code: public Tunnel getTunnel( String destinationHostname, int destinationPort ) {
          return tunnelsByDestination.get(
                  hostnamePortKey( destinationHostname, destinationPort ) );
      }
docs: Returns the tunnel matching the supplied values, or <code>null</code> if
  there isn't one that matches.
  
  @param destinationHostname
  The tunnels destination hostname
  @param destinationPort
  The tunnels destination port
  
  @return The tunnel matching the supplied values