Doubts are emerging about Skype's claim that Microsoft's Patch Tuesday knocked its service offline for millions of users last week.
Analysts and rivals said they were dubious of Skype's explanation that the voice-over-IP service's 48-hour outage was triggered by restarts after Microsoft's monthly security updates were delivered.
"Why this particular Tuesday?" asked Doug Williams, an analyst with JupiterResearch. "That doesn't really fly."
Skype's blackout was caused by a software glitch provoked, said Skype, by machines rebooting after they had applied the Patch Tuesday updates.
"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," Skype said in a statement.
"I'm leery of that explanation on two counts," said Aron Rosenberg, chief technology officer of SightSpeed, a competitor to Skype. "First, the timing of the patches."
Although Microsoft rolls out its monthly security updates before noon, US Pacific time, on Patch Tuesday, those updates are by default downloaded and installed at 3am in the PC's local time, often over a period of a day or two. "At the very least, then, systems would have rebooted time zone by time zone, not all at once," said Rosenberg said.
However, there may be a connection to the 3 am default reboot. According to Skype's statistics, the outage began sometime between Wednesday at 10:30 pm and 3:05 am Thursday, US Pacific time. Between those times, the number of connected users dropped by 50 percent.
Second, said Rosenberg, is the fact that Microsoft has been releasing its security fixes on the second Tuesday of each month since October 2003. If the problem was triggered by Windows Update, as Skype claimed, why hadn't it happened before?