In the Internet model, the application layer is an abstraction layer reserved for communications protocols and methods designed for process-to-process communications across an Internet Protocol (IP) computer network. Application layer protocols use the underlying transport layer protocols to establish process-to-process connections via ports.
In the OSI model, the definition of its application layer is narrower in scope. The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it.[2]
It separates functionality above the transport layer at two additional levels, the session layer and the presentation layer. OSI specifies strict modular separation of functionality at these layers and provides protocol implementations for each layer. The interface – responsible for displaying the information received to the user.
The application layer consists of what most users think of as programs. The application does the actual work at hand. Although each application is different, some applications are so useful that they have become standardized.
The Internet has defined standards for:
File transfer (FTP):
Connect to a remote machine and send or fetch an arbitrary file. FTP deals with authentication, listing a directory contents, ascii or binary files, etc.
Remote login (telnet):
A remote terminal protocol that allows a user at one site to establish a TCP connection to another site, and then pass keystrokes from the local host to the remote host.
Mail (SMTP):
Allow a mail delivery agent on a local machine to connect to a mail delivery agent on a remote machine and deliver mail.
News (NNTP):
Allows communication between a news server and a news client.
Web (HTTP):
Base protocol for communication on the World Wide Web.
