Linked The 9th Link: Achilles’ Heel The 10th Link: Viruses and Fads Amber Cornelius Dawn Moore Mark Strausser

The 9th Link: Achilles' Heel Easy to forget how dependent we are on modern technology Summer of 1996 Electricity totally down between the crest of the Rockies and the Pacific Coast In financial terms, blackout was more devastating than the Great Northeast Blackout of 1965 30 million people without power for 13 hours Today’s power grid is much more connected than the 1965 power grid

The 9th Link: Achilles' Heel Power grid used to be comprised of individual islands with only weak ties to rest of the grid Blackouts caused people to panic Result of panics was that formerly islanded power systems began to link up Gave rise to the largest man-made structure on Earth, containing enough wire to reach to the moon and back

The 9th Link: Achilles' Heel Huge electric grid was now so interconnected that a single disturbance could be detected thousands of miles away 1996 Blackout highlighted the vulnerability of system Connectivity made power system more robust and efficient However, errors could now cascade through entire system Connectivity caused vulnerability

The 9th Link: Achilles' Heel Man-made systems are corrupted by errors and failures Vehicles Computer circuitry Natural systems have a unique ability to survive in a wide range of conditions and errors Ecosystem Between 3 and 100 million species go extinct per year, but causes little harm to overall system

The 9th Link: Achilles' Heel Robustness Comes from Latin word meaning “oak” Signifies strength and longevity Symbolizes nature’s ability to maintain networks through interconnectivity Increasingly investigated topic in many fields Robustness is the ultimate goal for man-made networks and structures Copying nature’s choice of a universal network structure

The 9th Link: Achilles' Heel In 1999, Defense Advanced Research Projects Agency (DARPA) called for proposals to study fault-tolerant networks “the program will focus primarily on the development of new network technologies that will allow the networks of the future to be resistant to attacks and continue to provide network services”

The 9th Link: Achilles' Heel Node Failures Can easily break a network into isolated islands Such fragmentation is well-known property of networks affected by failures How long will it take to break a network if we remove random nodes? Decades of research on random networks indicates that it is not a gradual process Removing a few nodes has little impact If critical number of nodes are removed, the system abruptly breaks

The 9th Link: Achilles' Heel In January 2000, the DARPA proposal motivated a series of computer experiments to test the Internet’s resilience to router failures Using an Internet map and computer simulation, started removing randomly selected nodes Expected Internet to break apart when critical number of nodes reached The Internet refused to break apart Removed as many as 80% of nodes, and remaining 20% formed a tightly interlinked cluster

The 9th Link: Achilles' Heel Realized that the Internet, unlike other human made structures, showed high degree of robustness Frequent and unavoidable breakdowns of routers rarely cause significant disruptions of service Soon became clear that this robustness was not unique to the Internet Any scale-free network can tolerate removal of random nodes and not break apart Internet World Wide Web Cell networks Social networks

The 9th Link: Achilles' Heel Source of amazing robustness in scale-free networks? Hub: highly connected nodes keep networks together Failures, however, do not discriminate between nodes Affect large hubs and small nodes with same probability Small nodes more likely to be affected, as they number many, many more than large hubs

The 9th Link: Achilles' Heel Scale-free networks where the degree exponent is less than or equal to three have no critical threshold Most networks of interest have a degree exponent less than three These scale-free networks will only break apart when all nodes have been removed Or for all practical purposes, never

The 9th Link: Achilles' Heel Summer of 1997 National Security Agency (NSA) called for a war game to test the security of the US electronic infrastructure Hired between 25 and 50 computer specialists to execute a coordinated attack on the nation’s unclassified systems Power grid 911 systems

The 9th Link: Achilles' Heel Operation Eligible Receiver Illustrated that such assaults by moderately sophisticated adversaries were plausible and potentially devastating Capable of toppling US military communication systems and other critical infrastructures completely Demonstrated frightening vulnerabilities in US economic and security systems Attacks intuitively aimed to decimate the hubs

The 9th Link: Achilles' Heel The author embarked on a new set of experiments that mimicked the actions of the attackers Targeted the hubs of the network instead of randomly selecting nodes Consequences were immediately evident Removal of one hub did not break system Removal of several hubs, disruptions were clear Large chunks of nodes were falling off of the network Removing even more hubs collapsed the network entirely

The 9th Link: Achilles' Heel The response of scale-free networks under attack is similar to that of random networks under failures Same collapse witnessed when: removing highly connected proteins from a yeast cell deleting highly connected nodes from food webs Crucial difference is that it only takes disabling a few hubs for the scale-free network to collapse into tiny fragments

The 9th Link: Achilles' Heel DARPA refused Barabási’s paper detailing the error and attack tolerance of complex networks Nature featured it on their front cover In 2000, no one could see foresee the important role that scale-free networks would play in our understanding of attack survivability and fault tolerance The fact that the Internet was a scale-free network was only known to a few researchers Consequences were clearly unexplored

The 9th Link: Achilles' Heel Robustness of scale-free network comes at cost of fragility under attack Although they are vulnerable to attack at their hubs, several of the largest hubs must be simultaneously removed to crush them Would require several hundred Internet routers to be attacked disabled at the same time It might appear that the Internet’s topology harbors strong defenses against both random breakdowns and malicious assaults Unfortunately, this is not really the case

The 9th Link: Achilles' Heel 1996 blackout turned out to not be the result of an organized attack Blackout was the result of a cascading failure A cascading failure is a failure in a system of interconnected parts in which the failure of a part can trigger the failure of successive parts Local failure shifts load or responsibilities to other nodes If negligible load, can be absorbed If load is too much, node again shifts load or it fails Magnitude and reach of failure depends on the centrality and capacity of nodes removed in the first round

The 9th Link: Achilles' Heel Cascading failures are not unique to power grids Internet Routers do not break, they merely form a queue and drop packets if they can’t process them fast enough End result is denial-of-service Removal of several large nodes could result in the same catastrophic disruption on the Internet as the power line failing in the 1996 blackout caused Economy 1997: International Monetary Fund puts pressure on banks, banks call loans in from companies Led to a cascade of bank and corporation failures Ecosystem Removal of a specific species can lead to a significant reorganization of the ecosystem

The 9th Link: Achilles' Heel Duncan Watts, Columbia University Discovered that most cascades are not instantaneous Failure can go unnoticed for a long time before starting a landslide Cascading failures Understanding is limited They are a dynamic property of complex networks Barabási expects that there are still undiscovered laws that govern how cascading failures work Discovery of those laws would have profound implications for many fields, from the Internet to marketing

The 9th Link: Achilles' Heel Error tolerance we’ve discussed is good news Network robustness allows Humans to recover from minor malfunctions and irritations Internet router errors to not really be noticed The ecosystem to continue on even as species disappear Price for this is extreme vulnerability to attacks All complex systems have their Achilles’ Heel With increased awareness and research, understanding of these issues will definitely improve over time