npub17f…ntr0s

📅 Original date posted:2022-09-29
📝 Original message:Hi Gloria, Greg,
> I interpret most of the discussion around limitations as ideas for
> future improvements rather than criticisms of the proposal
As far as I'm concerned, definitely!
My current understanding is that the main change/improvement that would
make sense here is restricting the whole v3 package's size (instead of
just the child) via committing to a specific value in the taproot annex
(also note that it's probably not just the v3 package's size, it should
be the whole unconfirmed package including potential v2 unconfirmed
ancestors).
While I think this would be very valuable and would like to see this
happen, I believe that can be done in a second, separate step since this
would make relay policy stricter (some v3 transactions that previously
propagated wouldn't propagate under this new rule). As long as you are
able to find a path to miners through upgraded peers that use this annex
approach, you should be able to resolve ACP pinning issues?
I'm curious to know how other people feel about that: is it ok to do
later or should we try to implement this for the first release of v3
transactions?
The other change mentioned (making OP_TRUE standard and allowing outputs
that are below dust) can be added later, as those won't be standard until
we start allowing them, so there shouldn't be any backwards-compatibility
issue with postponing this change. But maybe it's still worth having from
the get-go, even though it may take a bit more time? Again, I'm curious to
have other people's opinion here, I'd be happy to get all of those directly
in the first release of v3 transactions, but I don't know how much
implementation will have to go into that.
> For clarification, package RBF is ParentTx*s*(plural), and
ChildTx(singular),
> so it might be a bit more complicated than we're thinking
Right, good catch, this does require new logic to handle this case.
As Gloria points out, this should be doable, and is definitely worth
adding (those CSV 1 on every other output are really hacky, glad to
find a way to get rid of them).
Thanks,
Bastien
Le lun. 26 sept. 2022 à 18:48, Gloria Zhao <gloriajzhao at gmail.com> a écrit :
> Hi Greg, Antoine, Bastien,
>
> Thanks very much for the feedback! I interpret most of the discussion
> around limitations as ideas for future improvements rather than criticisms
> of the proposal (please correct me if I'm wrong). I'll try to respond to as
> much as possible.
>
> Also I realize that I didn't contextualize this proposal clearly enough;
> it is very tailored for LN Penalty and definitely doesn't close all pinning
> attacks possible (sorry for confusing anyone). I also agree that some bits
> can be a little ugly or tack-on; I would definitely prefer a comprehensive
> RBF revamp to fix all our problems and enable other fee-bumping strategies
> such as
> sign-ANYONECANPAY-then-bring-your-own-fees-by-adding-inputs-at-broadcast. I
> was hoping to get some ideas with the "RBF Improvements" post in January,
> but it doesn't seem like we're much closer to a workable proposal. I think
> this is a minimally-invasive step that works for Lightning today, a small
> fix similar to CPFP carve out.
>
> > As you likely know from previous discussions the biggest scenario this
> does not fix in my estimation is ANYONECANPAY situations. If the parent
> transaction can be "inflated" by tacking on additional inputs, this means
> the total weight of the parent tx lowers the effective feerate of the
> package.
>
> (For more context to other readers I wrote an explanation for this in
> "SIGHASH_ANYONECANPAY Pinning" section of RBF ML post). Yes, this
> unfortunately doesn't fix any of the existing pinning attacks for single
> transaction RBF but also doesn't make them worse. This boils down to adding
> an incentive compatibility rule that ensures you can't replace a
> transaction with something that will confirm slower. Package RBF has an
> ancestor feerate-based rule for this (note it is quite conservative and not
> perfect).
>
> So in the scenario above with the "inflated" parent that was signed ACP,
> the replacement would be rejected because the package ancestor feerate is
> lower than the feerate of what is being replaced. But it is imperfect
> (explained below) and thus I wouldn't recommend it for single transaction
> replacement. So that attack still exists for single transactions, yes.
>
> The strategy of using ACP to bring-your-own-fees has its own challenges
> but hopefully has no current use cases as you say. AFAIK LN Penalty is not
> affected by this since it doesn't use ACP, though obviously I agree we
> should fix it for the future.
>
> So when I said "this is intended for fee-bumping presigned txns in
> contracting protocols," I should have said "this is intended for
> fee-bumping presigned txns specifically using CPFP and anchor outputs."
> Apologies for forgetting to contextualize, I've been sitting on this for
> too long.
>
> > The other scenario it doesn't really fix is where HTLC/commitment-like
> transactions are being resolved in a batch, but due to relative time
> constraints, you may want to accelerate some and not others. Now you must
> pay higher rates to replace all of the transaction bumps. This is a
> "self-pin" and "get good at utxos noob" type problem, but it's something
> that axing rule#3 in favor of a Replace-by-ancestor-feerate system would
> get us.
>
> I understand you to mean "if you don't have enough UTXOs and you're forced
> to batch-bump, you over-pay because you need to bring them all to the
> highest target feerate." Isn't this kind of separate, wallet-related
> problem? Contracting or not, surely every wallet needs to have enough UTXOs
> to not batch transactions that shouldn't be batched... I don't see how a
> replace-by-ancestor-feerate policy would make any difference for this?
>
> Also in general I'd like to reiterate that ancestor feerate is not a
> panacea to all our RBF incentive compatibility concerns. Like individual
> feerate, unless we run the mining algorithm, it cannot tell us exactly how
> quickly this transaction would be mined.
>
> We're estimating the incentive compatibility of the original
> transaction(s) and replacement transaction(s), with the goal of not letting
> a transaction replace something that would have been more incentive
> compatible to mine. As such, we don't want to overestimate how good the
> replacement is, and we don't want to underestimate how good the original
> transactions are. This rule "The minimum between package feerate and
> ancestor feerate of the child is not lower than the individual feerates of
> all directly conflicting transactions and the ancestor feerates of all
> original transactions" is a conservative estimate.
>
> > Would kind of be nice if package RBF would detect a "sibling output
> spend" conflict, and knock it out of the mempool via the other replacement
> rules? Getting rid of the requirement to 1 block csv lock every output
> would be quite nice from a smart contracting composability point of view.
>
> Interesting, so when a transaction hits a mempool tx's descendant limit,
> we consider evicting one of its descendants in favor of this transaction,
> based on the RBF rules.
> Cool idea! After chewing on this for a bit, I think this *also* just boils
> down to the fact that RBF should require replacements to be better mining
> candidates. As in, if we added this policy and it can make us evict the
> sibling and accept a transaction with a bunch of low-feerate ancestor junk,
> it would be a new pinning vector.
>
> > If you're a miner and you receive a non-V3, second descendant of an
> unconfirmed V3 transaction, if the offered fee is in the top mempool
> backlog, I think you would have an interest to accept such a transaction.
>
> > So I'm not sure if those two rules are compatible with miners
> incentives...
>
> The same argument can be made for the 26th descendant of a mempool
> transaction; it's also not entirely incentive-compatible to reject it, but
> that is not the *only* design goal in mempool policy. Of course, the
> difference here is that the 25-descendant limit rule is a sensible DoS
> protection, while this 1-descendant limit rule is more of a "help the
> Bitcoin ecosystem" policy, just like CPFP carve-out, dust limit, etc. I can
> of course understand why not everyone would be in favor of this, but I do
> think it's worth it.
>
> > > 4. A V3 transaction that has an unconfirmed V3 ancestor cannot be
> > > larger than 1000 virtual bytes.
>
> > If I understand correctly the 1000 vb upper bound rational, it would be
> to constraint the pinning counterparty to attach a high fee to a child due
> to the limited size, if they would like this transaction to be stuck in the
> network mempools. By doing so this child has high odds to confirm.
>
> Yeah exactly, the "Rule 3 pin" is done by adding a child that's high-fee
> (so you have to pay that much to evict it). Because they *don't* want this
> tx to confirm, normally, this child would be really large. If they only
> have 1000vB for the child, they can't increase the replacement cost without
> also fee-bumping the transaction to make it confirm faster.
>
> > As of today, I think yes you can already fingerprint LN transactions on
> the spec-defined amount value of the anchor outputs, 330 sats. There is
> always one of them on post-anchor commitment transactions. And sadly I
> would say we'll always have tricky fingerprints leaking from unilateral LN
> closures such as HTLC/PTLC timelocks...
>
> > I agree with you, this isn't worse than today, unilateral closes will
> probably always be identifiable on-chain.
>
> Great to hear that there is no privacy worsening!
>
> Best,
> Gloria
>
> On Mon, Sep 26, 2022 at 5:02 PM Greg Sanders <gsanders87 at gmail.com> wrote:
>
>> Bastien,
>>
>> > This may be already covered by the current package RBF logic, in that
>> scenario we are simply replacing [ParentTx, ChildTx1] with
>> [ParentTx, ChildTx2] that pays more fees, right?
>>
>> For clarification, package RBF is ParentTx*s*(plural), and
>> ChildTx(singular), so it might be a bit more complicated than we're
>> thinking, and currently the V3 proposal would first de-duplicate the
>> ParentTx based on what is in the mempool, then look at the "rest" of the
>> transactions as a package, then individually. Not the same, not sure how
>> different. I'll defer to experts.
>>
>> Best,
>> Greg
>>
>> On Mon, Sep 26, 2022 at 11:48 AM Bastien TEINTURIER via bitcoin-dev <
>> bitcoin-dev at lists.linuxfoundation.org> wrote:
>>
>>> Thanks Gloria for this great post.
>>>
>>> This is very valuable work for L2 contracts, and will greatly improve
>>> their security model.
>>>
>>> > "Only 1 anchor output? What if I need to bump counterparty's
>>> commitment tx in mempool?"
>>> > You won't need to fee-bump a counterparty's commitment tx using CPFP.
>>> > You would just package RBF it by attaching a high-feerate child to
>>> > your commitment tx.
>>>
>>> Note that we can also very easily make that single anchor spendable by
>>> both participants (or even anyone), so if you see your counterparty's
>>> commitment in your mempool, you can bump it without publishing your
>>> own commitment, which is quite desirable (your own commitment tx has
>>> CSV delays on your outputs, whereas your counterparty's commitment tx
>>> doesn't).
>>>
>>> > "Is this a privacy issue, i.e. doesn't it allow fingerprinting LN
>>> transactions based on nVersion?"
>>>
>>> I agree with you, this isn't worse than today, unilateral closes will
>>> probably always be identifiable on-chain.
>>>
>>> > Would kind of be nice if package RBF would detect a "sibling output
>>> spend"
>>> > conflict, and knock it out of the mempool via the other replacement
>>> rules?
>>> > Getting rid of the requirement to 1 block csv lock every output would
>>> be
>>> > quite nice from a smart contracting composability point of view.
>>>
>>> +1, that would be very neat!
>>>
>>> This may be already covered by the current package RBF logic, in that
>>> scenario we are simply replacing [ParentTx, ChildTx1] with
>>> [ParentTx, ChildTx2] that pays more fees, right?
>>>
>>> > 1) I do think that we should seriously consider allowing OP_TRUE to
>>> become
>>> > a standard script type as part of this policy update. If pinning is
>>> solved,
>>> > then there's no reason to require all those extra bytes for "binding"
>>> an
>>> > anchor to a specific wallet/user. We can save quite a few bytes by
>>> having
>>> > the input be empty of witness data.
>>> > 2) If we allow for a single dust-value(0 on up) output which is
>>> immediately
>>> > spent by the package, anchors become even easier to to design. No
>>> value has
>>> > to be "sapped" from contract participants to make an anchor output.
>>> There's
>>> > more complications for this, such as making sure the parent
>>> transaction is
>>> > dropped if the child spend is dropped, but maybe it's worth the
>>> squeeze.
>>>
>>> I also think both of these could be quite useful. This would probably
>>> always
>>> be used in combination with a parent transaction that pays 0 fees, so the
>>> 0-value output would always be spent in the same block.
>>>
>>> But this means we could end up with 0-value outputs in the utxo set, if
>>> for
>>> some reason the parent tx is CPFP-ed via another output than the 0-value
>>> one,
>>> which would be a utxo set bloat issue. But I'd argue that we're probably
>>> already creating utxo set bloat with the 330 sat anchor outputs
>>> (especially
>>> since we use two of them, but only one is usually spent), so it would
>>> probably be *better* than what we're doing today.
>>>
>>> Thanks,
>>> Bastien
>>>
>>> Le lun. 26 sept. 2022 à 03:22, Antoine Riard via bitcoin-dev <
>>> bitcoin-dev at lists.linuxfoundation.org> a écrit :
>>>
>>>> Hi Gloria,
>>>>
>>>> Thanks for the progress on package RBF, few early questions.
>>>>
>>>> > 2. Any descendant of an unconfirmed V3 transaction must also be V3.
>>>>
>>>> > 3. An unconfirmed V3 transaction cannot have more than 1 descendant.
>>>>
>>>> If you're a miner and you receive a non-V3, second descendant of an
>>>> unconfirmed V3 transaction, if the offered fee is in the top mempool
>>>> backlog, I think you would have an interest to accept such a transaction.
>>>>
>>>> So I'm not sure if those two rules are compatible with miners
>>>> incentives...
>>>>
>>>> > 4. A V3 transaction that has an unconfirmed V3 ancestor cannot be
>>>> > larger than 1000 virtual bytes.
>>>>
>>>> If I understand correctly the 1000 vb upper bound rational, it would be
>>>> to constraint the pinning counterparty to attach a high fee to a child due
>>>> to the limited size, if they would like this transaction to be stuck in the
>>>> network mempools. By doing so this child has high odds to confirm.
>>>>
>>>> I still wonder if this compatible with miner incentives in period of
>>>> empty mempools, in the sense that if you've already a V3 transaction of
>>>> size 100Kvb offering 2 sat/vb, it's more interesting than a V3 replacement
>>>> candidate of size 1000 vb offering 10 sat/vb. It could be argued the former
>>>> should be conserved.
>>>>
>>>> (That said, the hard thing with any replacement strategy we might evict
>>>> a parent transaction *now* to which is attached a high-feerate child
>>>> *latter* making for a utxo considered the best ancestor set. Maybe in the
>>>> long-term miners should keep every transaction ever accepted...)
>>>>
>>>> > (Lower bound) the smaller this limit, the fewer UTXOs a child may use
>>>> > to fund this fee-bump. For example, only allowing the V3 child to have
>>>> > 2 inputs would require L2 protocols to manage a wallet with high-value
>>>> > UTXOs and make batched fee-bumping impossible. However, as the
>>>> > fee-bumping child only needs to fund fees (as opposed to payments),
>>>> > just a few UTXOs should suffice.
>>>>
>>>> Reminder for L2 devs, batched fee-bumping of time-sensitive
>>>> confirmations of commitment transactions is unsafe, as the counterparty
>>>> could enter in a "cat-and-mouse" game to replace one of the batch element
>>>> at each block to delay confirmation of the remaining elements in the batch,
>>>> I think.
>>>>
>>>> On the other hand, I wonder if we wouldn't want a higher bound. LN
>>>> wallets are likely to have one big UTXO in their fee-bumping reserve pool,
>>>> as the cost of acquiring UTXO is non-null and in the optimistic case, you
>>>> don't need to do unilateral closure. Let's say you close dozens of channels
>>>> at the same time, a UTXO pool management strategy might be to fan-out the
>>>> first spends UTXOs in N fan-out outputs ready to feed the remaining
>>>> in-flight channels.
>>>>
>>>> > 1. The rule around unconfirmed inputs was
>>>> > originally "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."
>>>>
>>>> Note, I think we would like this new RBF rule to also apply to single
>>>> transaction package, e.g second-stage HTLC transactions, where a
>>>> counterparty pins a HTLC-preimage by abusing rule 3. In that case, the
>>>> honest LN node should be able to broadcast a "at least as high ancestor
>>>> feerate" HTLC-timeout transaction. With `option_anchor_outputs" there is no
>>>> unconfirmed ancestor to replace, as the commitment transaction, whatever
>>>> the party it is originating from, should already be confirmed.
>>>>
>>>> > "Is this a privacy issue, i.e. doesn't it allow fingerprinting LN
>>>> transactions based on nVersion?"
>>>>
>>>> As of today, I think yes you can already fingerprint LN transactions on
>>>> the spec-defined amount value of the anchor outputs, 330 sats. There is
>>>> always one of them on post-anchor commitment transactions. And sadly I
>>>> would say we'll always have tricky fingerprints leaking from unilateral LN
>>>> closures such as HTLC/PTLC timelocks...
>>>>
>>>> > "Can a V2 transaction replace a V3 transaction and vice versa?"
>>>>
>>>> IIUC, a V3 package could replace a V2 package, with the benefit of the
>>>> new package RBF rules applied. I think this would be a significant
>>>> advantage for LN, as for the current ~85k of opened channels, the old V2
>>>> states shouldn't be pinning vectors. Currently, commitment transactions
>>>> signal replaceability.
>>>>
>>>> Le ven. 23 sept. 2022 à 11:26, Gloria Zhao via bitcoin-dev <
>>>> bitcoin-dev at lists.linuxfoundation.org> a écrit :
>>>>
>>>>> Hi everyone,
>>>>>
>>>>> I'm writing to propose a very simple set of mempool/transaction relay
>>>>> policies intended to aid L2/contract protocols. I realized that
>>>>> the previously proposed Package Mempool Accept package RBF [1]
>>>>> had a few remaining problems after digging into the RBF logic more [2].
>>>>> This additional set of policies solves them without requiring a huge
>>>>> RBF overhaul.
>>>>>
>>>>> I've written an implementation (and docs) for Bitcoin Core:
>>>>> https://github.com/bitcoin/bitcoin/pull/25038
>>>>>
>>>>> (You may notice that this proposal incorporates feedback on the PR -
>>>>> thanks Suhas Daftuar, Gregory Sanders, Bastien Teinturier, Anthony Towns,
>>>>> and others.)
>>>>>
>>>>> If you are interested in using package RBF/relay to bump presigned
>>>>> transactions, I think you may be interested in reviewing this proposal.
>>>>> This should solve Rule 3 pinning and perhaps allow us
>>>>> to get rid of CPFP carve-out (yay!). I'm keen to hear if people find
>>>>> the 1-anchor-output, 1000vB child limit too restrictive. Also, if you
>>>>> find a
>>>>> pinning attack or something that makes it unusable for you, I would
>>>>> really really like to know.
>>>>>
>>>>> Note that transactions with nVersion=3 ("V3 transactions") are
>>>>> currently non-standard in Bitcoin Core. That means **anything that was
>>>>> standard before this policy change would still be standard
>>>>> afterwards.** If you don't want your transactions to be subject to
>>>>> these rules, just continue whatever you're doing and don't use
>>>>> nVersion=3. AFAICT this shouldn't break anything, but let me know if
>>>>> this would be disruptive for you?
>>>>>
>>>>> **New Policies:**
>>>>>
>>>>> This includes:
>>>>> - a set of additional policy rules applying to V3 transactions
>>>>> - modifications to package RBF rules
>>>>>
>>>>> **V3 transactions:**
>>>>>
>>>>> Existing standardness rules apply to V3 (e.g. min/max tx weight,
>>>>> standard output types, cleanstack, etc.). The following additional
>>>>> rules apply to V3:
>>>>>
>>>>> 1. A V3 transaction can be replaced, even if it does not signal BIP125
>>>>> replaceability. (It must also meet the other RBF rules around fees,
>>>>> etc. for replacement to happen).
>>>>>
>>>>> 2. Any descendant of an unconfirmed V3 transaction must also be V3.
>>>>>
>>>>> *Rationale*: Combined with Rule 1, this gives us the property of
>>>>> "inherited" replaceability signaling when descendants of unconfirmed
>>>>> transactions are created. Additionally, checking whether a transaction
>>>>> signals replaceability this way does not require mempool traversal,
>>>>> and does not change based on what transactions are mined. It also
>>>>> makes subsequent rules about descendant limits much easier to check.
>>>>>
>>>>> *Note*: The descendant of a *confirmed* V3 transaction does not need
>>>>> to be V3.
>>>>>
>>>>> 3. An unconfirmed V3 transaction cannot have more than 1 descendant.
>>>>>
>>>>> *Rationale*: (Upper bound) the larger the descendant limit, the more
>>>>> transactions may need to be replaced. This is a problematic pinning
>>>>> attack, i.e., a malicious counterparty prevents the transaction from
>>>>> being replaced by adding many descendant transactions that aren't
>>>>> fee-bumping.
>>>>>
>>>>> (Lower bound) at least 1 descendant is required to allow CPFP of the
>>>>> presigned transaction. The contract protocol can create presigned
>>>>> transactions paying 0 fees and 1 output for attaching a CPFP at
>>>>> broadcast time ("anchor output"). Without package RBF, multiple anchor
>>>>> outputs would be required to allow each counterparty to fee-bump any
>>>>> presigned transaction. With package RBF, since the presigned
>>>>> transactions can replace each other, 1 anchor output is sufficient.
>>>>>
>>>>> 4. A V3 transaction that has an unconfirmed V3 ancestor cannot be
>>>>> larger than 1000 virtual bytes.
>>>>>
>>>>> *Rationale*: (Upper bound) the larger the descendant size limit, the
>>>>> more vbytes may need to be replaced. With default limits, if the child
>>>>> is e.g. 100,000vB, that might be an additional 100,000sats (at
>>>>> 1sat/vbyte) or more, depending on the feerate.
>>>>>
>>>>> (Lower bound) the smaller this limit, the fewer UTXOs a child may use
>>>>> to fund this fee-bump. For example, only allowing the V3 child to have
>>>>> 2 inputs would require L2 protocols to manage a wallet with high-value
>>>>> UTXOs and make batched fee-bumping impossible. However, as the
>>>>> fee-bumping child only needs to fund fees (as opposed to payments),
>>>>> just a few UTXOs should suffice.
>>>>>
>>>>> With a limit of 1000 virtual bytes, depending on the output types, the
>>>>> child can have 6-15 UTXOs, which should be enough to fund a fee-bump
>>>>> without requiring a carefully-managed UTXO pool. With 1000 virtual
>>>>> bytes as the descendant limit, the cost to replace a V3 transaction
>>>>> has much lower variance.
>>>>>
>>>>> *Rationale*: This makes the rule very easily "tacked on" to existing
>>>>> logic for policy and wallets. A transaction may be up to 100KvB on its
>>>>> own (`MAX_STANDARD_TX_WEIGHT`) and 101KvB with descendants
>>>>> (`DEFAULT_DESCENDANT_SIZE_LIMIT_KVB`). If an existing V3 transaction
>>>>> in the mempool is 100KvB, its descendant can only be 1000vB, even if
>>>>> the policy is 10KvB.
>>>>>
>>>>> **Package RBF modifications:**
>>>>>
>>>>> 1. The rule around unconfirmed inputs was
>>>>> originally "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."
>>>>>
>>>>> The package may still include new unconfirmed inputs. However,
>>>>> the new rule is modified to be "The minimum between package feerate
>>>>> and ancestor feerate of the child is not lower than the individual
>>>>> feerates of all directly conflicting transactions and the ancestor
>>>>> feerates of all original transactions."
>>>>>
>>>>> *Rationale*: We are attempting to ensure that the replacement
>>>>> transactions are not less incentive-compatible to mine. However, a
>>>>> package/transaction's ancestor feerate is not perfectly representative
>>>>> of its incentive compatibility; it may overestimate (some subset of
>>>>> the ancestors could be included by itself if it has other high-feerate
>>>>> descendants or are themselves higher feerate than this
>>>>> package/transaction). Instead, we use the minimum between the package
>>>>> feerate and ancestor feerate of the child as a more conservative value
>>>>> than what was proposed originally.
>>>>>
>>>>> 2. A new rule is added, requiring that all package transactions with
>>>>> mempool conflicts to be V3. This also means the "sponsoring"
>>>>> child transaction must be V3.
>>>>>
>>>>> *Note*: Combined with the V3 rules, this means the package must be
>>>>> a child-with-parents package. Since package validation is only
>>>>> attempted if the transactions do not pay sufficient fees to be
>>>>> accepted on their own, this effectively means that only V3
>>>>> transactions can pay to replace their ancestors' conflicts, and only
>>>>> V3 transactions' replacements may be paid for by a descendant.
>>>>>
>>>>> *Rationale*: The fee-related rules are economically rational for
>>>>> ancestor packages, but not necessarily other types of packages.
>>>>> A child-with-parents package is a type of ancestor package. It
>>>>> may be fine to allow any ancestor package, but it's more difficult
>>>>> to account for all of the possibilities. For example, it gets much
>>>>> harder to see that we're applying the descendant limits correctly if
>>>>> the package has a gnarly, many-generation, non-tree shape. I'm also
>>>>> not sure if this policy is 100% incentive-compatible if the sponsor
>>>>> is not a direct descendant of the sponsee.
>>>>>
>>>>> Please see doc/policy/version3_transactions.md and
>>>>> doc/policy/packages.md in the PR for the full set of rules.
>>>>>
>>>>> **Intended usage for LN:**
>>>>>
>>>>> Commitment transactions should be V3 and have 1 anchor output. They
>>>>> can be signed with 0 fees (or 1sat/vbyte) once package relay is
>>>>> deployed
>>>>> on a significant portion of the network. If the commitment tx must
>>>>> be broadcast, determine the desired feerate at broadcast time and
>>>>> spend the anchor output in a high feerate transaction. I'm going to
>>>>> call the broadcasted commitment tx "the parent" and the attached
>>>>> fee-bumping tx "the child."
>>>>>
>>>>> - This child must be V3.
>>>>> - This child must be at most 1000vB. Note this restricts the
>>>>> number of inputs you can use to fund the fee bump. Depending
>>>>> on the output types, this is around 6-15.
>>>>> - One child may fund fees for multiple commitment tx ("batched
>>>>> fee-bumping").
>>>>> - To do a second fee-bump to add more fees, replace the
>>>>> *child* with a higher-feerate tx. Do not try to attach a grandchild.
>>>>>
>>>>> Otherwise, never try to spend from an unconfirmed V3 transaction. The
>>>>> descendant limits for V3 transactions are very restrictive.
>>>>>
>>>>> **Expected Questions:**
>>>>>
>>>>> "Does this fix Rule 3 Pinning?"
>>>>> Yes. The V3 descendant limit restricts both you and your counterparty.
>>>>> Assuming nodes adopted this policy, you may reasonably assume that you
>>>>> only need to replace the commitment transaction + up to 1000vB.
>>>>>
>>>>> "Only 1 anchor output? What if I need to bump counterparty's
>>>>> commitment tx in mempool?"
>>>>> You won't need to fee-bump a counterparty's commitment tx using CPFP.
>>>>> You would just package RBF it by attaching a high-feerate child to
>>>>> your commitment tx.
>>>>>
>>>>> "Is this a privacy issue, i.e. doesn't it allow fingerprinting LN
>>>>> transactions based on nVersion?"
>>>>> Indeed it may be unrealistic to assume V3 transactions will be in
>>>>> widespread use outside of L2. IIUC, unilateral closes are already
>>>>> obvious LN transactions because of the HTLC inputs. For e.g.
>>>>> cooperative closes and opens, I think it makes sense to continue using
>>>>> V2. So, unless I'm missing something, this shouldn't make it worse.
>>>>>
>>>>> "So a V3 transaction that doesn't signal BIP125 replaceability is
>>>>> replaceable? Is that a backward compatibility issue?"
>>>>> Yes it's replaceable. It's not an issue AFAICT because,
>>>>> under previous policy, the V3 transaction wouldn't have been
>>>>> in the mempool in the first place.
>>>>>
>>>>> "Can a V2 transaction replace a V3 transaction and vice versa?"
>>>>> Yes, otherwise someone can use V3 transactions to censor V2
>>>>> transactions spending shared inputs. Note if the
>>>>> original V3 transaction has an unconfirmed V3 parent, this would
>>>>> violate the "inherited V3" rule and would be rejected.
>>>>>
>>>>> Thanks for reading! Feedback and review would be much appreciated.
>>>>>
>>>>> [1]:
>>>>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-September/019464.html
>>>>> [2]:
>>>>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019817.html
>>>>>
>>>>> Best,
>>>>> Gloria
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📅 Original date posted:2021-09-20
📝 Original message:Hi Gloria,
Thanks for this detailed post!
The illustrations you provided are very useful for this kind of graph
topology problems.
The rules you lay out for package RBF look good to me at first glance
as there are some subtle improvements compared to BIP 125.
> 1. A package cannot exceed `MAX_PACKAGE_COUNT=25` count and
> `MAX_PACKAGE_SIZE=101KvB` total size [8]
I have a question regarding this rule, as your example 2C could be
concerning for LN (unless I didn't understand it correctly).
This also touches on the package RBF rule 5 ("The package cannot
replace more than 100 mempool transactions.")
In your example we have a parent transaction A already in the mempool
and an unrelated child B. We submit a package C + D where C spends
another of A's inputs. You're highlighting that this package may be
rejected because of the unrelated transaction(s) B.
The way I see this, an attacker can abuse this rule to ensure
transaction A stays pinned in the mempool without confirming by
broadcasting a set of child transactions that reach these limits
and pay low fees (where A would be a commit tx in LN).
We had to create the CPFP carve-out rule explicitly to work around
this limitation, and I think it would be necessary for package RBF
as well, because in such cases we do want to be able to submit a
package A + C where C pays high fees to speed up A's confirmation,
regardless of unrelated unconfirmed children of A...
We could submit only C to benefit from the existing CPFP carve-out
rule, but that wouldn't work if our local mempool doesn't have A yet,
but other remote mempools do.
Is my concern justified? Is this something that we should dig into a
bit deeper?
Thanks,
Bastien
Le jeu. 16 sept. 2021 à 09:55, Gloria Zhao via bitcoin-dev <
bitcoin-dev 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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