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8.2.2 Link credit Because a user may have multiple identities, we can no longer associate a separate credit balance with each identity. Otherwise, a malicious user could gain additional credit and send arbitrary amounts of unwanted communication simply by creating more identities. Instead, Ostra leverages the cost of forming new links in trust net- works to enforce a bound on each user. Specifically, each link in the trust network is assigned a link credit balance B, with an initial value of 0, and a link balance range [L, U ], with L ≤ 0 ≤ U and L ≤ B ≤ U . These are analogous to the user credit balance and range in the original design. We X→Y denote the balance and balance range for a link X ↔ Y from X’s perspective as BLU . X→Y For example, if the link has the state 3+6 , then X is currently owed 3 credits by Y, −5 and the balance can range between –5 and 6. The link balance represents the credit state between the user identities connected by the link. Ostra uses this balance to decide whether to issue tokens. It is important to note that the credit balance is symmetric. For example, if the link balance on the X→Y X↔Y linkis 1+3,thenXisowedonecreditbyY,or,fromY’sperspective,Y owes −2 X one credit (the latter can be denoted −1+2). Y→X −3 We map the user credit balance in the strawman design to a set of link credit balances on the user’s adjacent links in the trust network. For example, as shown in Figure 8.2, if a user has two links in the trust network, the user’s original credit balance is replaced with two separate credit balances, one on each link. However, we 163PDF Image | Online Social Networks: Measurement, Analysis, and Applications to Distributed Information Systems
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