Shiv Kalyanaraman , Performance of TCP / IP over ABR 1 Performance of TCP / IP over ABR 1

The Available Bit Rate (ABR) service has been developed to support data applications over Asynchronous Transfer Mode (ATM). It is hence interesting to study the performance of reliable data transport protocols like TCP over ABR. We study the eeect of running large unidirectional le transfer applications on TCP over ABR with an explicit rate algorithm (ERICA) implemented at the ATM switches. The study shows that it is possible to get maximum TCP throughput when there are enough buuers at the switches and the sources are rate-limited by ABR. However, when the number of buuers is smaller, there is a large reduction in throughput even though the Cell Loss Ratio (CLR) is very small and the sources are window-limited by TCP. The primary reason for the large throughput degradation is the TCP timer granularity (which aaects the timeout interval) and retransmission of duplicate packets which are discarded at the destination. We show that switch buuers should not be dimensioned based on the ABR Source parameter Transient Buuer Exposure (TBE). When ABR capacity is varied, we nd that CLR exhibits high variance and is not a good indicator of TCP performance. Further, larger buuers and a larger number of TCP sources increase TCP throughput. Even when buuers are small, dropping of End of Message (EOM) cells should be avoided. This avoids merging of packets at the destination AAL5 and improves fairness.