According to the Internet Engineering Task Force (IETF), which supporting protocol does File Transfer Protocol...

The File Transfer Protocol (FTP) uses the Transmission Control Protocol (TCP) for reliable communication. TCP is one of the core protocols of the Internet Protocol Suite and provides reliable, ordered, and error-checked delivery of a stream of data between applications running on hosts communicating via an IP network. FTP relies on TCP to ensure that all data packets are delivered accurately and in the correct sequence, which is essential for the reliable transfer of files between a client and server.

References

IETF RFC 959 - File Transfer Protocol (FTP)

TCP/IP Protocol Suite Documentation