Abstract: Prior studies have shown that XCP, while showing high potentials in effective congestion control, may have significant performance problems if it misestimates the bottleneck link capacity. To understand the magnitude of this implication and ultimately to contain it, we have conducted a control theoretic analysis of the XCP protocol and studied its properties in the presence of capacity estimation errors. With a revised fluid model, we discovered that XCP will not settle at zero steady-state error. However, we found that the steady-state error is bounded by the estimation error, and this bound can be exploited in XCP router queue size planning. This analysis explains what we have observed experimentally in an implementation study.