Many Internet Service Providers tune the configuration of th e Border Gateway Protocol on their routers to control their traffic. Content p roviders often need to control their outgoing traffic while access providers need t o control their incoming traffic. We show, by means of measurements and simulations, t hat controlling the flow of the incoming interdomain traffic is a difficult problem . For this purpose, we first rely on detailed measurements to show the limitations o f AS-Path prepending. Then, we show by using large-scale simulations that the diffi culty of controlling the flow of the incoming traffic lies in the difficulty of predic ting which BGP route will be selected by distant ASes.