The framework of this paper is the design of complex communication protocols by simulation, based on formal description techniques (FDT). We propose a general-purpose scalable module, formally specified in Estelle Formal Description Technique, emulating a wide area network behaviour, to permit the simulation for validation of complex unicast or multicast transport protocols. It transfers connectionless data packets while randomly generating events, with selectable probabilities of packet loss, delay, jitter, duplication, or reordering. Finite transfer bandwidth rates are selectable at various inputs. The module can be used as a simulation environment library component.