Hubs key to Net viruses

By Kimberly Patch, Technology Research News

When the Internet linked distant computers 30 years ago, its founders were probably not thinking about protecting the machines from infecting each other. Today's exponentially larger Internet, however, is vulnerable to software viruses in much the same way that large, crowded human populations are more likely to fall prey to biological viruses.

The Internet has a scale-free structure, meaning it has a few pages, or nodes with many connections to other pages and many with just a few connections. Researchers looking into how bits of disruptive code spread on the Internet have found that this structure isn't conducive to the conventional practices of inoculating large populations.

The researchers did, however, find an inoculation strategy that promises to protect computers more effectively.

When they applied an immunization strategy that's commonly used for biological populations to a simulated scale-free network, it simply didn't work, said Alessandro Vespignani, a research scientist at the Abdus Salaam International Center for Theoretical Physics in Italy.

The researchers inoculated progressively larger numbers of nodes, expecting the epidemic to eventually die out, he said. It did not even when they inoculated more than 90 percent of the nodes, he said. "Surprisingly, in scale-free networks we observed that infection survived... in the presence of massive vaccination campaigns involving the majority of the population. We realized that random... schemes were practically useless in scale-free networks."

The Internet is generally more vulnerable than human populations because the connections among computers are both more numerous and structured differently than many of the human connections that allow viruses to spread. Scale-free networks have some nodes -- large portals, for instance -- that contain more connections to other pages than even the most widely-traveled people could possibly have with other people.

The researchers eventually caused the epidemic to die out by targeting nodes that had a high number of connections rather than inoculating individuals randomly.

Using this scheme, the researchers sharply lowered the network's vulnerability to epidemic attacks, Vespignani said. "We have tested this recipe on a real map of the Internet [with] a targeted immunization involving all the most-connected individuals. In this case, by immunizing [less] than one percent of the total population, the cyber infection cannot propagate," he said.

The research explains why, though antivirus software is very successful in protecting individual computers, it does not prevent computer infection from becoming endemic. "The 'I love you' virus is still in the top list of most frequent viruses more than a year after its introduction... because the global implementation of antivirus [software] is practically equivalent to a random... vaccination," Vespignani said.

Ironically, this scheme could also be useful in the biological world where some of the paths viruses take to propagate in a human population have some similarities to the Internet. A map of human sexual relations, for instance, has scale-free properties, said Vespignani. The research implies that epidemics spread this way could be prevented more effectively by targeted vaccination of the few promiscuous individuals, he said.

This type of targeted vaccination would also prove to be much cheaper than the random kind, Vespignani said. "Instead of massive vaccination campaigns, we can think of identifying the network connectivity hierarchy." Controlling the hubs that spread the infection more quickly is both more effective and requires relatively few inoculations, he said. "The strategy is... particularly convenient in terms of economical and practical resources."

The problem in both the Internet and biological networks that harbor a scale-free nature is identifying the large hubs, said Vespignani. "The difficulty... is... detailed knowledge of the network connectivity. This is not always possible for privacy and economical reasons. It is very difficult to obtain a complete map of the Internet because many providers do not want to share publicly their information. As well in the case of sexual diseases we have to rely on people's concerns about their own sexual habits," he said.

This strategy "looks reasonable. It is consistent with my experience," said Gene Spafford, a computer science professor at Purdue University. "I'm surprised no one else has noted this property in research... either in networks or in epidemiology," he said.

One complication that the model leaves out is the notion of workgroups, or local area networks where each machine is connected to all the other machines in that group, and an infection of one infects all the others, Spafford added.

It is hard to estimate when the research could be used to actually inoculate networks, said Vespignani. "The use of these results is strictly related to social factors -- individuals' privacy -- and the existence of control agencies." These make estimating the time frame difficult, he said.

Vespignani's research colleague was Romualdo Pastor-Satorras of the Technical University of Catalonia in Spain. The research was funded by the European Community, the Spanish Ministry of Education and Culture, the Abdus Salaam International Center for Theoretical Physics (ICTP) and the Technical University of Catalonia (UPC).

Timeline:   unknown
Funding:   Government, Private
TRN Categories:   Internet
Story Type:   News
Related Elements:  Technical paper, "Optimal Immunization of Complex Networks," posted in the Los Alamos physics archive at


November 7, 2001

Page One

Hubs key to Net viruses

Electrified water spins gold into wire

Virtual reality gets easier

Laser emits linked photons

Dye brightens micromachines


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