Early selective packet discard for alternating resource access of TCP over ATM-UBR

Abstract

We investigate packet discarding schemes for TCP over ATM with UBR service. In doing so, we tested the effective throughput of two existing schemes, Partial Packet Discard (PPD) and Early Packet Discard (EPD), as compared to the Random Cell Discard (RCD) scheme which discards any incoming cells after buffer overflow. We observed that PPD alleviates the effect of packet fragmentation so that it gets effective throughput enhancement over RCD, and EPD provides further enhancement over PPD. After closer investigation, we found that there is a sustained congestion problem other than packet fragmentation that causes the effective throughput to be degraded. We noted that sustained congestion resulted in the synchronization of TCP window expansion and shrinkage. To provide a solution for this problem, we propose the Early Selective Packet Discard (ESPD) policy, a strategy which makes sessions take turns in accessing network capacity by discarding packets from selected sessions rather than randomly. Our results shows that ESPD achieves throughput and fairness enhancement over EPD with only a modest increase in implementation complexity.