public Object up(Event evt) {
        switch(evt.getType()) {
            case Event.MSG:
                Message msg=(Message)evt.getArg();
                Address sender=msg.getSrc();

                Header hdr=msg.getHeader(this.id);
                if(hdr != null) {
                    update(sender); // updates the heartbeat entry for 'sender'
                    num_heartbeats_received++;
                    return null; // consume heartbeat message, do not pass to the layer above
                } else if(msg_counts_as_heartbeat) {
                    // message did not originate from FD_ALL layer, but still count as heartbeat
                    update(sender); // update when data is received too ? maybe a bit costly
                }
                break; // pass message to the layer above
        }
        return up_prot.up(evt); // pass up to the layer above us
    }