Sliding Window Protocols
Outbound frame has a sequence number from 0 - 2^n -1 (therefore n bits suffice)
Sender maintains a list of numbers in sequence corresponding to frames it is permitted to send. This set of frames falls within the so-called sending window.
Receiver maintains a similar list of frames it is permitted to accept called a receiving window.
Sending & receiving window can have diff sizes, upper/lower limits & can grow/shrink