Control connection

   In  the  standard  mode,  the  control  connection is initiated by the
   client.  The  connection  is required on port 5021 on the server side.
   This  port  can  be  changed  at  compile-time (#define CONTROLPORT in
   includes/config.h)  or/and at run-time (option -w). If the server port
   is  not the default one (5021), a client which want to connect to this
   server must specify the port (option -w).
   In  the  SSH  mode,  the  server  listens  on  port  std+1 (ie 5022 by
   default). It cannot be changed at run-time.

    Data connection

   Whereas  the  FTP  protocol implements one data connection only, BBFTP
   opens  multiple  data  connections  depending on the number of streams
   required (client option -p and server option -m).
     * Passive mode (client 3 against server 3 only)
       The  server  listens  on  ephemeral  ports  which  are sent to the
       client.   These  ports  can  be  chosen  in  a  range  defined  at
       compile-time or at run-time.
     * Non-passive mode (other configurations)
       The  server  initiates the data connection(s) after the client has
       sent  him  its  private  IP  address  and  the  listening port(s).
       This(these)  port(s)  is(are)  ephemeral  ports  (>1023).  (it  is
       possible to define a range of ports when starting the client using
       the  -D option) The server can then establish the connection(s) to
       the specified port(s) from its port std-1 (ie 5020 by default).