The Application Layer: Sending and Receiving Data

Welcome to our lesson about the application layer. We're almost done covering all aspects of our networking model, which means you've already learned how computers process electrical or optical signals to send communication across a cable at the physical layer. We've also covered how individual computers can address each other and send each other data using Ethernet at the data link layer. We've discussed how the network layer is used by computers and routers to communicate between different networks using IP. In our last lesson, we covered how the transportation layer ensures that data is received and sent by the proper applications. You're chock-full of layers of new information. Now, we can finally talk about how those actual applications send and receive data using the application layer.

📦 The Payload Section

Just like with every other layer, TCP segments have a generic data section to them. As you might have guessed, this payload section is actually the entire contents of whatever data applications want to send to each other. It can be the contents of a webpage if a web browser is connecting to a web server. It could be the streaming video content of your Netflix app on your PlayStation connecting to the Netflix servers. It could be the contents of a document your word processor is sending to a printer, and many more things.

🔄 Diverse Application Layer Protocols

There are a lot of protocols used at the application layer, and they are numerous and diverse. At the data link layer, the most common protocol is Ethernet. I should call out that wireless technologies do use other protocols at this layer. At the network layer, the use of IP is everywhere you look. At the transport layer, TCP and UDP cover most of the use cases. But at the application layer, there are just so many different protocols in use that it wouldn't make sense for us to cover them all.

🌐 Standardized Application Layer Protocols

Even so, one concept you can take away about application layer protocols is that they are standardized across application types. Let's dive a little deeper into web servers and web browsers for an example. There are lots of different web browsers such as Chrome, Safari, and many more. They all need to speak the same protocol. The same thing is true for web servers. In this case, the web browser would be the client, and the web server would be the server. The most popular web servers are Microsoft IIS, Apache, NGINX, but they also need to all speak the same protocol. This way, you ensure that no matter which browser you're using, you'd still be able to communicate with any server. For web traffic, the application layer protocol is known as HTTP. All of these different web browsers and web servers have to communicate using the same HTTP protocol specification in order to ensure interoperability. The same is true for most other classes of applications. You might have dozens of choices for an FTP client, but they all need to speak the FTP protocol in the same way.

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The application layer plays a crucial role in enabling communication between different applications. By following standardized protocols, applications can seamlessly exchange data and provide interoperability across various platforms and devices.

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