    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).
