Application Layer | Functions, Applications, And Their Protocols


What is Application Layer

The application layer is the topmost layer of open service interconnection OSI and TCP/IP (Transmission control protocol/Internet Protocol) layered model layers.

This is the layer that directly communicates with the user or user application.

It is very important to have this layer for all the applications coming into the communication system.

Any user may or may not communicate directly with the application, but the application layer is the place where the user interacts realistically.

Since this layer is at the top of the OSI model, there is no need to serve any other layer.

The application layer takes data from all the layers below it, such as the transport layer, and transmits it to the remote host.

When the application layer protocol of the sender communicates with the application layer protocol of the receiver, it passes the information to the transport layer.

Work is carried out by the transport layer with the help of all the layers below him.

Functions of Application Layer

The application layer has the following functions that are important:

  • Mail services
  • Network Virtual Terminal
  • Directory Services
  • File Transfer Access and Management

Mail Services –

This layer provides the basis for forwarding email and then storing data.

Network Virtual Terminal –

It allows the user to log in to a server or host. This makes the remote host’s terminal a software emulator.

The computer system of any user talks to that software terminal and then communicates with the terminal host.

Due to this, the host feels that he is talking to a host terminal of his own and the user knows by logging in easily.


Directory Services –

This layer provides information about different services worldwide.

File Transfer Access and Management –

From this layer, we use to access the file and then manage it. There is also a standard system for extracting and then reading files from a remote computer system.

Application Layer Issues

This layer also has its own problems which we are listing below:

There are some problems that occur when designing and implementing application layer protocols and are fixed by different different pattern languages.

  • From the pattern language of the application-level communication protocol.
  • By service design pattern.
  • From the pattern of enterprise application architecture.
  • By pattern-oriented software architecture.

Application Layer protocols

To understand the functioning of this layer, it is very important to understand its protocols. These protocols can be divided into two parts in the application layer case.

Which are used by the user. As an example; E-mail.
Which help the protocols used by the user:

Now we will learn about all those protocols one by one –

  • DNS Protocol
  • HTTP Protocol
  • FTP Protocol
  • POP Protocol
  • SMTP Protocol

DNS Protocol

The DNS protocol works on the client-server model. This protocol uses the UDP protocol for transport layer communication. It works in hierarchical domains based on the naming scheme.

First the DNS server has to be configured with Fully Qualified Domain Names (FQDN) and then the email addresses have to be mapped with its IP address.

In this way the DNS server is requested by the FQDN and then it responds by its IP address. It uses UDP port 53.

HTTP Protocol –

Hypertext transfer protocol is the most important protocols on which the entire internet is based. This is the basis of the World Wide Web ie www.

Hypertext transfer protocol is a very well-organized documentation system that connects all the pages of a text document together with the help of hyperlinks.

HTTP works on the client-server model. When the user wants to access any WEB page on the Internet, the client machine establishes a TCP connection to the server that is based on port 80.

After its process, the server accepts that request and when the user is able to access that web page.

FTP Protocol –

FTP (File Transfer Protocol), protocol is the most commonly used protocol for exchanging all files.

FTP uses the TCP/IP (Transmission control protocol/Internet Protocol) protocol for communication and uses port 21 of TCP.

FTP operates on the client-server protocol where it allows access to server resources at the request of the client.

It works on controlling the out of band.

POP Protocol –

POP is a retrieval protocol used by user agents (client email software) to extract mail from a mail server.

POP works on two modes:

Delete Mode – In this, the email is deleted after downloading it to the user’s computer.
Funnel mode – In this, the email is kept in the server even after it passes to the user.

SMTP Protocol –

SMTP (Simple Mail Transfer Protocol), protocol to transfer electronic mail from one user to another. This is done by the user client software used by the user, which we also call as user agents.

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