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Opt-in Replace-by-Fee RBF allows transactions to be flagged as replaceable until they are confirmed in a block. Transaction replacement was introduced by Satoshi in the first release of the Bitcoin software, but later removed due to denial-of-service problems.
Opt-in RBF solves this issue by requiring transaction replacements to pay a higher fee. During the period when transactions are waiting to be confirmed, some wallets would like to be able to update those transactions in order to increase their fee which may help them get confirmed faster , compress multiple transactions into one, create background coinjoins to improve privacy , or to perform a number of other useful actions.
Opt-in RBF is a change to memory pool and network relay code that allows those wallets to optionally add a signal to their transactions which tells full nodes that those particular transactions may be updated replaced up until the point that they get confirmed in a new block. A transaction must be marked replaceable sequence number below MAX -1 in order for the opt-in RBF implementation to treat it as replaceable.
Opt-in RBF has no effect on transactions which are not using it and no one is forced to use it. It has no effect once a transaction has been confirmed. This means that users who care about unconfirmed transactions can continue to not use RBF. We currently do not have reason to believe that they are, at least not significantly, against fraudsters using the most effective tools and practices known.
But if they are or until it is more clear that they are not recipients can protect themselves by continuing to regard transactions with non-max sequence numbers as unsafe until they become confirmed.
What Alice saw first, Bob might see second. Sophisticated double spending attackers today use tools to map the network connectivity by making harmless looking conflicting spends and seeing which versions show up at which merchants and in which blocks. This allows them to craft two versions of the same transaction, one which they send to their victim and one which they send to miners for confirmation.
The presence of the non-replaceable payment to the merchant prevents his node from learning about a double-spend until it shows up in a block mined by a miner that was merely mining the thing they saw first.
This simple, common pattern is sometimes further amplified by additional techniques such as using unconfirmed transaction chains, low fees, or non-standard transactions. As a result, the vast majority of the security for unconfirmed transactions comes not from within the Bitcoin system, but from external factors such as the large numbers of honest Bitcoin users who would never attempt to defraud their vendors, the tolerance among vendors for small amounts of fraud, the ability or threat of vendors resorting to the legal system or other types of recourse, and other factors which have nothing to do with the design of the Bitcoin protocol.
Also, because RBF can sometimes eliminate long confirmation delays, some merchants which previously felt forced to accept unconfirmed transactions to prevent unfortunate delays will no longer need to do so, which reduces their exposure to fraud. However, no one using opt-in RBF is insisting that you agree with the above view on unconfirmed transaction security.
Transaction replacement for unconfirmed transactions was a feature in the very first release of Bitcoin. Transactions could mark themselves as replaceable by setting a non-maximal sequence number. This was later disabled because it was possible for an attacker to use up all the bandwidth among full nodes at only a small cost to himself, creating a denial-of-service vulnerability.
In addition, there was no incentive for miners to follow the convention and accept a replacement, and there could even be an incentive to violate the convention if an earlier version of a transaction paid a higher fee. These two factors kept replacement from being restored for several years, but in Peter Todd proposed requiring replacements to pay a strictly greater fee rate and to require that the replacement increase the fee rate by at least the minimum required to relay a new transaction.
This eliminated both the denial of service and incentive compatibility problems. But this is the kind of proposal only a game theorist could really love. Not in the slightest.
After extensive informal discussion stemming back months, the PR was opened on October 22nd. It was subsequently discussed in at least four Bitcoin development weekly meetings , , , and In the PR discussion, 19 people commented including people working on at least three different wallet brands and 14 people explicitly ACKed the change, including at least one person who had been very outspoken in the past against full RBF.
No clearly negative feedback was provided in the PR or elsewhere that we are aware of while the PR was open. There are already some nodes on the network which perform RBF in various ways and there have been for some time, probably years.
This may be sped up by the development of applications that make interesting use of it. This can save a large number of bytes and transaction fees even though the replacement will have pay a higher fee than the original. Opt-in RBF can also be used to implement more advanced cooperative stability schemes such as transaction cut-through. Various smart contract cases also need replacement, but they usually use locktime to create stronger ordering and work around the historic unavailability of replacement; these were presumably the motivation for supporting replacement in the Bitcoin protocol in its original design.
Although interesting for more reasons than just adjusting fees, the ability to adjust fees should not be understated. Replacements can be pre-computed, time locked, and given to an always-online remote server for later broadcast with no risk that the server could steal any user funds at worst it could fail to broadcast.
At block a wallet wants to make a transaction set to confirm within 3 blocks. The wallet authors a transaction locked at with its best estimate of a three block confirmation fee, at the same time it also authors replacement transactions locktimed for heights , , , … each paying say 1. These can be handed to a node that accepts advanced locktime transactions. Even if miners know higher fees will be paid in the future, rationally they still prefer the one they can include now—since, if they wait, another miner will likely take the fees.
Opt-In RBF is signaled using the nSequence field, a field that is specifically intended to cover replaceability. Many programs and platforms that trust unconfirmed transactions in general already regard low sequence numbered transactions as suspect and ignore them until they confirm. There are dozens of other ways that transactions can change their speed or reliability of confirmation and increase the probability of a successful fraudulent double spend:. These criteria are frequently changing and depend on node specific policy that is configurable by users.
Parties attempting to estimate the risk of an unconfirmed payment must track all these things and more, and they must be proactive in responding to changes. Opt-in RBF is a highly communicated addition known months in advance that overlays on already detected behavior. Opt-in RBF is opt-in. After all, the transaction has been explicitly marked as replaceable, and even without RBF, nLocktime may result in a conflict getting confirmed first. The same sorts of situations exist already for senders using non-standard transaction features or spending unconfirmed outputs, which makes transactions objectively more double spendable—but in those cases there is no fix to get the transaction through quickly.
RBF is a feature for consenting adults. Nothing stops miners from replacing even non-opt-in transactions right now, and some miners have previously experimented with RBF on their own.
There are many other logs for bitcoin-dev and bitcoin-core-dev. Satoshi originally introduced unconfirmed transaction replacement but this was disabled due to a DOS attack which Peter Todd fixed by requiring a higher fee for each replacement. Unconfirmed transactions can always be double spent, with or without RBF.
Wallets only have to update if they want to make use of opt-in RBF. This gives wallets a new dimension along which they can innovate and provide beneficial features to users. This prevents double spends, but it has three problems when it actually gets used:. It reduces privacy because it almost always increases the value of the change output, publicly marking that output as change.
Child pays for parent is a way of adding fees to a transaction by making an another transaction that depends on the first. What is opt-in RBF? Is it a new feature? What is it used for? How does it work? Does opt-in RBF make fraudulent spends significantly more successful? Are opt-in transactions themselves more useful tools to dedicated fraudsters, assuming people accept them without confirmation?
Who invented unconfirmed transaction replacement? Was the opt-in RBF pull request controversial? When and how does this change get activated? Is opt-in RBF only useful for adjusting fees? Would a wallet need to stay online to issue replacements with higher fees? What if I think that RBF is just awful? I heard Opt-in RBF was added with little or no discussion Can this be used to double spend against unprepared old wallets?
Will all wallets have to update?