OSI – reference model

The Open Systems Interconnection (OSI) reference model was created by the Internet Standards Organization (ISO) and it is a reference model used for communication. Each of the seven different layers has different protocols and responsibilities. The Security+ exam focuses mainly on layers 2, 3, and 7:

Figure 1: OSI reference model

Here is a brief summary of each layer:

• Application layer: Applications, such as HTTP for accessing the web or SMTP for accessing email, are launched here. The Web Application Firewall (WAF), whose job it is to protect web-based applications, operates at this layer.
• Presentation layer: The presentation layer formats data into the character code, such as Unicode or ASCII. Encryption also works at this layer.
• Session layer: This is where you would create a session, such as logging in and logging out.
• Transport layer: Ports are either TCP or UDP. TCP is connection-orientated as it uses a three-way handshake and acknowledges the receipt of each packet...