It is essential for Cyber Security Professionals to have a solid understanding of how computers communicate. There is much more happening behind the scenes of computer networks than what can be observed when using applications.
The OSI ("Open Systems Interconnection") model represents an easy and intuitive way to standardize the different parts required to communicate across networks.
The model makes it clear what is required to communicate on a network by splitting the requirements into multiple layers.
This is what the OSI Model looks like:
| Layer | What it does |
|---|---|
| 7 - Application | Where humans process data and information |
| 6 - Presentation | Ensures data is in a usable format |
| 5 - Session | Capable of maintaining connections |
| 4 - Transport | Data is forwarded to a service capable of handling requests |
| 3 - Network Layer | Responsible for which path packets should travel on a network |
| 2 - Data Link | Responsible for which physical devices packets should go to |
| 1 - Physical | The physical infrastructure to transport data |
The top 3 layers are typically implemented in software within the Operating System:
| Layer | Where it is implemented |
|---|---|
| 7 - Application | Software |
| 6 - Presentation | Software |
| 5 - Session | Software |
The bottom 3 layers are typically implemented in hardware within devices on the network, e.g. Switches, Routers and Firewalls:
| Layer | Where it is implemented |
|---|---|
| 3 - Network Layer | Hardware |
| 2 - Data Link | Hardware |
| 1 - Physical | Hardware |
Layer 4, the Transport layer, connects the software with the hardware layers.
SDN ("Software Defined Networking") is technology which allows more layers of the hardware to be implemented via software.
The business logic and functionality of the application lies here. This is what the users use to interact with services across a network. Most developers create applications on the Application Layer.
Most of the applications you use are on the Application Layer, with the complexity of the other layers hidden.
Examples of Layer 7 Applications:
There are many applications which uses these protocols like Google Chrome, Microsoft Skype and FileZilla.
You are accessing this class via Layer 7!
Typically an unseen layer, but is responsible of adapting, transforming and translating data. This is to ensure the application and layers beneath can understand one another.
This layer's responsibility is handling connections between the application and the layers below. It involves establishing, maintaining and terminating connections, otherwise referred to as sessions.
Common protocols which represent the Session Layer well are:
The layer which allows applications to be represented on the network.
Some well known applications on this layer:
A layer responsible of routing packets between networks via routers.
On this layer, the following protocols reside:
Link networks, as the name implies, consist of protocols designed to send packets through the actual links (physical connections) that network nodes are connected to. A simpler way of thinking of it is that the Link Layer is responsible for moving data from physical over to logical (to the network layer).
Protocols on this layer include:
Physical layer represents the signaling which allows bits and bytes to transfer between a physical medium. It can be transferred via radio or signals over a cable, using electrical signals or light, for example fiber.
Examples of the Physical Layer protocols includes: