此ftp客户端根据FTP协议（rfc959）编写，不采用现成的FTP API。所以对于想理解FTP协议并编写代码实现协议的朋友是个不错的参考！-This ftp client under the FTP protocol (rfc959) to prepare, do not use ready-made FTP API. So would like to understand the FTP protocol and write code to achieve agreement Friend