File Transfer Protocol (FTP) is a standard application-layer protocol used for transferring files between hosts on a TCP/IP network. It was designed for reliability and efficiency, especially across systems with differing file systems. FTP operates over TCP port 21 for control and establishes a separate data connection for file transfers.

OSI Layer: Application (Layer 7)

Default Ports:

• Control Channel: TCP/21

• Data Channel: TCP/20 (Active Mode) or random high port (Passive Mode)

  Encryption: None (plaintext)

Basic FTP Workflow

1. Client Initiates a Connection
   • Connects to the FTP server on port 21.
2. Authentication
   • Provides credentials using USER and PASS commands.
   • Anonymous login is also supported (USER anonymous, with an optional or empty password).
3. Command Exchange
   • After successful login, the user can issue FTP commands such as:
     • LIST – list directory contents.
     • RETR – retrieve (download) a file.
     • STOR – store (upload) a file.
4. File Transfer
   • Initiates a second TCP connection for data transfer.
   • Transfers can be in ASCII (text) or binary mode.

FTP Modes

Active Mode

• The client connects to port 21 and sends a PORT command.
• Server initiates the data connection from port 20 to the client.
• Might be blocked by firewalls on the client side.

Passive Mode (PASV)

• Server provides a random high-numbered port.
• Client initiates both control and data connections.
• More firewall-friendly, commonly used.