BIP 125 (RBF Signaling) — Glossary

TERM_DEF // UPGRADES_BIPS / BIP_125_RBF_SIGNALING

BIP 125
(RBF SIGNALING)

BIP 125 (RBF Signaling). The opt-in Replace-by-Fee signaling spec via sequence numbers.

This page sits in the Upgrades & BIPs section — The proposal process and the major upgrades that shaped Bitcoin. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.

WHAT_BIP_125_RBF_SIGNALING_IS

BIP 125 (RBF Signaling) — at a glance

UPGRADES

BIP 125 — "RBF Signaling" is a proposal in the Mempool family, first published in 2015 with status final. The opt-in Replace-by-Fee signaling spec via sequence numbers. Its technical mechanism: replace-by-fee signalling via sequence < 0xfffffffe; replacement must pay strictly higher feerate. every modern mempool implements RBF; SegWit + 2024 deployments enabled it by default

Why it exists

DESIGN

BIP 125 (RBF Signaling) exists to solve a specific problem: let a sender replace a stuck tx with a higher-fee version. Without a written, numbered spec, every wallet and node implementer would interpret the requirement differently — and Bitcoin's value depends on every implementation agreeing exactly. BIP-125 is the single source of truth for this concern, so any new client built today can match the behaviour of every client built since 2015.

HOW_IT_WORKS

Mechanism

HOW IT WORKS

Mechanically, BIP-125 replace-by-fee signalling via sequence < 0xfffffffe; replacement must pay strictly higher feerate. Adoption today: every modern mempool implements RBF; SegWit + 2024 deployments enabled it by default.

1. Author drafts BIP-125 against the BIP-2 template — abstract, motivation, specification, rationale. 2. The text + a reference implementation are posted on the bitcoin-dev list and as a PR to bitcoin/bips. 3. Reviewers tear it apart: ambiguities, security concerns, edge cases, interaction with prior BIPs. 4. Once stable, the BIP editor merges it; it gets a number (BIP-125) and the status "draft" or "proposed". 5. Implementations land in Bitcoin Core / wallets / other clients. For consensus changes: activation parameters chosen. 6. After deployment + adoption, BIP-125 is promoted to "final"; deprecated proposals get "replaced" or "withdrawn".

WORKED_EXAMPLE

BIP-125 — quick reference card

EXAMPLE

BIP number : 125 Title : RBF Signaling Area : Mempool First published: 2015 Status : final Fork class : no Motivation : let a sender replace a stuck tx with a higher-fee version. Mechanism : replace-by-fee signalling via sequence < 0xfffffffe; replacement must pay strictly higher feerate. Where it shows up : every modern mempool implements RBF; SegWit + 2024 deployments enabled it by default. Read the spec : https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki

KEY_PROPERTIES

OPT-IN

No authority can force a BIP on anyone. Adoption depends entirely on whether the change is genuinely useful.

NUMBERED

Stable numeric reference — BIP-125 means the same thing across every wallet, miner, and node that has read this spec.

NON-CONSENSUS

BIP-125 is not a consensus rule — clients can implement it without coordinating with the network at large.

DEPLOYED

Implementations of BIP-125 are in production. The behaviour is observable on mainnet today.

COMMON_PITFALLS

Things that catch people out

PITFALLS

Don't confuse "draft" / "proposed" / "final" status — read the BIP header before relying on it in production.
The BIP number is just an editorial counter — it doesn't imply correctness or stability. Always cross-reference with the latest bitcoin/bips repo.

RELATED_CONCEPTS

Other terms from Upgrades & BIPs — click any to read its page:

TERMINOLOGY_INDEX

TERMINOLOGY

BIP 125 (RBF Signaling)

The opt-in Replace-by-Fee signaling spec via sequence numbers.

Segregated Witness (SegWit, BIP 141)

The 2017 upgrade separating witness-data/">witness data, fixing tx malleability, and introducing weight units.

Taproot (BIP 341)

The 2021 upgrade introducing key-aggregable schnorr-signatures/">Schnorr signatures and Merkleized script trees.

Tapscript (BIP 342)

Script-language updates accompanying Taproot — new opcodes, removed limits.

Schnorr (BIP 340)

The Schnorr signature spec adopted by Taproot.

BIP Process

The community workflow for proposing, discussing, and tracking changes to Bitcoin.

Soft Fork Activation

The mechanism (versionbits, MASF, UASF, Speedy Trial) by which soft forks turn on.

MASF (Miner-Activated Soft Fork)

Activation triggered by miner signaling in block versions.