Many peer-to-peer communities, including private BitTorrent Communities that serve hundreds of thousands of users, utilize
credit-based or sharing ratio enforcement schemes to incentivize their members to contribute. In this paper, we analyze the performance
of such communities from both the system-level and the user-level perspectives. We show that both credit-based and sharing ratio
enforcement policies can lead to system-wide “crunches” or “crashes” where the system seizes completely due to too little or to too
much credit, respectively. We explore the conditions that lead to these system pathologies and present a theoretical model that predicts
if a community will eventually crunch or crash. We apply this analysis to design an adaptive credit system that automatically adjusts
credit policies to maintain sustainability. Given private communities that are sustainable, it has been demonstrated that they are greatly
oversupplied in terms of excessively high seeder-to-leecher ratios. We further analyze the user-level performance by studying the
effects of oversupply. We show that although achieving an increase in the average downloading speed, the phenomenon of oversupply
has three undesired effects: long seeding times, low upload capacity utilizations, and an unfair playing field for late entrants into swarms.
To alleviate these problems, we propose four different strategies, which have been inspired by ideas in social sciences and economics.
We evaluate these strategies through simulations and demonstrate their positive effects.

Many private BitTorrent communities employ Sharing Ratio Enforcement (SRE) schemes to incentivize users to contribute their upload resources. It has been demonstrated that communities that use SRE are greatly oversupplied, i.e., they have much higher seeder-to-leecher ratios than communities in which SRE is not employed. The first order effect of oversupply under SRE is a positive increase in the average downloading speed. However, users are forced to seed for extremely long times to maintain adequate sharing ratios to be able to start new downloads. In this paper, we propose a fluid model to study the effects of oversupply under SRE, which predicts the average downloading speed, the average seeding time, and the average upload capacity utilization for users in communities that employ SRE. We notice that the phenomenon of oversupply has two undesired negative effects: a) Peers are forced to seed for long times, even though their seeding efforts are often not very productive (in terms of low upload capacity utilization); and b) SRE discriminates against peers with low bandwidth capacities and forces them to seed for longer durations than peers with high capacities. To alleviate these problems, we propose four different strategies for SRE, which have been inspired by ideas in social sciences and economics. We evaluate these strategies through simulations. Our results indicate that these new strategies release users from needlessly long seeding durations, while also being fair towards peers with low capacities and maintaining high system-wide downloading speeds.