Comcast has also questioned whether the FCC has the authority to regulate its network management techniques.
But commissioners voting against Comcast said the FCC has consistently asserted authority to regulate broadband networks, and Friday's decision focuses narrowly on one provider's specific actions.
"If we aren't going to stop a company that is looking inside its subscribers' communications ... blocking that communication when it uses a particular application regardless of whether there is congestion on the network, hiding what it is doing by making consumers think the problem is their own, and lying about it to the public, what would we stop?" Martin said.
Comcast interfered with people watching movie trailers, updating online game clients and downloading open-source software available on P-to-P networks, said Dana Shaffer, chief of the FCC's Wireline Competition Bureau.
"Comcast's practices are not minimally intrusive, but invasive," she said.
The Associated Press, in late 2007, reported that Comcast was slowing BitTorrent and some other traffic without telling its customers. Consumer rights groups Public Knowledge and Free Press, along with online video distributor Vuze, filed complaints with the FCC.
Comcast has said it throttles P-to-P traffic only during times of peak congestion, but Martin and a study from the Max Planck Institute for Software Systems in Germany have contended that Comcast slows BitTorrent traffic during off-peak hours as well.
Only 6 to 7 per cent of Comcast subscribers use P-to-P services in a typical week, but one-half to two-thirds of the upstream traffic on its network comes from P-to-P, Fitzmaurice said. About 90 per cent of P-to-P sessions on the Comcast network are unaffected by traffic management, she added.