Card 09/ 09
Recap: The OSI Model
Seven layers, one header each, and the habit of asking which layer a fault lives at.
The OSI model is a map of network communication in seven layers, and every later topic is one part of that map.
- Seven layers, one job each, from Physical at 1 to Application at 7. Each depends only on the layer below, so one can change without the others.
- Encapsulation: on the way down each layer wraps the data in its own header; on the way up each removes it. A layer reads only its own header.
- The units: bits, frames, packets, segments or datagrams, and messages. The name says which header is on the outside.
- TCP/IP has four layers and is what the internet runs on. The top three OSI layers fold into its application layer.
- A device sits at the highest layer it reads to decide: switch 2, router 3, load balancer 4 or 7.
- Some protocols span layers. Describe TLS, ARP and QUIC by what they run on and what they provide.
| Layer | Holds | Lives for |
|---|---|---|
| 7 Application | Messages: HTTP, DNS | The program's service |
| 6 Presentation | Formats and encodings | Agreeing what the bytes mean |
| 5 Session | Conversations | Keeping an exchange together |
| 4 Transport | Segments and datagrams: TCP, UDP | Reaching the right program |
| 3 Network | Packets: IP; routers | Crossing networks |
| 2 Data Link | Frames: Ethernet; switches | Reaching the next device |
| 1 Physical | Bits; hubs and cables | Moving the signal |
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