Inside the Skype outage

Skype, the popular Voice over IP (VoIP) service, has been quite busy while we’ve all been eating leftover turkey and drinking champagne.

It was only late November that the service, which has over 500 million registered users and was only recently surpassed in size by Facebook, announced that it had hit a new record of 25 million concurrent users online. However, just before Christmas, the Skype network spent a total of over 24 hours unavailable and offline.

Skype uses peer to peer technology to route call data over the internet. Everyone who is running a Skype client is running a node in the peer to peer network, routing packets from others’ calls across the world.

So how do you bring down a peer to peer network? The problem was caused when a set of designated “super-nodes” in the network – peers that are entrusted with increased responsibility, for functions such as lookups and routing multi-user voice data – became unavailable. This caused the offline messaging feature in Skype to become available. In response to this, a specific Windows version of Skype – of which 50% of users at the time were using – crashed. This in turn ended up bringing down around a third of the super-nodes, and as Skype users attempted restarting their applications the remaining super-nodes were flooded and became unavailable. This made the service unavailable to users who were using versions of Skype not affected by the offline messaging bug.

Eventually the situation was rescued by Skype engineers working round-the-clock to bring up a large number of extra capacity super-nodes. These eventually enabled the network to recover, but it took a significant amount of time for it to do so.

Meanwhile, the Chinese government has announced that they intend to make illegal all VoIP services that are not owned by the two state-owned telecoms providers China Telecom and China Unicom in a move seen more widely as an economic than a state snooping play.

All in all, a busy few weeks for the popular communication application.

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