Gloria Zhao [ARCHIVE] / is on Nostr

📅 Original date posted:2022-02-07
📝 Original message:Hi everyone,
Thanks for giving your attention to the post! I haven't had time to write
responses to everything, but sending my thoughts about what has been most
noteworthy to me:
@jeremy:
> A final point is that a verifiable delay function could be used over,
e.g., each of the N COutpoints individually to rate-limit transaction
replacement. The VDF period can be made shorter / eliminated depending on
the feerate increase.
Thanks for the suggestion! In general, I don't think rate limiting by
outpoint/prevout is a safe option, as it is particularly dangerous for L2
applications with shared prevouts. For example, the prevout that LN channel
counterparties conflict on is the output from their shared funding tx. Any
kind of limit on spending this prevout can be monopolized by a spammy
attacker. For example, if you only allow 1 per minute, the attacker will
just race to take up that slot every minute to prevent the honest party's
transaction from being accepted.
This is similar to the pinning attack based on monopolizing the
transaction's descendant limit, except we can't carve out an exemption
because we wouldn't know whose replacement we're looking at.
@tbast:
> The way I understand it, limiting the impact on descendant transactions
is only important for DoS protection, not for incentive compatibility.
> I believe it's completely ok to require increasing both the fees and
feerate if we don't take descendants into account, because you control your
ancestor set - whereas the descendant set may be completely out of your
control.
Ignoring descendants of direct conflicts would certainly make our lives
much easier! Unfortunately, I don't think we can do this since they can be
fee bumps, i.e., in AJ's example. Considering descendants is important for
both incentive compatibility and DoS.
If the replacement transaction has a higher feerate than its direct
conflict, but the direct conflict also has high feerate descendants, we
might end up with lower fees and/or feerates by accepting the replacement.
@aj:
> I wonder sometimes if it could be sufficient to just have a relay rate
limit and prioritise by ancestor feerate though. Maybe something like:
>
> - instead of adding txs to each peers setInventoryTxToSend immediately,
> set a mempool flag "relayed=false"
>
> - on a time delay, add the top N (by fee rate) "relayed=false" txs to
> each peer's setInventoryTxToSend and mark them as "relayed=true";
> calculate how much kB those txs were, and do this again after
> SIZE/RATELIMIT seconds
>
> - don't include "relayed=false" txs when building blocks?
Wow cool! I think outbound tx relay size-based rate-limiting and
prioritizing tx relay by feerate are great ideas for preventing spammers
from wasting bandwidth network-wide. I agree, this would slow the low
feerate spam down, preventing a huge network-wide bandwidth spike. And it
would allow high feerate transactions to propagate as they should,
regardless of how busy traffic is. Combined with inbound tx request
rate-limiting, might this be sufficient to prevent DoS regardless of the
fee-based replacement policies?
One point that I'm not 100% clear on: is it ok to prioritize the
transactions by ancestor feerate in this scheme? As I described in the
original post, this can be quite different from the actual feerate we would
consider a transaction in a block for. The transaction could have a high
feerate sibling bumping its ancestor.
For example, A (1sat/vB) has 2 children: B (49sat/vB) and C (5sat/vB). If
we just received C, it would be incorrect to give it a priority equal to
its ancestor feerate (3sat/vB) because if we constructed a block template
now, B would bump A, and C's new ancestor feerate is 5sat/vB.
Then, if we imagine that top N is >5sat/vB, we're not relaying C. If we
also exclude C when building blocks, we're missing out on good fees.
> - keep high-feerate evicted txs around for a while in case they get
> mined by someone else to improve compact block relay, a la the
> orphan pool?
Replaced transactions are already added to vExtraTxnForCompact :D
@ariard
> Deployment of Taproot opens interesting possibilities in the
vaults/payment channels design space, where the tapscripts can commit to
different set of timelocks/quorum of keys. Even if the pre-signed states
stay symmetric, whoever is the publisher, the feerate cost to spend can
fluctuate.
Indeed, perhaps with taproot we may legitimately have
same-txid-different-witness transactions as a normal thing rather than rare
edge case. But as with everything enabled by taproot, I wouldn't count our
tapscript eggs until a concrete use case hatches and/or an application
actually implements it.
> How this new replacement rule would behave if you have a parent in the
"replace-by-feerate" half but the child is in the "replace-by-fee" one ?
Thanks for considering my suggestion! This particular scenario is not
possible, since a child cannot be considered for the next block without its
parent. But if the original transactions are found both in and outside the
next block, I think it would be fine to just require both are met.
> Overall, I think there is the deployment issue to warn of. Moving to a
new set of RBF rules implies for a lot of Bitcoin applications to rewrite
their RBF logics.
I agree that transitioning as painlessly as possible would be a huge
priority in any kind of upgrade to mempool policy. I'm very interested in
hearing wallet devs' feedback on this.
I'm also not actually clear on what backwards compatibility in this
scenario would look like. I imagine it to mean we run both sets of RBF
rules and accept the replacement if it passes either one. Or do we only
accept the replacement if it passes both?
For wallets, AJ's "All you need is for there to be *a* path that follows
the new relay rules and gets from your node/wallet to perhaps 10% of
hashpower" makes sense to me (which would be the former). For merchants who
care more about making sure the original transaction isn't replaceable,
would they prefer that either policy is sufficient to prevent a replacement
(more in line with the latter)? Or is that covered by signaling / am I
overthinking this?
Thanks,
Gloria
On Mon, Feb 7, 2022 at 10:24 AM Bastien TEINTURIER via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:
> Good morning,
>
> > The tricky question is what happens when X arrives on its own and it
> > might be that no one ever sends a replacement for B,C,D)
>
> It feels ok to me, but this is definitely arguable.
>
> It covers the fact that B,C,D could have been fake transactions whose
> sole purpose was to do a pinning attack: in that case the attacker would
> have found a way to ensure these transactions don't confirm anyway (or
> pay minimal/negligible fees).
>
> If these transactions were legitimate, I believe that their owners would
> remake them at some point (because these transactions reflect a business
> relationship that needed to happen, so it should very likely still
> happen). It's probably hard to verify because the new corresponding
> transactions may have nothing in common with the first, but I think the
> simplifications it offers for wallets is worth it (which is just my
> opinion and needs more scrutiny/feedback).
>
> > But if your backlog's feerate does drop off, *and* that matters, then
> > I don't think you can ignore the impact of the descendent transactions
> > that you might not get a replacement for.
>
> That is because you're only taking into account the current backlog, and
> not taking into account the fact that new items will be added to it soon
> to replace the evicted descendants. But I agree that this is a bet: we
> can't predict the future and guarantee these replacements will come.
>
> It is really a trade-off, ignoring descendents provides a much simpler
> contract that doesn't vary from one mempool to another, but when your
> backlog isn't full enough, you may lose some future profits if
> transactions don't come in later.
>
> > I think "Y% higher" rather than just "higher" is only useful for
> > rate-limiting, not incentive compatibility. (Though maybe it helps
> > stabilise a greedy algorithm in some cases?)
>
> That's true. I claimed these policies only address incentives, but using
> a percentage increase addresses rate-limiting a bit as well (I couldn't
> resist trying to do at least something for it!). I find it a very easy
> mechanism to implement, while choosing an absolute value is hard (it's
> always easier to think in relatives than absolutes).
>
> > This is why I think it is important to understand the rationales for
> introducing the rules in the first place
>
> I completely agree. As you mentioned, we are still in brainstorming
> phase, once (if?) we start to converge on what could be better policies,
> we do need to clearly explain each policy's expected goal. That will let
> future Bastien writing code in 2030 clearly highlight why the 2022 rules
> don't make sense anymore!
>
> Cheers,
> Bastien
>
> Le sam. 5 févr. 2022 à 14:22, Michael Folkson <
> michaelfolkson at protonmail.com> a écrit :
>
>> Thanks for this Bastien (and Gloria for initially posting about this).
>>
>> I sympathetically skimmed the eclair PR (
>> https://github.com/ACINQ/eclair/pull/2113) dealing with replaceable
>> transactions fee bumping.
>>
>> There will continue to be a (hopefully) friendly tug of war on this
>> probably for the rest of Bitcoin's existence. I am sure people like Luke,
>> Prayank etc will (rightfully) continue to raise that Lightning and other
>> second layer protocols shouldn't demand that policy rules be changed if
>> there is a reason (e.g. DoS vector) for those rules on the base network.
>> But if there are rules that have no upside, introduce unnecessary
>> complexity for no reason and make Lightning implementers like Bastien's
>> life miserable attempting to deal with them I really hope we can make
>> progress on removing or simplifying them.
>>
>> This is why I think it is important to understand the rationales for
>> introducing the rules in the first place (and why it is safe to remove them
>> if indeed it is) and being as rigorous as possible on the rationales for
>> introducing additional rules. It sounds like from Gloria's initial post we
>> are still at a brainstorming phase (which is fine) but knowing what we know
>> today I really hope we can learn from the mistakes of the original BIP 125,
>> namely the Core implementation not matching the BIP and the sparse
>> rationales for the rules. As Bastien says this is not criticizing the
>> original BIP 125 authors, 7 years is a long time especially in Bitcoin
>> world and they probably weren't thinking about Bastien sitting down to
>> write an eclair PR in late 2021 (and reviewers of that PR) when they wrote
>> the BIP in 2015.
>>
>> --
>> Michael Folkson
>> Email: michaelfolkson at protonmail.com
>> Keybase: michaelfolkson
>> PGP: 43ED C999 9F85 1D40 EAF4 9835 92D6 0159 214C FEE3
>>
>>
>>
>> ------- Original Message -------
>> On Monday, January 31st, 2022 at 3:57 PM, Bastien TEINTURIER via
>> bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> wrote:
>>
>> Hi Gloria,
>>
>> Many thanks for raising awareness on these issues and constantly pushing
>> towards finding a better model. This work will highly improve the
>> security of any multi-party contract trying to build on top of bitcoin
>> (because most multi-party contracts will need to have timeout conditions
>> and participants will need to make some transactions confirm before a
>> timeout happens - otherwise they may lose funds).
>>
>> For starters, let me quickly explain why the current rules are hard to
>> work with in the context of lightning (but I believe most L2 protocols
>> will have the same issues). Feel free to skip this part if you are
>> already convinced.
>>
>> ## Motivation
>>
>> The biggest pain point is BIP 125 rule 2.
>> If I need to increase the fees of a time-sensitive transaction because
>> the feerate has been rising since I broadcast it, I may need to also pay
>> high fees just to produce a confirmed utxo that I can use. I'm actually
>> paying a high fee twice instead of once (and needlessly using on-chain
>> space, our scarcest asset, because we could have avoided that additional
>> transaction!).
>>
>> It also has some annoying "non-determinism".
>> Imagine that my transaction has been evicted from my mempool because its
>> feerate was too low. I could think "Great, that means I don't have to
>> apply BIP 125 restrictions, I can just fund this transaction as if it
>> were a new one!". But actually I do, because my transaction could still
>> be in miner's mempools and I have no way of knowing it...this means that
>> whenever I have broadcast a transaction, I must assume that I will
>> always need to abide by whatever replacement rules the network applies.
>>
>> Fortunately, as far as I understand it, this rule only exists because of
>> a previous implementation detail of bitcoin core, so there's simply no
>> good reason to keep it.
>>
>> The second biggest pain point is rule 3. It prevents me from efficiently
>> using my capital while it's unconfirmed. Whenever I'm using a big utxo
>> to fund a transaction, I will get a big change output, and it would
>> really be a waste to be unable to use that change output to fund other
>> transactions. In order to be capital-efficient, I will end up creating
>> descendant trees for my time-sensitive transactions. But as Gloria
>> explained, replacing all my children will cost me an absurdly large
>> amount of fees. So what I'm actually planning to do instead is to RBF
>> one of the descendants high enough to get the whole tree confirmed.
>> But if those descendants' timeouts were far in the future, that's a
>> waste, I paid a lot more fees for them than I should have. I'd like to
>> just replace my transaction and republish the invalidated children
>> independently.
>>
>> Rule 4 doesn't hurt as much as the two previous ones, I don't have too
>> much to say about it.
>>
>> To be fair to the BIP 125 authors, all of these scenarios were very hard
>> to forecast at the time this BIP was created. We needed years to build
>> on those rules to get a better understanding of their limitations and if
>> the rationale behind them made sense in the long term.
>>
>> ## Proposals
>>
>> I believe that now is a good time to re-think those, and I really like
>> Gloria's categorization of the design constraints.
>>
>> I'd like to propose a different way of looking at descendants that makes
>> it easier to design the new rules. The way I understand it, limiting the
>> impact on descendant transactions is only important for DoS protection,
>> not for incentive compatibility. I would argue that after evictions,
>> descendant transactions will be submitted again (because they represent
>> transactions that people actually want to make), so evicting them does
>> not have a negative impact on mining incentives (in a world where blocks
>> are full most of the time).
>>
>> I'm curious to hear other people's thoughts on that. If it makes sense,
>> I would propose the following very simple rules:
>>
>> 1. The transaction's ancestor absolute fees must be X% higher than the
>> previous transaction's ancestor fees
>> 2. The transaction's ancestor feerate must be Y% higher than the
>> previous transaction's ancestor feerate
>>
>> I believe it's completely ok to require increasing both the fees and
>> feerate if we don't take descendants into account, because you control
>> your ancestor set - whereas the descendant set may be completely out of
>> your control.
>>
>> This is very easy to use by wallets, because the ancestor set is easy to
>> obtain. And an important point is that the ancestor set is the same in
>> every mempool, whereas the descendant set is not (your mempool may have
>> rejected the last descendants, while other people's mempools may still
>> contain them).
>>
>> Because of that reason, I'd like to avoid having a rule that relies on
>> some size of the replaced descendant set: it may be valid in your
>> mempool but invalid in someone else's, which makes it exploitable for
>> pinning attacks.
>>
>> I believe these rules are incentive compatible (again, if you accept
>> the fact that the descendants will be re-submitted and mined as well,
>> so their fees aren't lost).
>>
>> Can we choose X and Y so that these two rules are also DoS-resistant?
>> Unfortunately I'm not sure, so maybe we'll need to add a third rule to
>> address that. But before we do, can someone detail what it costs for a
>> node to evict a descendant tree? Given that bitcoin core doesn't allow
>> chains of more than 25 transactions, the maximum number of transactions
>> being replaced will be bounded by 25 * N (where N is the number of
>> outputs of the transaction being replaced). If it's just O(n) pruning of
>> a graph, maybe that's ok? Or maybe we make X or Y depend on the number
>> of outputs of the transaction being replaced (this would need very
>> careful thoughts)?
>>
>> If you made it this far, thanks for reading!
>> A couple of comments on the previous messages:
>>
>> > Currently, if we see a transaction
>> > that has the same txid as one in the mempool, we reject it as a
>> > duplicate, even if the feerate is much higher. It's unclear to me if
>> > we have a very strong reason to change this, but noting it as a
>> > limitation of our current replacement policy.
>>
>> I don't see a strong reason from an L2 protocol's point of view yet, but
>> there are many unkown unknowns. But from a miner incentive's point of
>> view, we should keep the transaction with the higher feerate, shouldn't
>> we? In that case it's also a more efficient use of on-chain space, which
>> is a win, right?
>>
>> > We might have a more-or-less long transition period during which we
>> support both...
>>
>> Yes, this is a long term thing.
>> Even if bitcoin core releases a new version with updated RBF rules, as a
>> wallet you'll need to keep using the old rules for a long time if you
>> want to be safe.
>>
>> But it's all the more reason to try to ship this as soon as possible,
>> this way maybe our grand-children will be able to benefit from it ;)
>> (just kidding on the timespan obviously).
>>
>> Cheers,
>> Bastien
>>
>> Le lun. 31 janv. 2022 à 00:11, Antoine Riard via bitcoin-dev <
>> bitcoin-dev at lists.linuxfoundation.org> a écrit :
>>
>>> Hi Gloria,
>>>
>>> Thanks for this RBF sum up. Few thoughts and more context comments if it
>>> can help other readers.
>>>
>>> > For starters, the absolute fee pinning attack is especially
>>> > problematic if we apply the same rules (i.e. Rule #3 and #4) in
>>> > Package RBF. Imagine that Alice (honest) and Bob (adversary) share a
>>> > LN channel. The mempool is rather full, so their pre-negotiated
>>> > commitment transactions' feerates would not be considered high
>>> > priority by miners. Bob broadcasts his commitment transaction and
>>> > attaches a very large child (100KvB with 100,000sat in fees) to his
>>> > anchor output. Alice broadcasts her commitment transaction with a
>>> > fee-bumping child (200vB with 50,000sat fees which is a generous
>>> > 250sat/vB), but this does not meet the absolute fee requirement. She
>>> > would need to add another 50,000sat to replace Bob's commitment
>>> > transaction.
>>>
>>> Solving LN pinning attacks, what we're aiming for is enabling a fair
>>> feerate bid between the counterparties, thus either forcing the adversary
>>> to overbid or to disengage from the confirmation competition. If the
>>> replace-by-feerate rule is adopted, there shouldn't be an incentive for Bob
>>> to
>>> pick up the first option. Though if he does, that's a winning outcome
>>> for Alice, as one of the commitment transactions confirms and her
>>> time-sensitive second-stage HTLC can be subsequently confirmed.
>>>
>>> > It's unclear to me if
>>> > we have a very strong reason to change this, but noting it as a
>>> > limitation of our current replacement policy. See [#24007][12].
>>>
>>> Deployment of Taproot opens interesting possibilities in the
>>> vaults/payment channels design space, where the tapscripts can commit to
>>> different set of timelocks/quorum of keys. Even if the pre-signed states
>>> stay symmetric, whoever is the publisher, the feerate cost to spend can
>>> fluctuate.
>>>
>>> > While this isn't completely broken, and the user interface is
>>> > secondary to the safety of the mempool policy
>>>
>>> I think with L2s transaction broadcast backend, the stability and
>>> clarity of the RBF user interface is primary. What we could be worried
>>> about is a too-much complex interface easing the way for an attacker to
>>> trigger your L2 node to issue policy-invalid chain of transactions.
>>> Especially, when we consider that an attacker might have leverage on chain
>>> of transactions composition ("force broadcast of commitment A then
>>> commitment B, knowing they will share a CPFP") or even transactions size
>>> ("overload commitment A with HTLCs").
>>>
>>> > * If the original transaction is in the top {0.75MvB, 1MvB} of the
>>> > mempool, apply the current rules (absolute fees must increase and
>>> > pay for the replacement transaction's new bandwidth). Otherwise, use a
>>> > feerate-only rule.
>>>
>>> How this new replacement rule would behave if you have a parent in the
>>> "replace-by-feerate" half but the child is in the "replace-by-fee" one ?
>>>
>>> If we allow the replacement of the parent based on the feerate, we might
>>> decrease the top block absolute fees.
>>>
>>> If we block the replacement of the parent based on the feerate because
>>> the replacement absolute fees aren't above the replaced package, we still
>>> preclude a pinning vector. The child might be low-feerate junk and even
>>> attached to a low ancestor-score branch.
>>>
>>> If I'm correct on this limitation, maybe we could turn off the
>>> "replace-by-fee" behavior as soon as the mempool is fulfilled with a few
>>> blocks ?
>>>
>>> > * Rate-limit how many replacements we allow per prevout.
>>>
>>> Depending on how it is implemented, though I would be concerned it
>>> introduces a new pinning vector in the context of shared-utxo. If it's a
>>> hardcoded constant, it could be exhausted by an adversary starting at the
>>> lowest acceptable feerate then slowly increasing while still not reaching
>>> the top of the mempool. Same if it's time-based or block-based, no
>>> guarantee the replacement slot is honestly used by your counterparty.
>>>
>>> Further, an above-the-average replacement frequency might just be the
>>> reflection of your confirmation strategy reacting to block schedule or
>>> mempools historical data. As long as the feerate penalty is paid, I lean to
>>> allow replacement.
>>>
>>> (One solution could be to associate per-user "tag" to the LN
>>> transactions, where each "tag" would have its own replacement slots, but
>>> privacy?)
>>>
>>> > * Rate-limit transaction validation in general, per peer.
>>>
>>> I think we could improve on the Core's new transaction requester logic.
>>> Maybe we could bind the peer announced flow based on the feerate score
>>> (modulo validation time) of the previously validated transactions from that
>>> peer ? That said, while related to RBF, it sounds to me that enhancing
>>> Core's rate-limiting transaction strategy is a whole discussion in itself
>>> [0]. Especially ensuring it's tolerant to the specific requirements of LN &
>>> consorts.
>>>
>>> > What should they be? We can do some arithmetic to see what happens if
>>> > you start with the biggest/lowest feerate transaction and do a bunch
>>> > of replacements. Maybe we end up with values that are high enough to
>>> > prevent abuse and make sense for applications/users that do RBF.
>>>
>>> That's a good question.
>>>
>>> One observation is that the attacker can always renew the set of DoSy
>>> utxos to pursue the attack. So maybe we could pick up constants scaled on
>>> the block size ? That way an attacker would have to burn fees, thus
>>> deterring them from launching an attack. Even if the attackers are miners,
>>> they have to renounce their income to acquire new DoSy utxos. If a low-fee
>>> period, we could scale up the constants ?
>>>
>>>
>>> Overall, I think there is the deployment issue to warn of. Moving to a
>>> new set of RBF rules implies for a lot of Bitcoin applications to rewrite
>>> their RBF logics. We might have a more-or-less long transition period
>>> during which we support both...
>>>
>>> Cheers,
>>> Antoine
>>>
>>> [0] https://github.com/bitcoin/bitcoin/pull/21224
>>>
>>> Le jeu. 27 janv. 2022 à 09:10, Gloria Zhao via bitcoin-dev <
>>> bitcoin-dev at lists.linuxfoundation.org> a écrit :
>>>
>>>> Hi everyone,
>>>>
>>>> This post discusses limitations of current Bitcoin Core RBF policy and
>>>> attempts to start a conversation about how we can improve it,
>>>> summarizing some ideas that have been discussed. Please reply if you
>>>> have any new input on issues to be solved and ideas for improvement!
>>>>
>>>> Just in case I've screwed up the text wrapping again, another copy can
>>>> be
>>>> found here:
>>>> https://gist.github.com/glozow/25d9662c52453bd08b4b4b1d3783b9ff
>>>>
>>>> ## Background
>>>>
>>>> Please feel free to skip this section if you are already familiar
>>>> with RBF.
>>>>
>>>> Nodes may receive *conflicting* unconfirmed transactions, aka
>>>> "double spends" of the same inputs. Instead of always keeping the
>>>> first transaction, since v0.12, Bitcoin Core mempool policy has
>>>> included a set of Replace-by-Fee (RBF) criteria that allows the second
>>>> transaction to replace the first one and any descendants it may have.
>>>>
>>>> Bitcoin Core RBF policy was previously documented as BIP 125.
>>>> The current RBF policy is documented [here][1]. In summary:
>>>>
>>>> 1. The directly conflicting transactions all signal replaceability
>>>> explicitly.
>>>>
>>>> 2. The replacement transaction only includes an unconfirmed input if
>>>> that input was included in one of the directly conflicting
>>>> transactions.
>>>>
>>>> 3. The replacement transaction pays an absolute fee of at least the
>>>> sum paid by the original transactions.
>>>>
>>>> 4. The additional fees pays for the replacement transaction's
>>>> bandwidth at or above the rate set by the node's *incremental relay
>>>> feerate*.
>>>>
>>>> 5. The sum of all directly conflicting transactions' descendant counts
>>>> (number of transactions inclusive of itself and its descendants)
>>>> does not exceed 100.
>>>>
>>>> We can split these rules into 3 categories/goals:
>>>>
>>>> - **Allow Opting Out**: Some applications/businesses are unable to
>>>> handle transactions that are replaceable (e.g. merchants that use
>>>> zero-confirmation transactions). We (try to) help these businesses by
>>>> honoring BIP125 signaling; we won't replace transactions that have not
>>>> opted in.
>>>>
>>>> - **Incentive Compatibility**: Ensure that our RBF policy would not
>>>> accept replacement transactions which would decrease fee profits
>>>> of a miner. In general, if our mempool policy deviates from what is
>>>> economically rational, it's likely that the transactions in our
>>>> mempool will not match the ones in miners' mempools, making our
>>>> fee estimation, compact block relay, and other mempool-dependent
>>>> functions unreliable. Incentive-incompatible policy may also
>>>> encourage transaction submission through routes other than the p2p
>>>> network, harming censorship-resistance and privacy of Bitcoin payments.
>>>>
>>>> - **DoS Protection**: Limit two types of DoS attacks on the node's
>>>> mempool: (1) the number of times a transaction can be replaced and
>>>> (2) the volume of transactions that can be evicted during a
>>>> replacement.
>>>>
>>>> Even more abstract: our goal is to make a replacement policy that
>>>> results in a useful interface for users and safe policy for
>>>> node operators.
>>>>
>>>> ## Motivation
>>>>
>>>> There are a number of known problems with the current RBF policy.
>>>> Many of these shortcomings exist due to mempool limitations at the
>>>> time RBF was implemented or result from new types of Bitcoin usage;
>>>> they are not criticisms of the original design.
>>>>
>>>> ### Pinning Attacks
>>>>
>>>> The most pressing concern is that attackers may take advantage of
>>>> limitations in RBF policy to prevent other users' transactions from
>>>> being mined or getting accepted as a replacement.
>>>>
>>>> #### SIGHASH_ANYONECANPAY Pinning
>>>>
>>>> BIP125#2 can be bypassed by creating intermediary transactions to be
>>>> replaced together. Anyone can simply split a 1-input 1-output
>>>> transaction off from the replacement transaction, then broadcast the
>>>> transaction as is. This can always be done, and quite cheaply. More
>>>> details in [this comment][2].
>>>>
>>>> In general, if a transaction is signed with SIGHASH\_ANYONECANPAY,
>>>> anybody can just attach a low feerate parent to this transaction and
>>>> lower its ancestor feerate. Even if you require SIGHASH\_ALL which
>>>> prevents an attacker from changing any outputs, the input can be a
>>>> very low amount (e.g. just above the dust limit) from a low-fee
>>>> ancestor and still bring down the ancestor feerate of the transaction.
>>>>
>>>> TLDR: if your transaction is signed with SIGHASH\_ANYONECANPAY and
>>>> signals replaceability, regardless of the feerate you broadcast at, an
>>>> attacker can lower its mining priority by adding an ancestor.
>>>>
>>>> #### Absolute Fee
>>>>
>>>> The restriction of requiring replacement transactions to increase the
>>>> absolute fee of the mempool has been described as "bonkers." If the
>>>> original transaction has a very large descendant that pays a large
>>>> amount of fees, even if it has a low feerate, the replacement
>>>> transaction must now pay those fees in order to meet Rule #3.
>>>>
>>>> #### Package RBF
>>>>
>>>> There are a number of reasons why, in order to enable Package RBF, we
>>>> cannot use the same criteria.
>>>>
>>>> For starters, the absolute fee pinning attack is especially
>>>> problematic if we apply the same rules (i.e. Rule #3 and #4) in
>>>> Package RBF. Imagine that Alice (honest) and Bob (adversary) share a
>>>> LN channel. The mempool is rather full, so their pre-negotiated
>>>> commitment transactions' feerates would not be considered high
>>>> priority by miners. Bob broadcasts his commitment transaction and
>>>> attaches a very large child (100KvB with 100,000sat in fees) to his
>>>> anchor output. Alice broadcasts her commitment transaction with a
>>>> fee-bumping child (200vB with 50,000sat fees which is a generous
>>>> 250sat/vB), but this does not meet the absolute fee requirement. She
>>>> would need to add another 50,000sat to replace Bob's commitment
>>>> transaction.
>>>>
>>>> Disallowing new unconfirmed inputs (Rule #2) in Package RBF would be
>>>> broken for packages containing transactions already in the mempool,
>>>> explained [here][7].
>>>>
>>>> Note: I originally [proposed][6] Package RBF using the same Rule #3
>>>> and #4 before I realized how significant this pinning attack is. I'm
>>>> retracting that proposal, and a new set of Package RBF rules would
>>>> follow from whatever the new individual RBF rules end up being.
>>>>
>>>> #### Same Txid Different Witness
>>>>
>>>> Two transactions with the same non-witness data but different
>>>> witnesses have the same txid but different wtxid, and the same fee but
>>>> not necessarily the same feerate. Currently, if we see a transaction
>>>> that has the same txid as one in the mempool, we reject it as a
>>>> duplicate, even if the feerate is much higher. It's unclear to me if
>>>> we have a very strong reason to change this, but noting it as a
>>>> limitation of our current replacement policy. See [#24007][12].
>>>>
>>>> ### User Interface
>>>>
>>>> #### Using Unconfirmed UTXOs to Fund Replacements
>>>>
>>>> The restriction of only allowing confirmed UTXOs for funding a
>>>> fee-bump (Rule #2) can hurt users trying to fee-bump their
>>>> transactions and complicate wallet implementations. If the original
>>>> transaction's output value isn't sufficient to fund a fee-bump and/or
>>>> all of the user's other UTXOs are unconfirmed, they might not be able
>>>> to fund a replacement transaction. Wallet developers also need to
>>>> treat self-owned unconfirmed UTXOs as unusable for fee-bumping, which
>>>> adds complexity to wallet logic. For example, see BDK issues [#144][4]
>>>> and [#414][5].
>>>>
>>>> #### Interface Not Suitable for Coin Selection
>>>>
>>>> Currently, a user cannot simply create a replacement transaction
>>>> targeting a specific feerate or meeting a minimum fee amount and
>>>> expect to meet the RBF criteria. The fee amount depends on the size of
>>>> the replacement transaction, and feerate is almost irrelevant.
>>>>
>>>> Bitcoin Core's `bumpfee` doesn't use the RBF rules when funding the
>>>> replacement. It [estimates][13] a feerate which is "wallet incremental
>>>> relay fee" (a conservative overestimation of the node's incremental
>>>> relay fee) higher than the original transaction, selects coins for
>>>> that feerate, and hopes that it meets the RBF rules. It never fails
>>>> Rule #3 and #4 because it uses all original inputs and refuses to
>>>> bump a transaction with mempool descendants.
>>>>
>>>> This is suboptimal, but is designed to work with the coin selection
>>>> engine: select a feerate first, and then add fees to cover it.
>>>> Following the exact RBF rules would require working the other way
>>>> around: based on how much fees we've added to the transaction and its
>>>> current size, calculate the feerate to see if we meet Rule #4.
>>>>
>>>> While this isn't completely broken, and the user interface is
>>>> secondary to the safety of the mempool policy, we can do much better.
>>>> A much more user-friendly interface would depend *only* on the
>>>> fee and size of the original transactions.
>>>>
>>>> ### Updates to Mempool and Mining
>>>>
>>>> Since RBF was first implemented, a number of improvements have been
>>>> made to mempool and mining logic. For example, we now use ancestor
>>>> feerates in mining (allowing CPFP), and keep track of ancestor
>>>> packages in the mempool.
>>>>
>>>> ## Ideas for Improvements
>>>>
>>>> ### Goals
>>>>
>>>> To summarize, these seem to be desired changes, in order of priority:
>>>>
>>>> 1. Remove Rule #3. The replacement should not be *required* to pay
>>>> higher absolute fees.
>>>>
>>>> 2. Make it impossible for a replacement transaction to have a lower
>>>> mining score than the original transaction(s). This would eliminate
>>>> the `SIGHASH\_ANYONECANPAY` pinning attack.
>>>>
>>>> 3. Remove Rule #2. Adding new unconfirmed inputs should be allowed.
>>>>
>>>> 4. Create a more helpful interface that helps wallet fund replacement
>>>> transactions that aim for a feerate and fee.
>>>>
>>>> ### A Different Model for Fees
>>>>
>>>> For incentive compatibility, I believe there are different
>>>> formulations we should consider. Most importantly, if we want to get
>>>> rid of the absolute fee rule, we can no longer think of it as "the
>>>> transaction needs to pay for its own bandwidth," since we won't always
>>>> be getting additional fees. That means we need a new method of
>>>> rate-limiting replacements that doesn't require additional fees every
>>>> time.
>>>>
>>>> While it makes sense to think about monetary costs when launching a
>>>> specific type of attack, given that the fees are paid to the miner and
>>>> not to the mempool operators, maybe it doesn't make much sense to
>>>> think about "paying for bandwidth". Maybe we should implement
>>>> transaction validation rate-limiting differently, e.g. building it
>>>> into the P2P layer instead of the mempool policy layer.
>>>>
>>>> Recently, Suhas gave a [formulation][8] for incentive compatibility
>>>> that made sense to me: "are the fees expected to be paid in the next
>>>> (N?) blocks higher or lower if we process this transaction?"
>>>>
>>>> I started by thinking about this where N=1 or `1 + p`.
>>>> Here, a rational miner is looking at what fees they would
>>>> collect in the next block, and then some proportion `p` of the rest of
>>>> the blocks based on their hashrate. We're assuming `p` isn't *so high*
>>>> that they would be okay with lower absolute fees in the next 1 block.
>>>> We're also assuming `p` isn't *so low* that the miner doesn't care
>>>> about what's left of the mempool after this block.
>>>>
>>>> A tweak to this formulation is "if we process this transaction, would
>>>> the fees in the next 1 block higher or lower, and is the feerate
>>>> density of the rest of the mempool higher or lower?" This is pretty
>>>> similar, where N=1, but we consider the rest of the mempool by feerate
>>>> rather than fees.
>>>>
>>>> ### Mining Score of a Mempool Transaction
>>>>
>>>> We are often interested in finding out what
>>>> the "mining score" of a transaction in the mempool is. That is, when
>>>> the transaction is considered in block template building, what is the
>>>> feerate it is considered at?
>>>>
>>>> Obviously, it's not the transaction's individual feerate. Bitcoin Core
>>>> [mining code sorts][14] transactions by their ancestor feerate and
>>>> includes them packages at a time, keeping track of how this affects the
>>>> package feerates of remaining transactions in the mempool.
>>>>
>>>> *ancestor feerate*: Ancestor feerate is easily accessible information,
>>>> but it's not accurate either, because it doesn't take into account the
>>>> fact that subsets of a transaction's ancestor set can be included
>>>> without it. For example, ancestors may have high feerates on their own
>>>> or we may have [high feerate siblings][8].
>>>>
>>>> TLDR: *Looking at the current ancestor feerate of a transaction is
>>>> insufficient to tell us what feerate it will be considered at when
>>>> building a block template in the future.*
>>>>
>>>> *min(individual feerate, ancestor feerate)*: Another
>>>> heuristic that is simple to calculate based on current mempool tooling
>>>> is to use the [minimum of a transaction's individual score and its
>>>> ancestor score][10] as a conservative measure. But this can
>>>> overestimate as well (see the example below).
>>>>
>>>> *min ancestor feerate(tx + possible ancestor subsets)* We can also
>>>> take the minimum of every possible ancestor subset, but this can be
>>>> computationally expensive since there can be lots and lots of ancestor
>>>> subsets.
>>>>
>>>> *max ancestor feerate(tx + possible descendant subsets)*: Another idea
>>>> is to use the [maximum ancestor score of the transaction + each of its
>>>> descendants][9]. This doesn't work either; it has the same blindspot
>>>> of ancestor subsets being mined on their own.
>>>>
>>>> #### Mining Score Example
>>>>
>>>> Here's an example illustrating why mining score is tricky to
>>>> efficiently calculate for mempool transactions:
>>>>
>>>> Let's say you have same-size transactions A (21sat/vB), B (1sat/vB),
>>>> C(9sat/vB), D(5sat/vB).
>>>> The layout is: grandparent A, parent B, and two children C and D.
>>>>
>>>> ```
>>>> A
>>>> ^
>>>> B
>>>> ^ ^
>>>> C D
>>>> ```
>>>>
>>>> A miner using ancestor packages to build block templates will first
>>>> include A with a mining score of 21. Next, the miner will include B and
>>>> C with a mining score of 6. This leaves D, with a mining score of 5.
>>>>
>>>> Note: in this case, mining by ancestor feerate results in the most
>>>> rational decisions, but [a candidate set-based approach][10] which
>>>> makes ancestor feerate much less relevant could
>>>> be more advantageous in other situations.
>>>>
>>>> Here is a chart showing the "true" mining score alongside the values
>>>> calculating using imperfect heuristics described above. All of them
>>>> can overestimate or underestimate.
>>>>
>>>> ```
>>>> A B C D
>>>> mining score | 21 | 6 | 6 | 5 |
>>>> ancestor feerate | 21 | 11 | 10.3 | 9 |
>>>> min(individual, ancestor) | 21 | 1 | 9 | 5 |
>>>> min(tx + ancestor subsets) | 21 | 1 | 5 | 3 |
>>>> max(tx + descendants subsets) | 21 | 9 | 9 | 5 |
>>>>
>>>> ```
>>>>
>>>> Possibly the best solution for finding the "mining score" of a
>>>> transaction is to build a block template, see what feerate each
>>>> package is included at. Perhaps at some cutoff, remaining mempool
>>>> transactions can be estimated using some heuristic that leans
>>>> {overestimating, underestimating} depending on the situation.
>>>>
>>>> Mining score seems to be relevant in multiple places: Murch and I
>>>> recently [found][3] that it would be very important in
>>>> "ancestor-aware" funding of transactions (the wallet doesn't
>>>> incorporate ancestor fees when using unconfirmed transactions in coin
>>>> selection, which is a bug we want to fix).
>>>>
>>>> In general, it would be nice to know the exact mining priority of
>>>> one's unconfirmed transaction is. I can think of a few block/mempool
>>>> explorers who might want to display this information for users.
>>>>
>>>> ### RBF Improvement Proposals
>>>>
>>>> After speaking to quite a few people, here are some suggestions
>>>> for improvements that I have heard:
>>>>
>>>> * The ancestor score of the replacement must be {5, 10, N}% higher
>>>> than that of every original transaction.
>>>>
>>>> * The ancestor score of the replacement must be 1sat/vB higher than
>>>> that of every original transaction.
>>>>
>>>> * If the original transaction is in the top {0.75MvB, 1MvB} of the
>>>> mempool, apply the current rules (absolute fees must increase and
>>>> pay for the replacement transaction's new bandwidth). Otherwise, use a
>>>> feerate-only rule.
>>>>
>>>> * If fees don't increase, the size of the replacement transaction must
>>>> decrease by at least N%.
>>>>
>>>> * Rate-limit how many replacements we allow per prevout.
>>>>
>>>> * Rate-limit transaction validation in general, per peer.
>>>>
>>>> Perhaps some others on the mailing list can chime in to throw other
>>>> ideas into the ring and/or combine some of these rules into a sensible
>>>> policy.
>>>>
>>>> #### Replace by Feerate Only
>>>>
>>>> I don't think there's going to be a single-line feerate-based
>>>> rule that can incorporate everything we need.
>>>> On one hand, a feerate-only approach helps eliminate the issues
>>>> associated with Rule #3. On the other hand, I believe the main concern
>>>> with a feerate-only approach is how to rate limit replacements. We
>>>> don't want to enable an attack such as:
>>>>
>>>> 1. Attacker broadcasts large, low-feerate transaction, and attaches a
>>>> chain of descendants.
>>>>
>>>> 2. The attacker replaces the transaction with a smaller but higher
>>>> feerate transaction, attaching a new chain of descendants.
>>>>
>>>> 3. Repeat 1000 times.
>>>>
>>>> #### Fees in Next Block and Feerate for the Rest of the Mempool
>>>>
>>>> Perhaps we can look at replacements like this:
>>>>
>>>> 1. Calculate the directly conflicting transactions and, with their
>>>> descendants, the original transactions. Check signaling. Limit the
>>>> total volume (e.g. can't be more than 100 total or 1MvB or something).
>>>>
>>>> 2. Find which original transactions would be in the next ~1 block. The
>>>> replacement must pay at least this amount + X% in absolute fees. This
>>>> guarantees that the fees of the next block doesn't decrease.
>>>>
>>>> 3. Find which transactions would be left in the mempool after that ~1
>>>> block. The replacement's feerate must be Y% higher than the maximum
>>>> mining score of these transactions. This guarantees that you now have
>>>> only *better* candidates in your after-this-block mempool than you did
>>>> before, even if the size and fees the transactions decrease.
>>>>
>>>> 4. Now you have two numbers: a minimum absolute fee amount and a
>>>> minimum feerate. Check to see if the replacement(s) meet these
>>>> minimums. Also, a wallet would be able to ask the node "What fee and
>>>> feerate would I need to put on a transaction replacing this?" and use
>>>> this information to fund a replacement transaction, without needing to
>>>> guess or overshoot.
>>>>
>>>> Obviously, there are some magic numbers missing here. X and Y are
>>>> TBD constants to ensure we have some kind of rate limiting for the
>>>> number of replacements allowed using some set of fees.
>>>>
>>>> What should they be? We can do some arithmetic to see what happens if
>>>> you start with the biggest/lowest feerate transaction and do a bunch
>>>> of replacements. Maybe we end up with values that are high enough to
>>>> prevent abuse and make sense for applications/users that do RBF.
>>>>
>>>> ### Mempool Changes Need for Implementation
>>>>
>>>> As described in the mining score section above,
>>>> we may want additional tooling to more accurately assess
>>>> the economic gain of replacing transactions in our mempool.
>>>>
>>>> A few options have been discussed:
>>>>
>>>> * Calculate block templates on the fly when we need to consider a
>>>> replacement. However, since replacements are [quite common][11]
>>>> and the information might be useful for other things as well,
>>>> it may be worth it to cache a block template.
>>>>
>>>> * Keep a persistent block template so that we know what transactions
>>>> we would put in the next block. We need to remember the feerate
>>>> at which each transaction was included in the template, because an
>>>> ancestor package may be included in the same block template in
>>>> multiple subsets. Transactions included earlier alter the ancestor
>>>> feerate of the remaining transactions in the package. We also need
>>>> to keep track of the new feerates of transactions left over.
>>>>
>>>> * Divide the mempool into two layers, "high feerate" and "low
>>>> feerate." The high feerate layer contains ~1 block of packages with
>>>> the highest ancestor feerates, and the low feerate layer contains
>>>> everything else. At the edge of a block, we have a Knapsacky problem
>>>> where the next highest ancestor feerate package might not fit, so we
>>>> would probably want the high feerate layer ~2MvB or something to avoid
>>>> underestimating the fees.
>>>>
>>>> ## Acknowledgements
>>>>
>>>> Thank you to everyone whose RBF-related suggestions, grievances,
>>>> criticisms and ideas were incorporated in this document:
>>>> Andrew Chow, Matt Corallo, Suhas Daftuar, Christian Decker,
>>>> Mark Erhardt, Lloyd Fournier, Lisa Neigut, John Newbery,
>>>> Antoine Poinsot, Antoine Riard, Larry Ruane,
>>>> S3RK and Bastien Teinturier.
>>>>
>>>> Thanks for reading!
>>>>
>>>> Best,
>>>> Gloria
>>>>
>>>> [1]:
>>>> https://github.com/bitcoin/bitcoin/blob/master/doc/policy/mempool-replacements.md
>>>> [2]:
>>>> https://github.com/bitcoin/bitcoin/pull/23121#issuecomment-929475999
>>>> [3]:
>>>> https://github.com/Xekyo/bitcoin/commit/d754b0242ec69d42c570418aebf9c1335af0b8ea
>>>> [4]: https://github.com/bitcoindevkit/bdk/issues/144
>>>> [5]: https://github.com/bitcoindevkit/bdk/issues/414
>>>> [6]:
>>>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-September/019464.html
>>>> [7]:
>>>> https://gist.github.com/glozow/dc4e9d5c5b14ade7cdfac40f43adb18a#new-unconfirmed-inputs-rule-2
>>>> [8]:
>>>> https://github.com/bitcoin/bitcoin/pull/23121#discussion_r777131366
>>>> [9]:
>>>> https://github.com/bitcoin/bitcoin/pull/22290#issuecomment-865887922
>>>> [10]:
>>>> https://gist.github.com/Xekyo/5cb413fe9f26dbce57abfd344ebbfaf2#file-candidate-set-based-block-building-md
>>>> [11]:
>>>> https://github.com/bitcoin/bitcoin/pull/22539#issuecomment-885763670
>>>> [12]: https://github.com/bitcoin/bitcoin/pull/24007
>>>> [13]:
>>>> https://github.com/bitcoin/bitcoin/blob/1a369f006fd0bec373b95001ed84b480e852f191/src/wallet/feebumper.cpp#L114
>>>> [14]:
>>>> https://github.com/bitcoin/bitcoin/blob/cf5bb048e80d4cde8828787b266b7f5f2e3b6d7b/src/node/miner.cpp#L310-L320
>>>> _______________________________________________
>>>> bitcoin-dev mailing list
>>>> bitcoin-dev at lists.linuxfoundation.org
>>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>>
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev at lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>
>> _______________________________________________
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>
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📅 Original date posted:2021-09-23
📝 Original message:Hi Antoine,
Thanks as always for your input. I'm glad we agree on so much!
In summary, it seems that the decisions that might still need
attention/input from devs on this mailing list are:
1. Whether we should start with multiple-parent-1-child or 1-parent-1-child.
2. Whether it's ok to require that the child not have conflicts with
mempool transactions.
Responding to your comments...
> IIUC, you have package A+B, during the dedup phase early in
`AcceptMultipleTransactions` if you observe same-txid-different-wtixd A'
and A' is higher feerate than A, you trim A and replace by A' ?
> I think this approach is safe, the one who appears unsafe to me is when
A' has a _lower_ feerate, even if A' is already accepted by our mempool ?
In that case iirc that would be a pinning.
Right, the fact that we essentially always choose the first-seen witness is
an unfortunate limitation that exists already. Adding package mempool
accept doesn't worsen this, but the procedure in the future is to replace
the witness when it makes sense economically. We can also add logic to
allow package feerate to pay for witness replacements as well. This is
pretty far into the future, though.
> It sounds uneconomical for an attacker but I think it's not when you
consider than you can "batch" attack against multiple honest
counterparties. E.g, Mallory broadcast A' + B' + C' + D' where A' conflicts
with Alice's honest package P1, B' conflicts with Bob's honest package P2,
C' conflicts with Caroll's honest package P3. And D' is a high-fee child of
A' + B' + C'.
> If D' is higher-fee than P1 or P2 or P3 but inferior to the sum of HTLCs
confirmed by P1+P2+P3, I think it's lucrative for the attacker ?
I could be misunderstanding, but an attacker wouldn't be able to
batch-attack like this. Alice's package only conflicts with A' + D', not A'
+ B' + C' + D'. She only needs to pay for evicting 2 transactions.
> Do we assume that broadcasted packages are "honest" by default and that
the parent(s) always need the child to pass the fee checks, that way saving
the processing of individual transactions which are expected to fail in 99%
of cases or more ad hoc composition of packages at relay ?
> I think this point is quite dependent on the p2p packages format/logic
we'll end up on and that we should feel free to revisit it later ?
I think it's the opposite; there's no way for us to assume that p2p
packages will be "honest." I'd like to have two things before we expose on
P2P: (1) ensure that the amount of resources potentially allocated for
package validation isn't disproportionately higher than that of single
transaction validation and (2) only use package validation when we're
unsatisifed with the single validation result, e.g. we might get better
fees.
Yes, let's revisit this later :)
> Yes, if you receive A+B, and A is already in-mempoo, I agree you can
discard its feerate as B should pay for all fees checked on its own. Where
I'm unclear is when you have in-mempool A+B and receive A+B'. Should B'
have a fee high enough to cover the bandwidth penalty replacement
(`PaysForRBF`, 2nd check) of both A+B' or only B' ?
B' only needs to pay for itself in this case.
> > Do we want the child to be able to replace mempool transactions as well?
> If we mean when you have replaceable A+B then A'+B' try to replace with a
higher-feerate ? I think that's exactly the case we need for Lightning as
A+B is coming from Alice and A'+B' is coming from Bob :/
Let me clarify this because I can see that my wording was ambiguous, and
then please let me know if it fits Lightning's needs?
In my proposal, I wrote "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." What
I meant was: the package can replace mempool transactions if any of the
parents conflict with mempool transactions. The child cannot not conflict
with any mempool transactions.
The Lightning use case this attempts to address is: Alice and Mallory are
LN counterparties, and have packages A+B and A'+B', respectively. A and A'
are their commitment transactions and conflict with each other; they have
shared inputs and different txids.
B spends Alice's anchor output from A. B' spends Mallory's anchor output
from A'. Thus, B and B' do not conflict with each other.
Alice can broadcast her package, A+B, to replace Mallory's package, A'+B',
since B doesn't conflict with the mempool.
Would this be ok?
> The second option, a child of A', In the LN case I think the CPFP is
attached on one's anchor output.
While it would be nice to have full RBF, malleability of the child won't
block RBF here. If we're trying to replace A', we only require that A'
signals replaceability, and don't mind if its child doesn't.
> > B has an ancestor score of 10sat/vb and D has an
> > ancestor score of ~2.9sat/vb. Since D's ancestor score is lower than
B's,
> > it fails the proposed package RBF Rule #2, so this package would be
> > rejected. Does this meet your expectations?
> Well what sounds odd to me, in my example, we fail D even if it has a
higher-fee than B. Like A+B absolute fees are 2000 sats and A+C+D absolute
fees are 4500 sats ?
Yes, A+C+D pays 2500sat more in fees, but it is also 1000vB larger. A miner
should prefer to utilize their block space more effectively.
> Is this compatible with a model where a miner prioritizes absolute fees
over ancestor score, in the case that mempools aren't full-enough to
fulfill a block ?
No, because we don't use that model.
Thanks,
Gloria
On Thu, Sep 23, 2021 at 5:29 AM Antoine Riard <antoine.riard at gmail.com>
wrote:
> > Correct, if B+C is too low feerate to be accepted, we will reject it. I
> > prefer this because it is incentive compatible: A can be mined by itself,
> > so there's no reason to prefer A+B+C instead of A.
> > As another way of looking at this, consider the case where we do accept
> > A+B+C and it sits at the "bottom" of our mempool. If our mempool reaches
> > capacity, we evict the lowest descendant feerate transactions, which are
> > B+C in this case. This gives us the same resulting mempool, with A and
> not
> > B+C.
>
> I agree here. Doing otherwise, we might evict other transactions mempool
> in `MempoolAccept::Finalize` with a higher-feerate than B+C while those
> evicted transactions are the most compelling for block construction.
>
> I thought at first missing this acceptance requirement would break a
> fee-bumping scheme like Parent-Pay-For-Child where a high-fee parent is
> attached to a child signed with SIGHASH_ANYONECANPAY but in this case the
> child fee is capturing the parent value. I can't think of other fee-bumping
> schemes potentially affected. If they do exist I would say they're wrong in
> their design assumptions.
>
> > If or when we have witness replacement, the logic is: if the individual
> > transaction is enough to replace the mempool one, the replacement will
> > happen during the preceding individual transaction acceptance, and
> > deduplication logic will work. Otherwise, we will try to deduplicate by
> > wtxid, see that we need a package witness replacement, and use the
> package
> > feerate to evaluate whether this is economically rational.
>
> IIUC, you have package A+B, during the dedup phase early in
> `AcceptMultipleTransactions` if you observe same-txid-different-wtixd A'
> and A' is higher feerate than A, you trim A and replace by A' ?
>
> I think this approach is safe, the one who appears unsafe to me is when A'
> has a _lower_ feerate, even if A' is already accepted by our mempool ? In
> that case iirc that would be a pinning.
>
> Good to see progress on witness replacement before we see usage of Taproot
> tree in the context of multi-party, where a malicious counterparty inflates
> its witness to jam a honest spending.
>
> (Note, the commit linked currently points nowhere :))
>
>
> > Please note that A may replace A' even if A' has higher fees than A
> > individually, because the proposed package RBF utilizes the fees and size
> > of the entire package. This just requires E to pay enough fees, although
> > this can be pretty high if there are also potential B' and C' competing
> > commitment transactions that we don't know about.
>
> Ah right, if the package acceptance waives `PaysMoreThanConflicts` for the
> individual check on A, the honest package should replace the pinning
> attempt. I've not fully parsed the proposed implementation yet.
>
> Though note, I think it's still unsafe for a Lightning
> multi-commitment-broadcast-as-one-package as a malicious A' might have an
> absolute fee higher than E. It sounds uneconomical for
> an attacker but I think it's not when you consider than you can "batch"
> attack against multiple honest counterparties. E.g, Mallory broadcast A' +
> B' + C' + D' where A' conflicts with Alice's honest package P1, B'
> conflicts with Bob's honest package P2, C' conflicts with Caroll's honest
> package P3. And D' is a high-fee child of A' + B' + C'.
>
> If D' is higher-fee than P1 or P2 or P3 but inferior to the sum of HTLCs
> confirmed by P1+P2+P3, I think it's lucrative for the attacker ?
>
> > So far, my understanding is that multi-parent-1-child is desired for
> > batched fee-bumping (
> > https://github.com/bitcoin/bitcoin/pull/22674#issuecomment-897951289)
> and
> > I've also seen your response which I have less context on (
> > https://github.com/bitcoin/bitcoin/pull/22674#issuecomment-900352202).
> That
> > being said, I am happy to create a new proposal for 1 parent + 1 child
> > (which would be slightly simpler) and plan for moving to
> > multi-parent-1-child later if that is preferred. I am very interested in
> > hearing feedback on that approach.
>
> I think batched fee-bumping is okay as long as you don't have
> time-sensitive outputs encumbering your commitment transactions. For the
> reasons mentioned above, I think that's unsafe.
>
> What I'm worried about is L2 developers, potentially not aware about all
> the mempool subtleties blurring the difference and always batching their
> broadcast by default.
>
> IMO, a good thing by restraining to 1-parent + 1 child, we artificially
> constraint L2 design space for now and minimize risks of unsafe usage of
> the package API :)
>
> I think that's a point where it would be relevant to have the opinion of
> more L2 devs.
>
> > I think there is a misunderstanding here - let me describe what I'm
> > proposing we'd do in this situation: we'll try individual submission for
> A,
> > see that it fails due to "insufficient fees." Then, we'll try package
> > validation for A+B and use package RBF. If A+B pays enough, it can still
> > replace A'. If A fails for a bad signature, we won't look at B or A+B.
> Does
> > this meet your expectations?
>
> Yes there was a misunderstanding, I think this approach is correct, it's
> more a question of performance. Do we assume that broadcasted packages are
> "honest" by default and that the parent(s) always need the child to pass
> the fee checks, that way saving the processing of individual transactions
> which are expected to fail in 99% of cases or more ad hoc composition of
> packages at relay ?
>
> I think this point is quite dependent on the p2p packages format/logic
> we'll end up on and that we should feel free to revisit it later ?
>
>
> > What problem are you trying to solve by the package feerate *after* dedup
> rule ?
> > My understanding is that an in-package transaction might be already in
> the mempool. Therefore, to compute a correct RBF penalty replacement, the
> vsize of this transaction could be discarded lowering the cost of package
> RBF.
>
> > I'm proposing that, when a transaction has already been submitted to
> > mempool, we would ignore both its fees and vsize when calculating package
> > feerate.
>
> Yes, if you receive A+B, and A is already in-mempoo, I agree you can
> discard its feerate as B should pay for all fees checked on its own. Where
> I'm unclear is when you have in-mempool A+B and receive A+B'. Should B'
> have a fee high enough to cover the bandwidth penalty replacement
> (`PaysForRBF`, 2nd check) of both A+B' or only B' ?
>
> If you have a second-layer like current Lightning, you might have a
> counterparty commitment to replace and should always expect to have to pay
> for parent replacement bandwidth.
>
> Where a potential discount sounds interesting is when you have an univoque
> state on the first-stage of transactions. E.g DLC's funding transaction
> which might be CPFP by any participant iirc.
>
> > Note that, if C' conflicts with C, it also conflicts with D, since D is a
> > descendant of C and would thus need to be evicted along with it.
>
> Ah once again I think it's a misunderstanding without the code under my
> eyes! If we do C' `PreChecks`, solve the conflicts provoked by it, i.e mark
> for potential eviction D and don't consider it for future conflicts in the
> rest of the package, I think D' `PreChecks` should be good ?
>
> > More generally, this example is surprising to me because I didn't think
> > packages would be used to fee-bump replaceable transactions. Do we want
> the
> > child to be able to replace mempool transactions as well?
>
> If we mean when you have replaceable A+B then A'+B' try to replace with a
> higher-feerate ? I think that's exactly the case we need for Lightning as
> A+B is coming from Alice and A'+B' is coming from Bob :/
>
> > I'm not sure what you mean? Let's say we have a package of parent A +
> child
> > B, where A is supposed to replace a mempool transaction A'. Are you
> saying
> > that counterparties are able to malleate the package child B, or a child
> of
> > A'?
>
> The second option, a child of A', In the LN case I think the CPFP is
> attached on one's anchor output.
>
> I think it's good if we assume the
> solve-conflicts-after-parent's`'PreChecks` mentioned above or fixing
> inherited signaling or full-rbf ?
>
> > Sorry, I don't understand what you mean by "preserve the package
> > integrity?" Could you elaborate?
>
> After thinking the relaxation about the "new" unconfirmed input is not
> linked to trimming but I would say more to the multi-parent support.
>
> Let's say you have A+B trying to replace C+D where B is also spending
> already in-mempool E. To succeed, you need to waive the no-new-unconfirmed
> input as D isn't spending E.
>
> So good, I think we agree on the problem description here.
>
> > I am in agreement with your calculations but unsure if we disagree on the
> > expected outcome. Yes, B has an ancestor score of 10sat/vb and D has an
> > ancestor score of ~2.9sat/vb. Since D's ancestor score is lower than B's,
> > it fails the proposed package RBF Rule #2, so this package would be
> > rejected. Does this meet your expectations?
>
> Well what sounds odd to me, in my example, we fail D even if it has a
> higher-fee than B. Like A+B absolute fees are 2000 sats and A+C+D absolute
> fees are 4500 sats ?
>
> Is this compatible with a model where a miner prioritizes absolute fees
> over ancestor score, in the case that mempools aren't full-enough to
> fulfill a block ?
>
> Let me know if I can clarify a point.
>
> Antoine
>
> Le lun. 20 sept. 2021 à 11:10, Gloria Zhao <gloriajzhao at gmail.com> a
> écrit :
>
>>
>> Hi Antoine,
>>
>> First of all, thank you for the thorough review. I appreciate your
>> insight on LN requirements.
>>
>> > IIUC, you have a package A+B+C submitted for acceptance and A is
>> already in your mempool. You trim out A from the package and then evaluate
>> B+C.
>>
>> > I think this might be an issue if A is the higher-fee element of the
>> ABC package. B+C package fees might be under the mempool min fee and will
>> be rejected, potentially breaking the acceptance expectations of the
>> package issuer ?
>>
>> Correct, if B+C is too low feerate to be accepted, we will reject it. I
>> prefer this because it is incentive compatible: A can be mined by itself,
>> so there's no reason to prefer A+B+C instead of A.
>> As another way of looking at this, consider the case where we do accept
>> A+B+C and it sits at the "bottom" of our mempool. If our mempool reaches
>> capacity, we evict the lowest descendant feerate transactions, which are
>> B+C in this case. This gives us the same resulting mempool, with A and not
>> B+C.
>>
>>
>> > Further, I think the dedup should be done on wtxid, as you might have
>> multiple valid witnesses. Though with varying vsizes and as such offering
>> different feerates.
>>
>> I agree that variations of the same package with different witnesses is a
>> case that must be handled. I consider witness replacement to be a project
>> that can be done in parallel to package mempool acceptance because being
>> able to accept packages does not worsen the problem of a
>> same-txid-different-witness "pinning" attack.
>>
>> If or when we have witness replacement, the logic is: if the individual
>> transaction is enough to replace the mempool one, the replacement will
>> happen during the preceding individual transaction acceptance, and
>> deduplication logic will work. Otherwise, we will try to deduplicate by
>> wtxid, see that we need a package witness replacement, and use the package
>> feerate to evaluate whether this is economically rational.
>>
>> See the #22290 "handle package transactions already in mempool" commit (
>> https://github.com/bitcoin/bitcoin/pull/22290/commits/fea75a2237b46cf76145242fecad7e274bfcb5ff),
>> which handles the case of same-txid-different-witness by simply using the
>> transaction in the mempool for now, with TODOs for what I just described.
>>
>>
>> > I'm not clearly understanding the accepted topologies. By "parent and
>> child to share a parent", do you mean the set of transactions A, B, C,
>> where B is spending A and C is spending A and B would be correct ?
>>
>> Yes, that is what I meant. Yes, that would a valid package under these
>> rules.
>>
>> > If yes, is there a width-limit introduced or we fallback on
>> MAX_PACKAGE_COUNT=25 ?
>>
>> No, there is no limit on connectivity other than "child with all
>> unconfirmed parents." We will enforce MAX_PACKAGE_COUNT=25 and child's
>> in-mempool + in-package ancestor limits.
>>
>>
>> > Considering the current Core's mempool acceptance rules, I think CPFP
>> batching is unsafe for LN time-sensitive closure. A malicious tx-relay
>> jamming successful on one channel commitment transaction would contamine
>> the remaining commitments sharing the same package.
>>
>> > E.g, you broadcast the package A+B+C+D+E where A,B,C,D are commitment
>> transactions and E a shared CPFP. If a malicious A' transaction has a
>> better feerate than A, the whole package acceptance will fail. Even if A'
>> confirms in the following block,
>> the propagation and confirmation of B+C+D have been delayed. This could
>> carry on a loss of funds.
>>
>> Please note that A may replace A' even if A' has higher fees than A
>> individually, because the proposed package RBF utilizes the fees and size
>> of the entire package. This just requires E to pay enough fees, although
>> this can be pretty high if there are also potential B' and C' competing
>> commitment transactions that we don't know about.
>>
>>
>> > IMHO, I'm leaning towards deploying during a first phase
>> 1-parent/1-child. I think it's the most conservative step still improving
>> second-layer safety.
>>
>> So far, my understanding is that multi-parent-1-child is desired for
>> batched fee-bumping (
>> https://github.com/bitcoin/bitcoin/pull/22674#issuecomment-897951289)
>> and I've also seen your response which I have less context on (
>> https://github.com/bitcoin/bitcoin/pull/22674#issuecomment-900352202).
>> That being said, I am happy to create a new proposal for 1 parent + 1 child
>> (which would be slightly simpler) and plan for moving to
>> multi-parent-1-child later if that is preferred. I am very interested in
>> hearing feedback on that approach.
>>
>>
>> > If A+B is submitted to replace A', where A pays 0 sats, B pays 200 sats
>> and A' pays 100 sats. If we apply the individual RBF on A, A+B acceptance
>> fails. For this reason I think the individual RBF should be bypassed and
>> only the package RBF apply ?
>>
>> I think there is a misunderstanding here - let me describe what I'm
>> proposing we'd do in this situation: we'll try individual submission for A,
>> see that it fails due to "insufficient fees." Then, we'll try package
>> validation for A+B and use package RBF. If A+B pays enough, it can still
>> replace A'. If A fails for a bad signature, we won't look at B or A+B. Does
>> this meet your expectations?
>>
>>
>> > What problem are you trying to solve by the package feerate *after*
>> dedup rule ?
>> > My understanding is that an in-package transaction might be already in
>> the mempool. Therefore, to compute a correct RBF penalty replacement, the
>> vsize of this transaction could be discarded lowering the cost of package
>> RBF.
>>
>> I'm proposing that, when a transaction has already been submitted to
>> mempool, we would ignore both its fees and vsize when calculating package
>> feerate. In example G2, we shouldn't count M1 fees after its submission to
>> mempool, since M1's fees have already been used to pay for its individual
>> bandwidth, and it shouldn't be used again to pay for P2 and P3's bandwidth.
>> We also shouldn't count its vsize, since it has already been paid for.
>>
>>
>> > I think this is a footgunish API, as if a package issuer send the
>> multiple-parent-one-child package A,B,C,D where D is the child of A,B,C.
>> Then try to broadcast the higher-feerate C'+D' package, it should be
>> rejected. So it's breaking the naive broadcaster assumption that a
>> higher-feerate/higher-fee package always replaces ?
>>
>> Note that, if C' conflicts with C, it also conflicts with D, since D is a
>> descendant of C and would thus need to be evicted along with it.
>> Implicitly, D' would not be in conflict with D.
>> More generally, this example is surprising to me because I didn't think
>> packages would be used to fee-bump replaceable transactions. Do we want the
>> child to be able to replace mempool transactions as well? This can be
>> implemented with a bit of additional logic.
>>
>> > I think this is unsafe for L2s if counterparties have malleability of
>> the child transaction. They can block your package replacement by
>> opting-out from RBF signaling. IIRC, LN's "anchor output" presents such an
>> ability.
>>
>> I'm not sure what you mean? Let's say we have a package of parent A +
>> child B, where A is supposed to replace a mempool transaction A'. Are you
>> saying that counterparties are able to malleate the package child B, or a
>> child of A'? If they can malleate a child of A', that shouldn't matter as
>> long as A' is signaling replacement. This would be handled identically with
>> full RBF and what Core currently implements.
>>
>> > I think this is an issue brought by the trimming during the dedup
>> phase. If we preserve the package integrity, only re-using the tx-level
>> checks results of already in-mempool transactions to gain in CPU time we
>> won't have this issue. Package childs can add unconfirmed inputs as long as
>> they're in-package, the bip125 rule2 is only evaluated against parents ?
>>
>> Sorry, I don't understand what you mean by "preserve the package
>> integrity?" Could you elaborate?
>>
>> > Let's say you have in-mempool A, B where A pays 10 sat/vb for 100
>> vbytes and B pays 10 sat/vb for 100 vbytes. You have the candidate
>> replacement D spending both A and C where D pays 15sat/vb for 100 vbytes
>> and C pays 1 sat/vb for 1000 vbytes.
>>
>> > Package A + B ancestor score is 10 sat/vb.
>>
>> > D has a higher feerate/absolute fee than B.
>>
>> > Package A + C + D ancestor score is ~ 3 sat/vb ((A's 1000 sats + C's
>> 1000 sats + D's 1500 sats) / A's 100 vb + C's 1000 vb + D's 100 vb)
>>
>> I am in agreement with your calculations but unsure if we disagree on the
>> expected outcome. Yes, B has an ancestor score of 10sat/vb and D has an
>> ancestor score of ~2.9sat/vb. Since D's ancestor score is lower than B's,
>> it fails the proposed package RBF Rule #2, so this package would be
>> rejected. Does this meet your expectations?
>>
>> Thank you for linking to projects that might be interested in package
>> relay :)
>>
>> Thanks,
>> Gloria
>>
>> On Mon, Sep 20, 2021 at 12:16 AM Antoine Riard <antoine.riard at gmail.com>
>> wrote:
>>
>>> Hi Gloria,
>>>
>>> > 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.
>>>
>>> IIUC, you have a package A+B+C submitted for acceptance and A is already
>>> in your mempool. You trim out A from the package and then evaluate B+C.
>>>
>>> I think this might be an issue if A is the higher-fee element of the ABC
>>> package. B+C package fees might be under the mempool min fee and will be
>>> rejected, potentially breaking the acceptance expectations of the package
>>> issuer ?
>>>
>>> Further, I think the dedup should be done on wtxid, as you might have
>>> multiple valid witnesses. Though with varying vsizes and as such offering
>>> different feerates.
>>>
>>> E.g you're going to evaluate the package A+B and A' is already in your
>>> mempool with a bigger valid witness. You trim A based on txid, then you
>>> evaluate A'+B, which fails the fee checks. However, evaluating A+B would
>>> have been a success.
>>>
>>> AFAICT, the dedup rationale would be to save on CPU time/IO disk, to
>>> avoid repeated signatures verification and parent UTXOs fetches ? Can we
>>> achieve the same goal by bypassing tx-level checks for already-in txn while
>>> conserving the package integrity for package-level checks ?
>>>
>>> > 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.
>>>
>>> I'm not clearly understanding the accepted topologies. By "parent and
>>> child to share a parent", do you mean the set of transactions A, B, C,
>>> where B is spending A and C is spending A and B would be correct ?
>>>
>>> If yes, is there a width-limit introduced or we fallback on
>>> MAX_PACKAGE_COUNT=25 ?
>>>
>>> IIRC, one rationale to come with this topology limitation was to lower
>>> the DoS risks when potentially deploying p2p packages.
>>>
>>> Considering the current Core's mempool acceptance rules, I think CPFP
>>> batching is unsafe for LN time-sensitive closure. A malicious tx-relay
>>> jamming successful on one channel commitment transaction would contamine
>>> the remaining commitments sharing the same package.
>>>
>>> E.g, you broadcast the package A+B+C+D+E where A,B,C,D are commitment
>>> transactions and E a shared CPFP. If a malicious A' transaction has a
>>> better feerate than A, the whole package acceptance will fail. Even if A'
>>> confirms in the following block,
>>> the propagation and confirmation of B+C+D have been delayed. This could
>>> carry on a loss of funds.
>>>
>>> That said, if you're broadcasting commitment transactions without
>>> time-sensitive HTLC outputs, I think the batching is effectively a fee
>>> saving as you don't have to duplicate the CPFP.
>>>
>>> IMHO, I'm leaning towards deploying during a first phase
>>> 1-parent/1-child. I think it's the most conservative step still improving
>>> second-layer safety.
>>>
>>> > *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.
>>>
>>> I'm unsure about the logical order of the checks proposed.
>>>
>>> If A+B is submitted to replace A', where A pays 0 sats, B pays 200 sats
>>> and A' pays 100 sats. If we apply the individual RBF on A, A+B acceptance
>>> fails. For this reason I think the individual RBF should be bypassed and
>>> only the package RBF apply ?
>>>
>>> Note this situation is plausible, with current LN design, your
>>> counterparty can have a commitment transaction with a better fee just by
>>> selecting a higher `dust_limit_satoshis` than yours.
>>>
>>> > 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.
>>>
>>> What problem are you trying to solve by the package feerate *after*
>>> dedup rule ?
>>>
>>> My understanding is that an in-package transaction might be already in
>>> the mempool. Therefore, to compute a correct RBF penalty replacement, the
>>> vsize of this transaction could be discarded lowering the cost of package
>>> RBF.
>>>
>>> If we keep a "safe" dedup mechanism (see my point above), I think this
>>> discount is justified, as the validation cost of node operators is paid for
>>> ?
>>>
>>> > The child cannot replace mempool transactions.
>>>
>>> Let's say you issue package A+B, then package C+B', where B' is a child
>>> of both A and C. This rule fails the acceptance of C+B' ?
>>>
>>> I think this is a footgunish API, as if a package issuer send the
>>> multiple-parent-one-child package A,B,C,D where D is the child of A,B,C.
>>> Then try to broadcast the higher-feerate C'+D' package, it should be
>>> rejected. So it's breaking the naive broadcaster assumption that a
>>> higher-feerate/higher-fee package always replaces ? And it might be unsafe
>>> in protocols where states are symmetric. E.g a malicious counterparty
>>> broadcasts first S+A, then you honestly broadcast S+B, where B pays better
>>> fees.
>>>
>>> > All mempool transactions to be replaced must signal replaceability.
>>>
>>> I think this is unsafe for L2s if counterparties have malleability of
>>> the child transaction. They can block your package replacement by
>>> opting-out from RBF signaling. IIRC, LN's "anchor output" presents such an
>>> ability.
>>>
>>> I think it's better to either fix inherited signaling or move towards
>>> full-rbf.
>>>
>>> > if a package parent has already been submitted, it would
>>> > look
>>> >like the child is spending a "new" unconfirmed input.
>>>
>>> I think this is an issue brought by the trimming during the dedup phase.
>>> If we preserve the package integrity, only re-using the tx-level checks
>>> results of already in-mempool transactions to gain in CPU time we won't
>>> have this issue. Package childs can add unconfirmed inputs as long as
>>> they're in-package, the bip125 rule2 is only evaluated against parents ?
>>>
>>> > 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.
>>>
>>> I'm not sure if this holds...
>>>
>>> Let's say you have in-mempool A, B where A pays 10 sat/vb for 100 vbytes
>>> and B pays 10 sat/vb for 100 vbytes. You have the candidate replacement D
>>> spending both A and C where D pays 15sat/vb for 100 vbytes and C pays 1
>>> sat/vb for 1000 vbytes.
>>>
>>> Package A + B ancestor score is 10 sat/vb.
>>>
>>> D has a higher feerate/absolute fee than B.
>>>
>>> Package A + C + D ancestor score is ~ 3 sat/vb ((A's 1000 sats + C's
>>> 1000 sats + D's 1500 sats) /
>>> A's 100 vb + C's 1000 vb + D's 100 vb)
>>>
>>> Overall, this is a review through the lenses of LN requirements. I think
>>> other L2 protocols/applications
>>> could be candidates to using package accept/relay such as:
>>> * https://github.com/lightninglabs/pool
>>> * https://github.com/discreetlogcontracts/dlcspecs
>>> * https://github.com/bitcoin-teleport/teleport-transactions/
>>> * https://github.com/sapio-lang/sapio
>>> *
>>> https://github.com/commerceblock/mercury/blob/master/doc/statechains.md
>>> * https://github.com/revault/practical-revault
>>>
>>> Thanks for rolling forward the ball on this subject.
>>>
>>> Antoine
>>>
>>> Le jeu. 16 sept. 2021 à 03:55, Gloria Zhao via bitcoin-dev <
>>> bitcoin-dev at 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 at lists.linuxfoundation.org
>>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>>
>>>
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