A line-rate, real-time-traffic detector classifies a network traffic flow as real-time when it determines the smoothness of the packet arrival rate of the network traffic flow is bounded by an empirically derived bound. In some embodiments, to improve performance, a tighter smoothness bound is applied to the smoothness calculations performed on a first set of packet arrival times, while a looser smoothness bound is applied to a second set of packet arrival times, the second set inclusive of and larger than the first.