It apparently took two days, but Skype is now back online after having been down for most people for the better part of two days. Skype posted an explanation of what caused the outage.

The statement, plus my analysis, after the jump.

On Thursday, 16th August 2007, the Skype peer-to-peer network became unstable and suffered a critical disruption. The disruption was initiated by a massive restart of our user's computers across the globe within a very short timeframe as they re-booted after receiving a routine software update.

The abnormally 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.

Tom Keating was right: some Microsoft patches were causing problems with Skype. Not in the sense they conflicted with Skype, but the fact that these patches required a reboot to be applied. With all the end user computers rebooting, i.e. many of the Skype Supernodes, there was nowhere to log into.

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 issue has now been identified explicitly within Skype. We can confirm categorically that no malicious activities were attributed or that our users' security was not, at any point, at risk.

This disruption was unprecedented in terms of its impact and scope. We would like to point out that very few technologies or communications networks today are guaranteed to operate without interruptions.

This is what happens when you provide a service using the resources of others whom you are not in control of. What if, instead of a "patch Tuesday" sort of deal, a widespread virus took out the Skype supernodes, or their ISPs found a way to effectively filter Skype traffic? The possibilities here are endless and unless Skype rolls out supernodes they are in direct control of, this kind of thing is going to happen again.

The flaw with Skype isn't peer-to-peer, it's how they implement peer-to-peer. SightSpeed also uses peer-to-peer, but it's very different. You're still talking to a central server controlled by SightSpeed, but the bulk of the data between any two parties actually goes between the two parties directly.

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.

The Skype community of users has been incredibly supportive and we are very grateful for all their good wishes.

It's great that Skype fixed whatever problem they had in their algorithm, but I think it behooves everyone to have a backup plan in case Skype goes away.