Let It Flow: Resilient Asymmetric Load Balancing with Flowlet Switching

Datacenter networks require efficient multi-path load balancing to achieve high bisection bandwidth. Despite much progress in recent years towards addressing this challenge, a load balancing design that is both simple to implement and resilient to network asymmetry has remained elusive. In this paper, we show that flowlet switching, an idea first proposed more than a decade ago, is a powerful technique for resilient load balancing with asymmetry. Flowlets have a remarkable elasticity property: their size changes automatically based on traffic conditions on their path. We use this insight to develop LetFlow, a very simple load balancing scheme that is resilient to asymmetry. LetFlow simply picks paths at random for flowlets and lets their elasticity naturally balance the traffic on different paths. Our extensive evaluation with real hardware and packet-level simulations shows that LetFlow is very effective. Despite being much simpler, it performs significantly better than other traffic oblivious schemes like WCMP and Presto in asymmetric scenarios, while achieving average flow completions time within 10-20% of CONGA in testbed experiments and 2x of CONGA in simulated topologies with large asymmetry and heavy traffic load.