BA-05 · LESSON 06

Mempool policy versus consensus

Nodes can reject relay or mining of a consensus-valid transaction under local policy without making it invalid forever.

45 min Difficulty 2/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · rbf replacementRBF replaces one unconfirmed transaction with a fee-paying conflictSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
ORIGINALInput X, fee 1,000 sats
The transaction remains unconfirmed in the node's mempool.
02
REPLACEMENTInput X, fee 2,000
Spending the same input creates the required conflict.
03
POLICYHigher absolute fee and required increment
Relay policy evaluates the replacement package and any transactions it removes.
04
MEMPOOLRemove original, accept replacement
Another node can have a different mempool view.
05
CHAINConfirmation resolves conflict
RBF is mempool policy. Consensus ultimately permits only one spend of input X in the selected chain.

Rule shown: A replacement must conflict with the original and satisfy the node's replacement policy, including absolute fee and incremental relay requirements.

STAGE 1 OF 5ORIGINAL: Input X, fee 1,000 sats

The transaction remains unconfirmed in the node's mempool.

Governing relationship: A replacement must conflict with the original and satisfy the node's replacement policy, including absolute fee and incremental relay requirements. Complete path: 1. ORIGINAL, Input X, fee 1,000 sats. The transaction remains unconfirmed in the node's mempool. 2. REPLACEMENT, Input X, fee 2,000. Spending the same input creates the required conflict. 3. POLICY, Higher absolute fee and required increment. Relay policy evaluates the replacement package and any transactions it removes. 4. MEMPOOL, Remove original, accept replacement. Another node can have a different mempool view. 5. CHAIN, Confirmation resolves conflict. RBF is mempool policy. Consensus ultimately permits only one spend of input X in the selected chain.

1. State transitionWhat exists, what changes, and what results

BEFORE

Transaction T satisfies consensus Script and value rules but exceeds a node's configured policy limit for an unconfirmed chain.

MECHANISM

The node verifies spendable inputs, values, scripts, lock conditions, and other rules that blocks must obey.

AFTER

Relay refusal predicts propagation, not permanent consensus invalidity.

Text equivalent: Starting from Transaction T satisfies consensus Script and value rules but exceeds a node's configured policy limit for an unconfirmed chain. The mechanism begins when The node verifies spendable inputs, values, scripts, lock conditions, and other rules that blocks must obey. The resulting lesson is Relay refusal predicts propagation, not permanent consensus invalidity.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Check consensus validity

The node verifies spendable inputs, values, scripts, lock conditions, and other rules that blocks must obey.

Complete path: 1. Check consensus validity: The node verifies spendable inputs, values, scripts, lock conditions, and other rules that blocks must obey. 2. Check mempool context: Unconfirmed conflicts, ancestor and descendant state, fee conditions, and mempool-specific rules depend on the node's current pool. 3. Apply standardness policy: Nodes commonly relay a restricted set of script forms, sizes, and outputs to limit abuse and improve predictable resource use. 4. Accept, reject, or mine directly: A policy-rejected transaction is not necessarily consensus-invalid. A miner can receive it privately and include it if the resulting block remains valid. 5. Reevaluate after confirmation: Once included in a valid block, nodes judge the transaction by consensus rules, not by whether their mempool would have accepted it earlier.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

Concrete facts first determine consensus validity, then each node's local relay decision and the miner's selection, with block acceptance returning to consensus rules.

CHANGED CONDITION

Consensus failure forbids valid-block inclusion; policy failure only blocks that local mempool path and cannot justify rejecting an otherwise valid mined block.

Important distinction: A transaction rejected by one node's mempool can never confirm. It may reach a miner through another node or direct submission and confirm if it is consensus-valid. Policy and mining choices affect likelihood.

Text equivalent: The accepted path is enforced by Concrete facts first determine consensus validity, then each node's local relay decision and the miner's selection, with block acceptance returning to consensus rules. If the required condition changes, Consensus failure forbids valid-block inclusion; policy failure only blocks that local mempool path and cannot justify rejecting an otherwise valid mined block.

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Learn the idea

Consensus rules decide whether a transaction may appear in a valid Bitcoin block. Mempool and relay policy decide whether a particular node stores or forwards an unconfirmed transaction. Policy is local and can change without a consensus fork. A transaction can be consensus-valid but nonstandard and therefore difficult to relay or mine.

GUIDED EXPLANATION1/5 ideas inspected

Check consensus validity

The node verifies spendable inputs, values, scripts, lock conditions, and other rules that blocks must obey.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“A transaction rejected by one node's mempool can never confirm.”

It may reach a miner through another node or direct submission and confirm if it is consensus-valid. Policy and mining choices affect likelihood.

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