You're trying to log on to a Web site and it's not working. You try again and again. But persistence doesn't pay off. The site you want is inexplicably, frustratingly, out of reach.

The other computer might just be turned off, but the causes could be more mysterious. At any given moment, a proportion of computer traffic ends up being routed into information black holes. These are situations where a path between two computers does exist, but messages -- a request to visit a Web site, an outgoing e-mail -- get lost along the way.

A University of Washington system named Hubble looks for these black holes and maps them on a Web site, providing an ever-changing constellation of the Internet's weak points.

The Hubble map lets visitors see a map of problems worldwide or type in a specific Web page or network address to check its status. The work is being presented next week in San Francisco at the Usenix Symposium on Networked Systems Design and Implementation.

"There's an assumption that if you have a working Internet connection then you have access to the entire Internet," said first author Ethan Katz-Bassett, a UW doctoral student in computer science and engineering. "We found that's not the case."


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