Why the internet's oldest file transfer protocol still uses two separate connections

Cultures · Published 29 September 2026 Swipe more like this →

In short: The File Transfer Protocol, or FTP, has been moving data since 1971. Despite the rise of modern, secure alternatives, its architecture remains rooted in the limitations of the ancient Network Control Protocol. This legacy design forces the protocol to use two distinct ports, creating persistent challenges for modern network firewalls.

The File Transfer Protocol was first defined in 1971 by Abhay Bhushan in RFC 114. In its original environment, FTP operated on the Network Control Protocol (NCP), a system that required two simplex connections for two-way communication. Each application was assigned an odd and an even port to manage these streams. Although the later transition to TCP/IP introduced duplex ports—allowing a single port to handle both sending and receiving—FTP was never updated to use only one. It maintained its dual-port structure to ensure backwards compatibility, a design choice that persists today.

This dual-port requirement creates significant friction with modern security infrastructure. In the default active mode, the server initiates a data connection back to the client. However, most firewalls and Network Address Translators (NATs) are configured to block incoming connections from the internet, effectively breaking the protocol. To bypass this, passive mode was introduced, where the client initiates the data connection to the server. While this is the standard for modern clients, it remains a workaround for a protocol designed for a different era of networking.

FTP’s evolution has been marked by attempts to modernize its capabilities while retaining its core identity. While the current specification, RFC 959, dates back to 1985, subsequent amendments have added support for IPv6 and security extensions. Despite these updates, the protocol’s lack of native encryption for credentials has led major browsers like Chrome and Firefox to drop support for it entirely. Today, FTP is often replaced by secure alternatives like SFTP or FTPS, yet it remains a foundational piece of internet history, still utilized for tasks like software distribution via anonymous logins.

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