OSI Model
The Open Systems Interconnection (OSI) model is a conceptual framework that describes how data moves through a network in seven abstraction layers. It was standardized by ISO as ISO/IEC 7498-1 (1994) and remains the canonical reference for teaching networking, even though real-world protocols (like the Internet suite) do not map cleanly onto all seven layers.
The seven layers
| Layer | Name | Role |
|---|---|---|
| 7 | Application | Network services directly to end-user applications (HTTP, SMTP, FTP) |
| 6 | Presentation | Data translation, encryption, compression (TLS/SSL, MIME) |
| 5 | Session | Session establishment, maintenance, termination (NetBIOS, PPTP) |
| 4 | Transport | Reliable end-to-end delivery, segmentation, flow control (TCP, UDP) |
| 3 | Network | Logical addressing and routing (IP, ICMP, OSPF) |
| 2 | Data Link | Physical addressing and error detection on the local link (Ethernet, ARP, PPP) |
| 1 | Physical | Raw bit transmission over physical media (cables, radio, fiber) |
Key properties
- Encapsulation — Each layer wraps the layer above it with its own header (and sometimes trailer), creating a protocol data unit (PDU).
- Peer communication — Each layer on the sender communicates logically with the same layer on the receiver.
- Abstraction — A layer only needs to know how to interact with the layers immediately above and below it.
Security relevance
- Defense in depth — Security controls can be placed at multiple layers (firewalls at L3/L4, TLS at L6, application filtering at L7).
- Attack surface — Different attacks target different layers: ARP spoofing at L2, IP spoofing at L3, TCP hijacking at L4, protocol exploits at L7.
- Troubleshooting — The model helps isolate whether a failure is physical, local-link, routing, transport, or application-level.
Related: tcp (Layer 4), arp (Layer 2/3 boundary), smtp (Layer 7).
Sources
- ISO/IEC 7498-1:1994 — Information technology — Open Systems Interconnection — Basic Reference Model: The Basic Model
- wikipedia-osi-model