In peer to peer content distribution networks, nodes can cooperate by using portion of their bandwidth to transfer data to other nodes in the network. A fundamental problem with such environments is that nodes have no incentive to cooperate. This thesis introduces a credit-based market among the nodes. In the market nodes buy and sell bandwidth at a price set by the selling node. Consequently, the thesis proves that the proposed credit-based system will increase the total benefit to the social welfare at a unique equilibrium point. Thus, maximum cooperation is gained by applying the proposed credit system. Finally an iterative algorithm is introduced for nodes to update their prices. Simulation results show that the mentioned algorithm, applied simultaneously by all nodes, will converge to the equilibrium state.