Example implementations relate to generating a packet processing pipeline definition. According to an example, an SDN controller includes a flow class module to receive flow class registrations specifying flow classes. The flow classes specify characteristics of expected network flows. The controller also includes a pipeline generating module to generate a packet processing pipeline definition to accommodate the expected network flows, the packet processing pipeline definition including table definitions for the flow classes. The SDN controller further includes a communication interface to send the packet processing pipeline definition to a switch.