Abstract: Examples disclosed herein relate to controlling an unknown flow inflow to an SDN controller in a software defined network (SDN). In an example, an optimizer may be provided, between a switch and an SDN controller, to intercept an unknown flow from the switch to the SDN controller, in a software defined network. A portion of a data packet from each data packet in a plurality of data packets from the unknown flow may be aggregated at the optimizer. Only the aggregated portion of the data packet from each data packet may be sent, from the optimizer to the SDN controller, in a single package.