【My Study Note】Application Layer
Application Layer
This article is about how those actual applications send and receive data using the application layer. Just like with every other layer, TCP segments have a generic data section to them. That payload section is actually the entire contents of whatever data applications want to send to each other.
It could be the contents of a web page if a web browser is connecting to a web server. This could be the streaming video content of your Netflix app on your PlayStation connecting with the Netflix servers. It could be the contents of a document your word processor is sending to a printer. And many more things.
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. At the network layer, 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, it wouldn’t make sense for us to cover them.
Even so, one concept you can take away about application layer protocols is that they’re still standardized across application types. There are lots of different web browsers. You could be using Chrome, Internet Explorer, Safari, you name it.
They’ll need to speak the 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, and nginx. But they also need to speak the same protocol.
This way, you ensure that no matter which browser you’re using, you’d still be able to be speak to 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 specifications in order to ensure interoperability.
The same is true for most other classes of application. You might have dozens of choices for an FTP client, but they all need to speak the FTP protocol in the same way.