Did Patch Tuesday really kill Skype?

Doubts are emerging about Skype's claim that Microsoft's Patch Tuesday knocked its service offline for millions of users last week.

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While he scoffed at Skype's explanation, Rosenberg also noted that the service's infrastructure may make it vulnerable to problems experienced by a minority of systems on the network. Like the Kazaa file-sharing network, created by the same people as Skype - the peer-to-peer service uses "supernodes" to detect online Skype users, establish connections between users, and help route traffic.

The supernodes, which are computers that Skype identifies as having surplus bandwidth and processor cycles, serve as the directory servers and traffic cops of the network. If too many go offline in a short time - whether from restarts or simply by being switched off - Skype could suffer.

Skype's explanation hinted as much. "Normally Skype's peer-to-peer network has an inbuilt ability to self-heal," said spokesman Villu Arak. "However, this event revealed a previously unseen software bug within the network resource allocation algorithm which prevented the self-healing function from working quickly."

What Skype describes as self-healing, said Rosenberg, is simply the ability of Skype to switch a user from one supernode to another, necessary, say, when the first supernode goes offline. If too many of those supernodes dropped off the network simultaneously, Skype might have had trouble switching users to other supernodes.

In other words, there would have been too many nodes - normal users - chasing too few supernodes to allow regular users to log on. Skype itself described it as "a chain reaction that had a critical impact."

"Skype is unusual in that one of its key components, the supernodes, are always going up and down," Rosenberg said. "Because it relies on the supernodes working, if Skype's [network] software wasn't load balancing across time zones, they could have had a massive loss of supernodes [when systems rebooted]," he added.

Microsoft also dismissed the idea that there was anything out of the ordinary in last Tuesday's updates that might have triggered the Skype crash. "Windows Update is a routine service Microsoft provides to its users to receive software updates, including last Tuesday's security updates, which were not unique," said a company spokeswoman. "As indicated in Skype's blog, their specific disruption was caused by a bug in their software."

Tallies of Microsoft's recent monthly updates seem to back up the company's claim that last week's were not unique, at least in the number which demanded restarts. Although five of August's nine updates required a reboot, that number wasn't out of line with July's four of six, or even February's five out of 12.

Skype did not reply to a request for comment, and additional information about the impact of Windows Update-generated restarts on its network.

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