Oh I know. The point is something hard that the sender needs to do. Ideally it wouldn't waste lots of power, but it must consume some scarce resource, or else i

Oh I know. The point is something hard that the sender needs to do. Ideally it wouldn't waste lots of power, but it must consume some scarce resource, or else it wouldn't cost anything, and the cost should be borne by the sender, so e.g. a RAM-intensive hash function wouldn't be right.