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Suppose, A and B are malicious attackers. Both want to send unlimited fake transactions to each other. I know that the transactions won't be accepted in the blockchain. In fact, they don't want their transactions to be mined. But, is there any way to take a countermeasure against A and B? And, what will be the cost?
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The countermeasure is built in the system. All non malicious nodes would instantly discard the fake transactions. I don't understand the problem statement though, like why would any one want to broadcast fake transactions without any motive that it gets mined.
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Either the transactions are valid (including offer a sufficient fee) or not. If they are fully valid, then they will get mined and A and B will pay lots of fees. If they are not fully valid, then everybody will ignore them and they will do no harm whatsoever.
I think OP is trying to imagine some kind of denial of service attack. – Osias Jota – 2018-03-19T01:14:03.900
and soon enough the fake node will be blacklisted by many other nodes. – Will Gu – 2018-05-05T06:06:05.753