The OSI Model in Practice

Seven layers, seven control sets

The OSI reference model breaks communication into seven layers so that problems — and controls — can be isolated. On the exam you will be asked what layer a device, protocol, or attack operates at.

7 Application — HTTP, SMTP, DNS, SNMP. Controls: application proxies, WAFs.
6 Presentation — formatting, ASCII/JPEG, encryption and compression services.
5 Session — establishes and tears down dialogs; RPC, NFS.
4 Transport — TCP (reliable, connection-oriented) and UDP (best effort). Controls: TLS, port filtering.
3 Network — IP routing, ICMP. Devices: routers. Controls: IPSec, ACLs.
2 Data Link — MAC addressing, framing. Devices: switches, bridges. Controls: 802.1X, port security.
1 Physical — bits on the wire. Devices: hubs, repeaters, cabling.

Memory aid

All People Seem To Need Data Processing (layers 7 to 1). The TCP/IP model collapses these into Application, Transport, Internet, and Link.