The packet losses imposed by IP networks can cause long and erratic recovery delays, since senders must often use conservative loss detection and retransmission mechanisms. This paper proposes a model to explain and predict loss rates for TCP traffic. Based on that model, the paper describes a new router buffering algorithm, flow-proportional queuing (FPQ), that handles heavy TCP loads without imposing high loss rates. FPQ controls TCP by varying the router's queue length in proportion to the number of active TCP connections. Simulation results show that FPQ produces the same average transfer delays as existing schemes, but makes the delays more predictable and fairer.