In 1995, the first warning was raised: The throngs of people swarming to the Internet would overwhelm the system in 1996. For more than a decade, that fear has proven untrue.

Until right about now. The growing popularity of video on the Net has driven a traffic increase that's putting strains on service providers, particularly cable companies. To deal with it, they have had to change the way they convey Internet data.

And they've done this in secret, raising concerns by Web companies, consumer groups and the chairman of the Federal Communications Commission that the nature of the Internet is being altered in ways that are difficult to divine.

But as traffic grows, there are signs that these subtle and secret controls are insufficient, and will give way to more overt measures. For instance, we could find ourselves paying not just for the speed of our connection, but for how much we download. Already, some ISPs are hindering file-sharing traffic, and AT&T Inc. is talking about blocking pirated content.

The issue is coming to a head this year, as the FCC is investigating complaints from consumer groups and legal scholars that Comcast Corp., the country's largest cable ISP, secretly hampered file sharing by its subscribers. File sharing, which allows Internet users to download movies, music and software among each other with software like BitTorrent and KaZaa, has been a haven for piracy, though legal uses are proliferating as well.

By interfering with traffic, the groups said, Comcast is determining what will and won't work, violating the Internet's unwritten tradition of "net neutrality" the principle that traffic be treated equally.

The FCC has adopted a broad policy that Internet service providers can't block specific applications. But its interpretation of that statement is unclear because it hasn't had to rule on a similar case. Crucially, the policy makes an exemption for "reasonable traffic management," which Comcast says its practices fall under.

The FCC case will be closely watched by ISPs, because it appears that most of them use some kind of traffic management, slowing down less time-sensitive traffic, like file sharing, to keep Web surfing snappy. Whereas earlier doom scenarios for the Internet mostly concerned the "highways" that move traffic around the country, the chokepoints that are appearing are actually close to our homes. It's your neighbors who are the problem.

"The increasing use of bandwidth by a minority is an increasingly important issue for all ISPs," said Time Warner Cable Inc. spokesman Alex Dudley.

Time Warner Cable reserves the right to limit the bandwidth available to applications like file sharing and manage traffic in other ways. But it won't say what it does, for fear that competitors could attack that in their marketing.

Internet service providers and consumer advocates agree that some form of network management, also called "traffic shaping," can be good for all. Not all Web traffic has the same level of urgency. It makes sense for the service providers to give priority to a voice call, which needs a steady stream of quickly delivered data, over a movie download.

This is unusual territory for telecommunications providers in the old telephone network, as some phone calls aren't generally prioritized over others. Prioritization makes the Internet more like the postal system, where you pay for delivery speed and quality of service.

The goal for ISPs is to ensure that "some bandwidth hog can't knock your mom off, who's just trying to get her e-mail," said Rob Malan, founder of Arbor Networks, a company in Lexington, Mass., that supplies the gear ISPs use to identify and prioritize traffic. The company is quick to point out that its products don't work in the way that's drawn FCC scrutiny to Comcast.

But the heavy veil of secrecy ISPs lay over their practices make it very hard to evaluate what they are doing.

Arbor Networks said it has 160 customers worldwide for traffic prioritization. It has been able to reveal the names of some overseas clients, but no U.S. customer is willing to be identified.

When users complained to Comcast about file-sharing not working, the company would not acknowledge the problem. Only after an Associated Press investigation brought attention to the issue in October did Comcast disclose it was temporarily blocking some file-sharing attempts. It updated its online Terms of Service on Jan. 25., without telling customers, to include a statement about how it may limit file-sharing.

It's difficult to have a public discussion without knowing the facts, so Lauren Weinstein of the nonprofit People for Internet Responsibility has formed a Network Neutrality Squad to figure out what is actually happening on the Internet in the U.S.

"It's an open society, and frankly, I don't buy into the premise that you have to have these secret rules ... to protect these networks," Weinstein said.

Weinstein's squad is developing software tools that it will distribute for free to people who want to test what their ISP is doing, and collect the data. Its supporters include some of the Internet's royalty: Vint Cerf, "chief Internet evangelist" at Google and one of the inventors of an underlying Internet technology, and Craig Newmark, founder of Craigslist.

FCC Chairman Kevin Martin has seized on the secrecy issue as well.

"The question is going to arise: Are they reasonable network practices?" Martin told an audience at a technology trade show in January. "When they have reasonable network practices, they should disclose those and make those public."

Comcast also has drawn fire for imposing secret limits on how much its subscribers can download in a month, then cutting off subscribers who don't heed warnings. Its rationale is that it doesn't want to scare the vast majority of its customers, who get nowhere close to the limits. (User estimates and hints from the company put the download caps somewhere around 100 gigabytes per month, equivalent to a dozen high-definition movies.) Other ISPs are also reported to have secret limits.