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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
Notes: