BIP 125: Opt-in Full Replace-by-Fee Signaling
Authors: David A. Harding, Peter Todd Status: Final Type: Standards Track Created: 2015-11-03
Abstract
BIP 125 defines a signaling mechanism that allows transaction creators to indicate that their unconfirmed transactions may be replaced by a new version paying a higher fee. Transactions signal opt-in RBF by setting any input’s nSequence value to less than 0xfffffffe.
Background
The concept of transaction replacement was present in Bitcoin’s original design. Satoshi Nakamoto’s early codebase included a transaction replacement mechanism based on sequence numbers, but it was disabled in 2010 due to denial-of-service concerns. Peter Todd championed the reintroduction of transaction replacement with a fee-based policy, arguing that the ability to replace transactions was an inherent property of the Bitcoin network that should be acknowledged rather than denied.
Key Rules
For a replacement transaction to be accepted:
- The original transaction must signal replaceability (nSequence < 0xfffffffe)
- The replacement must pay a higher absolute fee than the original
- The replacement must pay a sufficient fee rate
- The replacement must not contain any new unconfirmed inputs
- The number of original transactions evicted must not exceed 100
Controversy
RBF was one of the most contentious policy changes in Bitcoin’s history. Critics argued it undermined zero-confirmation transactions used by merchants for in-person payments. Proponents, led by Peter Todd, argued that zero-confirmation transactions were never truly secure and that RBF provided a more honest security model while improving fee market efficiency.
Adoption
RBF signaling support was merged into Bitcoin Core 0.12.0 (released February 2016). Over time, RBF became widely adopted, and in Bitcoin Core 24.0 (2022), full RBF (without opt-in signaling) was introduced as a configurable option.
Significance
BIP 125 formalized the economic reality that miners are incentivized to include higher-fee transactions. It enabled practical fee bumping for stuck transactions and became a foundational mechanism for efficient use of Bitcoin block space.