The drive towards richer and more interactive web content places increasingly stringent requirements on datacenter networks. These networks typically function by splitting up an incoming query into multiple (smaller) requests, and then aggregating the individual responses to assemble the final result. As a result, a huge fraction—as high as 80%—of the network flows are short and latency-sensitive. The speed with which such networks respond to packet drops limits their ability to meet high-percentile flow completion time SLOs. Indirect notifications indicating packet drops (e.g., duplicates in an end-to-end acknowledgement sequence) are an important limitation to the agility of response to packet drops.
This paper proposes FastLane, an in-network drop notification mechanism. FastLane enhances switches to send high-priority drop notifications to sources, thus informing sources as quickly as possible. Consequently, sources can retransmit packets sooner and throttle transmission rates earlier, thus reducing high-percentile flow completion times. We demonstrate, through simulation and implementation, that FastLane reduces 99.9th percentile completion times of short flows by up to 81%. These benefits come at minimal cost— safeguards ensure that FastLane consume no more than 1% of bandwidth and 2.5% of buffers.