On the Performance of Early Packet Discard

In a previous paper, one of the authors gave a worstcase analysis for the early packet discard (EPD) technique formaintaining packet integrity during overload in ATM switches.This analysis showed that to ensure 100% goodput during overloadunder worst case conditions requires a buffer with enoughstorage for one maximum length packet from every active virtualcircuit. This paper refines that analysis, using assumptions thatare closer to what we expect to see in practice, and examineshow EPD performs when the buffer is not large enough to achieve100% goodput.We show that 100% goodput can be achieved withsubstantially smaller buffers than predicted by the worst caseanalysis, although the required buffer space can be significantwhen the link speed is substantially higher than the rate ofthe individual virtual circuits. We also show that high goodputscan be achieved with more modest buffer sizes, but that EPDexhibits anomalies with respect to buffer capacity, in that thereare situations in which increasing the amount of buffering cancause the goodput to decrease. These results are validated bycomparison with simulation.