Software-Defined Networks (SDNs) introduce interesting new opportunities in how network routes can be defined, verified, and changed over time. Yet despite the logicallycentralized perspective offered, an SDN still needs to be considered a distributed system: rule updates communicated from the controller to the individual switches traverse an asynchronous network and may arrive out-of-order, and hence lead to (temporary or permanent) inconsistencies. Accordingly, the consistent network update problem has recently received much attention. Motivated by the advent of tightly synchronized SDNs, we in this paper initiate the study of algorithms for consistent network updates in “timed SDNs”—SDNs in which individual node updates can be scheduled at specific times. This paper presents Chronus, which is based on provably congestion- and loop-free update scheduling algorithms, and avoids the flow table space headroom required by existing twophase update approaches. We formulate the Minimum Update Time Problem (MUTP) as an optimization program. We propose a tree algorithm to check the feasibility and a greedy algorithm to find a update sequence in polynomial time. Extensive experiments on Mininet and numerical simulations show that Chronus can substantially reduce transient congestion by 75% and save over 60% of the rules compared to the state of the art. Index Terms—SDN, network updates, clock synchronization, congestion-free, loop-free.

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