
    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).
