Efficient and safe network updates with suffix causal consistency

Abstract

Though centrally managed by a controller, a software-defined network (SDN) can still encounter routing inconsistencies among its switches due to the non-atomic updates to their forwarding tables. In this paper, we propose a new method to rectify these inconsistencies that is inspired by causal consistency, a consistency model for shared-memory systems. Applied to SDNs, causal consistency would imply that once a packet is matched to (“reads”) a forwarding rule in a switch, it can be matched in downstream switches only to rules that are equally or more up-to-date. We propose and analyze a relaxed but functionally equivalent version of this property called suffix causal consistency (SCC) and evaluate an implementation of SCC in Open vSwitch and P4 switches, in conjunction with the Ryu and P4Runtime controllers. Our results show that SCC provides greater efficiency than competing consistent-update alternatives while offering consistency that is strong enough to ensure high-level routing properties (black-hole freedom, bounded looping, etc.).