Approximating Fair Queueing On Reconfigurable Switches

Congestion control today is predominantly achieved via end-to-end mechanisms with little support from the network. As a result, end-hosts must cooperate to achieve optimal throughput and fairness, leading to inefficiencies and poor performance isolation. While router mechanisms such as Fair Queuing guarantee fair bandwidth allocation to all participants and have proven to be optimal in some respects, they require complex flow classification, buffer allocation, and scheduling on a per-packet basis. These factors make them expensive to implement in high-speed switches.In this paper, we use emerging reconfigurable switches to develop an approximate form of Fair Queueing that operates at line-rate. We leverage configurable per-packet processing and the ability to maintain mutable state inside switches to achieve fair bandwidth allocation across all traversing flows. Further, present our design for a new dequeuing scheduler, called Rotating Strict Priority scheduler that lets us transmit packets from multiple queues in approximate sorted order. Our hardware emulation and software simulations on a large leaf-spine topology show that our scheme closely approximates ideal Fair Queueing, improving the average flow completion times for short flows by 2-4x and 99th tail latency by 4-8x relative to TCP and DCTCP.