CISSP 11.1 - OSI Model (Part 2 of 2)

This episode of the ISC2 Certified Information Systems Security Professional (CISSP) exam prep series continues the Domain 4 walk down the network stack, working through the lower layers and the gear that lives at each one. Every control you deploy — encryption, routing rules, switch settings, cabling — attaches to one of these layers, and naming the layer tells you exactly what a device can see and protect.

Frequently Asked Questions

What does the presentation layer handle?

It is the translator: it reshapes data into a standard format any compliant system can read, and it is also where encryption and compression traditionally sit. On internet protocol networks there is no real presentation layer, because reformatting for transport is rarely needed. Encryption still happens, just in other places, such as before data enters the stack, at the transport layer, inside a VPN, over wireless, or in bulk at the physical layer.

What does the session layer manage?

It is the conversation coordinator: it opens, maintains, and closes the dialogue between two machines, sets checkpoints so a broken exchange can resume, and decides the direction of flow. That direction comes in three modes — simplex is one-way only, half-duplex is two-way but one side at a time, and full-duplex is two-way at once. On internet protocol networks there is no distinct session layer; those duties get handled by the transport protocol, or not at all.

What does the transport layer guarantee?

It manages the integrity and control of the connection, establishing a logical link between two nodes and defining how big each segment can be, how integrity is checked, and how lost data is spotted. It handles sequencing, error checking, flow control, and multiplexing, like a shipping manager who numbers every carton and reorders whatever went missing. Its main occupants are the reliable transport protocol, its lightweight connectionless cousin, and transport layer security.

What does the network layer decide?

It handles logical addressing and routing: the header here carries the source and destination IP addresses, and the layer chooses the path but does not promise delivery. The device that rules this layer is the router, which reads the destination IP address and picks the best path by speed, hop count, and preference using its routing table.

What do the data link and physical layers do?

The data link layer formats the frame for the specific network technology, such as Ethernet, and stamps on the source and destination MAC addresses — 48-bit hardware addresses whose first half identifies the manufacturer. Its device is the switch, which forwards each frame out the correct port by destination MAC address. The physical layer converts frames into bits as electricity, light, or radio, and is home to network cards, hubs, repeaters, and amplifiers.

Reference: This article is based on concepts discussed in CISSP 11.1 - OSI Model (Part 2 of 2).