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Peer-to-Peer by Andy Oram

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Case study 1: Freenet

The small-world effect is fundamental to Freenet’s operation. As with Milgram’s letters, Freenet queries are forwarded from one peer to the next according to local decisions about which potential recipient might make the most progress towards the target. Unlike Milgram’s letters, however, Freenet messages are not targeted to a specific named peer but toward any peer having a desired file in its data store.

To take a concrete example, suppose I were trying to obtain a copy of Peer-to-Peer. Using Milgram’s method, I could do this by trying to get a letter to Tim O’Reilly asking for a copy of the book. I might begin by passing it to my friend Dan (who lives in Boston), who might pass it to his friend James (who works in computers), who might pass it to his friend Andy (who works for Tim), who could pass it to Tim himself. Using Freenet’s algorithm, I don’t try to contact a particular person. Instead, I might ask my friend Alison (who I know has other O’Reilly books) if she has a copy. If she didn’t, she might similarly ask her friend Helena, and so on. Freenet’s routing is based on evaluating peers’ bookshelves rather than their contacts—any peer owning a copy can reply, not just Tim O’Reilly specifically.

For the Freenet algorithm to work, we need two properties to hold. First, the Freenet graph must be connected, so that it is possible for any request to eventually reach some peer where the data is stored. (This assumes, of course, that the data does exist on ...

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