Re: [bitcoin-dev] Proposal: Package Mempool Accept and Package RBF

[Gloria Zhao <gloriajzhao@gmail.com>]

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Hi Bastien,

Thank you for your feedback!

> In your example we have a parent transaction A already in the mempool
> and an unrelated child B. We submit a package C + D where C spends
> another of A's inputs. You're highlighting that this package may be
> rejected because of the unrelated transaction(s) B.

> The way I see this, an attacker can abuse this rule to ensure
> transaction A stays pinned in the mempool without confirming by
> broadcasting a set of child transactions that reach these limits
> and pay low fees (where A would be a commit tx in LN).

I believe you are describing a pinning attack in which your adversarial
counterparty attempts to monopolize the mempool descendant limit of the
shared  transaction A in order to prevent you from submitting a fee-bumping
child C; I've tried to illustrate this as diagram A here:
https://user-images.githubusercontent.com/25183001/134159860-068080d0-bbb6-4356-ae74-00df00644c74.png
(please let me know if I'm misunderstanding).

I believe this attack is mitigated as long as we attempt to submit
transactions individually (and thus take advantage of CPFP carve out)
before attempting package validation. So, in scenario A2, even if the
mempool receives a package with A+C, it would deduplicate A, submit C as an
individual transaction, and allow it due to the CPFP carve out exemption. A
more general goal is: if a transaction would propagate successfully on its
own now, it should still propagate regardless of whether it is included in
a package. The best way to ensure this, as far as I can tell, is to always
try to submit them individually first.

I would note that this proposal doesn't accommodate something like diagram
B, where C is getting CPFP carve out and wants to bring a +1 (e.g. C has
very low fees and is bumped by D). I don't think this is a use case since C
should be the one fee-bumping A, but since we're talking about limitations
around the CPFP carve out, this is it.

Let me know if this addresses your concerns?

Thanks,
Gloria

On Mon, Sep 20, 2021 at 10:19 AM Bastien TEINTURIER <bastien@acinq.fr>
wrote:

> Hi Gloria,
>
> Thanks for this detailed post!
>
> The illustrations you provided are very useful for this kind of graph
> topology problems.
>
> The rules you lay out for package RBF look good to me at first glance
> as there are some subtle improvements compared to BIP 125.
>
> > 1. A package cannot exceed `MAX_PACKAGE_COUNT=25` count and
> > `MAX_PACKAGE_SIZE=101KvB` total size [8]
>
> I have a question regarding this rule, as your example 2C could be
> concerning for LN (unless I didn't understand it correctly).
>
> This also touches on the package RBF rule 5 ("The package cannot
> replace more than 100 mempool transactions.")
>
> In your example we have a parent transaction A already in the mempool
> and an unrelated child B. We submit a package C + D where C spends
> another of A's inputs. You're highlighting that this package may be
> rejected because of the unrelated transaction(s) B.
>
> The way I see this, an attacker can abuse this rule to ensure
> transaction A stays pinned in the mempool without confirming by
> broadcasting a set of child transactions that reach these limits
> and pay low fees (where A would be a commit tx in LN).
>
> We had to create the CPFP carve-out rule explicitly to work around
> this limitation, and I think it would be necessary for package RBF
> as well, because in such cases we do want to be able to submit a
> package A + C where C pays high fees to speed up A's confirmation,
> regardless of unrelated unconfirmed children of A...
>
> We could submit only C to benefit from the existing CPFP carve-out
> rule, but that wouldn't work if our local mempool doesn't have A yet,
> but other remote mempools do.
>
> Is my concern justified? Is this something that we should dig into a
> bit deeper?
>
> Thanks,
> Bastien
>
> Le jeu. 16 sept. 2021 à 09:55, Gloria Zhao via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> a écrit :
>
>> Hi there,
>>
>> I'm writing to propose a set of mempool policy changes to enable package
>> validation (in preparation for package relay) in Bitcoin Core. These
>> would not
>> be consensus or P2P protocol changes. However, since mempool policy
>> significantly affects transaction propagation, I believe this is relevant
>> for
>> the mailing list.
>>
>> My proposal enables packages consisting of multiple parents and 1 child.
>> If you
>> develop software that relies on specific transaction relay assumptions
>> and/or
>> are interested in using package relay in the future, I'm very interested
>> to hear
>> your feedback on the utility or restrictiveness of these package policies
>> for
>> your use cases.
>>
>> A draft implementation of this proposal can be found in [Bitcoin Core
>> PR#22290][1].
>>
>> An illustrated version of this post can be found at
>> https://gist.github.com/glozow/dc4e9d5c5b14ade7cdfac40f43adb18a.
>> I have also linked the images below.
>>
>> ## Background
>>
>> Feel free to skip this section if you are already familiar with mempool
>> policy
>> and package relay terminology.
>>
>> ### Terminology Clarifications
>>
>> * Package = an ordered list of related transactions, representable by a
>> Directed
>>   Acyclic Graph.
>> * Package Feerate = the total modified fees divided by the total virtual
>> size of
>>   all transactions in the package.
>>     - Modified fees = a transaction's base fees + fee delta applied by
>> the user
>>       with `prioritisetransaction`. As such, we expect this to vary across
>> mempools.
>>     - Virtual Size = the maximum of virtual sizes calculated using [BIP141
>>       virtual size][2] and sigop weight. [Implemented here in Bitcoin
>> Core][3].
>>     - Note that feerate is not necessarily based on the base fees and
>> serialized
>>       size.
>>
>> * Fee-Bumping = user/wallet actions that take advantage of miner
>> incentives to
>>   boost a transaction's candidacy for inclusion in a block, including
>> Child Pays
>> for Parent (CPFP) and [BIP125][12] Replace-by-Fee (RBF). Our intention in
>> mempool policy is to recognize when the new transaction is more
>> economical to
>> mine than the original one(s) but not open DoS vectors, so there are some
>> limitations.
>>
>> ### Policy
>>
>> The purpose of the mempool is to store the best (to be most
>> incentive-compatible
>> with miners, highest feerate) candidates for inclusion in a block. Miners
>> use
>> the mempool to build block templates. The mempool is also useful as a
>> cache for
>> boosting block relay and validation performance, aiding transaction
>> relay, and
>> generating feerate estimations.
>>
>> Ideally, all consensus-valid transactions paying reasonable fees should
>> make it
>> to miners through normal transaction relay, without any special
>> connectivity or
>> relationships with miners. On the other hand, nodes do not have unlimited
>> resources, and a P2P network designed to let any honest node broadcast
>> their
>> transactions also exposes the transaction validation engine to DoS
>> attacks from
>> malicious peers.
>>
>> As such, for unconfirmed transactions we are considering for our mempool,
>> we
>> apply a set of validation rules in addition to consensus, primarily to
>> protect
>> us from resource exhaustion and aid our efforts to keep the highest fee
>> transactions. We call this mempool _policy_: a set of (configurable,
>> node-specific) rules that transactions must abide by in order to be
>> accepted
>> into our mempool. Transaction "Standardness" rules and mempool
>> restrictions such
>> as "too-long-mempool-chain" are both examples of policy.
>>
>> ### Package Relay and Package Mempool Accept
>>
>> In transaction relay, we currently consider transactions one at a time for
>> submission to the mempool. This creates a limitation in the node's
>> ability to
>> determine which transactions have the highest feerates, since we cannot
>> take
>> into account descendants (i.e. cannot use CPFP) until all the
>> transactions are
>> in the mempool. Similarly, we cannot use a transaction's descendants when
>> considering it for RBF. When an individual transaction does not meet the
>> mempool
>> minimum feerate and the user isn't able to create a replacement
>> transaction
>> directly, it will not be accepted by mempools.
>>
>> This limitation presents a security issue for applications and users
>> relying on
>> time-sensitive transactions. For example, Lightning and other protocols
>> create
>> UTXOs with multiple spending paths, where one counterparty's spending
>> path opens
>> up after a timelock, and users are protected from cheating scenarios as
>> long as
>> they redeem on-chain in time. A key security assumption is that all
>> parties'
>> transactions will propagate and confirm in a timely manner. This
>> assumption can
>> be broken if fee-bumping does not work as intended.
>>
>> The end goal for Package Relay is to consider multiple transactions at
>> the same
>> time, e.g. a transaction with its high-fee child. This may help us better
>> determine whether transactions should be accepted to our mempool,
>> especially if
>> they don't meet fee requirements individually or are better RBF
>> candidates as a
>> package. A combination of changes to mempool validation logic, policy, and
>> transaction relay allows us to better propagate the transactions with the
>> highest package feerates to miners, and makes fee-bumping tools more
>> powerful
>> for users.
>>
>> The "relay" part of Package Relay suggests P2P messaging changes, but a
>> large
>> part of the changes are in the mempool's package validation logic. We
>> call this
>> *Package Mempool Accept*.
>>
>> ### Previous Work
>>
>> * Given that mempool validation is DoS-sensitive and complex, it would be
>>   dangerous to haphazardly tack on package validation logic. Many efforts
>> have
>> been made to make mempool validation less opaque (see [#16400][4],
>> [#21062][5],
>> [#22675][6], [#22796][7]).
>> * [#20833][8] Added basic capabilities for package validation, test
>> accepts only
>>   (no submission to mempool).
>> * [#21800][9] Implemented package ancestor/descendant limit checks for
>> arbitrary
>>   packages. Still test accepts only.
>> * Previous package relay proposals (see [#16401][10], [#19621][11]).
>>
>> ### Existing Package Rules
>>
>> These are in master as introduced in [#20833][8] and [#21800][9]. I'll
>> consider
>> them as "given" in the rest of this document, though they can be changed,
>> since
>> package validation is test-accept only right now.
>>
>> 1. A package cannot exceed `MAX_PACKAGE_COUNT=25` count and
>> `MAX_PACKAGE_SIZE=101KvB` total size [8]
>>
>>    *Rationale*: This is already enforced as mempool ancestor/descendant
>> limits.
>> Presumably, transactions in a package are all related, so exceeding this
>> limit
>> would mean that the package can either be split up or it wouldn't pass
>> this
>> mempool policy.
>>
>> 2. Packages must be topologically sorted: if any dependencies exist
>> between
>> transactions, parents must appear somewhere before children. [8]
>>
>> 3. A package cannot have conflicting transactions, i.e. none of them can
>> spend
>> the same inputs. This also means there cannot be duplicate transactions.
>> [8]
>>
>> 4. When packages are evaluated against ancestor/descendant limits in a
>> test
>> accept, the union of all of their descendants and ancestors is
>> considered. This
>> is essentially a "worst case" heuristic where every transaction in the
>> package
>> is treated as each other's ancestor and descendant. [8]
>> Packages for which ancestor/descendant limits are accurately captured by
>> this
>> heuristic: [19]
>>
>> There are also limitations such as the fact that CPFP carve out is not
>> applied
>> to package transactions. #20833 also disables RBF in package validation;
>> this
>> proposal overrides that to allow packages to use RBF.
>>
>> ## Proposed Changes
>>
>> The next step in the Package Mempool Accept project is to implement
>> submission
>> to mempool, initially through RPC only. This allows us to test the
>> submission
>> logic before exposing it on P2P.
>>
>> ### Summary
>>
>> - Packages may contain already-in-mempool transactions.
>> - Packages are 2 generations, Multi-Parent-1-Child.
>> - Fee-related checks use the package feerate. This means that wallets can
>> create a package that utilizes CPFP.
>> - Parents are allowed to RBF mempool transactions with a set of rules
>> similar
>>   to BIP125. This enables a combination of CPFP and RBF, where a
>> transaction's descendant fees pay for replacing mempool conflicts.
>>
>> There is a draft implementation in [#22290][1]. It is WIP, but feedback is
>> always welcome.
>>
>> ### Details
>>
>> #### Packages May Contain Already-in-Mempool Transactions
>>
>> A package may contain transactions that are already in the mempool. We
>> remove
>> ("deduplicate") those transactions from the package for the purposes of
>> package
>> mempool acceptance. If a package is empty after deduplication, we do
>> nothing.
>>
>> *Rationale*: Mempools vary across the network. It's possible for a parent
>> to be
>> accepted to the mempool of a peer on its own due to differences in policy
>> and
>> fee market fluctuations. We should not reject or penalize the entire
>> package for
>> an individual transaction as that could be a censorship vector.
>>
>> #### Packages Are Multi-Parent-1-Child
>>
>> Only packages of a specific topology are permitted. Namely, a package is
>> exactly
>> 1 child with all of its unconfirmed parents. After deduplication, the
>> package
>> may be exactly the same, empty, 1 child, 1 child with just some of its
>> unconfirmed parents, etc. Note that it's possible for the parents to be
>> indirect
>> descendants/ancestors of one another, or for parent and child to share a
>> parent,
>> so we cannot make any other topology assumptions.
>>
>> *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 also easier to reason about and simplifies the
>> validation
>> logic greatly. Multi-parent-1-child allows us to think of the package as
>> one big
>> transaction, where:
>>
>> - Inputs = all the inputs of parents + inputs of the child that come from
>>   confirmed UTXOs
>> - Outputs = all the outputs of the child + all outputs of the parents that
>>   aren't spent by other transactions in the package
>>
>> Examples of packages that follow this rule (variations of example A show
>> some
>> possibilities after deduplication): ![image][15]
>>
>> #### Fee-Related Checks Use Package Feerate
>>
>> Package Feerate = the total modified fees divided by the total virtual
>> size of
>> all transactions in the package.
>>
>> To meet the two feerate requirements of a mempool, i.e., the
>> pre-configured
>> minimum relay feerate (`minRelayTxFee`) and dynamic mempool minimum
>> feerate, the
>> total package feerate is used instead of the individual feerate. The
>> individual
>> transactions are allowed to be below feerate requirements if the package
>> meets
>> the feerate requirements. For example, the parent(s) in the package can
>> have 0
>> fees but be paid for by the child.
>>
>> *Rationale*: This can be thought of as "CPFP within a package," solving
>> the
>> issue of a parent not meeting minimum fees on its own. This allows L2
>> applications to adjust their fees at broadcast time instead of
>> overshooting or
>> risking getting stuck/pinned.
>>
>> We use the package feerate of the package *after deduplication*.
>>
>> *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 for both its individual RBF and package RBF.
>>
>> Examples F and G [14] show the same package, but P1 is submitted
>> individually before
>> the package in example G. In example F, we can see that the 300vB package
>> pays
>> an additional 200sat in fees, which is not enough to pay for its own
>> bandwidth
>> (BIP125#4). In example G, we can see that P1 pays enough to replace M1,
>> but
>> using P1's fees again during package submission would make it look like a
>> 300sat
>> increase for a 200vB package. Even including its fees and size would not
>> be
>> sufficient in this example, since the 300sat looks like enough for the
>> 300vB
>> package. The calculcation after deduplication is 100sat increase for a
>> package
>> of size 200vB, which correctly fails BIP125#4. Assume all transactions
>> have a
>> size of 100vB.
>>
>> #### Package RBF
>>
>> If a package meets feerate requirements as a package, the parents in the
>> transaction are allowed to replace-by-fee mempool transactions. The child
>> cannot
>> replace mempool transactions. Multiple transactions can replace the same
>> transaction, but in order to be valid, none of the transactions can try to
>> replace an ancestor of another transaction in the same package (which
>> would thus
>> make its inputs unavailable).
>>
>> *Rationale*: Even if we are using package feerate, a package will not
>> propagate
>> as intended if RBF still requires each individual transaction to meet the
>> feerate requirements.
>>
>> We use a set of rules slightly modified from BIP125 as follows:
>>
>> ##### Signaling (Rule #1)
>>
>> All mempool transactions to be replaced must signal replaceability.
>>
>> *Rationale*: Package RBF signaling logic should be the same for package
>> RBF and
>> single transaction acceptance. This would be updated if single transaction
>> validation moves to full RBF.
>>
>> ##### New Unconfirmed Inputs (Rule #2)
>>
>> A package may include new unconfirmed inputs, but the ancestor feerate of
>> the
>> child must be at least as high as the ancestor feerates of every
>> transaction
>> being replaced. This is contrary to BIP125#2, which states "The
>> replacement
>> transaction may only include an unconfirmed input if that input was
>> included in
>> one of the original transactions. (An unconfirmed input spends an output
>> from a
>> currently-unconfirmed transaction.)"
>>
>> *Rationale*: The purpose of BIP125#2 is to ensure that the replacement
>> transaction has a higher ancestor score than the original transaction(s)
>> (see
>> [comment][13]). Example H [16] shows how adding a new unconfirmed input
>> can lower the
>> ancestor score of the replacement transaction. P1 is trying to replace
>> M1, and
>> spends an unconfirmed output of M2. P1 pays 800sat, M1 pays 600sat, and
>> M2 pays
>> 100sat. Assume all transactions have a size of 100vB. While, in
>> isolation, P1
>> looks like a better mining candidate than M1, it must be mined with M2,
>> so its
>> ancestor feerate is actually 4.5sat/vB.  This is lower than M1's ancestor
>> feerate, which is 6sat/vB.
>>
>> In package RBF, the rule analogous to BIP125#2 would be "none of the
>> transactions in the package can spend new unconfirmed inputs." Example J
>> [17] shows
>> why, if any of the package transactions have ancestors, package feerate
>> is no
>> longer accurate. Even though M2 and M3 are not ancestors of P1 (which is
>> the
>> replacement transaction in an RBF), we're actually interested in the
>> entire
>> package. A miner should mine M1 which is 5sat/vB instead of M2, M3, P1,
>> P2, and
>> P3, which is only 4sat/vB. The Package RBF rule cannot be loosened to
>> only allow
>> the child to have new unconfirmed inputs, either, because it can still
>> cause us
>> to overestimate the package's ancestor score.
>>
>> However, enforcing a rule analogous to BIP125#2 would not only make
>> Package RBF
>> less useful, but would also break Package RBF for packages with parents
>> already
>> in the mempool: if a package parent has already been submitted, it would
>> look
>> like the child is spending a "new" unconfirmed input. In example K [18],
>> we're
>> looking to replace M1 with the entire package including P1, P2, and P3.
>> We must
>> consider the case where one of the parents is already in the mempool (in
>> this
>> case, P2), which means we must allow P3 to have new unconfirmed inputs.
>> However,
>> M2 lowers the ancestor score of P3 to 4.3sat/vB, so we should not replace
>> M1
>> with this package.
>>
>> Thus, the package RBF rule regarding new unconfirmed inputs is less
>> strict than
>> BIP125#2. However, we still achieve the same goal of requiring the
>> replacement
>> transactions to have a ancestor score at least as high as the original
>> ones. As
>> a result, the entire package is required to be a higher feerate mining
>> candidate
>> than each of the replaced transactions.
>>
>> Another note: the [comment][13] above the BIP125#2 code in the original
>> RBF
>> implementation suggests that the rule was intended to be temporary.
>>
>> ##### Absolute Fee (Rule #3)
>>
>> The package must increase the absolute fee of the mempool, i.e. the total
>> fees
>> of the package must be higher than the absolute fees of the mempool
>> transactions
>> it replaces. Combined with the CPFP rule above, this differs from BIP125
>> Rule #3
>> - an individual transaction in the package may have lower fees than the
>>   transaction(s) it is replacing. In fact, it may have 0 fees, and the
>> child
>> pays for RBF.
>>
>> ##### Feerate (Rule #4)
>>
>> The package must pay for its own bandwidth; the package feerate must be
>> higher
>> than the replaced transactions by at least minimum relay feerate
>> (`incrementalRelayFee`). Combined with the CPFP rule above, this differs
>> from
>> BIP125 Rule #4 - an individual transaction in the package can have a lower
>> feerate than the transaction(s) it is replacing. In fact, it may have 0
>> fees,
>> and the child pays for RBF.
>>
>> ##### Total Number of Replaced Transactions (Rule #5)
>>
>> The package cannot replace more than 100 mempool transactions. This is
>> identical
>> to BIP125 Rule #5.
>>
>> ### Expected FAQs
>>
>> 1. Is it possible for only some of the package to make it into the
>> mempool?
>>
>>    Yes, it is. However, since we evict transactions from the mempool by
>> descendant score and the package child is supposed to be sponsoring the
>> fees of
>> its parents, the most common scenario would be all-or-nothing. This is
>> incentive-compatible. In fact, to be conservative, package validation
>> should
>> begin by trying to submit all of the transactions individually, and only
>> use the
>> package mempool acceptance logic if the parents fail due to low feerate.
>>
>> 2. Should we allow packages to contain already-confirmed transactions?
>>
>>     No, for practical reasons. In mempool validation, we actually aren't
>> able to
>> tell with 100% confidence if we are looking at a transaction that has
>> already
>> confirmed, because we look up inputs using a UTXO set. If we have
>> historical
>> block data, it's possible to look for it, but this is inefficient, not
>> always
>> possible for pruning nodes, and unnecessary because we're not going to do
>> anything with the transaction anyway. As such, we already have the
>> expectation
>> that transaction relay is somewhat "stateful" i.e. nobody should be
>> relaying
>> transactions that have already been confirmed. Similarly, we shouldn't be
>> relaying packages that contain already-confirmed transactions.
>>
>> [1]: https://github.com/bitcoin/bitcoin/pull/22290
>> [2]:
>> https://github.com/bitcoin/bips/blob/1f0b563738199ca60d32b4ba779797fc97d040fe/bip-0141.mediawiki#transaction-size-calculations
>> [3]:
>> https://github.com/bitcoin/bitcoin/blob/94f83534e4b771944af7d9ed0f40746f392eb75e/src/policy/policy.cpp#L282
>> [4]: https://github.com/bitcoin/bitcoin/pull/16400
>> [5]: https://github.com/bitcoin/bitcoin/pull/21062
>> [6]: https://github.com/bitcoin/bitcoin/pull/22675
>> [7]: https://github.com/bitcoin/bitcoin/pull/22796
>> [8]: https://github.com/bitcoin/bitcoin/pull/20833
>> [9]: https://github.com/bitcoin/bitcoin/pull/21800
>> [10]: https://github.com/bitcoin/bitcoin/pull/16401
>> [11]: https://github.com/bitcoin/bitcoin/pull/19621
>> [12]: https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki
>> [13]:
>> https://github.com/bitcoin/bitcoin/pull/6871/files#diff-34d21af3c614ea3cee120df276c9c4ae95053830d7f1d3deaf009a4625409ad2R1101-R1104
>> [14]:
>> https://user-images.githubusercontent.com/25183001/133567078-075a971c-0619-4339-9168-b41fd2b90c28.png
>> [15]:
>> https://user-images.githubusercontent.com/25183001/132856734-fc17da75-f875-44bb-b954-cb7a1725cc0d.png
>> [16]:
>> https://user-images.githubusercontent.com/25183001/133567347-a3e2e4a8-ae9c-49f8-abb9-81e8e0aba224.png
>> [17]:
>> https://user-images.githubusercontent.com/25183001/133567370-21566d0e-36c8-4831-b1a8-706634540af3.png
>> [18]:
>> https://user-images.githubusercontent.com/25183001/133567444-bfff1142-439f-4547-800a-2ba2b0242bcb.png
>> [19]:
>> https://user-images.githubusercontent.com/25183001/133456219-0bb447cb-dcb4-4a31-b9c1-7d86205b68bc.png
>> [20]:
>> https://user-images.githubusercontent.com/25183001/132857787-7b7c6f56-af96-44c8-8d78-983719888c19.png
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>

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