Set the ContentOffset property when downloading a file from an FTP server. This offset indicates the position in the server's file that marks the start of the data to be downloaded. The offset is specified as the number of bytes from the start of the file; the offset of the first byte is zero.
Setting ContentOffset causes the FtpWebRequest to send a restart (REST) command to the server. This command is ignored by most FTP server implementations if you are uploading data to the server.