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Unformatted text preview: and a loop (step 7) before locating the desired file. •  requester'cannot' differentiate'provider'from' maintain a central index of files, so that users can send requests directly to information holda'cache'holder'or'a'forwarding' ers. Unfortunately, centralization creates a sinpeer'(allows'for'anonymity)' gle point of failure that is easy to attack. For example, if you were trying to phone Michael Jordan, the simplest way to get his number would ordinarily be to call directory assistance. However, because directory assistance is centralized, your access can be easily blocked if Jordan or someone else decides to remove his directory entry, or if the service goes down. Systems like Gnutella broadcast queries to every •  content'is'divided'into'N'' cessful, each node in the chain passes the file back upstream and creates a new entry in its routing table associating the data holder with the requested key. Depending on its distance from the holder, each node might also cache a copy locally. To conceal the identity of the data holder, nodes will occasionally alter reply messages, setting the holder tags to point to themselves before passing them back up the chain. Later requests will still locate the data because the node retains the true data holder’s identity in its own routing table and •  forwards queries to the correct holder. Routing tables are never revealed to other nodes. •  To limit resource usage, the requester gives each query a time-to-live  limit that is decremented at • each node. If the TTL expires, the query fails, although the user can try again with a higher TTL •  (up to some maximum). Because the TTL can give clues about where in the chain the requester is, Freenet offers the option of enhancing security by adding an...
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