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File Transfer Protocol (FTP)

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1. Overview and Design Philosophy

The File Transfer Protocol (FTP) (RFC 959) is an Application Layer protocol designed to transfer files reliably, securely, and efficiently between two computers across a TCP/IP network.

Unlike most client-server protocols (which use a single communication channel), FTP is famous for its Dual-Connection Architecture (Out-of-Band Control): it uses two completely separate parallel TCP connections for a single user session:

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2. The Two Connections: Control vs. Data

Why Use Two Separate Connections?

1. Out-of-Band Signaling: Control commands travel on a clean, dedicated channel uninterrupted by massive multi-gigabyte file payloads.
2. Interactive Abort: If a user initiates a 10 GB file download and wants to cancel it mid-transfer, they can immediately send an ABOR command across the Control Connection. In single-connection protocols (like HTTP/1.0), the client must download or terminate the entire socket!

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3. Active Mode vs. Passive Mode (The Firewall Problem)

FTP operates in two distinct operational modes, differing in which entity initiates the Data Connection:

1. Active Mode (Default):

• The client tells the server its IP address and an arbitrary local port ($M$) using the PORT command.
• The server initiates the data connection from its own TCP Port 20 back to the client's port $M$.
• Problem: Modern client NAT routers and personal firewalls block unsolicited inbound connection attempts from the outside Internet!

2. Passive Mode (PASV):

• The client sends a PASV command across the control connection.
• The server opens an unprivileged ephemeral port (e.g., $32000$) and returns this port number to the client.
• The client initiates the data connection outbound to the server's ephemeral port.
• Advantage: Because both the Control and Data connections originate outbound from the client, client firewalls allow the connections without interference.

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4. Common FTP Commands and Status Codes

Standard Commands:

• USER <username>: Submits user identity.
• PASS <password>: Submits authentication password.
• LIST: Requests a list of files in the current remote directory.
• RETR <filename>: Retrieve (download) a file from server to client.
• STOR <filename>: Store (upload) a file from client to server.
• QUIT: Terminates the session.

3-Digit Status Response Codes:

• 1xx: Positive Preliminary reply (e.g., 150 Opening BINARY mode data connection).
• 2xx: Positive Completion reply (e.g., 220 Service ready, 230 User logged in, 226 Transfer complete).
• 3xx: Positive Intermediate reply (e.g., 331 Password required).
• 4xx: Transient Negative reply (e.g., 425 Can't open data connection).
• 5xx: Permanent Negative error (e.g., 530 Login incorrect, 550 File not found).

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5. Security Limitations and Modern Replacements

Standard FTP transmits all credentials (usernames and passwords) and file contents in unencrypted cleartext.

• FTPS (FTP Secure): Adds TLS encryption to standard FTP (RFC 4217).
• SFTP (SSH File Transfer Protocol): A completely different, modern protocol running over SSH (TCP Port 22) that provides encrypted file transfers over a single multiplexed connection.

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