seqno=-1;
    private void removeAll() {        NakReceiverWindow win;        synchronized(sent_msgs) {            sent_msgs.clear();        }        synchronized(received_msgs) {            for(Iterator it=received_msgs.values().iterator(); it.hasNext();) {                win=(NakReceiverWindow)it.next();                win.destroy();            }            received_msgs.clear();        }    }