http://ttevhjjsjxz6uqqcjkbig5cycd7n7xv7cmd6f5fcvrqaaa7f3bj36wad.onion/technology/security/pow/index.html
This is far
more expensive for the attacker, as compared to the cost to a client of buying
a slightly more expensive ticket, in order to "jump the line". This key idea is why this defense works as a deterrent, doesn't burn energy for
no reason, and can still provide access under an active attack.