Using ReversePTP to distribute time in Software Defined Networks

Accurate time can be a useful tool in Software Defined Networks (SDN), allowing to coordinate network updates and topology changes, and to timestamp events and notifications. Moreover, accurate time is used in various environments in which software defined networking is being considered, making accurate time distribution an essential feature of SDNs. Accurate timekeeping requires a clock synchronization method, such as the Precision Time Protocol (PTP). Contrary to the centralized SDN paradigm, PTP is by nature a distributed protocol, in which every node is required to run a complex clock servo algorithm. We introduce ReversePTP, a clock synchronization protocol for SDN. ReversePTP is based on PTP, but is conceptually reversed; in ReversePTP all nodes (switches) in the network distribute timing information to a single node, the controller, that tracks the state of all the clocks in the network. Hence, all computations and bookkeeping are performed by the controller, whereas the `dumb´ switches are only required to send it their current time periodically. In accordance with the SDN paradigm, the controller is the `brain´, making ReversePTP flexible and programmable from an SDN programmer´s perspective. We present the ReversePTP architecture, and discuss how SDN applications that require accurate time can use ReversePTP. Our experimental evaluation of a network with 34 ReversePTP-enabled nodes shows that ReversePTP can be effectively used for coordinating events in networks at the same level of accuracy as provided by the conventional PTP.