Bahl, ShikhaIn order to assess the performance of a connection it is important to determine the bandwidth offered by the slowest node along the path, also known as bottleneck bandwidth. In past, many reasearchers have used a packet-pair technique in order to estimate the bottleneck bandwidth. In real networks, however, the measurements made using the packet-pair technique do not always reflect the correct estimate of the bottleneck service time due to the presence of cross traffic. While several reasons for the observed variability have been reported, the exact nature of the impact of cross traffic on the observations has not been studied In this paper we present a model to explain how the measured difference of the reception time for a packet-pair can be related to the characteristic of the service time and cross traffic that the pair found along the path. (Also cross-referenced as UMIACS-TR-98-42)en-USTechnical Report