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From Wikipedia:

Most private trackers monitor how much users upload or download, and in most situations, enforce a minimum upload-to-download ratio.

How exactly can a tracker figure out how much data was uploaded and downloaded by each user?

My understanding is that a BitTorrent tracker is merely a registry of users that are currently downloading/seeding and that peers, once connected, transfer data directly. So I wouldn't think that the tracker would know anything about the amount of data transferred, much less, where it came from.

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up vote 4 down vote accepted

The BitTorrent clients report back to the tracker what they are doing. There are some clients that lie but this can be detected by the fact the reports from the two sides consistently disagree. That's why you'll see some site banning the lying clients.

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My understanding is that a BitTorrent tracker is merely a registry of users that are currently downloading/seeding

If you take a look at the specification, you'll see that clients are required to report back a bit more information than just that. Two of those things are how much data has been downloaded and uploaded.

The client can, of course, fake this, which is why tracker staff take other means to determine if you are "stat-hacking".

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Some private trackers will identify users by IP address but most give each user a different .torrent file where the announce URL inside the .torrent file is unique for each user to identify them when connecting to the tracker.

Combined with the fact that bittorrent clients constantly inform the tracker about how much data they have downloaded and uploaded, this allows the tracker to record a users stats between sessions and even dynamic IP changes.

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Private trackers do, Public trackers do not. You have to connect to the tracker server to initiate/maintain the torrent download/upload, its called a tracker for a reason.

Most private trackers are not 100% accurate when collecting your U/D stats.

There are also Trackerless torrents, (DHT)


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