FTP is based on the client-server model of communication between computers: one computer runs a server program that makes information available to other computers. The other computers run client programs that request information and receive replies from the server.
To access an FTP server, users must be able to connect to the Internet or an intranet (via a modem or local area network) with an FTP client program.
A client-server session establishes two connections: a control channel that stays open for the entire session and a data channel that opens and closes to transfer data such as folder listings and files to or from the server as requested by the client. Normally, the control channel occurs on port 21, but WS_FTP Server can be configured to accept connections on any port.
The server runs continuously in the background and listens on a specified port (the standard port is 21) for a connection request from a client. When a client requests a connection, the server verifies the username and password and, if valid, listens to the control channel for the next command.
After a user logs on, his or her access to the host's folders and files is determined by permissions assigned to folders.
Note: The Internet Engineering Task Force (IETF) publishes Requests for Comments (RFCs) for all Internet standards. Each RFC defines a standard. You can view RFCs online by connecting to the IEFT Web site.