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Comment by mynewwork | Hacker News Reader

Does that mean someone can denial-of-service all of bitcoin just by moving a balance between a couple wallets repeatedly? Can one person (or group)'s activity crowd out other people so their transactions can't be added to the blockchain? What prevents that from happening?

25,000/hour is a shockingly small number to me. I feel like I must be misunderstanding something, or else someone from 4chan would have already DOS'd bitcoin for fun.

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Transactions normally include a small fee, to incentivize miners and disincentivize abuse. You don't have to pay the fee, but if there are more transactions being broadcast than the blockchain can accommodate, the fee-paying ones get priority. (It's a little more complicated than that: when you generate a transaction, you choose whatever fee you want, and they get prioritized based on a combination of the fee, the transaction size in KB, and how long it's been since those coins were last moved.)

Current transaction fees are typically in the range of 0.1-0.5mBTC, so in order to continuously flood out everyone else, you'd need to spend more than 2.5BTC/hour in fees.

It is definitely a small number. Visa is capable of handling 80 million+ per hour, for example. However, transactions are prioritized by transaction fee so you would have to pay a fee higher than all other transactions to block them out. Currently this would cost you something like $100k per day, but if you started to do this you would rapidly increase the cost of transactions, so the cost to perform this attack would grow much more expensive.

There already is a transaction fee - miners will ignore your transaction if you pay too little or nothing.

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