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What Is Replace-By-Fee (RBF) In Bitcoin Transactions?

Last Updated December 2, 2023 1:12 PM
Andrew Kamsky
Last Updated December 2, 2023 1:12 PM

Key Takeaways

  • Replace-by-fee (RBF) in Bitcoin allows users to increase transaction fees after sending a Bitcoin transfer to increase the probability of faster network confirmation by miners.
  • RBF enhances transaction efficiency by enabling fee adjustments in response to network congestion.
  • Using RBF, while introducing risks like double-spending and user confusion, offers benefits like faster confirmations and fee adjustments.
  • RBF integration in hardware wallets like Trezor combines transaction flexibility with secure hardware protection.

Replace-by-fee (RBF) is a protocol within the Bitcoin network that allows senders of Bitcoin to increase the fee of an already broadcasted but unconfirmed transaction.

This is accomplished by creating a new transaction that is essentially a duplicate of the original, with a higher fee. By duplicating a transaction with a higher fee, miners are incentivized to confirm the new transaction over the original one quicker than the previous which had a lower fee.

It’s important to note that the RBF doesn’t allow the sender to recall or cancel a transaction. Instead, RBF lets the sender of Bitcoin create a new transaction with a higher fee. The higher fee makes it more likely to be picked up and confirmed by miners. Once a transaction is approved, the transaction cannot be replaced or altered.

The Role Of Replace-By-Fee (RBF) Mechanism In Bitcoin Transactions

Before the introduction of RBF, once a transaction was broadcast to the network, it couldn’t be altered, leading to potential delays if the included fee was too low versus other transactions in the mempool that held higher fees.

 The primary purpose of RBF in the Bitcoin network is to address the issue of transaction replaceability. RBF, therefore, allows users to “bump” the transaction fee ahead of other transactions, encouraging faster confirmation by miners.

How To Replace A Bitcoin Transaction By Fee In The Mempool?

In 2016, a release of the Bitcoin Core 0.12  outlined the RBF update in the Bitcoin protocol, explaining that the RBF feature allowed for the replacement of existing transactions in the mempool of Bitcoin Core 0.12 nodes under specific conditions.

Key Features Of The Opt-In Replace-by-fee Protocol

Transaction Replacement Criteria

For a transaction to be eligible for replacement, the transaction must have specific settings in the inputs of the transaction that someone is seeking to replace under the RBF protocol. 

These inputs must be explicitly configured to make the transaction eligible for replacement according to the rules in the Bitcoin Improvement Proposal (BIP) 125. The transaction can only be replaced if it meets the criteria outlined in BIP 125, which includes paying a sufficient fee.

Control For Node Operators

Node operators, referred to as the individuals or entities that run a Bitcoin node, have the option to disable transaction replacement. 

This is achieved through the command line option -mempoolreplacement=0. While this setting prevents the replacement of transactions, it does not block the initial acceptance of transactions marked as replaceable under BIP 125.

Implications For Wallet Users

For wallet users, it’s important to note that the 0.12 version of Bitcoin Core does not support the creation of replaceable transactions. However, users can identify if a transaction is replaceable under BIP 125 through updated RPC calls like “get transaction” and “list transactions,” which include a field indicating the transaction’s replaceability status.

What Is The Bitcoin Improvement Proposal (BIP) 125?

BIP 125 was proposed to address the issue of stuck transactions due to low fees. Before its implementation, if a user sent a transaction with a low fee to be attractive to miners, it could remain unconfirmed for a long time. BIP 125 provides a way for users to “unstuck” these transactions.

BIP 125, therefore, lays out the rules for transaction replacement in the Bitcoin network, ensuring a standardized approach. This BIP 125 proposal maintains consistency and predictability in handling transactions, especially when replacing existing transactions in the memory pool.

What is A Bitcoin Transaction Fee?

A Bitcoin transaction fee is a small amount of Bitcoin included in a transaction that is paid to the miner who mines the block containing that transaction. A fee incentivizes miners to have the transaction in a block, especially when the network is congested with many transactions. 

Unlike traditional financial systems where institutions set fees, the market determines Bitcoin transaction fees. Users decide how much they will pay, and miners choose which transactions to include in blocks. Typically, the higher the fee attached to a transaction, the faster it is likely to be confirmed; this means miners prioritize transactions with higher fees to maximize miner profits. 

Fees also play a crucial role in the Bitcoin ecosystem by helping secure the network, as they are a part of the rewards miners receive, alongside the block reward, for validating and recording transactions on the blockchain.

Technical Aspects Of Implementing Replace-By-Fee (RBF) In Bitcoin Transactions

Implementing RBF in Bitcoin transactions involves certain technical considerations. It requires the initial transaction to be flagged as replaceable, typically by setting the sequence number of the transaction inputs to a specific value. This signals to the network that the transaction may be replaced in the future.

What Is The Replace-By-Fee (RBF) Flag In Bitcoin?

The Replace-by-fee (RBF) flag in Bitcoin is a feature that allows a sender to increase the fee of an already transmitted but unconfirmed transaction. Only when a transaction is flagged as RBF it signals to the network that the sender may later replace said transaction with a new one that includes a higher fee.

The RBF flag adds flexibility to Bitcoin transactions, enabling users to adjust fees in response to fluctuating network conditions. However, it’s important to note that not all transactions are RBF-enabled so the transaction will need an opt-in feature. A transaction must be explicitly marked as replaceable when created for the RBF protocol to be applicable.

Time-Based Replace-By-Fee (RBF) And Replacement Policies

Time-based RBF is a specific implementation where the replacement transaction can only occur within a particular time frame. By adopting a time-based approach, replacement policies ensure network stability and predictability while offering flexibility to users transacting on the Bitcoin network.

Differentiating Between First-Seen-Safe (FSS) And Replace-By-Fee (Full-RBF) Approaches

Within the RBF framework, there are two main approaches: 

First-Seen-Safe (FSS)

FSS allows the replacement of a transaction only if the new transaction does not conflict with any other transaction already seen by the network. The FSS method is a more conservative approach to transaction replacement in the Bitcoin network. 

This FSS method offers limited flexibility for fee adjustment without altering core transaction details. This method is deemed valid and is favored because of its enhanced security and predictability, making it suitable for users and applications requiring transaction integrity and consistent processing.

Full-Replace-By-Fee (RBF) 

Full-RBFpermits the replacement of transactions regardless of such conflicts, outlined above, offering greater flexibility at the cost of predictability. This means that under Full-RBF, a new transaction could change more than just the fee; it could alter the recipient or other details of the transaction. 

This greater flexibility offered by Full-RBF comes at the cost of reduced predictability and potentially increased risk of double-spending, as it does not follow the ‘first-seen’ rule that FSS adheres to.

Benefits And Risks Associated With Replace-By-Fee Feature in Bitcoin Transactions

While RBF offers significant advantages in terms of flexibility and efficiency in the Bitcoin network, it also introduces complexities and risks that need to be carefully managed by users and merchants alike.

Benefits Associated With Replace-By-Fee (RBF)

Increased flexibility

RBF allows users to adjust transaction fees in response to fluctuating network conditions, providing greater control over transaction processing times.

Faster confirmation

By increasing the fee, users can prioritize individual transactions broadcasted to the Bitcoin blockchain, leading to a potential for users to benefit from quicker confirmations, especially during periods of high network congestion.

Error correction

RBF enables users to rectify mistakes in unconfirmed transactions, such as incorrect fees or even wrong recipient addresses, by replacing them with a corrected version.

Network Efficiency

RBF can contribute to overall network efficiency. By allowing fee adjustments, it helps in aligning transaction fees with the current demand for block space, optimizing block space utilization.

Risks Associated With Replace-By-Fee (RBF)

Double spending risk

RBF introduces a potential risk for double spending, as it allows the sender to replace an unconfirmed transaction with another one, possibly to a different recipient.

User confusion

The concept and mechanics of RBF can be confusing for users who are not familiar with it, leading to misunderstandings about the status and finality of the investor made transactions.

Merchant challenges

For merchants accepting Bitcoin, RBF can create complications, as they need to adjust their payment systems to account for the possibility of transaction replacement and the associated risks.

Potential for fraud

There is a risk of fraudulent activities, as malicious actors might exploit RBF to deceive recipients by initially sending a transaction with a low fee and then replacing it after receiving goods or services.

Transaction security

While RBF offers flexibility, it also raises concerns about transaction security. The ability to replace transactions can be exploited if not properly understood and managed. Users need to be made aware of the security implications, especially in transactions where immediate finality is critical to the underlying act of transacting Bitcoin.

Step-By-Step Guide To Replace-By-Fee (RBF) Functionality In Trezor Suite Hardware Wallets

The integration of RBF functionality into Trezor hardware wallets, particularly through the Trezor Suite software interface, a non-custodial wallet, marks a significant enhancement in Bitcoin transaction management. Trezor Suite offers a user-friendly interface, simplifying the initiation and management of RBF transactions, making it accessible to users regardless of technical expertise required in Bitcoin. 

Trezor Replace-By-Fee
Trezor Replace-By-Fee

The RBF feature, illustrated above, in Trezor Suite enables users to speed up the transaction confirmation process. Importantly, this increased flexibility in transaction management does not come at the cost of security. The integration of RBF in Trezor ensures that while users have the option to modify transaction fees, user private keys remain securely stored within the hardware wallet, safeguarding investor funds throughout the process.

Trezor Opt-In Replace-By-Fee (RBF) And Bump Fee Feature

Opt-in RBF is a user-friendly implementation where transactions are only replaceable if explicitly marked as such.

"Bump Fee" Function Trezor
“Bump Fee” Function Trezor

The “Bump Fee” feature, illustrated above, in many wallets is a direct application of this, allowing users to increase the fee on investor transactions after they have been broadcasted.


RBF in Bitcoin transactions is a protocol enhancing the flexibility and efficiency of the Bitcoin network and empowering users to adjust transaction fees post-broadcast, thereby addressing issues of delayed confirmations due to low fees.

While RBF offers significant benefits like faster transaction confirmations, error correction, and improved network efficiency, it also introduces risks such as potential double-spending and user confusion. 

The integration of RBF functionality in hardware wallets like Trezor, particularly with features like Opt-in RBF and “Bump Fee,” underscores the ongoing evolution of cryptocurrency wallets toward offering more user-friendly and secure transaction management tools.

As the Bitcoin ecosystem continues to mature, understanding and effectively utilizing protocols like RBF will benefit users, investors, and merchants navigating this dynamic digital currency landscape.


What is replace-by-fee (RBF) in Bitcoin transactions?

Replace-by-fee (RBF) is a Bitcoin protocol allowing users to increase the fee of an unconfirmed transaction, enhancing the likelihood of faster confirmation by incentivizing miners to confirm the transaction ahead of those transactions with a lower transaction fee.

How does RBF improve Bitcoin transaction efficiency?

RBF improves efficiency by allowing users to adjust fees in response to network congestion, potentially speeding up transaction confirmations during busy periods.

Are there risks associated with using RBF in Bitcoin?

Yes, RBF introduces risks like potential double spending and user confusion, especially for those unfamiliar with its mechanics and implications for transaction finality.

How does RBF integration in hardware wallets like Trezor benefit users?

RBF integration in wallets like Trezor offers a secure, user-friendly way to manage transaction fees, combining flexibility with the security of hardware wallet protection.

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