How I feel this.
Every transaction, 1 address -> 1 address, take a 0.0000 0001 BTC.
Attacker try to flood network with transactions. Presuppose, he have 1 BTC. Flood could looks like 1M of addresses. 1 000 000 * Fee( 0.0000 0001 ) = 0.01 BTC. Attacker could make 100 attacks with 1M addresses, thereafter he out of money.
Will network of bitcoin overloaded ? What a math behind current Fee?