Comcast Corp. told the Federal Communications Commission in formal comments Tuesday that hampering some file-sharing by its subscribers was a justifiable way to keep Web traffic flowing for everyone.
The comments are the fullest accounting yet of how Comcast manages its network.
Comcast's network management is the subject of formal complaints to the FCC from consumer groups and law professors.
The groups say Comcast has breached the principle, known as "'Net Neutrality," of treating all Internet traffic equally.
They also say the company was hampering movie downloading services because they might compete with Comcast's cable TV business.
Comcast says it must curb some file-sharing traffic because some subscribers would otherwise hog the cables with their uploads and slow traffic in their neighborhood.
The company — the country's second-largest Internet service provider — also said it was justified in using "reset" packets to break off communications between two computers.
Comcast sometimes inserts these packets in the data stream to kill a file-sharing session. The move "fools" each computer into believing the other computer wants to end the connection.
The return addresses of Comcast's packets indicate they're from one of the file-sharing computers when they are in fact from Comcast.
An Associated Press story in October that brought attention to Comcast's practices likened its use of reset packets to an operator breaking into a telephone conversation and telling each participant in the voice of the other: "Sorry, I have to hang up. Good bye."
In its comments Tuesday, Comcast called that analogy "inflammatory hyberbole" and cites ZDNet.com blogger George Ou, who wrote that the reset packet method is a common way to stifle traffic where alternatives don't exist.
It said the messages are like a busy signal for a fax machine.
In their complaint with the FCC, advocacy groups Free Press and Public Knowledge said Comcast was "forging" the return addresses of the reset packets.
File-sharing programs generally try to re-establish a connection after receiving a reset packet, and Comcast may let subsequent attempts through.
More likely, though, the computer that is requesting a file goes elsewhere on the Internet to find it.
Before the AP story ran, Comcast would acknowledge only that it was managing traffic and gave no details, even to its subscribers.
On Jan. 25, it updated its online Acceptable Use Policy to specify that it reserved the right to break off file-sharing connections on congested cables.
The FCC's reaction to subscriber complaints will be an important test of how far the agency will let Web providers go to manage their traffic.
Even Comcast's detractors generally agree proper traffic management can improve the Internet experience for everyone.
FCC Chairman Kevin Martin is supportive of network management, but has said providers like Comcast should be open about their practices.