Application Layer Protocols: Types & Uses

Instructor: Meghalee Goswami
This lesson explains the basics of application layer protocols. It also elucidates some of the most commonly used protocols in the OSI Model application layer.

Understanding the Application Layer

The application layer is the top-most layer in the OSI Model and is used for establishing process-to-process communication and user services in a network. It is the interface between user applications and the underlying network. Whether you open a web page in a browser or read an email, you are interacting with the application layer of the network. In short, it is a layer which involves human interaction with applications and software to connect users together across the globe.

Application Layer Protocols Defined

A protocol is set if rules used to communicate between systems in a network. Although the application layer is the medium through which you are able to communicate with other users, a set of protocols are required to assist with this communication. For example, if you have to open a web page, you need the HTTP or HTTPS protocols. Similarly you would require POP3 or IMAP and SMTP for sending and receiving emails. Let us take a look at the various types of protocols with their uses.

Types of Application Layer Protocols

Application layer protocols can be broadly categorized as is shown in Figure 1.

Figure 1: Application Layer Protocols

Browsing Protocols

Hypertext Transfer Protocol (HTTP) and Hypertext Transfer Protocol Secure (HTTPS) are used when you attempt to open any web page online. You have probably seen that whenever you open a web page in a browser, http or https is always mentioned in the URL or web site address.

File Transfer Protocols

File Transfer Protocol (FTP) and Trivial File Transfer Protocol (TFTP) are used for transferring small and large files across a network. The only difference between the two protocols is that FTP allows authentication and TFTP does not.

Email Protocols

Post Office Protocol v3 (POP3) and Internet Message Access Protocol (IMAP) are used for retrieving an email from a server. The difference between them lies with how the original message is treated. POP3 downloads a copy of the email from the server and deletes the original. IMAP downloads a copy from the server but does not remove the email from the server until the user himself deletes it. Simple Mail Transfer Protocol (SMTP) is the protocol used to send emails from one account to another.

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