Study: Cox, Comcast Internet subscribers blocked

Comcast maintains that hampering such programs helps ensure that traffic other than file-sharing is not impeded by a few big users. The company adds that is "delaying" file transfers rather than blocking them. Even that will end later this year, Comcast said in March, as it pledged to stop selectively targeting file-sharing.

Much of the FCC's attention to the matter has been focused on Comcast's secrecy. Before the AP's investigation, Comcast acknowledged only in the most general terms that it was managing traffic.

At least since 2006, Cox's subscriber agreement has noted that the company engages in "protocol filtering," which means it treats different types of Internet traffic, like Web surfing, e-mail and file-sharing, differently.

Cox said in a written statement Thursday that it takes such steps "to ensure the best possible online experience for our customers." But Cox denied that protocol filtering amounts to discrimination of any specific services.

The blocking observed by Gummadi's group occurs when a subscriber has downloaded a file using the BitTorrent application and tries to upload it, or share it with others, over the Internet. The main victims are the other Internet subscribers, who will not be able to download a file if a complete version is not available from someone else's computer.

Persistent attempts by file-sharing software to get through an Internet service provider's filtering may succeed after several minutes, as experienced in the AP's test last year. But Gummadi's test did not look at the duration of the traffic blocks.

The percentage of blocked connections for Comcast and Cox subscribers did not appear to correspond to periods of high congestion, despite Comcast's assertions to the FCC that the filtering only happens at certain times. Subscribers were roughly equally likely to be blocked at all times of day and night. FCC Chairman Kevin Martin told Congress in April that testimony collected by the agency indicated that Comcast's filter was active even when there was no congestion.

Robb Topolski, a former Intel Corp. engineer who noticed blocking on his home Comcast connection last year and brought it to notice, said Gummadi's work was the most authoritative study so far of this type of traffic interference.

Gummadi acknowledged that the test did not conclusively show that Cox or Comcast were blocking traffic, since neither company carried data all the way from their subscribers to his servers in Germany. In theory, Internet backbone carriers that take the traffic the rest of the way could be disrupting the connections, but there is no reason to believe they're doing so.

Apart from Comcast and Cox, Gummadi found signs of interference at seven other U.S. Internet providers, all of them cable companies. But the number of blocked connections was too low to conclusively say their subscribers are being targeted, and Gummadi withheld the companies' names.