A new protocol called generalized sliding window protocol is described and formally specified using the Specification and Description Language (SDL). The protocol generality was achieved by appropriately combining the classical go-back-n and selective-repeat protocols that both have traditionally been referred to as sliding window protocols. The proposed protocol is valid for any combination of send and receive window widths. The stop-and-wait, go-back-n and selective-repeat protocols are shown to be its special cases. Simulations show that the protocol efficiency initially increases with the increase of the receive window width, but can saturate or even reach its maximum value at a receive window width that is less than the send window width. This offers more freedom to choose a suitable compromise between buffer lengths and protocol efficiency, and an optimal efficiency may be achieved using less resources than with the selective-repeat protocol. The described protocol can serve as a starting point for the further development of new protocols, but can also be used as a simple, unified and theoretically sound protocol to be taught to telecommunications students.