Skype engineers worked non-stop over several hours yesterday to fix the peer-to-peer IP telephony's site's worst outage.
On its Twitter feed at 6am Skype said the service was gradually returning to normal. "We expect it may take several hours for everyone to be able to sign in again."
Skype blamed the problem on failure of its supernodes.
Supernodes are normal Skype clients on the internet that broker the communication between two Skype clients. Effectively, they act like a massive online telephone directory, which users' Skype clients use to look up other Skype users.
Any user can become a supernode, which should makes Skype resilient, but somehow the supernode infrastructure failed yesterday.
Writing on the Disruptive Telephony blog, Dan York, best practices chair of the voice over internet protocol (VOIP) security alliance (VOIPSA), said, "My guess would be that there might have been "cascading failures".
"If there was, say, a software update affecting some supernodes, as those supernodes dropped offline, the increased load of Skype clients trying to connect to online supernodes might have caused some of them to then drop offline. Or when a supernode came back online, it may have been overwhelmed by the quantity of connection requests and soon failed again."
Skpye last went offline in August 2007, following a Windows update.