue is not the same class of problem as a dropped TCP connection or a broken cable.

OSI Model

Layers Working Together

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