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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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”

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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