FTP多线程序下载和断点续传

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　　REST (RESTART) The REST command is used to continue a session that has been interrupted. The REST command has an argument, an integer, that represents the position in the file where transfer should begin. For example, to restart a transfer at byte 4096 in a file, the client would issue the following commands to the server:
REST 4096
RETR /documents/file.zip
It is important to understand that the command that follows the REST command must be a transfer of some kind, either a STOR or a RECV.
　　
　　RESTART (REST)
    The argument field represents the server marker at which
    file transfer is to be restarted.  This command does not
    cause file transfer but skips over the file to the specified
    data checkpoint.  This command shall be immediately followed
    by the appropriate FTP service command which shall cause
    file transfer to resume.
　　RETRIEVE (RETR)
    This command causes the server-DTP to transfer a copy of the
    file, specified in the pathname, to the server- or user-DTP
    at the other end of the data connection.  The status and
    contents of the file at the server site shall be unaffected.
　　
　　有了断点续传的支持，就可以将要上传或下载的文件分割成多个段，每个段用一个线程进行下载。(具体例子可参考《Windows应用程序捆绑核心编程》---多线程下载和断点续传一章)