File transfer service FTP, FTP protocol includes two parts, FTP server and FTP client. Among them, FTP server is used to store files, and users can use FTP client to access resources located on FTP server through FTP protocol.

FTP establishes a control connection and a data connection, so there are two tcp connections. By default, FTP protocol uses two TCP ports, 20 and 2 1, where 20 is used to transmit data and 2 1 is used to transmit control information. However, whether to use 20 as the data transmission port is related to the transmission method used by FTP. If active mode is adopted, the data transmission port is 20; > if passive mode is adopted; The high random port of 1024 should be decided by the server and the client through negotiation.