Time Loss Loss Threshold Threshold Timeout Timeout

In this paper we analyse congestion in TCP over ATM-UBR, in particular the association between the congestion measures at the ATM physical and TCP transport layers. In addition to the commonly used metrics, cell loss ratio (CLR) and packet retransmission ratio (PRR) two new indicators, protocol level congestion association factor (PLCAF) and CLR-PRR correlation coe cient are used. CLR and PRR are good measures of congestion at their respective layers, but do not address the relation between cell loss and its e ect on TCP congestion control scheme. Both PLCAF and CLR-PRR correlation coe cient are derived to better capture the association between congestion measures at ATM and TCP levels. It is shown through simulation of multiple TCP connections over ATM-UBR service under selected number of di erent operating conditions that the two new indicators can be successfully used for the analysis of congestion dynamics in TCP over ATM-UBR.