OSI Model - The Upper Layers

The lines are a little blurry.....

In the next series of post I will be diving into each OSI layer. As part of the deep dive I will outline the overall goal of each layer and any protocols that map to each layer.

When the OSI model is referenced, most of the magic happens down in layers 4-1. That is where networking guys spend most of their time dealing with routing protocols, packet captures, bad cabling, or poor WiFi. But before we get to the fun stuff, we need to cover the upper layers 7-5.

Layer 7 - Application Layer

The application layer directly interacts with the end users. If a user wants to visit a web page or copy a file to a remote server, they will need to use a network-aware application. A key point is that the application layer does not refer to a specific application running on a computer. This layer refers to the software included in the application and operating system that allows the application to be network-aware.

Layer 7 Protocol examples:

• HTTP (Hypertext Transfer Protocol

  • used for web browsing.

• HTTPS (Hypertext Transfer Protocol Secure)

  • extension of HTTP that provides security mechanisms like encryption of HTTP traffic

• FTP (File Transfer Protocol)

  • used to transfer files between a client and a server

• SMTP (Simple Mail Transfer Protocol

  • used for sending emails

• POP3 (Post Office Protocol 3)

  • used for receiving emails

Data that is destined for the network is passed down from Layer 7 to Layer 6, the presentation layer. It is important to understand that each layer only interacts with the same layer on the other end of the interaction. Layer 7 data is passed down the stack of the sending device, sent across the network, and then decapsulated up the stack on the receiving device. The Layer 7 data that was transferred is read by the specific application running on the receiving device.

Layer 6 - Presentation Layer

The goal of the presentation layer is to transform the data into a format that the application will accept. I like to say that the goal is to make the data presentable. This layer is also referred to as the translation layer, as it serves as the translator for the network. The presentation layer’s primary responsibility is to provide the data format and encryption services to package and secure the data so the receiver can use it once it is received. Data formats include file types such as HTML, doc, jpg, or pdf.

Here are a few processes performed at Layer 6:

• Compression / Decompression

• Encryption / Decryption

• Code Translation

  • Example: EBCDIC to ASCII

Layer 6 Protocols:

• SSL (Secure Sockets Layer)

• TLS (Transport Layer Security)

Layer 5 - Session Layer

The goal of the session layer is to set up, maintain, synchronize, and tear down sessions between two end-user applications. A session is an individual conversation between two devices, so this layer's job is to open up the line of communication between the devices. A typical device will have hundreds of concurrent sessions. If you are on a Windows computer, open up a command prompt and run the command “netstat.” The output will provide a list of all active connections, including open and connected ports.

Layer 5 protocols:

• NetBIOS (Network Basic Input Output System)

• PAP (Password Authentication Protocol)

• PPTP (Point to Point Tunneling Protocol)

• RPC (Remote Procedure Call)

• L2TP (Layer 2 Tunneling Protocol)

When we cover the TCP/IP model, you will notice that OSI layers 7-5 all map to the single Application Layer. The lines between Layers 7-5 are blurred, but their overall goal is to format the application data and make it ready to send using the lower layer technologies. In the next post, I’ll break down the Transport Layer.

Let me know if I missed anything #HappyLearning