A method of providing state synchronization between a controller and a switch in a communications network, wherein the controller provides flow data for routing flows through the switch and transmits, in flow modification messages, the flow data to the switch for storage in flow tables, the state synchronization comprising ensuring that flow data in the flow tables of the switch are consistent with the flow data provided by the controller. The method comprises, by the controller (19), providing (21) an identifier in each flow modification message. The method further comprises, upon reestablishment of communications (22) between the controller and the switch after a loss of connectivity receiving (23) a resynchronization message from the switch, wherein the resynchronization message comprises an identifier indication capable of being used to identify an identifier of a most recently received message, determining (24) if all transmitted messages have been received by the switch by comparing the identifier indication with an identifier of a most recently transmitted flow modification message, identifying (25) flow modification messages transmitted after the most recently received message as “un-received” messages and retransmitting (26) un-received messages.