In this illustration we see a series of multicolored rectangles positioned side-by-side like a line of dominoes. A key identifies what the various colors represent: dark blue = Already ACK'd; light blue = Send, not yet ACK'd; gray = Usable, not yet sent; white = Not usable. Going from left to right, we see six dark blue rectangles followed by eight light blue rectangles. The first light blue rectangle is labeled "base." Next we see six gray rectangles; the first is labeled "nextseqnum." Finally, we see seven white rectangles. The area between the first light blue rectangle and the last gray rectangle is labeled "Window size, N."