Restarting FTP Uploads and Downloads (IIS 6.0)
If your connection closes unexpectedly while uploading or downloading a file using FTP, it might not be necessary to transfer the entire file again, because the IIS -based FTP service supports restarting failed FTP uploads and downloads. If your FTP client software supports FTP restart, you can reestablish the FTP connection and restart the file transfer; and the file transfer will automatically pick up where it left off.
The command-line FTP client included with Microsoft Windows does not support restarting downloads. You must use a third-party command-line FTP client for that functionality. If you are using a command-line FTP client, uploads can be resumed by using the append command to add data to an existing file. Downloads can be restarted by first issuing a rest command with the desired offset, and then issuing the retr command.
The IIS implementation of FTP restart is limited to an offset of less than four gigabytes.