P2P (peer-to-peer) overlays have attracted many researchers due to increase in Internet based P2P applications. A P2P overlay is a distributed system in which the independent nodes participate at their will to share resources in distributed manner. P2P overlays are designed for wired based communication systems but today P2P applications are becoming popular in wireless networks even in multi-hop wireless networks. Routing mechanism in P2P overlays is based on IP infrastructure and many protocols are deployed successfully for efficient and fair P2P applications. However, there are many challenges in designing and deployment of efficient and fair protocols for the P2P overlays. These overlays suffer from many challenges such as dynamic overlay management, lack of robust trust model, counterfeit content distribution, free riding, poor resource search scalability, security, etc. The churn rate of nodes (join and leave of nodes) in the P2P overlay makes overlay management and resource searching more challenging. The free riding nature of the nodes in a P2P overlay is undesirable and it creates unfairness in the P2P overlay. There are several mechanisms proposed such as Eigen Trust model, tit-for-tat policy, point-based incentive policy, Page Rank policy, layered taxation, advertisement of incentive, etc. for encouraging fairness in the P2P overlays. I have presented in-depth survey over free riding behaviour, its effect, and existing mechanisms to reduce free riding in structured P2P overlays.
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