BitTorrent

BitTorrent is by far the most popular P2P protocol in the 2004-2005 time span. Its only claim to legality is its popularity for distributing releases (as well as a few minor nice uses), and it has quickly risen to the top of any net/sysadmin's hit list as the first thing to squash if you start having bandwidth issues.

I expect s to start actively blocking it any day now.

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