FTP Dual-Channel Architecture Simulation

What is File Transfer Protocol (FTP)?

File Transfer Protocol (FTP) is an Application Layer protocol running over TCP. It utilizes an out-of-band architecture splitting session logic into two discrete channels: a Control Channel (Port 21) for commands/responses, and a separate Data Channel (Port 20 or High Random Ports) for raw file streams and directory listings.

Active Mode vs. Passive Mode FTP

• Active Mode (PORT): The client opens a control socket (e.g., TCP 1026 -> 21) and sends a PORT command telling the server which local IP/port to connect to. The FTP server then initiates the data connection from its Port 20 back to the client. This frequently fails behind client NATs/Firewalls.
• Passive Mode (PASV): The client sends a PASV command. The server opens an unprivileged ephemeral data port (e.g., TCP 50001) and returns it to the client. The client initiates both control and data connections outward, making it firewall-friendly for clients.

FTP Command & Response Codes

• 220: Service ready for new user.
• 331: User name okay, password required.
• 230: User logged in, proceed.
• 200 PORT command successful: Active mode channel established.
• 227 Entering Passive Mode (10,0,0,100,195,81): IP and calculated port (195*256 + 81 = 50001).
• 150 / 226: File status okay; opening/closing data connection.

Secure FTP Alternatives

• FTPS (FTP over SSL/TLS): Encrypts FTP control and data streams using TLS while preserving the dual-port FTP model (Port 990 implicit / Port 21 explicit).
• SFTP (SSH File Transfer Protocol): Entirely unrelated to legacy FTP. Runs completely inside a single SSH tunnel on TCP Port 22.