Abstract. The viability of overlay multicasting has been established by prev ious research. However, in order to apply overlay multicast to Internet -scale distributed systems, such as the Grid and Peer-to-Peer systems, the issue of effectively enforcing fairness among peers so as to optimize overall performance remains as a challenge. This paper argues that simply appl ying a multiple-tree scheme does not provide sufficient fairness, in terms of performance. Instead, we believe that a better way to define fairness, for performance’s sake, is to fa ctor in peers’ proportional contributions as it provides the opportunity to support many simultaneous multicasting sessions. This paper then presents a protocol, called FairOM (Fair Overlay Multicast), to enforce proportional co ntributions among peers in Internet -scale distributed system s. By exploiting the notion of staged spare capacity group and deploying a two -phase multicast forest construction process, FairOM enforces proportio...