My notes on the 7 Layers of the OSI model. Taken during CISSP revision.

HOST layers

User

The thing between the keyboard and the chair, and only interacts directly with the application. They don’t directly use the lower levels.

Application (layer 7)

The thing the user interacts with. The ‘program’ or ‘app’ or ‘exe’.

Aspects that live in layer 7 are authentication, authorisation, web browsers, http, and general end-user services.

Layer 7 deals with DATA.

Presentation (layer 6)

Data representation. Encryption and compression. Translating formats to a suitable mode for use on the network.

Encrypting data for network transmission. Dealing with MIME types. MPEG, ASCII, SSL, TLS, JPEG and other image formats.

Anything that translates between network needs and application need is in the presentation layer.

Layer 6 deals with DATA.

Session (layer 5)

Inter-host communication between two hosts.

Logical ports.

Management of the ‘session’ or connection on the behalf of applications. Network sockets. Network APIs. Connections to SQL databases. NFS and SMB connections. SIP.

Authentication, permissions, and session restoration.

Layer 5 deals with DATA.

Transport (layer 4)

End-to-End connections with error and flow control.

Transport protocols such as TCP and UDP, with error recovery and flow control.

Layer 4 deals with SEGMENTS.

MEDIA layers

Network (layer 3)

Path determination.

Switches and routers. The management of pathways through the network of data links. The technology that makes only the appropriate data links used based on packet destination.

Virtual circuits, address handling, routing and forwarding.

IP, IPSec, ICMP.

The common example of this is ‘IP’ or Internet Protocol. Address schemes in use are IPv4 and IPv6.

Layer 3 deals with PACKETS.

Physical addressing.

Things that connect to the physical layer, with MAC addresses. Examples: Wifi cards, 802.11, Ethernet.

Encoding and decoding of data packets happens in layer 2.

Layer 2 deals with FRAMES.

Physical (layer 1)

Signalling and binary transmission.

Wires. Radio waves. Carrier pigeons. Electro/mechanical objects in the real world used as the medium for transferring data.

Layer 1 deals with BITS.

Mnemonics

Ok I might have gotten carried away here, but writing these helps me remember. The ‘sausage pizza’ is found elsewhere and commonly used.

Alsatian Puppies Salivate Thinking Nice Delicious Popcorn

Allow Purple Saws To Notch Dangerous Powercables

Any Partridge Seen Thrice Never Disappoints People

Army Personnel Stand Tall Near Deputy President

A Plucky Stockholder Takes Nearly Double Percentages

(inverted) Please Do Not Throw Sausage Pizza Away