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Package Mempool Accept

Definitions

A package is an ordered list of transactions, representable by a connected Directed Acyclic Graph (a directed edge exists between a transaction that spends the output of another transaction).

For every transaction t in a topologically sorted package, if any of its parents are present in the package, they appear somewhere in the list before t.

A child-with-unconfirmed-parents package is a topologically sorted package that consists of exactly one child and all of its unconfirmed parents (no other transactions may be present). The last transaction in the package is the child, and its package can be canonically defined based on the current state: each of its inputs must be available in the UTXO set as of the current chain tip or some preceding transaction in the package.

Package Mempool Acceptance Rules

The following rules are enforced for all packages:

  • Packages cannot exceed MAX_PACKAGE_COUNT=25 count and MAX_PACKAGE_SIZE=101KvB total size (#20833)

    • Rationale: This is already enforced as mempool ancestor/descendant limits. If transactions in a package are all related, exceeding this limit would mean that the package can either be split up or it wouldn't pass individual mempool policy.

    • Note that, if these mempool limits change, package limits should be reconsidered. Users may also configure their mempool limits differently.

  • Packages must be topologically sorted. (#20833)

  • Packages cannot have conflicting transactions, i.e. no two transactions in a package can spend the same inputs. Packages cannot have duplicate transactions. (#20833)

  • No transaction in a package can conflict with a mempool transaction. BIP125 Replace By Fee is currently disabled for packages. (#20833)

    • Package RBF may be enabled in the future.

  • When packages are evaluated against ancestor/descendant limits, the union of all transactions' descendants and ancestors is considered. (#21800)

    • Rationale: This is essentially a "worst case" heuristic intended for packages that are heavily connected, i.e. some transaction in the package is the ancestor or descendant of all the other transactions.

The following rules are only enforced for packages to be submitted to the mempool (not enforced for test accepts):

  • Packages must be child-with-unconfirmed-parents packages. This also means packages must contain at least 2 transactions. (#22674)

    • Rationale: This allows for fee-bumping by CPFP. Allowing multiple parents makes it possible to fee-bump a batch of transactions. Restricting packages to a defined topology is easier to reason about and simplifies the validation logic greatly.

    • Warning: Batched fee-bumping may be unsafe for some use cases. Users and application developers should take caution if utilizing multi-parent packages.

  • Transactions in the package that have the same txid as another transaction already in the mempool will be removed from the package prior to submission ("deduplication").

    • Rationale: Node operators are free to set their mempool policies however they please, nodes may receive transactions in different orders, and malicious counterparties may try to take advantage of policy differences to pin or delay propagation of transactions. As such, it's possible for some package transaction(s) to already be in the mempool, and there is no need to repeat validation for those transactions or double-count them in fees.

    • Rationale: We want to prevent potential censorship vectors. We should not reject entire packages because we already have one of the transactions. Also, if an attacker first broadcasts a competing package or transaction with a mutated witness, even though the two same-txid-different-witness transactions are conflicting and cannot replace each other, the honest package should still be considered for acceptance.