Recent Results on Sizing Router Buffers

Today all Internet routers are built with buffers that hold packets in times of congestion. These buffers can typically store between 250ms to one second worth of data. According to a widely used “rule-of-thumb”, a link needs a buffer of size B = RTT × C, where RTT is the average round-trip time of a flow passing across the link, and C is the data rate of the link. For example, a 10Gb/s router linecard needs approximately 250ms × 10Gb/s = 2.5Gbits of buffers; and the amount of buffering grows linearly with the line-rate. Such large buffers are challenging for router manufacturers, who must use large, slow, off-chip DRAMs. They also lead to high latencies in case of congestion. Recent research suggest that the rule-of-thumb (B = RTT ×C) is now outdated and incorrect for routers serving highly aggregated traffic. According to these new results, a link with n flows requires no more than B = (RTT × C)/ √ n, for long-lived or short-lived TCP flows. These lower buffer requirements have been verified in simulation and laboratory experiments as well as on real networks with live traffic. The consequences on router design are enormous: a 2.5Gb/s link carrying 10,000 flows could reduce its buffers by 99% with negligible difference in throughput, and a 10Gb/s link carrying 50,000 flows requires only 10Mbits of buffering, which can easily be implemented using fast, on-chip SRAM.