The Federal Communications Commission (FCC) went to court once again on Friday to defend its rules that ban the slowing or blocking of Internet traffic by Internet service providers (ISPs), in a landmark case that advocates of “net neutrality” fear could open the door for favoritism on the Internet and dramatically alter how consumers access data.

For several years, ISPs including Verizon, Comcast and AT&T have been fighting to strike down the FCC’s regulations on net neutrality — the principle that all Internet traffic should be treated equally. The companies say they should be able to regulate traffic and sell higher speeds to those who can afford it. The ISPs have twice managed to convince the same federal court in Washington, D.C., that the FCC’s rules went too far, and the rules were struck down.

The latest case deals with the FCC’s newest set of rules, approved in February. The key element in those rules, which the agency argues are necessary to protect the interests of consumers, is the FCC’s reclassification of broadband Internet as a telecommunications service — rather than an information service — thereby exposing it to greater agency regulation.

Net neutrality advocates who attended Friday’s hearing were optimistic that their argument stood on firm legal ground.

“There is very strong Supreme Court precedent which says the FCC has discretion in this area,” said John Bergmayer, a senior staff attorney for Public Knowledge, a Washington, D.C.-based public interest group that works on net neutrality.

The worst fear of net neutrality advocates is that without the FCC's Open Internet rules in place, ISPs would be free to charge major technology companies like Google or Facebook for speedier access to content, edging out smaller content providers. They also argue that net neutrality is key to innovation, giving entrepreneurs the same access to Internet users enjoyed by tech giants like Google.

Some companies, such as Netflix, have come out strongly in favor of Open Internet, fearful that an ISP could slow service to Netflix as an incentive to drive traffic to its own streaming service — like Comcast’s Xfinity.

“The FCC rules protect consumers’ ability to use broadband to access content. And it’s not up to the ISP to interfere," Bergmayer said. "If you want to use a certain streaming service, your decision shouldn't be based on, ‘Well the ISP is blocking one so I’ll use the other.'"

The ISPs argue that the FCC has no right to interfere in how they manage these information pipelines, since the ISPs built them. More generally, the telecom industry makes the case that net neutrality discourages further investment in infrastructure.

AT&T Senior Vice President Bob Quinn said this week that since the FCC’s Open Internet rules came into effect, the company has had to “shelve” the development of certain projects for fear they would violate the rules. He did not provide any details.

But Michael Burstein, law professor at Yeshiva University’s Cardozo School of Law, who filed briefs in support of the FCC, said the case wouldn’t hinge on whether net neutrality is a good or bad idea, but on whether the agency had the authority to make the rules and whether they violate the First Amendment.

Judge David Tatel, who is considered the “swing vote” in the circuit’s three-judge panel, wrote the majority opinion in a 2010 ruling against the FCC after it cited Comcast for slowing Internet traffic speeds to some customers who were downloading very large data files from peer-to-peer networks. The FCC then issued its new rules to block broadband providers from slowing or discriminating against Internet traffic; Tatel wrote the opinion that blocked those rules in 2014.

Congress has considered writing its own rules on net neutrality, but hasn't voted on any proposals. The appeals court ruled in 2014 that the FCC had used the wrong legal reasoning to issue the rules, but said that without net neutrality, "broadband providers represent a threat to Internet openness."

The net neutrality rules are backed by the American Civil Liberties Union, library groups and several large online content companies, including Twitter, Medium, Yelp and Reddit. The companies filed a joint legal brief warning that freedom of speech online "is at risk if broadband providers get to block or limit access to those voices they decide to disfavor — and that is exactly what petitioners are fighting for the right to do."

With Reuters