So miners should change the algorithm to choose the right transaction when a double-spend is detected: choose the one that pays more fees.
Good point. I assume that miners will choose to mine the version of a transaction with the highest fee-per-kilobyte, since that will give them the best profit, but actually changing the code to implement that policy has been controversial when I've brought it up before.To fight transaction spam, I think the relaying logic will need to get smarter, too. A large, expensive-to-verify double-spend should be way down on the "stuff that should be relayed when there is enough bandwidth" list.
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(B) How to allow a honest user to replace a transaction because of, for example, low fees specified in the first one?
You have to mark transaction in the pool to be automatically removed after some time, with some type of priority queue structure to allow fast search and removal of old transactions.
Marks in prevouts are lightweight, and easy to wipe out. Just erase them all every 10 minutes.
You have to mark transaction in the pool to be automatically removed after some time, with some type of priority queue structure to allow fast search and removal of old transactions.
Marks in prevouts are lightweight, and easy to wipe out. Just erase them all every 10 minutes.
That's a different issue, and a new feature. I think the best way to implement that feature is "child pays for parent" (see https://github.com/bitcoin/bitcoin/pull/1647 for a proposed implementation), and then the user can broadcast a high-fee pay-to-self child transaction to get the parent accepted into a block.