Skype crash: Software bug and server overloads blamed

Details of what caused the service to be unusable for millions of users prior to Christmas have been posted on the firm's blog.

The two events combined to create a cascade of problems that managed to knock out much of the network underpinning the phone service.

Skype is assessing how its network is built to stop the problem recurring.

Traffic cascade

Writing on the Skype blog, Lars Rabbe, chief information officer at the company, said the problems started on 22 December, when some of its servers that handle instant messaging started getting overloaded.

This meant that the responses they sent to Windows machines running Skype were slightly delayed. Unfortunately, a bug in one version of Skype for Windows meant this delay caused the program to crash.

About 50% of all Skype users ran the buggy 5.0.0.152 version of the software, said Mr Rabbe.

This caused problems for Skype because of the way the network supporting it is organised. Some of the data travelling round Skype's network are passed through all those machines logged on to the service.

Those participating machines act as what Skype calls "supernodes" and carry out some of the administrative tasks of the global network and help to ensure calls get through.

With a huge number of these machines offline because of the crash, the rest of the network quickly became overloaded.

Mr Rabbe wrote that the disappearance of the supernodes meant the remaining ones were swamped by traffic.

"The initial crashes happened just before our usual daily peak-hour and very shortly after the initial crash," wrote Mr Rabbe, "which resulted in traffic to the supernodes that was about 100 times what would normally be expected at that time of day."

Traffic levels were so high that they blew through the safe operating specifications supernodes usually use. As a result, more supernodes shut down; Kazinform cites BBC News.

See www.bbc.co.uk for full version