The Supernode Connection

In section 3 of its latest Heartbeat posting on the problem, Skype refers to problems being caused by its supernodes rebooting, without explaining exactly what a 'supernode' is. So, what exactly is a 'supernode'? As Skype discusses in the universities section of its security resource center, any PC running Skype that is not behind a firewall and is left running for several days at a time can be automatically promoted to supernode status.

Skype's supernodes are used to enable Skype users to find each other through storing directory information. Promoting PCs directly on the Internet to supernodes enables Skype to provide service to computers behind firewalls and routers that use NAT (network address translation). So, if you use Skype and your PC is behind a firewall or connected to a router, be sure to thank those brave Skype users whose PCs are outside those protections.

The flaw discovered by Skype last week affected Skype's ability to recover from having a large number of its supernodes offline because of the automatic rebooting triggered by Windows Update. In other words, if your PC is a supernode, it's part of the infrastructure of the Skype P2P service, not merely a client. For users who've been wondering if Skype is really a distributed network rather than merely a P2P network (scroll down and read the comments), the fact that some clients are also supernodes that assist other clients, and the fact that some clients are merely clients, suggests Skype has some of the characteristics of a distributed network.

Don't Wanna Be a Supernode? You Do Have a Choice

You didn't know your PC could become a supernode? You granted permission for your system to be elevated to supernode status when you accepted the Skype end-user license agreement, which reads (in part):

Utilization of Your Computer. Skype Software may utilize the processor and bandwidth of the computer (or other applicable device) you are utilizing, for the limited purpose of facilitating the communication between Skype Software users.

However, you can opt out of supernode status if you want to. Becoming a supernode has been known to saturate Fast Ethernet networks and to overwhelm low-end routers connected to supernodes, so Skype provides a variety of ways (open the 'How can we prevent our network from hosting supernodes?' section) to disable the option to become a supernode.

Supernodes and Skype - a Few Questions for the Future

Now that Skype users understand more about how the service works, it's going to be interesting to see what happens with the supernodes that make it all work. Will some Skype users opt to disable supernode services or move their systems behind a firewall or NAT router to preserve bandwidth? Will other Skype users set up an "always-on" system that's not protected to do their share to provide supernode service for the network? Skype's software changes are designed to prevent a recurrence of last week's service meltdown, but what happens to Skype if users realize that their systems are part of the infrastructure - and don't like the idea? Stay tuned.

Edited for minor grammar problems.