Most users run into an Internet black hole when the Web site they want to access won't load or their e-mail seems to get swallowed up in cyberspace.

According to a research team at the University of Washington, something like that is happening a lot more often, and over much longer times, than anyone ever thought. And they've created an interactive, online map, updated every 15 minutes, that shows the Internet's trouble spots. Colored flags show problem areas, and list the IP addresses affected.

The map is the fruit of an Internet monitoring system, dubbed Hubble after the black-hole-searching space telescope. Launched September 17, 2007, Hubble uses several techniques to uncover and identify what are called reachability problems -- when one Internet address can't reach another, even when their physical link is operational. Traffic seems to simply disappear into a black hole.

"We found reachability problems to be more common, widespread and longer lasting than we had expected," according to the UW researchers in an online report. During one three-week period, Hubble found more than 31,000 reachability problems involving more than 10,000 Internet prefixes (with each prefix covering a group of IP addresses). More than 21,000 were reachable by some computers but not others. Nearly 5,000 were completely unreachable. Of the prefixes with problems, 58 per cent experienced only a single "reachability event," but 25 per cent experienced three or more, and 193 experienced at least 20.

Many were resolved in less than an hour, but more than 60 per cent (more than 19,000 events) lasted more than two hours, and nearly 10 per cent (2,940) lasted at least 24 hours. The median duration for partial-reachability events was 2.75 hours, for complete unreachability events, 3.5 hours. Nearly 1,700 prefixes were partially unreachable for more than 24 hours.

The UW researchers are presenting a paper on Hubble at the Usenix Symposium on Network Systems Design and Implementation in the US. The University of Washington has posted a short online article about the project.

You would think that ISPs would have some really sophisticated way of detecting a black hole and locating the region or router that's responsible for it. They don't.

In an online paper, the Hubble team notes that network administrators typically resort to mailing lists, such as those of the Internet Security Operations Task Force and the North American Network Operators Group. Unexplained outages repeatedly prompt pleas for help "with users asking whether others can reach their prefixes, or posting when they are unable to reach certain destinations, to ask if others see the same problem or know the cause," according to the Hubble document.

The extent and durability of these problems undermine a basic tenet of the Internet, that every address is reachable from every other address -- at least in theory. Since Hubble began, the system has uncovered nearly 885,000 black holes.

The ongoing data collection by Hubble is done with test messages and probes through PlanetLab, a global network of just over 800 academic, industry, and government computers organized for development of new network services. Hubble uses about 100 of these computers, in some 40 countries, to continually probe the Internet. According to the UW team, Hubble is able to reliably monitor about 90 per cent of the Internet.

The University of Washington Hubble team includes Ethan Katz-Bassett, Harsha V. Madhyastha, John P. John, Arvind Krishnamurthy, Thomas Anderson, and David Wetherall. All are members of the Department of Computer Science and Engineering (Wetherall is listed as also being with Intel Research).