Several systems model communications but do not really address reliability issues, e.g., STREAMS [18] and the x-Kernel [17]. AVOCA [24] defines the notion of protocol objects, but not in the sense BAST does it; furthermore, it mainly applies to high-performance communication subsystems. Other systems offer reliable distributed communications, either based on groups as elemental addressing facilities, e.g., CONSUL [15], ISIS [1] and HORUS [23], or based on transactions, e.g., ARJUNA [22].