Some private BitTorrent trackers monitor their members to ensure users upload as much as they download. Supporters say monitoring the upload/download ratio encourages sharing, resulting in faster download speeds. Those who do not reach the minimum ratio are normally banned from using the tracker.

To monitor user ratios’, trackers depend on clients reporting their true upload and download statistics. Programmers are increasingly exploiting this vulnerability by developing software which falsifies upload and download reports to the tracker.

The exploit was first widely publicized over a year ago, but no solution has yet been found. Only standard code is sent to the tracker, which is impossible to verify using the current BitTorrent protocol.

As ratio cheating software is becoming easier to use and more readily available, the pressure is on Bram Cohen and BitTorrent Inc. to update the protocol to catch ratio “cheats”.

However, Bram Cohen has told Zeropaid that he stands by the comments he made in 2005, arguing that ratio monitoring is destructive to sharing.

“[Leechers are] engaging in perfectly reasonable and non-destructive behavior and the site is trying to punish him for it, thus fostering the creation of clients which lie about their statistics. This is the site’s fault, and the result could do serious damage to the value of BitTorrent statistics generally. Sites which do this are being extremely destructive, and the way they grandstand about how they’re fostering sharing really ticks me off,” he said.

Bram argues that the tit-for-tat nature of protocol is sufficient enough to stop destructive leeching. The BitTorrent protocol is robust enough to handle file sharers who limit their upload and do not seed after the file has finished downloading.

“Even if almost everyone quit the instant their download was completed you’d still have decent download rates, they’d just be closer to everyone’s upload rates,” he explains.

By definition this means download speeds will be slower, which could easily be classed as “destructive”.

Still, according to Cohen, the alternative of monitoring ratios is worse.

“What typically happens in a single torrent is that at the beginning upload and download ratios are reasonably correlated, then over time people finish downloading, and some of them leave, but a significant number of seeds remain. After a while there’s a period of time where there are many more seeders than downloaders. Anyone who joins the torrent at this late time will be generally get a download rate limited by their download capacity, and anyone who tries to upload to them will only be able to do so at a low rate. Such people will have very out of whack upload/download ratios, but they’re downloading from otherwise unutilized upload resources, and hence not being anywhere near the drain on the system that their total upload/download ratio indicates,” he explains.

“Just a little bit of threatening to ban people can get the overall balance to be very heavily weighted on the side of uploading, making it difficult for people to accomplish a reasonable amount of upload even if they try.”

To encourage sharing beyond the tit-for-tat system, Cohen suggests that user ratios should not be monitored at the peak of each swarm, or if a torrent is heavily seeded. Alternatively, he suggests adapting an alternative method for calculating the ratio, which takes into consideration the health of a torrent:

“When a client reports new downloads to the tracker, the tracker can multiply the amount by (number of current peers total – number of current seeds) / (number of current peers total) and add that to the ‘total downloaded’. This results in most people having a ‘ratio’ of more than 1, but that isn’t actually a problem unless you’re more interested in mathematical purity than practical behavior.”

So far, the BitTorrent world has not been turned on its head by ratio cheating software, but there is no accounting for the future. Without support from BitTorrent Inc. to upgrade the protocol, private tracker administrators who want to keep ratio monitoring may be forced to develop a new protocol.

*Editor’s Note: This story is a follow up to a previous article

BitTorrent Ratio Exploit Software on the Increase