The Disruptive Force of a Global Cascade

Skype is a peer-to-peer network.  When the behavior of nodes in a network is interdependent, the network is subject to global cascades.  From the Skype Heartbeat blog (emphasis added):

On Thursday, 16th August 2007, the Skype peer-to-peer network became unstable and suffered a critical disruption. The disruption was triggered by a massive restart of our users’ computers across the globe within a very short timeframe as they re-booted after receiving a routine set of patches through Windows Update.

The high number of restarts affected Skype’s network resources. This caused a flood of log-in requests, which, combined with the lack of peer-to-peer network resources, prompted a chain reaction that had a critical impact.

Normally Skype’s peer-to-peer network has an inbuilt ability to self-heal, however, this event revealed a previously unseen software bug within the network resource allocation algorithm which prevented the self-healing function from working quickly. Regrettably, as a result of this disruption, Skype was unavailable to the majority of its users for approximately two days.

The folks at Skype have gone out of their way to explain that its service was not a victim to any kind of targeted attack or hack:

We can confirm categorically that no malicious activities were attributed or that our users’ security was not, at any point, at risk.

In fact, peer-to-peer networks are robust in the face of targeted attacks.  Ironically, Skype couldn't protect itself from its own customers.  Once more, the role of the consumer is under-appreciated.  On the one hand, Skype wants its users to use its service constantly.  Furthermore, the company wants its users to be open to automatic updates.  But, as a consequence, Skype contributed to the conditions that make this kind of disruptive cascade possible, even likely.