FTP 21 and 20 ports

FTP 21 and 20 ports: We all know that the FTP corresponding port should be 21, why another 20. Actually, we carry out FTP file transfer, the client first connected to the FTP server 21 port, the user's authentication, after the successful authentication, when we want to transfer files, the server will open a port for 20来 to carry out data files, that is, Port 20 is the true transmission of the use of the port, Port 21 is only used for FTP login authentication. We usually download files, we will encounter download to 99%, the file does not complete, can not successfully download. In fact, because the file after downloading, but also in the 21-port to conduct user authentication, and we download the file if the time is too long, the client and the server's 21-port connection will be the server is considered a time-out connection and interrupted, that is the reason. The workaround is to set the response time for Port 21.