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Fast, Fair, and Efficient Flows in Networks

School of Business, Universidad Adolfo Ibáñez, Av. Presidente Errázuriz 3485, Las Condes, Santiago, Chile
Sloan School of Management, Massachusetts Institute of Technology, E53-361, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139
Graduate School of Business, Columbia University, Uris Hall, Room 418, 3022 Broadway, New York, New York 10027
correa{at}uai.cl
schulz{at}mit.edu
stier{at}gsb.columbia.edu

We study the problem of minimizing the maximum latency of flows in networks with congestion. We show that this problem is NP-hard, even when all arc latency functions are linear and there is a single source and sink. Still, an optimal flow and an equilibrium flow share a desirable property in this situation: All flow-carrying paths have the same length, i.e., these solutions are "fair," which is in general not true for optimal flows in networks with nonlinear latency functions. In addition, the maximum latency of the Nash equilibrium, which can be computed efficiently, is within a constant factor of that of an optimal solution. That is, the so-called price of anarchy is bounded. In contrast, we present a family of instances with multiple sources and a single sink for which the price of anarchy is unbounded, even in networks with linear latencies. Furthermore, we show that an s-t-flow that is optimal with respect to the average latency objective is near-optimal for the maximum latency objective, and it is close to being fair. Conversely, the average latency of a flow minimizing the maximum latency is also within a constant factor of that of a flow minimizing the average latency.

Subject classifications: networks/graphs; multicommodity; theory; games/group decisions; noncooperative; nonatomic; transportation; models; network.
History: Received November 2003; revision received February 2006; accepted February 2006.