private void moveWindow(Packet pkt){
		int seqNum = pkt.getSeqnum();
		if (windowBuffer.peek().getSeqnum() > pkt.getSeqnum()){
			return; //ack for an old packet
		}
		Packet head = windowBuffer.poll();
		while (head != null && head.getSeqnum() != seqNum){
			head = windowBuffer.poll();
		}
		aBase = (head == null) ? aNextSeqNum : head.getSeqnum() + 1;
	}