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TCP/IP Guide by Charles M. Kozierok

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FTP Control Connection Establishment, User Authentication, and Anonymous FTP Access

You just saw how FTP uses 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 ...

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