In a 57-page filing with the Federal Communications Commission, Comcast admitted temporarily blocking certain peer-to-peer traffic, characterizing its actions as "network management."

Coincidentally, the filing came a day before Congress took up a landmark, bipartisan "net neutrality" bill. The legislation would prevent network operators, like Comcast, from restricting the kinds of content that travels across their networks, or in any way discriminating against content providers.

In its filing, made Tuesday, Comcast said the "carefully limited measures" it takes are part of its effort to provide "a high-quality, reliable Internet experience" for its customers.

The filing was made in response to complaints leveled by consumer rights group Free Press and Web video company Vuze charging that Comcast is trying to stifle competition by blocking rival video-on-demand services.

F.C.C. Chairman Kevin Martin said last month that the commission would look into the allegations. Today is the last day for interested parties to comment on the matter.

Comcast says that it delays some video uploads during times of heavy traffic to prevent its network from being overwhelmed. It compared its actions to a traffic light on a freeway on-ramp. The light doesn't prevent cars from using the road, it just regulates traffic flow.

Free Press general counsel Marvin Ammori maintains that Comcast is using its control over the network to act as a gatekeeper for content its favors.

Congress entered the debate when Representatives Edward J. Markey, a Massachusetts Democrat, and Charles W. "Chip" Pickering, a Mississippi Republican, introduced the bill to guarantee net neutrality.

Telecom and cable companies oppose the legislation. "Network management is best left to the sound, good-faith judgment of the engineers and proprietors who run and own the networks," Comcast said in its filing.

Broadband companies contend that since the own the "pipes" — the high-capacity cables that carry Internet traffic — they should have a say over what moves through them. They also assert a right to charge more to give priority to some content and to block other content.

Net neutrality advocates counter that giving broadband companies the ability to discriminate against certain content providers would stifle innovation by making it harder for new services to grow. Discrimination could also allow cable and phone companies to quash potential video-on-demand competitors.

"The introduction of this legislation gives hope to the millions of Americans who want the public — not phone and cable companies — in control of the Internet," said Timothy Karr, campaign director of Free Press.

The alliance in favor of the bill is cuts a wide swath across the political and social landscape and includes MoveOn.org, the Gun Owners of America, the American Civil Liberties Union, and the Christian Coalition of America.

Teamsters General President Jim Hoffa, another supporter of the bill, said the issue is essential to the country. "Americans need to ask themselves: What good is free speech if a handful of powerful corporations have the ability to shut off or slow viewpoints they find objectionable?" he asked.

by Sam Gustin