The main functionality of application layer is getting data on to the network.

Application Layer Functionality and Protocols : - 

• When we try to  access information on a device, the data may not be physically stored on that device.
• If that is the case, a request must be made to the device where the data resides and we after that we can use the data.
• An application layer protocol defines how an application processes (clients and servers) , running on different end systems, pass messages to each other.
• In particular, an application layer protocol defines : -  The type , The syntax  The rule.
• Rules for determining when and how a process sends messages and responds to messages.