A Case for Spraying Packets in Software Middleboxes

The standard approach adopted by software middleboxes to use multiple cores has long been to direct packets to cores at flow granularity. This, however, has significant shortcomings. First, it is inefficient, since it cannot use all cores when there is a small number of concurrent flows---which happens frequently. Second, asymmetry in flow distribution causes unfairness even with a larger number of flows. Yet, the current trend of higher-speed links and core-richer CPUs only aggravates these problems. In this paper, we propose a natural alternative: that middleboxes should direct packets to cores at a finer granularity. Our system, Sprayer, solves the fundamental problems of per-flow solutions and addresses the new challenges of handling shared flow state that come with packet spraying. Sprayer builds on the observation that most middleboxes only update flow state when connections start or finish; ensuring that all control packets from the same TCP connection are processed in the same core. We show that, when compared to the per-flow alternative, Sprayer significantly improves fairness and seamlessly uses the entire capacity, even when there is a single flow.