How does FTP work and why does it use two connections?

FTP uses two TCP connections: a control connection on port 21 for commands/replies and a separate data connection on port 20 for the actual file transfer.

FTP (File Transfer Protocol):

A protocol developed to allow data transfers between a client and a server.

* A control connection on TCP 21 carries commands; a separate data connection on TCP 20 carries the files. *

Two Separate Connections:

Process:

1. Client establishes first connection to server on port 21 for control traffic (commands and responses)
2. Client establishes second connection on port 20 for actual data transfer
3. Data connection is created every time there is data to be transferred

Data Transfer Modes:

• Download (pull): Client downloads data from server
• Upload (push): Client uploads data to server

Client                          Server
  │                               │
  │──── Control (port 21) ───────→│
  │     Commands & responses      │
  │                               │
  │←─── Data (port 20) ──────────→│
  │     File transfer             │

Key insight: The two-connection model allows commands and data to flow independently, improving efficiency.

Go deeper:

• FTP, SFTP, TFTP Explained — PowerCert on the file-transfer protocols and their differences.

• File Transfer Protocol (Wikipedia) — control vs data connections, active vs passive mode.