Cisco Networking: OSI Model Layer 7 - Application

The application layer is the highest level in the Open System Interconnection (OSI) model and is the level that is closest to you — or furthest away from you if you are at the other end of the connection. The application layer effectively moves data between your computer and the server.

It is comprised of the server application that reads and writes files, such as the Apache Web Server or Microsoft’s Internet Information Services (IIS), as well as the application you use at the client computer to write or read that data (such as an e-mail program).

Following the flow of data in the following figure, the communication process between the application layer on a client computer and server is illustrated.

  1. The process starts at the application layer with someone retrieving and opening an e-mail in an e-mail program, such as Microsoft Outlook.

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    Outlook is the client program which operates the OSI model’s application layer. Outlook communicates with application layer processes or programs being used on a server. In this example, when you start Outlook, it starts the process to request all e-mail from that server for the user.

    Any program may perform functions for other layers, such as the presentation layer by formatting the data in ASCII, Unicode, or Multipurpose Internet Mail Extensions (MIME), which is where things get muddy in comparing reality to ideals.

  2. The server’s Post Office Protocol version 3 (POP3) service responds to the request for the user’s e-mail.

    The POP3 service runs at the application layer, and will retrieve the e-mail from the hard drive and prepares the e-mail message to be sent to the user’s computer.

  3. The data passes through the presentation, session, transport, network, data link, and physical layers, across the physical media, and backs up through all those layers on the client side of the connection.

  4. The data arrives at the application layer on the destination computer, and is then displayed using a program such as Outlook.

For most of the components on the Internet you deal with, you likely already refer to the application-level components:

  • Hypertext Transfer Protocol (HTTP): Protocol for transferring data from web servers to web browser applications like Microsoft Internet Explorer.

  • Simple Mail Transfer Protocol (SMTP): Protocol for sending mail from one computer to another. This could be from an e-mail client, like Microsoft Outlook, to an e-mail server, or from one e-mail server to another.

  • Post Office Protocol version 3 (POP3): Protocol for e-mail client application, like Microsoft Outlook, to retrieve e-mail from an e-mail server.

  • Network Time Protocol (NTP): Standard protocol for synchronizing time between computers and devices over the Internet.

  • Secure Shell (SSH): Remote terminal access protocol that encrypts all data sent over the network.

  • File Transfer Protocol (FTP): Transfers files to and from servers over the Internet.

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