Garbage Collection of Forwarding Rules in Software Defined Networks

Abstract

Software defined networking has brought new interesting challenges by externalizing the task of controlling the network to some generic computer software. In particular, the controller software can modify the network routes by introducing new forwarding rules and deleting old ones at a distributed set of switches, a challenge that has received lots of attention in the last six years. In this article, we survey the different techniques to update rules, sometimes relying on redundant paths to reroute the traffic during the update, sometimes activating rules at distinct switches in a specific order, to avoid looping packets. This state of the art helps understand another, often overlooked, problem that consists of determining the appropriate point in the update where it is safe to garbage collect old rules. To illustrate the difficulty of the problem, we list the previously proposed assumptions, like the upper bound on the transmission delay of every packet through the network. Finally, we propose a solution that alleviates these assumptions and speeds up the original two-phase rule update by about 80 percent through the use of dedicated clean-up packets that detect the absence of in-flight packets.