http://www.fmi.uni-passau.de/archive/doc/unix/unixhelp/Unixhelp/tasks_ftp3.2.4.html (Einblicke ins Internet, ~06/1995) Managing the file transfer queue
Once a transfer has been initiated, it is given a number
to identify it and placed in the transfer queue. You can
then use this number to manage the file transfer.