As promised, Skype has let us in on what brought down their network last week: too many people rebooting their computers due to a Windows update.
In an entry on the Skype blog, Villu Arak explains that when the new Windows software update required users to reboot, this caused a strain on their system.
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.
He goes into more detail about how the system has the ability to normally self-heal, but an unknown bug in "the network resource allocation algorithm" prevented the safeguard from kicking in.
They went on to add:
We are very proud that over the four years of its operation, Skype has provided a technically resilient communications tool to millions of people worldwide. Skype has now identified and already introduced a number of improvements to its software to ensure that our users will not be similarly affected in the unlikely possibility of this combination of events recurring.