Software-Defined Networks (SDNs) introduce great flexibilities in how packet routes can be defined and changed over time, and enable a more fine-grained and adaptive traffic engineering. The recently introduced support for more accurate synchronization in SDNs further improves the degree of control an operator can have over the packets’ forwarding paths, and also allows to avoid disruptions and inconsistencies during network updates, i.e., during the rerouting of flows. However, how to optimally exploit such technology algorithmically — to efficiently schedule the update of multiple flows in such timed SDNs — while accounting for possible interference and congestion, is not well-understood today. We in this paper initiate the study of the fundamental problem of how to reroute the updates of multiple network flows in a synchronized SDN in a congestion-free manner. We rigorously prove that that the problem is NP-hard for flows of unit size and network links with unit delay. We also show that a greedy approach to update the network can delay the update significantly. Our main contribution is the first solution to this problem: Chronicle. Our approach is based on a time-extended network construction and resource dependency graph, which is implemented by Openflow 1.5 using the scheduled bundles feature. Evaluation results show that Chronicle can reduce the makespan by 63% and reduce the number of changed rules by 50% compared to state-of-the-art.

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