@Test(dataProvider = "createTimer")
  public void testRetransmitter(TimeScheduler timer) {
    try {
      NakReceiverWindow win = new NakReceiverWindow(sender, cmd, 0, timer);
      win.add(1, msg());
      win.add(2, msg());
      win.add(3, msg());
      win.add(5, msg());
      win.add(6, msg());
      System.out.println("win = " + win);
      int num_pending_xmits = win.getPendingXmits();
      assert num_pending_xmits == 1;

      win.add(4, msg());
      num_pending_xmits = win.getPendingXmits();
      assert num_pending_xmits == 0;
    } finally {
      timer.stop();
    }
  }