An apparatus for routing data packets includes a network interface, a memory, a general purpose processor and a flow classifier. The memory stores a flow structure. Every packet in one flow has identical values for a set of data fields in the packet. The memory stores instruction that cause the processor to receive missing flow data and to add the missing flow to the flow structure. The apparatus forwards a packet based on the flow. The flow classifier determines a particular flow and whether it is already stored in the flow structure. If not, then the classifier determines whether that flow has already been sent to the processor as missing data. If not, then the classifier stores into a different data structure data that indicates the flow has been sent to the processor but is not yet included in the flow data structure, and sends missing data to the processor.