Software :: BitTorrent Ration - How Does It Work

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 what I want. So, could anyone tell me in quick words or point me to a site explaining what the underlying mechanics of the system are?

Furthermore, are there possibilities to adapt the ratio generation to your bandwidth circumstances? Unfortunately, as most people in Germany, I am connected via ADSL, which allows for great download rates, but only minimal upload, which is why it takes me forever to equalize what I downloaded. Is this problem thought of in the concept of BT?

I am in no way interested in cheating, so please don't mistake this for an immoral question - if I'm stuck with this situation, fine. Either way, I would still like to read about the technical concepts. (In an plain way, if possible, scientific English is quite hard for me to understand.) [I am aware that this is not a "problem" with software, but seeing how BT is quite popular with Linux users