Skype protocol and supernodes

I have a question about the skype protocol. Supposedly, according to wiki, the supernodes in Skype are used in UDP hole punching. The supernodes are nodes without firewalls/NATs.

My question is, how is this reliable? Isn't the vast majority of internet users behind NAT?

And, if I was to create a P2P application using this technique, what happens if there are no peers w开发者_运维百科ithout firewalls? I don't understand how you can launch an application that relies on that there will be some peers eventually without NAT

Thanks

I can't comment on Skype specifically, but I have some experience with this (http://wiki.squeak.org/squeak/5629). We called our supernodes "big friendly giants" or BFGs :).

The idea behind supernodes is that while you hope that they pop up in the network, giving new users more options for NAT hole punching, you provide, as p2p network operator, a minimal set yourselves (could be just one or two machines, they are just needed for initial hole punching, real traffic will get re-routed directly anyway). As far as I'm aware, Skype does that as well - they run a minimum set of supernodes themselves.

When Skype had issues earlier this year, a lot of people tried to reconnect and thus the supernodes got overloaded, resulting in a domino effect. Skype added supernodes, but the amount of people trying to reconnect at that time was so massive that it took quite a while before the network rebuilt itself. It's quite funny - we had that with the above project as well - that a P2P network can be extremely resilient until it gets pushed over some edge and the whole thing crumbles.

[disclaimer: I work for eBay, former owner of Skype, but this is all my personal opinion and based on public information]

Read the papers on libjingle with discussion about services like STUN. When both parties are behind NAT an external service is often required to relay through or assist in punching a hole open on one or other side.

http://code.google.com/apis/talk/libjingle/important_concepts.html