Skype experienced an outage of approximately 24 hours last week after suffering a critical failure on its peer-to-peer platform. The initial flashpoint was understood to have occurred at 8am PST on December 22, which soon snowballed out of control to affect all users. The service was only restored in the morning of December 23 after the company pumped addition computing resources into the Skype network overnight to help stabilize and restore services.

In a detailed blog entry, Skype CIO Lars Rabble blamed the catastrophe on a bug in a version of the Windows Skype client run by the majority of Skype users. A delayed response from servers responsible for offline instant messaging (IM) was not handled properly by the client, causing the buggy version 5.0.0152 of the software to crash instead.

The reduction of so many 5.0.0152 instances effectively took 30 percent of Skype's supernodes capacity offline--which are user nodes tasked to function like a directory to route connections between clients, among other ancillary tasks. In a nutshell, a combination of factors which includes the increased loading on the remaining supernodes, the start of the traditional peak hour, and users restarted their crashed Windows client, resulted in unprecedented traffic to surviving supernodes that was "about 100 times what would normally be expected at that time of day." Only the activation of "several thousand" dedicated supernodes helped to eventually restore the server.

For now, Rabble wrote that Skype is committed in reviewing the capacity of core systems, as well as reviewing testing processes to detect bugs in its client software. Questions remain however, such as why the systems responsible for offline IM failed in the first place.

For more on this story:
- check out this article at Skype Blogs
- check out this article at Network World

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