We propose a method of dimensioning and managing the bandwidth of a link on which flows arriving on access links that have heterogeneous bandwidths are aggregated. We start by developing a formula that approximates the mean TCP file-transfer time of a flow in such a situation. This only requires the bandwidth of the access link carrying the flow and the bandwidth and utilization of the aggregation link, each of which is easy to set or measure. We then extend the approximation to handle various factors that affect actual TCP behavior, such as round-trip time and restrictions other than the access-link bandwidth and congestion of the aggregation link in the end-to-end path of the flow. To do this, we define the virtual access-link bandwidth as the file-transfer speed of the flow when utilization of the aggregation link is negligibly small. We apply the virtual access-link bandwidth in the approximation to estimate the TCP performance of the flow with increasing utilization of the aggregation link. We use this method of estimation as the basis for a method of dimensioning the bandwidth of the link such that TCP performance is maintained and a method of managing bandwidth by comparing measured link utilization with the estimated threshold that indicates degradation of TCP performance. We also use simulation to analyze the accuracy of the estimates produced by our method.