Application Layer

Application layer is the top most layer in OSI and TCP/IP layered model. This layer exists in both layered Models because of its significance, of interacting with user and user applications. This layer is for applications which are involved in communication system.

A user may or may not directly interacts with the applications. Application layer is where the actual communication is initiated and reflects. Because this layer is on the top of the layer stack, it does not serve any other layers. Application layer takes the help of Transport and all layers below it to communicate or transfer its data to the remote host.

When an application layer protocol wants to communicate with its peer application layer protocol on remote host, it hands over the data or information to the Transport layer. The transport layer does the rest with the help of all the layers below it.

There’is an ambiguity in understanding Application Layer and its protocol. Not every user application can be put into Application Layer. except those applications which interact with the communication system. For example, designing software or text-editor cannot be considered as application layer programs.

On the other hand, when we use a Web Browser, which is actually using Hyper Text Transfer Protocol (HTTP) to interact with the network. HTTP is Application Layer protocol.

Another example is File Transfer Protocol, which helps a user to transfer text based or binary files across the network. A user can use this protocol in either GUI based software like FileZilla or CuteFTP and the same user can use FTP in Command Line mode.

Hence, irrespective of which software you use, it is the protocol which is considered at Application Layer used by that software. DNS is a protocol which helps user application protocols such as HTTP to accomplish its work.

Functions of Application Layer

The functions of application layer are explained below −

• User Interface:  It represents the user interface to low-level layers and multiple application processes.
• Security:  It is responsible for the execution of the security tests at the user entity points.
• File Transfer: The application layer supports the file transfer access and management (FTAM). It enables customers to create files in a remote system to retrieve the documents from a remote system and to handle or control the files in a remote system.
• E-mail: It supports a basis for email forwarding and string.
• Database Access: It supports distributed database sources and global data about several objects and functions.
• Addressing: It is used for the connection between user and server. There is a requirement for addressing. When a user requests the server, the request includes the server address and its address. The server responds to the user request and the request consists of the destination address, i.e., client address.
• Directory Services: An application includes a distributed database that supports the global data about multiple objects and functions.