The OSI model outlines seven layers: application, presentation, session, transport, network, data link, and physical. The first three layers define and implement domain-specific exchanges between hosts. The last four layers provide for the more-or-less reliable end-to-end transmission between the local process and a remote.
Application protocols allow the network application to exchange domain-specific data with a network application running on a remote host. For a file transfer application, the domain-specific data is the files and their properties. In a daytime application, the domain-specific data is the current date and time.
An application protocol is a pre-agreed syntax of commands and responses that allow the two processes to exchange this domain-specific data. Our file transfer application uses the File Transfer Protocol (FTP), which was defined in RFC 959. The daytime application uses the Daytime Protocol (DTP), which is defined in RFC 867. Both protocols date back to the ARPA days of the Internet.
The application layer is also responsible for providing an interface to external systems, such as users. The application translates user input into application data, which is sent to the remote application according to the rules of the application protocol. When the user selects the “login” button, our file transfer application prompts the user for the server name, username, and password. It then translates this application data into an FTP ...