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66 lines
2.9 KiB
66 lines
2.9 KiB
3 years ago
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# Mempool Limits
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## Definitions
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Given any two transactions Tx0 and Tx1 where Tx1 spends an output of Tx0,
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Tx0 is a *parent* of Tx1 and Tx1 is a *child* of Tx0.
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A transaction's *ancestors* include, recursively, its parents, the parents of its parents, etc.
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A transaction's *descendants* include, recursively, its children, the children of its children, etc.
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A mempool entry's *ancestor count* is the total number of in-mempool (unconfirmed) transactions in
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its ancestor set, including itself.
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A mempool entry's *descendant count* is the total number of in-mempool (unconfirmed) transactions in
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its descendant set, including itself.
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A mempool entry's *ancestor size* is the aggregated virtual size of in-mempool (unconfirmed)
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transactions in its ancestor set, including itself.
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A mempool entry's *descendant size* is the aggregated virtual size of in-mempool (unconfirmed)
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transactions in its descendant set, including itself.
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Transactions submitted to the mempool must not exceed the ancestor and descendant limits (aka
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mempool *package limits*) set by the node (see `-limitancestorcount`, `-limitancestorsize`,
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`-limitdescendantcount`, `-limitdescendantsize`).
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## Exemptions
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### CPFP Carve Out
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**CPFP Carve Out** if a transaction candidate for submission to the
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mempool would cause some mempool entry to exceed its descendant limits, an exemption is made if all
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of the following conditions are met:
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1. The candidate transaction is no more than 10,000 virtual bytes.
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2. The candidate transaction has an ancestor count of 2 (itself and exactly 1 ancestor).
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3. The in-mempool transaction's descendant count, including the candidate transaction, would only
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exceed the limit by 1.
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*Rationale*: this rule was introduced to prevent pinning by domination of a transaction's descendant
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limits in two-party contract protocols such as LN. Also see the [mailing list
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post](https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-November/016518.html).
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This rule was introduced in [PR #15681](https://github.com/bitcoin/bitcoin/pull/15681).
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### Single-Conflict RBF Carve Out
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When a candidate transaction for submission to the mempool would replace mempool entries, it may
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also decrease the descendant count of other mempool entries. Since ancestor/descendant limits are
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calculated prior to removing the would-be-replaced transactions, they may be overestimated.
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An exemption is given for a candidate transaction that would replace mempool transactions and meets
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all of the following conditions:
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1. The candidate transaction has exactly 1 directly conflicting transaction.
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2. The candidate transaction does not spend any unconfirmed inputs that are not also spent by the
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directly conflicting transaction.
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The following discounts are given to account for the would-be-replaced transaction(s):
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1. The descendant count limit is temporarily increased by 1.
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2. The descendant size limit temporarily is increased by the virtual size of the to-be-replaced
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directly conflicting transaction.
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