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70 lines
3.8 KiB
70 lines
3.8 KiB
3 years ago
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# Mempool Replacements
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## Current Replace-by-Fee Policy
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A transaction conflicts with an in-mempool transaction ("directly conflicting transaction") if they
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spend one or more of the same inputs. A transaction may conflict with multiple in-mempool
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transactions.
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A transaction ("replacement transaction") may replace its directly conflicting transactions and
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their in-mempool descendants (together, "original transactions") if, in addition to passing all
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other consensus and policy rules, each of the following conditions are met:
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1. The directly conflicting transactions all signal replaceability explicitly. A transaction is
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signaling replaceability if any of its inputs have an nSequence number less than (0xffffffff - 1).
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*Rationale*: See [BIP125
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explanation](https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki#motivation).
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2. The replacement transaction only include an unconfirmed input if that input was included in
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one of the directly conflicting transactions. An unconfirmed input spends an output from a
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currently-unconfirmed transaction.
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*Rationale*: When RBF was originally implemented, the mempool did not keep track of
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ancestor feerates yet. This rule was suggested as a temporary restriction.
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3. The replacement transaction pays an absolute fee of at least the sum paid by the original
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transactions.
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*Rationale*: Only requiring the replacement transaction to have a higher feerate could allow an
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attacker to bypass node minimum relay feerate requirements and cause the network to repeatedly
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relay slightly smaller replacement transactions without adding any more fees. Additionally, if
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any of the original transactions would be included in the next block assembled by an economically
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rational miner, a replacement policy allowing the replacement transaction to decrease the absolute
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fees in the next block would be incentive-incompatible.
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4. The additional fees (difference between absolute fee paid by the replacement transaction and the
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sum paid by the original transactions) pays for the replacement transaction's bandwidth at or
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above the rate set by the node's incremental relay feerate. For example, if the incremental relay
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feerate is 1 satoshi/vB and the replacement transaction is 500 virtual bytes total, then the
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replacement pays a fee at least 500 satoshis higher than the sum of the original transactions.
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*Rationale*: Try to prevent DoS attacks where an attacker causes the network to repeatedly relay
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transactions each paying a tiny additional amount in fees, e.g. just 1 satoshi.
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5. The number of original transactions does not exceed 100. More precisely, the sum of all
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directly conflicting transactions' descendant counts (number of transactions inclusive of itself
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and its descendants) must not exceed 100; it is possible that this overestimates the true number
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of original transactions.
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*Rationale*: Try to prevent DoS attacks where an attacker is able to easily occupy and flush out
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significant portions of the node's mempool using replacements with multiple directly conflicting
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transactions, each with large descendant sets.
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This set of rules is similar but distinct from BIP125.
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## History
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* Opt-in full replace-by-fee (without inherited signaling) honoured in mempool and mining as of
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**v0.12.0** ([PR 6871](https://github.com/bitcoin/bitcoin/pull/6871)).
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* [BIP125](https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki) defined based on
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Bitcoin Core implementation.
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* The incremental relay feerate used to calculate the required additional fees is distinct from
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`minRelayTxFee` and configurable using `-incrementalrelayfee`
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([PR #9380](https://github.com/bitcoin/bitcoin/pull/9380)).
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* RBF enabled by default in the wallet GUI as of **v0.18.1** ([PR
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#11605](https://github.com/bitcoin/bitcoin/pull/11605)).
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