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_WEIGHT=404000 total weight (#20833)

    • Rationale: We want package size to be as small as possible to mitigate DoS via package validation. However, we want to make sure that the limit does not restrict ancestor packages that would be allowed if submitted individually.

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

    • Note that this is transaction weight, not "virtual" size as with other limits to allow simpler context-less checks.

  • 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)

  • Only limited package replacements are currently considered. (#28984)

    • Packages are 1-parent-1-child, with no in-mempool ancestors of the package.

    • All conflicting clusters (connected components of mempool transactions) must be clusters of up to size 2.

    • No more than MAX_REPLACEMENT_CANDIDATES transactions can be replaced, analogous to regular replacement rule 5).

    • Replacements must pay more total total fees at the incremental relay fee (analogous to regular replacement rules 3 and 4).

    • Parent feerate must be lower than package feerate.

    • Must improve feerate diagram. (#29242)

    • Rationale: Basic support for package RBF can be used by wallets by making chains of no longer than two, then directly conflicting those chains when needed. Combined with TRUC transactions this can result in more robust fee bumping. More general 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.

  • CPFP Carve Out is disabled in packaged contexts. (#21800)

    • Rationale: This carve out cannot be accurately applied when there are multiple transactions' ancestors and descendants being considered at the same time.

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.

Package Fees and Feerate

Package Feerate is the total modified fees (base fees + any fee delta from prioritisetransaction) divided by the total virtual size of all transactions in the package. If any transactions in the package are already in the mempool, they are not submitted again ("deduplicated") and are thus excluded from this calculation.

To meet the dynamic mempool minimum feerate, i.e., the feerate determined by the transactions evicted when the mempool reaches capacity (not the static minimum relay feerate), the total package feerate instead of individual feerate can be used. For example, if the mempool minimum feerate is 5sat/vB and a 1sat/vB parent transaction has a high-feerate child, it may be accepted if submitted as a package.

Rationale: This can be thought of as "CPFP within a package," solving the issue of a presigned transaction (i.e. in which a replacement transaction with a higher fee cannot be signed) being rejected from the mempool when transaction volume is high and the mempool minimum feerate rises.

Note: Package feerate cannot be used to meet the minimum relay feerate (-minrelaytxfee) requirement. For example, if the mempool minimum feerate is 5sat/vB and the minimum relay feerate is set to 5satvB, a 1sat/vB parent transaction with a high-feerate child will not be accepted, even if submitted as a package.

Rationale: Avoid situations in which the mempool contains non-bumped transactions below min relay feerate (which we consider to have pay 0 fees and thus receiving free relay). While package submission would ensure these transactions are bumped at the time of entry, it is not guaranteed that the transaction will always be bumped. For example, a later transaction could replace the fee-bumping child without still bumping the parent. These no-longer-bumped transactions should be removed during a replacement, but we do not have a DoS-resistant way of removing them or enforcing a limit on their quantity. Instead, prevent their entry into the mempool.

Implementation Note: Transactions within a package are always validated individually first, and package validation is used for the transactions that failed. Since package feerate is only calculated using transactions that are not in the mempool, this implementation detail affects the outcome of package validation.

Rationale: It would be incorrect to use the fees of transactions that are already in the mempool, as we do not want a transaction's fees to be double-counted.

Rationale: Packages are intended for incentive-compatible fee-bumping: transaction B is a "legitimate" fee-bump for transaction A only if B is a descendant of A and has a higher feerate than A. We want to prevent "parents pay for children" behavior; fees of parents should not help their children, since the parents can be mined without the child. More generally, if transaction A is not needed in order for transaction B to be mined, A's fees cannot help B. In a child-with-parents package, simply excluding any parent transactions that meet feerate requirements individually is sufficient to ensure this.

Rationale: We must not allow a low-feerate child to prevent its parent from being accepted; fees of children should not negatively impact their parents, since they are not necessary for the parents to be mined. More generally, if transaction B is not needed in order for transaction A to be mined, B's fees cannot harm A. In a child-with-parents package, simply validating parents individually first is sufficient to ensure this.

Rationale: As a principle, we want to avoid accidentally restricting policy in order to be backward-compatible for users and applications that rely on p2p transaction relay. Concretely, package validation should not prevent the acceptance of a transaction that would otherwise be policy-valid on its own. By always accepting a transaction that passes individual validation before trying package validation, we prevent any unintentional restriction of policy.