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155 can find themselves unable to communicate. To address this problem, credit balances in Ostra decay towards 0 at a constant rate d with 0 ≤ d ≤ 1. For example, Ostra may be configured so that each day, any outstanding credit (whether positive or negative) decays by 10%. This decay allows an imbalance between the credit earned and the credit lost by a user. The choice of d must be high enough to cover the expected imbalance but low enough to prevent considerable amounts of intentional unwanted communication. As we show as part of Ostra’s evaluation, a small value of d is sufficient to ensure that most legitimate users never exceed their credit range. With this refinement, Ostra ensures that each user can produce unwanted com- munication at a rate of at most d∗L+S (8.1) where S is the rate at which the user receives communication that she marks as unwanted. A denial of service attack is, however, still possible. Colluding malicious users can inundate a victim with large amounts of unwanted communication, causing the victim to acquire too much credit to receive any additional communication. For these users, the rate of decay may be too low to ensure that they do not exceed their credit balances. To prevent such attacks, we introduce a special account, C, that is not owned by any user and has no upper bound. Users with too much credit can transfer credit into C, thereby enabling them to receive further communication. Note thatPDF Image | Online Social Networks: Measurement, Analysis, and Applications to Distributed Information Systems
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