When you search for a popular music title, you ask your local ultrapeer to search for you. That ultrapeer asks its leaves if they have it, and then it asks other ultrapeers, who do the same thing.
The thing that stops people from controlling the entire network is that ultrapeers will only pass on searches a certain number of times. If the limit is 6(I don't know what it actually is), after the 6th ultrapeer "generation" doesn't find anything in its leaves, it gives up and *does not* pass on the search. So *no one* is *ever* able to search, control, or monitor even a decent fraction of the entire network, unless they already control almost all the ultrapeers in it.
That does mean that some people get caught, and the **AA is able to monitor small parts of the network, but it hinders them enough to make it very expensive. It also means that you have a good amount of protection; it could be years before you get caught sharing, and you might never get caught downloading. [ theymos's advice column | Ask theymos A Question ]
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