Web users worried about a natural Internet disaster are surfing up the wrong
The Internet would almost certainly survive even the worst imaginable
breakdown of routers and other essential equipment, a new study finds. But
the same structure that makes the Web resistant to this kind of random ruin
also increases its vulnerability to organized attacks.
"Random day-by-day errors will not crash the system," says physicist
Albert-Laszlo Barabasi. "No matter how many routers break down, those in the
system will not notice it."
Routers are specialized computers that link together the Internet's
smaller regional networks. They act as Internet traffic cops, deciding how
best to send each packet of electronic information to its destination. At
any given instant, an average of three of every 1,000 Internet routers
experience a technical glitch that puts them temporarily out of commission.
But even if 10 times this many routers failed, the performance of the
Internet would remain unchanged, Barabasi and fellow Notre Dame physicists
Reka Albert and Hawoong Jeong wrote last month in the journal Nature.
Another study, published online by Israeli researchers, suggests that
up to 99.9 percent of all routers would have to fail to cause the Internet
to break down completely.
But this resilience does not extend to organized attacks by hackers
bent on breaking the system, the Notre Dame study found. By disabling
routers with the greatest number of connections, Internet terrorists could
cause a complete network collapse.
This peculiar combination of stability and vulnerability is a product
of the Internet's self-organized structure, says Barabasi.
Until recently, he says, nobody really knew what the structure of the
Internet was. After 1995, when the National Science Foundation relinquished
its stewardship of the Internet, no one bothered to keep track of how the
Internet's millions of computers were connected.
But growing interest in improving the speed, performance and security
of Internet connections led to the formation in 1997 of an agency to help
map the rapidly growing network. The Cooperative Association for Internet
Data Analysis has now mapped enough connections to enable scientists like
Barabasi to come up with mathematical equations that describe the Internet's
flow of electronic information. These mathematical models can be used to
study the strength of the network's structure.
The model that best represents the Internet is what mathematicians call
a scale-free structure. Nodes, or points, in a scale-free network can be
connected to a large number of other nodes, or to just a few.
In some networks, explains IBM mathematician Yuhai Tu, each node has
roughly the same number of connections. The points of a Star of David form
just such a network, with each point connected to two others.
But the connections of the Internet look more like an airline route
map. A few "hub" nodes connect to a large number of other nodes, while a
much larger number of spoke sites have just a handful of links. On the
airline route map, cities would be nodes; on the Internet, routers are the
The Israeli study used a simplified model of this structure to assess
the Internet's staying power. Reuven Cohen and three other physicists from
Bar-Ilan University found at least in mathematical theory that the vast
majority of the connections in their simplified Internet remained intact
even if most of the routers failed. The study appears on the World Wide Web
In the Nature study, Barabasi and his team confirmed that this
theoretical resilience works for the real Internet as well.
Resilience, the Notre Dame scientists wrote, can be gauged using a
quantity called the network diameter. It's a measure of the distance that
information must travel to get to its destination: the minimum number of
links between two nodes. The diameter of an airline network, for example,
gives the fewest number of layovers a passenger could expect flying between
"The diameter of the Internet is around four," Barabasi says. That
means that the typical e-mail message will travel through four routers
before reaching its destination.
Randomly knock out 10 times the normal number of malfunctioning
routers, and the distance between nodes remains unchanged. But disable the
same number of the most-connected routers the hubs and the distance more
than triples. Instead of passing through four routers, the typical e-mail
message will go through more than 12.
Tripling the distance between nodes would not do much damage, says
Barabasi. Since e-mail and other bits of digital data take only a few
seconds to reach their destination, the delay would be small. But remove
just a few more router hubs, and the main network breaks down completely in
a process scientists call percolation.
"At percolation," says Cohen, "the network becomes mainly small islands
of a few sites which are not connected to the rest of the network."
It's the electronic equivalent of a snowstorm striking Chicago's O'Hare
and Dallas/Fort Worth airports. Like travelers, information would become
stranded at isolated outposts. Communication would cease.
The susceptibility of the Internet and other similarly structured
networks to percolation "seriously reduce(s) their attack survivability,"
the Notre Dame scientists write. "This could be exploited by those seeking
to damage these systems."
But not all scientists agree that such a catastrophe could occur.
Duncan Callaway of Cornell University in Ithaca, N.Y., and three other
scientists argue in a paper online that monitoring the Internet's diameter
is not the correct way to gauge network resilience.
Using a different measure, which they call long-range connectivity,
they found the Internet to be much more robust. Their results appear on the
World Wide Web at xxx.lanl.gov/abs/cond-mat/0007300.
Callaway and his colleagues removed the same central hubs that
Barabasi's team did. But while the main network crashed according to
Barabasi's measure, only about 15 percent of the nodes broke off for
Barabasi counters that the exact timing of the mathematical crash,
which he calls clustering, is not the real concern.
"The Internet would break down way before it starts clustering," he
says. "If you take out the bigger nodes, then all the messages have to
travel around it. All the messages would have to line up at the smaller
nodes that don't have the capacity."
Changing the structure of the Internet to defend against such
calculated attacks won't be easy, Barabasi says. Because the network has no
centralized control, its structure, or topology, remains in the hands of the
people who administer the individual routers, like Internet service
"As long as you give the freedom to local managers," he says, "the
topology will not change. But on the other hand, there are lots of clever
people out there. Maybe they can think of ways of fixing it."
Cohen, of the Israeli study, is quick to warn that all of the recent
results are based on mathematical models. These models make simplifying
assumptions that might not be applicable to the real Internet, he says.
Barabasi agrees. "If you really want to simulate how the Internet
breaks down, you have to simulate the dynamics," he says, referring to the
technological differences between different routers, and differences in the
actual jobs each router performs.
Barabasi's group plans to perform computer simulations that account for
these details. But, at this point, he says, "we don't have the data to
simulate an attack."
Asked what he believes is the true breaking point of the Internet,
Barabasi says: "Hopefully we won't ever find out."
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