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A highly recommend a careful read at David Beckemeyer’s blog post on yesterday’s and still ongoing Skype outage.

In my opinion, David made a lot of very good points, in particular he points out that reliability of large-scale systems doesn’t depend on whether they are P2P or not. I totally agree and this outage is a clear demonstration.

David says:

Basically, Skype depends on selected end users’ computers out there to act as database servers and perform other network services (”supernodes”). In order for the network to operate, a certain number of these supernodes must be active on the network. Given that these computers are in the hands of end users and not in any way under Skype’s physical control, they can go off-line at any time. Once Skype “hits the wall”, since these supernode boxes are also serving as Skype clients for end-users, if they start acting funny, the user of that machine might reboot or otherwise restart or shut down the Skype application. That’s one less supernode on the network… and so on, and so on. It could theoretically take DAYS for this pathology to end.

I’ve never been a fan of P2P architectures just because a system based on a P2P network “depends” on people’s nodes, and that’s very bad if you are providing a communication platform to your users. In particular, Skype uses a SuperNode peer-to-peer node, which exposes them to vulnerabilities not present in other P2P communication systems.

As Aron Rosemberg points out in this article:

A SuperNode Peer-to-Peer system is one in which you rely on your customers rather than your own servers to handle the majority of your traffic. SuperNodes are just normal computers which get promoted by the Skype software to serve as the traffic cops for their entire network. In theory this is a good idea, but the problem happens if your network starts to destabilize. Skype, as a company, has no physical or programmatic control over the most vital piece of its product. Skype instead is at the mercy of and vulnerable to the people who unknowingly run the SuperNodes.

P2P makes sense in many circumstances, but it becomes a big problem if hundred millions people rely on a VoIP service based on that technology.

david beckemeyer, luca filigheddu, outage, p2p vulnerabilites, skype