It's interesting that the recent Skype service downtime has hit the news in a fairly big way. On the front page of BBC news no less, and for a number of days.

Another recent indication of how mainstream Skype use has become is that when I recently helped reinstall a laptop for a friend's mum, she was very explicit in needing Skype installed too (as well as Flash). This was Ubuntu 10.10 (Maverick). The Flash was required for playing scrabble on Facebook (how things have changed).

So a few more people now see that Skype is potentially a very invasive piece of software - if you're on a public IP address at least. This is because of their use of (so called) supernodes (Dan York's excellent summary of how Skype works, with pictures).

Basically, Skype might use large amounts of your network bandwidth (and CPU) to route Skype calls via your PC. Of course you agreed to this when you clicked "I Agree" in their EULA, right?

So, maybe a recently software update pushed to the Windows clients caused a lot of clients to start crashing, and so a lot of supernodes to go offline. That's what Voxeo's CEO thinks happened anyway. This update was pushed, not requested or OK'd by the users by the way.

Skype's a very closed company and don't disclose much of their technology or cryptography. Their Linux client plays a distant second fiddle to Windows and they're not open source friendly. When I learned about the "supernode" concept a few years ago, and about how disruptive this traffic might be on a network, it made me very wary of Skype use. I still am.

Update 2011-01-01 :
Well, Skype's CIO, Lars Rabbe, posted a discussion and explanation of this outage on his blog. Turns out that a fault with a Windows software upgrade made the whole network unstable, with the supernode systems (or instability of them) causing most of the trouble. Dan York was right.