What is the essence of sliding window protocols?

A.at any instant of time, the sender maintains a set of sequence numbers corresponding to frames it is permitted to send said to fall within the sending window; similarly, the receiver maintains a receiving window

B.

we have windows of frames, which slide through time

C.

the utilization of pipelining and selective repeat

D.to preserve the frames currently in the senders window from being ultimately lost or damaged in transit