File Transfer Protocol (FTP)

FTP is an application-layer protocol (RFC 959) for transferring files between a client and a server over TCP. It uses two separate connections.

Control Connection

• Uses server port 21; opened first and stays open for the whole session
• Carries commands (USER, PASS, LIST, RETR, STOR, QUIT) and replies (status codes)
• Text (NVT ASCII) based; handles login/authentication

Data Connection

• Uses server port 20 (active mode)
• Opened for each file transfer and closed afterward
• Carries the actual file or directory listing

Modes

• Active: server initiates the data connection to the client.
• Passive: client initiates the data connection (works through firewalls/NAT).

Working

1. Client opens the control connection to port 21 and logs in.
2. Client sends a command (e.g. RETR file).
3. A data connection is set up and the file is transferred.
4. The data connection closes; the control connection remains for further commands until QUIT.

Using separate connections (out-of-band control) allows commands to be sent even during a transfer.