File Transfer Protocol (FTP) is a standard network protocol used to copy a file from one host to another over a TCP/IP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server applications, which solves the problem of different end host configurations (i.e., Operating System, file names).[1] FTP is used with user-based password authentication or with anonymous user access.

This SonicWALL signature identifies legitimate FTP control channel traffic over TCP port 21.