A network device that manages the flow rate of a stream of packets traveling within a network is converted from managing based on data rate to managing based on packet rate. In one embodiment, an interface receives from the device an actual length of a packet and provides to the device an effective length for the packet. A multiplexer generates the effective length by selecting the actual length during data rate mode and selecting a virtual length during packet rate mode. Various embodiments work with network devices that use various traffic management techniques. Such techniques include, but are not limited to: virtual time algorithms for determining excess packets; policing techniques that drop excess packets; and shaping techniques that buffer excess packets for possible later transmission.