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In the past few years, P2P file distribution applications (e.g., BitTorrent) are becoming so popular that they are the dominating source of Internet traffic. This creates significant problems to Internet Service Providers (ISPs), not only because of the added complexity in traffic engineering, but the increase of traffic, in particular on the cross-ISP links, implies congestion and a higher operating cost. In this paper, we consider an ISP-friendly file distribution protocol which uses the “exploiting-the-locality principle” (ELP) to reduce the cross-ISP traffic. To show its benefit, we derive an upper and lower bound of cross-ISP traffic for the protocols which rely on ELP and show that the cross-ISP traffic can be reduced significantly when the number of peers within an ISP increases. To carry out realistic study, we design and implement our ISP-friendly protocol (which is compatible with the current BitTorrent protocol) and carry out large scale experiments on PlanetLab to measure the reduction of the cross ISP-traffic and the file downloading time. More important, we also show how the proposed ISP-friendly protocol can handle the “black-hole” security attack. This paper sheds light on the merits and design direction of ISP-friendly content distribution protocols.