BA-05 · LESSON 09

CPFP and package economics

KEY DEFINITIONS

Terms used in this lesson

CPFP: Child Pays for Parent: a high-fee child transaction raises the combined fee rate of an unconfirmed package.
Proof-of-work target: The maximum header-hash value accepted for a block; lowering it makes valid hashes harder to find.

A child can pay enough fee for miners to include a low-fee parent when the package feerate is attractive.

35 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 · transaction bytesA transaction consumes prior outputs and commits new onesSelect any state to inspect its exact role.

RELATIONSHIP MAPInputs, rule, and outputs

01
OUTPOINTSTxid : output index
Every input references one specific prior transaction output.
02
AUTHORIZATIONWitness or scriptSig
Spending data satisfies the prior output's locking condition.
03
SERIALIZATIONCPFP and package economics
The CPFP and package economics mechanism determines which transaction fields or identifiers matter.
04
NEW OUTPUTSAmount and script
Payment and change outputs are newly created state.
05
IDENTIFIERSTxid and wtxid
Double-SHA256 identifiers commit to defined byte regions.

Rule shown: Inputs name exact outpoints; outputs allocate value; signatures authorize the committed spending context.

STAGE 1 OF 5OUTPOINTS: Txid : output index

Every input references one specific prior transaction output.

Governing relationship: Inputs name exact outpoints; outputs allocate value; signatures authorize the committed spending context. Complete path: 1. OUTPOINTS, Txid : output index. Every input references one specific prior transaction output. 2. AUTHORIZATION, Witness or scriptSig. Spending data satisfies the prior output's locking condition. 3. SERIALIZATION, CPFP and package economics. The CPFP and package economics mechanism determines which transaction fields or identifiers matter. 4. NEW OUTPUTS, Amount and script. Payment and change outputs are newly created state. 5. IDENTIFIERS, Txid and wtxid. Double-SHA256 identifiers commit to defined byte regions.

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

BEFORE

A 200-vB parent pays 200 sats. A 100-vB child pays 2,800 sats.

MECHANISM

The parent pays a low feerate and remains unconfirmed. The child cannot confirm without it because the child spends an output the parent creates.

AFTER

The child does not alter the parent. It changes the economics of including the dependency package.

Text equivalent: Starting from A 200-vB parent pays 200 sats. A 100-vB child pays 2,800 sats. The mechanism begins when The parent pays a low feerate and remains unconfirmed. The child cannot confirm without it because the child spends an output the parent creates. The resulting lesson is The child does not alter the parent. It changes the economics of including the dependency package.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 4

The parent is stuck

The parent pays a low feerate and remains unconfirmed. The child cannot confirm without it because the child spends an output the parent creates.

Complete path: 1. The parent is stuck: The parent pays a low feerate and remains unconfirmed. The child cannot confirm without it because the child spends an output the parent creates. 2. Create a high-fee child: A party controlling a parent output creates a child with a deliberately high fee. This may be the sender spending change or the recipient spending the payment output. 3. Miners evaluate the package: The useful number is the combined fee divided by the combined virtual size. The child must pay for its own weight and make up the parent’s fee deficit. 4. Both confirm in dependency order: A miner that accepts the package includes the parent first and the child after it in the same block, collecting both fees.

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

ACCEPTED PATH

The child must spend a real unconfirmed parent output controlled by the bumper, both transactions must remain valid, and their combined integer fee divided by combined virtual size must meet the selected package target and pinned Bitcoin Core v31.0 cluster li…

CHANGED CONDITION

A child with no parent dependency, no controlled output, an invalid member, excessive cluster count or size, or insufficient combined fee cannot rescue the parent through CPFP.

Important distinction: A high-fee child can confirm before its unconfirmed parent. It cannot. The child depends on the parent output, so the parent must appear earlier in the chain and in the block.

Text equivalent: The accepted path is enforced by The child must spend a real unconfirmed parent output controlled by the bumper, both transactions must remain valid, and their combined integer fee divided by combined virtual size must meet the selected package target and pinned Bitcoin Core v31.0 cluster limits. If the required condition changes, A child with no parent dependency, no controlled output, an invalid member, excessive cluster count or size, or insufficient combined fee cannot rescue the parent through CPFP.

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Learn the idea

Child Pays for Parent, or CPFP, is a fee-bumping strategy for an unconfirmed transaction you cannot or do not want to replace. You spend one of its outputs in a new child transaction and give the child a high enough fee that mining both transactions is attractive as a package.

GUIDED EXPLANATION1/4 ideas inspected

The parent is stuck

The parent pays a low feerate and remains unconfirmed. The child cannot confirm without it because the child spends an output the parent creates.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“A high-fee child can confirm before its unconfirmed parent.”

It cannot. The child depends on the parent output, so the parent must appear earlier in the chain and in the block.

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