While several algorithms have been created to actively measure the end-to-end available bandwidth of a network path, they require instrumentation at both ends of the path, and the traffic injected by these algorithms may affect the performance of other applications on the path. Our goal is to apply the self-induced congestion principle to passive traces of existing TCP traffic instead of actively probing the path. The primary challenge is that, unlike active algorithms, we have no control over the traffic pattern in the passive TCP traces. As part of the Wren bandwidth monitoring tool, we are developing techniques that use single-sided packet traces of existing application traffic to measure available bandwidth. In this paper, we describe our implementation of available bandwidth analysis using passive traces of TCP traffic and evaluate our approach using bursty traffic on a 100 Mb testbed.