Most of the world's Internet users lost access to YouTube for several hours Sunday after an attempt by Pakistan's government to block access domestically affected other countries.
The outage highlighted yet another of the Internet's vulnerabilities, coming less than a month after broken fiber-optic cables in the Mediterranean took Egypt off line and caused communications problems from the Middle East to India.
An Internet expert explained that Sunday's problems arose when a Pakistani telecommunications company accidentally identified itself to Internet computers as the world's fastest route to YouTube. But instead of serving up videos of skateboarding dogs, it sent the traffic into oblivion.
On Friday, the Pakistan Telecommunication Authority ordered 70 Internet service providers to block access to YouTube.com, because of anti-Islamic movies on the video-sharing site, which is owned by Google.
The authority did not specify what the offensive material was, but a PTA official said the ban concerned a trailer for an upcoming film by Dutch lawmaker Geert Wilders, who has said he plans to release a movie portraying Islam as fascist and prone to inciting violence against women and homosexuals.
The block was intended to cover only Pakistan, but extended to about two-thirds of the global Internet population, starting at 1:47 p.m. ET Sunday, according to Renesys Corp., a Manchester, N.H., firm that keeps track of the pathways of the Internet for telecommunications companies and other clients.
The greatest effect was in Asia, were the outage lasted for up to two hours, Renesys said.
YouTube confirmed the outage on Monday, saying it was caused by a network in Pakistan.
"We are investigating and working with others in the Internet community to prevent this from happening again," YouTube said in an e-mailed statement.
A YouTube spokeswoman did not immediately respond to an e-mailed question on whether the clips that offended Pakistan's government had been removed. Several clips with interviews of Wilders were still up on the site Monday afternoon.
Two apparent errors allowed the outage to propagate beyond Pakistan, according to Todd Underwood, vice president and general manager of Internet community services at Renesys.
Pakistan Telecom established a route that directed requests for YouTube videos from local Internet subscribers to a "black hole," where the data was discarded, according to Renesys. Pakistan Telecom's mistake was that it then published that route to its international data carrier, PCCW Ltd. of Hong Kong, Underwood said.
The second mistake was that PCCW accepted that route, Underwood said. It started directing requests from its customers for YouTube data to Pakistan. And since PCCW is one of the world's 20 largest data carriers, its routing table was passed along to other large carriers without any attempt at verification.
"Once a pretty big network gets an error like that, it propagates to most or all of the Internet very quickly," Underwood said. As he put it, Pakistan Telecom was impersonating YouTube to much of the world.
Pakistan Telecom and the Pakistan Telecommunications Authority were unavailable for comment on Monday night local time. Rex Stover, vice president of enterprise sales for PCCW Global in Herndon, Va., said the company is still trying to figure out what happened and why.