Why do we need application layer protocols?

Application layer protocols structure the rules and data formats for application to communicate over the network. It ensures the proper exchange of data.

Discuss some examples of application layer protocols.

Examples of application layer protocols are as follows: Email$($STMP$),$ FTP$($File transfer$)$ Domain Name System$($DNS$)$ and web browsing $($HTTP$).$

Let$$s consider one of the popular application layer protocols, HTTP to learn the principal operation of HTTP$.$ To carry on this discussion, we can do a small role play.

HTTP is a client$-$server protocol. Can you identify the key features of HTTP$?$

Identify a group member who could act as a web server and other three members can act as clients. So$,$ your network has one server and three clients.

Let$$s assume each client wants to access a web page stored in the web server and we use HTTP to communicate in the application layer level. You can now act as a server$-$client model communicating the right messages between each entity in sequence to get the information that each client wants. For example, Client $1$ could say: "Client $1$ initiating TCP connection with the Server". Then, the server replies with the correct response. You need to note down the correct messages in sequence.

You can maintain a timeline diagram as follows to show the messages that exchanged between the server and clients.Figure $1$: Client$-$server message exchange timeline

Figure $1$: Client$-$server message exchange timeline

Once you have done the above steps $($a to d$),$ now assume a Web cache which is also called a proxy server introduced to the network as shown in the following diagram.Figure $2$: Proxy server

Figure $2$: Proxy server

Let$$s do another role paying to understand the role of proxy server$/$ web cache. For this scenario, one group member who acted as a client can now act as a proxy server. You can assume that the proxy server has the objects that Client $1$ requested but does not have the objects requested by the Client $3.$

Now repeat Steps c and d for the scenario described in Step e$.$