Prior studies have shown that XCP D. Katabi et al., (2002), while showing high potential for effective congestion control, may have significant performance problems if it misestimates the bottleneck link capacity Y. Zhang et al., (2005). To explore the magnitude of this implication and ultimately to contain it, we have conducted a control theoretic analysis of the XCP protocol and have studied its properties in the presence of capacity estimation errors. Under such conditions, by using a revised fluid model, we have 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 preliminary analysis explains what we have observed experimentally in an implementation study of XCP.