Blocking of BitTorrent traffic is widespread among some ISPs in the US and Singapore, but not in Canada or any other country on the globe, says a new study released this week from German researchers.

The study was conducted between March 18 and May 15 by the German-based Max Planck Institute for Software Systems, with assistance with more than 8,000 users worldwide. Together, they implemented a tool called Glasnost to test whether their BitTorrent traffic was being manipulated.

Their results pinpointed Comcast and Cox as the leading culprits of torrent blocking in the US, though that may not come as a surprise to many of their customers.

The survey was limited strictly to hosts whose BitTorrent transfers to the institute's servers were blocked -- or interrupted by RST packets generated by an ISP -- as opposed to being throttled or rate-limited. For each country studied, the number of "distinct hosts" measured was compared against the number of hosts for which BitTorrent blocking was detected.

Interestingly, in spite of accusations of BitTorrent blocking recently filed with Canadian authorities against Bell Canada and other Canadian ISPs, the study found a total of only one blocked host out of 1,272 hosts measured in Canada.

That compares with Malaysia, where one of the 64 measured hosts turned out to be blocked. None of the hosts measured in the UK were blocked, though, and no blocking appeared in Australia, France, Germany, India, the Ukraine, Israel, United Arab Emirates (UAR), China, Japan, South Korea, or dozens of other countries studied.

In the US, however, 599 of the 2,714 measured hosts were blocked for BitTorrent transfers. The same held true for 26 of the 70 measured hosts in Singapore.

The study isolated the results for Comcast and Cox in the US, and for the StarHub ISP in Singapore. For Comcast, 491 of the 788 measure hosts were blocked, whereas for Cox, 82 of the 151 measured hosts were blocked. On StarHub, BitTorrent blocking took place on 26 of the 45 measured hosts.

All of the hosts observed to be blocking BitTorrent tracking did so in the upstream direction, while only a handful did so in downstream BitTorrent transfers.

In the US, Comcast has defended itself in FCC hearings by claiming that it "does not block any Web site, application, or Web protocol." But David L. Cohen, Comcast's executive VP, has admitted to "a limited form of network management," in which requests for file uploads are "delayed" but not "blocked," and only when periods of high network traffic and other conditions prevail.

Yet according to results of the institute's survey, the percentage of connections blocked -- as opposed to throttled -- by Comcast remains high at all times of the day.

"Our data suggests that the BitTorrent blocking is independent of the time of [day]," according to the institute's report.

The institute has released the source code for its Glasnost tool, which is downloadable from this page. The researchers plan to update their results page on BitTorrent traffic blocking as more data becomes available.