Ack. Number Lower bound

Much harder.
1st try: acks should move forward or contain valid data. No good for out of order dataless acks
2nd try: allow acks when the ack value is within a certain window of the next octet to be sent. This window should be larger then the maximum TCP window (chosen to be 66000)
No big deal: 'old' acks will do no harm anyway

Much harder.