Skype explains worldwide outage

FRANKFURT, Germany — A two-day outage that left millions of Skype users unable to use the popular Internet phone service was caused by an abnormally high number of restarts after people had downloaded a Windows security update, the company said Monday.

The worldwide outage, which began Thursday and ended Saturday, left millions of Skype users unable to log on to make phone calls or send instant messages.

Luxembourg-based Skype, part of online-auction giant eBay, has more than 220 million users in total but typically has 5 million to 6 million users online at any given time. In January, Skype reported that it had counted 9 million users online at one time.

In an update to users on Skype's Heartbeat blog, employee Villu Arak said the disruption was not because of hackers or any other malicious activity.

Instead, he said the disruption "was triggered by a massive restart of our users' computers across the globe within a very short time frame as they rebooted after receiving a routine set of patches through Windows Update," Arak wrote.

Microsoft released its monthly patches last Tuesday, and many computers are set to automatically download and install them. Installation requires a computer restart.

"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," Arak wrote.

Arak did not blame Microsoft for the troubles and said the outage ultimately rested with Skype.

Arak said Skype's network normally has an ability to heal itself in such cases, but a previously unknown glitch in Skype's software prevented that from occurring quickly enough.

In a statement, Microsoft described its patch as routine and reiterated that the disruption resulted from a bug in Skype software.

Users from Vietnam to Brazil to Germany to the United States had complained they could not log on and make phone calls or send instant messages.

The outage was a critical moment for the company, founded in 2003 by Niklas Zennstrom and Janus Friis, and was the first major outage since October 2005 when its service was down for a few hours.

"This disruption was unprecedented in terms of its impact and scope," Arak wrote.

"We would like to point out that very few technologies or communications networks today are guaranteed to operate without interruptions."