BitTorrent ration - how does it work?

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http://www.linuxquestions.org – Hi, when I just purged and reinstalled my Torrent client due to configuration issues, the program still knew what U/D ratio I had before purging. Also, I read that individual trackers also keep a record of a user's history, which seems likely as my client takes some time to report it to them when closing. Thus, I am wondering how this statistic is kept. How is BT able to identify a single machine, maybe in a private network, or even a single user on it? Unfortunately, Google only points me to sites telling me how to cheat programs and / or trackers, no matter what I search for, which is not (HowTos)