modeling an amount of delay observed by the at least one proxy server for the content fetched from the at least one content provider original site at time t0 as ret ??? ( P ) = congestion ( P + r ??? ( t 0 ) B ) ??? r ??? ( t 0 ) ??? d , wherein P is a pre-fetch bandwidth size, B is a total bandwidth available to the system, r(t)=(1?????(P))???u(t) is an amount of bandwidth the system uses for ret