Wickham's musings from Chalke

 
This is garnet stone's guess as to the cause:- his post at 18.39 on 17 August.

Skype's official reply:-
Skype's report look for "What happened on August 16" posted on August 20th.

Essentially there was a domino effect as supernode servers crashed, putting more load on other supernodes which then crashed, causing meltdown for the whole system.

Skype's official report differs from garnet stone's guess, but there may be some connection between them.

Skype said that Microsoft's automatic update caused millions of computers to restart at the same time. Millions of those (Skype usually has 300 million users online at any time) would have had Skype set to automatic login, so they tried to log in to Skype at the same time. This overloaded some supernodes which crashed.

The supernodes that crashed would have had other Skype users logged in besides those that were getting Microsoft automatic updates, so those extra Skype users like me and my neighbour also started trying to log in to Skype automatically.

The Skype login attempts were transferred to other supernodes, which also crashed until the whole peer to peer network was down and all Skype programs were continually trying to log in every few seconds.

The problem for Skype was then to start up a decentralised system when 300 million Skype programs were continually attempting to log in. Apparently Skype has some algorithms in the program on our computers which was designed to control this but a bug had lain hidden for years in all previous programs.

Skype say that this has been corrected in the version 3.5.0.214 issued after the crash but time will tell if it stops another similar situation.

Somehow Skype managed a gradual rebooting of supernodes and Skype programs started logging in after two days.

Supernodes aren't directly under the control of Skype; it seems that Skype identifies servers or computers using Skype which have large and continuously available bandwidth and uses them as supernodes (all users agree to being used but obviously personal computers without much bandwidth which are turned off quite often would never qualify). It seems that servers of major institutions are used, presumably with some kind of tacit agreement, as Skype is beneficial to them as it saves landline call charges.