On the Necessity of Time-based Updates in SDN

The usage of accurate time to schedule updates in software defined networks was recently proposed in [1]; time can be a powerful tool for applying network updates in a relatively simple manner and with a very brief period of inconsistency during the update. In the current paper we introduce the flow-swapping scenario, which demonstrates the necessity of time-based updates. We show that while traditional update approaches result in temporary packet loss, the time-based approach allows a smooth reconfiguration procedure. We introduce the lossless flow allocation problem, and formally show that given the online nature of resource allocation in SDN, scenarios that require simultaneous changes at multiple switches are bound to occur, with clock-based near-simultaneous scheduling offering an advantageous solution.