AP Tests Comcast's File-Sharing Filter
An AP reporter attempted to download, using file-sharing program BitTorrent, a copy of the King James Bible from two computers in the Philadelphia and San Francisco areas, both of which were connected to the Internet through Comcast cable modems.
We picked the Bible for the test because it's not protected by copyright and the file is a convenient size.
In two out of three tries, the transfer was blocked. In the third, the transfer started only after a 10-minute delay. When we tried to upload files that were in demand by a wider number of BitTorrent users, those connections were also blocked.
Not all Comcast-connected computers appear to be affected, however. In a test with a third Comcast-connected computer in the Boston area, we were unable to test with the Bible, apparently due to an unrelated error. When we attempted to upload a more widely disseminated file, there was no evidence of blocking.
The Bible test was conducted with three other Internet connections. One was provided by Time Warner Inc.'s Time Warner Cable, and the other came from Cablevision Systems Corp. The third was the business-class connection to the AP's headquarters, provided by AT&T Inc. and Cogent Communications Group Inc.
No signs of interference with file-sharing were detected in those tests.
Further analysis of the transfer attempt from the Comcast-connected computer in the San Francisco area revealed that the failure was due to "reset" packets that the two computers received, carrying the return address of the other computer.
Those packets tell the receiving computer to stop communicating with the sender. However, the traffic analyzer software running on each computer showed that neither computer actually sent the packets. That means they originated somewhere in between, with faked return addresses.
In tests analyzing the traffic received by a computer on Time Warner Cable that was trying to download a file from a large "swarm" of BitTorrent users, more than half of the reset packets received carried the return addresses of Comcast subscribers, even though Comcast's 12.4 million residential customers make up only about 20 percent of U.S. broadband subscribers. It was the only U.S. Internet service provider whose subscribers consistently appeared to send reset packets (which are occasionally generated legitimately).
Comcast subscriber Robb Topolski, who discovered the blocking earlier this year and traced it to reset packets, pointed out that a Canadian company called Sandvine Inc. sells equipment that promises to save bandwidth for Internet service providers by managing and redirecting file-sharing traffic.
BitTorrent Inc. President Ashwin Navin said that the interference method on Comcast's network is consistent with Sandvine's technology. Sandvine did not respond to a request for comment.
Comcast spokesman Charlie Douglas would not confirm that the company uses Sandvine equipment.
"We rarely disclose our vendors or our processes for operating our network for competitive reasons and to protect against network abuse," he said.
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