Now, at time t, there can be 2 clear demarcations of the frames available in the sender window side. One of them, consists of the frames numbered from k to n - 1 and the other set contains frames numbered from 0 to x, wherein, if we add up the total number of frames observed in both the sets, we get a total of n.
So, number of frames available in the 1st set = (n - 1) - (k) + 1 = n - k = A (let)
and number of frames available in the 2nd set = (x) - (0) + 1 = x + 1 = B (let)
Now, A + B = n.
Hence, n - k + x + 1 = n.
Solving it, we get that the value of x is :- x = k - 1.
Hence, according to me, the set of values found is :- k, k + 1, ..., n - 1, 0, 1, ..., k - 1.
Now, since k can be any value from 0 to n - 1, so a total of n possible sequences exist.
Someone please verify my answer and do let me know if I have erred anywhere...