By deploying Peer-to-Peer (P2P) caches at the edge of access ISPs

cross-ISP P2P traffic can be reduced.The problem is how to design an optimal cache capacity? Up to now

no optimal P2P cache capacity design method has been proposed yet.In this paper

an optimal P2P cache capacity design method is proposed based on the storage versus bandwidth cost tradeoff.In order to minimize the total traffic cost of access ISPs

the cache capacity design problem is formulated into an integer programming problem whose objective function is piecewise continuous.The formula of optimal cache capacity can be derived through theoretical derivation.Experimental results suggest that compared with several simple design rules including No Cache

Median and 20-80 Rule

ISPs can achieve significant cost saving using our method.For example

compared with 20-80 Rule that is most commonly used by ISPs nowadays

the cost saving can be as much as 7.5%.