Scheme harnesses Internet handshakes

By Eric Smalley, Technology Research News

Whenever you click on a link on a Web site, your computer sends a message to the site's Web server and the server responds. Billions of such network handshakes take place on the Internet every day. Although individually these handshakes are insignificant, large numbers of them can add up to an impressive amount of computer processing power.

A team of researchers at the University of Notre Dame has figured out a way to use Web server handshakes to compute small pieces of a mathematical problem by disguising the pieces as ordinary Web browser messages.

The researchers' parasitic computing scheme uses the processing power of unwitting Web servers by exploiting one of the most basic operations carried out by all computers connected to the Internet, the Transmission Control Protocol (TCP) checksum, said Vincent W. Freeh, an assistant professor of computer science and engineering at the University of Notre Dame.

TCP breaks messages from one computer to another into small pieces, or packets, sends them over the network and reassembles them on the receiving end. The TCP checksum adds up the number of bits in the message and attaches the result to the message. On the receiving end, the computer adds up the number of bits received and compares it to the checksum number to make sure the message arrived intact.

The researchers performed their experiment with a type of math problem that can only be solved by examining each possible solution until the right solution is found. They encoded each candidate solution as a Web browser request for a web page so that the TCP checksum was actually checking to see if the message contained the correct solution.

The Web servers that received requests treated the messages that contained failed solutions to the math problem as corrupted messages and discarded them. The Web servers treated messages that contained the correct solution as a request for a Web page that did not exist and sent the standard 'page not found' error message to the researchers' computer.

Although the parasitic computing scheme demonstrates a principle, it is not a useful tool because the amount of computer resources used to implement the scheme far exceeds the amount that would be needed to solve the problem on the researchers' computer by itself, said Freeh.

"For a general communication protocol, I think the probability [of developing an efficient version of the scheme] is very remote," he said. "By design, the receiver doesn't have to do that much. However, I think people are [already] exploiting specific Web sites."

Web servers that run interactive applications and process forms are good candidates for this kind of scheme, said Freeh. "This is where lots of host cycles can be gotten," he said.

The parasitic computing scheme raises the possibility that computers on the Internet can be used in ways their owners are unaware of, which raises ethical and legal issues about the use of publicly available computer resources.

Though the Web server resources used in the Notre Dame implementation were barely measurable, if the scheme were used aggressively it could have a similar effect to denial-of-service attacks in which one or more Web servers are flooded with messages and effectively shut down, said Ian Foster, a computer science professor at the University of Chicago and a senior scientist at Argonne National Laboratory.

Each tiny message in the parasitic computing scheme is by itself indistinguishable from any other Web page request, said Freeh. "The way to tell is by seeing many such messages and deducing what is happening," he said. The researchers have configured an intrusion detection system to detect their parasitic computing scheme and they are working on configuring the system to detect variations of the scheme, he said.

"The instance of parasitic computing that [the researchers] demonstrate... is totally inefficient, returning a minuscule amount of computation for great effort," said Foster.

The question is whether there are more efficient versions of such a scheme, he said. "Within the Internet infrastructure [it] seems very unlikely to me, given its fundamental simplicity." It's more likely, though still doubtful, that someone could develop an efficient scheme to exploit peer-to-peer networks like Gnutella, he said. "I wouldn't discount it totally, especially as these infrastructures evolve."

Even if computationally efficient versions of the scheme can be developed, it remains to be seen if it can perform useful work, said Miron Livny, a computer science professor at the University of Wisconsin.

"It's a creative idea [but] it's not clear to me how it will work if you really care about the result," said Livny. The the problem is the scheme counts on messages that do not generate a reply to indicate that the message did not containing the correct solution, but failing the TCP checksum is not the only reason a message might not be returned. "The biggest challenge in distributed systems is to understand why somebody is not responding, because there [are] many, many reasons why you didn't hear back," Livny said.

The researchers tested the reliability of their scheme by repeatedly sending out the correct solution. They got the correct answer back at rates that varied from about 99 out of 100 to about 16,999 out of 17,000 times , according to Freeh.

Freeh's research colleagues were Albert-Laszlo Barabsi, Hawoong Jeong and Jay B. Brockman of Notre Dame. They published the research in the August 30, 2001 issue of the journal Nature. The research was funded by the National Science Foundation (NSF).

Timeline:   Now
Funding:   Government
TRN Categories:   Internet; Distributed Computing
Story Type:   News
Related Elements:  Technical paper, "Parasitic Computing," Nature, August 30, 2001


September 12, 2001

Page One

Internet stays small world

Tools automate computer sharing

Nanotube kinks control current

Hydrogen chip to fuel handhelds

Scheme harnesses Internet handshakes


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