orm and none seems to follow
 RFCs whole-heartedly. We have no choice, but assume common denominator
 as following:
 (1) Use stream mode for data transfer. Block mode will be better for
     multiple file downloading and partial file downloading. However
     not every ftpd has block mode support.
 (2) Use passive mode for data connection.
     So Nutch will work if we run behind firewall.
 (3) Data connection is opened/closed per ftp command for the reasons
     listed in (1). There are ftp servers out the