With Skype's voice-over-IP service back to normal Monday after a two-day interruption, the company said the loss of service was caused when a mass of users made an overwhelming number of logon requests after they received Microsoft Windows Update patches.

"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," said Villu Arak on Skype's Heartbeat blog Monday. "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."

Skype's service failed Thursday as users around the world suddenly found they couldn't log on to the service to use its e-mail and instant message features. The company has more than 220 million users, who can talk to each other for free; there's a modest charge of about 3 cents a minute for Skype users to call non-Skype users over the company's non-Skype service. Skype is a unit of eBay, the online auction company.

Skype co-founder Niklas Zennstroem said in a message Monday that the VoIP service was back to normal in the early hours of Saturday. "We're back now and we're stronger," he said.

Noting that Skype's peer-to-peer network normally heals itself, Aral said a software bug within the network resource allocation algorithm kept the self-healing function from working as it should have. The interruption affected most Skype users for about two days, Arak added.

"The issue has now been identified explicitly within Skype," said Arak. "We can confirm categorically that no malicious activities were attributed." He also indicated that users' security was not compromised.

Telecom industry analyst Allan Sulkin said he was shocked to hear that Skype's resources were tapped out considering parent company eBay can handle millions of server requests every second.

"You just wouldn't see this type of outage coming from the PBX [classic telephone landline] providers," Sulking told InformationWeek. "Skype should have had a backup system in place to handle this sort of thing. However, I don't know if they think they are looking at themselves as a telecom player."

The interruption follows the collapse last month of VoIP provider SunRocket, which voluntarily shut down its service. Some 200,000 subscribers were left to find new telephone service when they discovered their SunRocket service was discontinued.