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FTP Data Transfer
Port 20 is primarily used as the data channel within the File Transfer Protocol (FTP), responsible for the actual transfer of files between client and server. Typically used alongside port 21, which handles control commands, this port facilitates unencrypted file transfers across networks, making it a fundamental component in traditional file sharing and server communication..
Port 20 operates as the default data channel in active mode FTP sessions. When an FTP client requests or sends a file, the server establishes a connection from its port 20 back to an ephemeral port on the client to facilitate data transfer. This usage alongside port 21 allows FTP to separate command exchange from data exchange, optimizing connection control and throughput.
In traditional network configurations, port 20 typically initiates outbound connections for data transmission after the client establishes the control connection on port 21. This design leverages the TCP protocol to ensure reliable, ordered delivery of files. Because port 20 is used for the file contents themselves, it handles potentially large data streams, emphasizing the need for both bandwidth considerations and proper flow control.
Modern implementations sometimes employ passive mode, where the client initiates both data and command connections, often avoiding restrictive firewall issues. However, regardless of mode, port 20 remains a core identifier for FTP data transfer sessions and is deeply ingrained in legacy file transfer systems.