Slidind windows - go back n
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This example models the go back n protocol.

The receiver requires frames to be received in the same order as sent.
The sender buffers the frames in the window in case it has to resend them.

When the receiver gets a frame:

If the frame is out of order or damaged , the receiver ignores the frame 
and sends a NAK for the frame it expected.
If the frame is the expected one , the receiver may send an ACK for the 
frame which means he has received the frame and all its previous frames.
If the frame is repeated , the receiver drops it.

When the sender gets an ACK/NAK:

If ack for frame j is received , the sender assumes all the frames before 
j were received properly and moves the window to j+1.
If NAK is received , the sender resends all frames in the window.