Pakistani authorities' efforts to block access to the video-sharing Web site YouTube from Internet addresses in their own country effectively shut down the site altogether last weekend.
The outage, which appears to have been the result of an error, blocked traffic to the site for about two hours Sunday.
On Friday, the Pakistan Telecommunications Authority ordered Internet service providers (ISPs) in Pakistan to block access to a YouTube video: a trailer for a film by Dutch politician Geert Wilders that is critical of the Koran.
The authority called the trailer "highly provocative and blasphemous," adding that it was "arousing deep anguish and distress across the Muslim world."
The authority said it blocked the site to forestall "more unrest and possible loss of life and property across the country."
Experts say the method at least one ISP used to block the site caused worldwide traffic to YouTube to be directed into a kind of Internet cul-de-sac in Pakistan.
YouTube confirmed the cause of the outage in an e-mailed statement yesterday. YouTube also confirmed that the outage originated in Pakistan.
"Traffic to YouTube was routed according to erroneous Internet protocols, and many users around the world could not access our site," YouTube said.
"We have determined that the source of these events was a network in Pakistan," YouTube's statement continued. "We are investigating and working with others in the Internet community to prevent this from happening again."
Marcus Sachs, director of the Internet Storm Center run by the nonprofit Sans Institute in Bethesda, said the outage was a byproduct of the open architecture of the Internet and the way in which Web traffic is routed from server to server.
"This looks like a mistake by a router engineer," said Mr. Sachs, and the error occurred when a "local announcement to computers using one ISP ... leaked out somehow and was propagated across the Internet generally."
The announcement was in the form of routing information provided under the so-called Border Gateway Protocol, or BGP.
BGP tables tell computers how to connect to Internet addresses their users want to visit, Mr. Sachs said.
To block YouTube, one ISP, Pakistan Telecommunication Co. Ltd. (PCTL), essentially placed false information in its BGP tables to divert customers trying to access YouTube into a kind of Internet dead-end, said Daniel Castro of the Information Technology and Innovation Foundation, a research institute based in the District.
But the false routing information was picked up by a large telecommunications company that provides network services to PCTL and distributed throughout the Internet, Mr. Castro explained.
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