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This paper describes and evaluates a probabilistic packet scheduling (PPS) algorithm for providing different levels of service to TCP flows. With our approach, each router defines a local currency in terms of tickets and assigns tickets to its inputs based on contractual agreements with its upstream routers. A flow is tagged with tickets to represent the relative share of bandwidth it should receive at each link. When multiple flows share the same bottleneck, the bandwidth that each flow obtains is proportional to the relative tickets assigned to that flow. Simulations show that PPS does a better job of proportionally allocating bandwidth than DiffServ and weighted CSFQ. In addition, PPS accommodates flows that cross multiple currency domains.