The current peer-to-peer (P2P) information sharing paradigm does not provide incentive and service differentiation for users. Since there is no motivation to share information or resources, this leads to the "free-riding" and the "tragedy of the commons" problems. In this paper, we address how one can incorporate incentive into the P2P information sharing paradigm so as to encourage users to share information and resources. Our mechanism (or protocol) provides service differentiation to users with different contribution values and connection types. The mechanism also has some desirable properties: (1) conservation of cumulative contribution and social utility in the P2P community, (2) maximization of social utility if all requesting clients have the same contribution value, and (3) incentive-based resource distribution. The resource distribution algorithm and the contribution update algorithm are computationally efficient and can be easily implemented. Experimental results illustrate the efficiency and fairness of our algorithms.