The MSS parameter ensures that we don't send segments that are too large—TCP is not allowed to create a segment larger than the MSS. Unfortunately, the basic sliding windows mechanism doesn't provide any minimum size of segment that can be transmitted. In fact, not only is it possible for a device to send very small, inefficient segments, the simplest implementation of flow control using unrestricted window size adjustments ensures that under conditions of heavy load, window size will become small, leading to significant performance reduction!