An anonymous reader writes "Ever wondered, how P2P software like Skype directly exchanges data — despite the fact, that both machines are sitting behind a firewall that only permits outgoing traffic? Read about the hole punching techniques, that make a firewall admin's nightmares come true."
They link to an article:
This is almost identical to what I am saying in my paper dated 6/3/2002
This was a technique I pioneered, we had it in our one of our livecam video streaming products back in 1997.
Now Skype is using it.
P2P Through Firewalls