Skype's popular internet telephone service went down on August 16th and was unavailable for between two and three days.

Skype relies on peer-to-peer technology, where each client is also a server, reducing the load on centralised servers. Peer-to-peer applications are meant to be resilient: if one peer fails, another can take its place. Skype's extreme outage shows this is not always the case.

So what happened?

Skype says the problem was triggered by a Microsoft patch, delivered by Windows Update, which caused an automatic reboot of many PCs. "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," says Skype's Villu Arak.

Some observers have speculated that Skype's entire distributed database became unsynchronised and had to be rebuilt, accounting for the long delay.

The incident prompts a number of awkward questions. The first is whether it is sensible for a company to make critical services dependent on its customers' machines - over which it has no control. Suddenly the business benefits of peer-to-peer seem more doubtful. Skype's nightmare weekend is also a warning to anyone considering greater dependence on VoIP, such as using it for business conferencing.

Another issue is that probably only a few Skype customers realised what they agreed to when they clicked OK in the licence agreement. In Skype's case, any machine may become a supernode, a server responsible for hundreds of other clients.

Similar considerations apply to the BBC's iPlayer application for TV over broadband and Sky's similar Anytime service, both of which use a peer-to-peer application called Kontiki. When you download a broadcast, you are downloading it from another user's PC, as well as allowing others to upload it from you.

Skype's troubles are good news for some. Blogger Om Malik reports that rival VoIP provider Gizmo Project saw a fourfold increase in sales during the outage. Gizmo Project uses the standard Session Initiation Protocol (SIP), in contrast to Skype's proprietary system. This means that unlike Skype, Gizmo Project software has multiple options.