The FTP operational model describes the distinct logical data and control channels that are established between an FTP client (user) and an FTP server. Before the data connection can be used to send actual files, the control connection must be established. A specific process is followed to set up this connection and thereby create the permanent FTP session between devices that can be used for transferring files.

As with other client/server protocols, the FTP server assumes a passive role in the control connection process. The server protocol interpreter (Server-PI) “listens” on the special well-known TCP port reserved for FTP control connections: port 21. The User-PI initiates the connection by opening a TCP connection from the user device to the server on this port. It uses an ephemeral port number as its source port in the TCP connection.

Once TCP has been set up, the control connection between the devices is established, allowing commands to be sent from the User-PI to the Server-PI, and reply codes to be sent back in response. The first order of business after the channel is operating is user authentication, which the FTP standard calls the login sequence. There are two purposes for this process:

• Access Control: The authentication process allows access to the server to be restricted to only authorized users. It also lets the server control what types of access each user has.
  
  
• Resource Selection: By identifying the user making the connection, the FTP server can make decisions about what resources to make available to the user.