npub1jd…3gh0m

📅 Original date posted:2022-09-29
📝 Original message:> 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).
For the record, it turns out ephemeral anchors + v3 solves this already, as
the anchor must be spent, and the parent tx may only have one child.
Somehow I missed this implication for a few months. It's great news if we
can directly source fees from any output claimable, including HTLCs!
On Thu, Sep 29, 2022 at 5:15 AM Bastien TEINTURIER <bastien at acinq.fr> wrote:
> 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
>>>>>> _______________________________________________
>>>>>> 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
>>>>>
>>>> _______________________________________________
>>>> bitcoin-dev mailing list
>>>> bitcoin-dev at lists.linuxfoundation.org
>>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>>
>>>
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📅 Original date posted:2022-05-17
📝 Original message:Hi Gloria,
Thanks for working on this important proposal!
Still a lot to digest, but I just had on area of comment/question:
> A child-with-unconfirmed-parents package sent between nodes must abide by
the rules below, otherwise the package is malformed and the sender should
be disconnected.
> However, if the child has confirmed parents, they must not be in the
package.
If my naive understanding is correct, this means things like otherwise
common situations such as a new block will result in disconnects, say when
the sender doesn't hear about a new block which makes the relay package
superfluous/irrelevant. Similar would be disconnection
when confirmed gets turned into unconfirmed, but those situations are
extremely uncommon. The other rules are entirely under the control
of the sender, which leads me to wonder if it's appropriate.
Cheers,
Greg
On Tue, May 17, 2022 at 12:09 PM Gloria Zhao via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:
> Hi everybody,
>
> I’m writing to propose a set of p2p protocol changes to enable package
> relay, soliciting feedback on the design and approach. Here is a link
> to the most up-to-date proposal:
>
> https://github.com/bitcoin/bips/pull/1324
>
> If you have concept or approach feedback, *please respond on the
> mailing list* to allow everybody to view and participate in the
> discussion. If you find a typo or inaccurate wording, please feel free
> to leave suggestions on the PR.
>
> I’m also working on an implementation for Bitcoin Core.
>
>
> The rest of this post will include the same contents as the proposal,
> with a bit of reordering and additional context. If you are not 100%
> up-to-date on package relay and find the proposal hard to follow, I
> hope you find this format more informative and persuasive.
>
>
> ==Background and Motivation==
>
> Users may create and broadcast transactions that depend upon, i.e.
> spend outputs of, unconfirmed transactions. A “package” is the
> widely-used term for a group of transactions representable by a
> connected Directed Acyclic Graph (where a directed edge exists between
> a transaction that spends the output of another transaction).
>
> Incentive-compatible mempool and miner policies help create a fair,
> fee-based market for block space. While miners maximize transaction
> fees in order to earn higher block rewards, non-mining users
> participating in transaction relay reap many benefits from employing
> policies that result in a mempool with the same contents, including
> faster compact block relay and more accurate fee estimation.
> Additionally, users may take advantage of mempool and miner policy to
> bump the priority of their transactions by attaching high-fee
> descendants (Child Pays for Parent or CPFP). Only considering
> transactions one at a time for submission to the mempool creates a
> limitation in the node's ability to determine which transactions have
> the highest feerates, since it cannot take into account descendants
> until all the transactions are in the mempool. Similarly, it cannot
> use a transaction's descendants when considering which of two
> conflicting transactions to keep (Replace by Fee or RBF).
>
> When a user's transaction does not meet a mempool's minimum feerate
> and they cannot create a replacement transaction directly, their
> transaction will simply be rejected by this mempool. They also cannot
> attach a descendant to pay for replacing a conflicting transaction.
> This limitation harms users' ability to fee-bump their transactions.
> Further, it presents a security issue in contracting protocols which
> rely on **presigned**, time-sensitive transactions to prevent cheating
> (HTLC-Timeout in LN Penalty [1] [2] [3], Unvault Cancel in Revault
> [4], Refund Transaction in Discreet Log Contracts [5], Updates in
> eltoo [6]). In other words, a key security assumption of many
> contracting protocols is that all parties can propagate and confirm
> transactions in a timely manner.
>
> In the past few years, increasing attention [0][1][2][3][6] has been
> brought to **pinning attacks**, a type of censorship in which the
> attacker uses mempool policy restrictions to prevent a transaction
> from being relayed or getting mined. TLDR: revocation transactions
> must meet a certain confirmation target to be effective, but their
> feerates are negotiated well ahead of broadcast time. If the
> forecasted feerate was too low and no fee-bumping options are
> available, attackers can steal money from their counterparties. I walk
> through a concrete example for stealing Lightning HTLC outputs at
> ~23:58 in this talk [7][8]. Note that most attacks are only possible
> when the market for blockspace at broadcast time demands much higher
> feerates than originally anticipated at signing time. Always
> overestimating fees may sidestep this issue temporarily (while mempool
> traffic is low and predictable), but this solution is not foolproof
> and wastes users' money. The feerate market can change due to sudden
> spikes in traffic (e.g. huge 12sat/vB dump a few days ago [9]) or
> sustained, high volume of Bitcoin payments (e.g. April 2021 and
> December 2017).
>
> The best solution is to enable nodes to consider packages of
> transactions as a unit, e.g. one or more low-fee parent transactions
> with a high-fee child, instead of separately. A package-aware mempool
> policy can help determine if it would actually be economically
> rational to accept a transaction to the mempool if it doesn't meet fee
> requirements individually. Network-wide adoption of these policies
> would create a more purely-feerate-based market for block space and
> allow contracting protocols to adjust fees (and therefore mining
> priority) at broadcast time. Some support for packages has existed in
> Bitcoin Core for years. Since v0.13, Bitcoin Core has used ancestor
> packages instead of individual transactions to evaluate the incentive
> compatibility of transactions in the mempool [10] and select them for
> inclusion in blocks [11].
>
> Package Relay, the concept of {announcing, requesting, downloading}
> packages between nodes on the p2p network, has also been discussed for
> many years. The earliest public mention I can find is from 2015 [12].
> The two most common use cases for package relay are fee-bumping
> otherwise-too-low-fee transactions and reducing the amount of orphans.
> It seems uncontroversial to say that everybody desires package relay
> conceptually, with varying degrees of urgency. Lots of work has been
> done by others over the past few years, from which I've taken
> inspiration from [13][14][15][16].
>
> My approach has been to split the project into two components: (1) Package
> Mempool Accept, which includes validation logic and mempool policy.
> (3) Package Relay, which includes the p2p protocol changes.
>
> Progress so far:
> After discussions with various developers of contracting protocols
> (with heavier emphasis towards LN), it was determined that a
> package containing a child with all of its unconfirmed parents
> (child-with-unconfirmed-parents or 1-child-multi-parent package) would
> be sufficient for their use case, i.e. fee-bumping presigned
> transactions. A child-with-unconfirmed-parents package has several
> properties that make many things easier to reason about.
>
> A few months ago, I proposed a set of policies for safe package
> validation and fee assessment for packages of this restricted
> topology [17]. A series of PRs implementing this proposal have
> been merged into Bitcoin Core [18].
>
> Theoretically, developing a safe and incentive-compatible package
> mempool acceptance policy is sufficient to solve this issue. Nodes
> could opportunistically accept packages (e.g. by trying combinations
> of transactions rejected from their mempools), but this practice would
> likely be inefficient at best and open new Denial of Service attacks
> at worst. Additional p2p messages may enable nodes to request and
> share package validation-related information with one another in a
> more communication-efficient way.
>
> Given that only package RBF remains for package mempool accept, and we
> can make progress on p2p and mempool in parallel, I think it’s
> appropriate to put forward a package relay proposal.
>
> ==Proposal==
>
> This proposal contains 2 components: a “generic” package relay
> protocol and an extension of it, child-with-unconfirmed-parents
> packages, as version 1 package relay. Another version of packages,
> “tx-with-unconfirmed-ancestors” can be created to extend package relay
> for eliminating orphans.
>
> ===Generic Package Relay===
>
> Two main ideas are introduced:
>
> Download and validate packages of transactions together.
>
> Provide information to help peers decide whether to request and/or how
> to validate transactions which are part of a package.
>
> ====Intended Protocol Flow====
>
> Due to the asynchronous nature of a distributed transaction relay
> network, nodes may not receive all of the information needed to
> validate a transaction at once. For example, after a node completes
> Initial Block Download (IBD) and first starts participating in
> transaction relay with an empty mempool, it is common to receive
> orphans. In such scenarios where a node is aware that it is missing
> information, a ''receiver-initiated'' dialogue is appropriate:
>
> 1. Receiver requests package information.
>
> 2. The sender provides package information, including the wtxids of
> the transactions in the package and anything else that might be
> relevant (e.g. total fees and size).
>
> 3. The reciever uses the package information to decide how to request
> and validate the transactions.
>
> Sometimes, no matter what order transactions are received by a node,
> validating them individually is insufficient. When the sender is aware
> of additional information that the receiver needs to accept a package,
> a proactive ''sender-initiated'' dialogue should be enabled:
>
> 1. Sender announces they have package information pertaining to a
> transaction that might otherwise be undesired on its own.
>
> 2. The receiver requests package information.
>
> 3. The sender provides package information, including the wtxids of
> the transactions in the package and anything else that might be
> relevant (e.g. total fees and size).
>
> 4. The reciever uses the package information to decide how to request
> and validate the transactions.
>
> Package relay is negotiated between two peers during the version
> handshake. Package relay requires both peers to support wtxid-based
> relay because package transactions are referenced by their wtxid.
>
> ====New Messages====
>
> Three new protocol messages are added for use in any version of
> package relay. Additionally, each version of package relay must define
> its own inv type and "pckginfo" message version, referred to in this
> document as "MSG_PCKG" and "pckginfo" respectively. See
> BIP-v1-packages for a concrete example.
>
> =====sendpackages=====
>
> {|
> | Field Name || Type || Size || Purpose
> |-
> |version || uint32_t || 4 || Denotes a package version supported by the
> node.
> |-
> |max_count || uint32_t || 4 ||Specifies the maximum number of transactions
> per package this node is
> willing to accept.
> |-
> |max_weight || uint32_t || 4 ||Specifies the maximum total weight per
> package this node is willing
> to accept.
> |-
> |}
>
> 1. The "sendpackages" message has the structure defined above, with
> pchCommand == "sendpackages".
>
> 2. During version handshake, nodes should send a "sendpackages"
> message indicate they support package relay and may request
> packages.
>
> 3. The message should contain a version supported by the node. Nodes
> should send a "sendpackages" message for each version they support.
>
> 4. The "sendpackages" message MUST be sent before sending a "verack"
> message. If a "sendpackages" message is received afer "verack", the
> sender should be disconnected.
>
> 5. If 'fRelay==false' in a peer's version message, the node must not
> send "sendpackages" to them. If a "sendpackages" message is
> received by a peer after sending `fRelay==false` in their version
> message, the sender should be disconnected.
>
> 6.. Upon receipt of a "sendpackages" message with a version that is
> not supported, a node must treat the peer as if it never received the
> message.
>
> 7. If both peers send "wtxidrelay" and "sendpackages" with the same
> version, the peers should announce, request, and send package
> information to each other.
>
> =====getpckgtxns=====
>
> {|
> | Field Name || Type || Size || Purpose
> |-
> |txns_length||CompactSize||1 or 3 bytes|| The number of transactions
> requested.
> |-
> |txns||List of wtxids||txns_length * 32|| The wtxids of each transaction
> in the package.
> |}
>
> 1. The "getpckgtxns" message has the structure defined above, with
> pchCommand == "getpckgtxns".
>
> 2. A "getpckgtxns" message should be used to request all or some of
> the transactions previously announced in a "pckginfo" message,
> specified by witness transactiosome id.
>
> 3. Upon receipt of a "getpckgtxns" message, a node must respond with
> either a "pckgtxns" containing the requested transactions or a
> "notfound" message indicating one or more of the transactions is
> unavailable. This allows the receiver to avoid downloading and storing
> transactions that cannot be validated immediately.
>
> 4. A "getpckgtxns" message should only be sent if both peers agreed to
> send packages in the version handshake. If a "getpckgtxns" message
> is received from a peer with which package relay was not negotiated,
> the sender should be disconnected.
>
> =====pckgtxns=====
>
> {|
> | Field Name || Type || Size || Purpose
> |-
> |txns_length||CompactSize||1 or 3 bytes|| The number of transactions
> provided.
> |-
> |txns||List of transactions||variable|| The transactions in the package.
> |}
>
> 1. The "pckgtxns" message has the structure defined above, with
> pchCommand == "pckgtxns".
>
> 2. A "pckgtxns" message should contain the transaction data requested
> using "getpckgtxns".
>
> 3. A "pckgtxns" message should only be sent to a peer that requested
> the package using "getpckgtxns". If a node receives an unsolicited
> package, the sender should be disconnected.
>
> 4. A "pckgtxns" message should only be sent if both peers agreed to
> send packages in the version handshake. If a "pckgtxns" message is
> received from a peer with which package relay was not negotiated, the
> sender should be disconnected.
>
> ===Version 1 Packages: child-with-unconfirmed-parents===
>
> This extends package relay for packages consisting of one transaction
> and all of its unconfirmed parents,by defining version 1 packages, a
> pckginfo1 message, and a MSG_PCKG1 inv type. It enables the use case
> in which a child pays for its otherwise-too-low-fee parents and their
> mempool conflict(s).
>
> ====Intended Protocol Flow====
>
> When relaying a package of low-fee parent(s) and high-fee child, the
> sender and receiver do the following:
>
> 1. Sender announces they have a child-with-unconfirmed-parents package
> for a child that pays for otherwise-too-low-fee parent(s) using
> "inv(MSG_PCKG1)".
>
> 2. The receiver requests package information using
> "getdata(MSG_PCKG1)".
>
> 3. The sender provides package information using "pckginfo1",
> including the blockhash of the sender's best block, the wtxids of
> the transactions in the package, their total fees and total weight.
>
> 4. The reciever uses the package information to decide how to request
> the transactions. For example, if the receiver already has some of
> the transactions in their mempool, they only request the missing ones.
> They could also decide not to request the package at all based on the
> fee information provided.
>
> 5. Upon receiving a "pckgtxns", the receiver submits the transactions
> together as a package.
>
> ====New Messages====
>
> A new inv type, "MSG_PCKG1", and new protocol message, "PCKGINFO1",
> are added.
>
> =====pckginfo1=====
>
> {|
> | Field Name || Type || Size || Purpose
> |-
> |blockhash || uint256 || 32 || The chain tip at which this package is
> defined.
> |-
> |pckg_fee||CAmount||4|| The sum total fees paid by all transactions in the
> package.
> |-
> |pckg_weight||int64_t||8|| The sum total weight of all transactions in the
> package.
> |-
> |txns_length||CompactSize||1 or 3 bytes|| The number of transactions
> provided.
> |-
> |txns||List of wtxids||txns_length * 32|| The wtxids of each transaction
> in the package.
> |}
>
>
> 1. The "pckginfo1" message has the structure defined above, with
> pchCommand == "pckginfo1".
>
> 2. A "pckginfo1" message contains information about a version 1
> package (defined below), referenced by the wtxid of the transaction
> it pertains to and the current blockhash.
>
> 3. Upon receipt of a "pckginfo1" message, the node should decide if it
> wants to validate the package, request transaction data if
> necessary, etc.
>
> 4. Upon receipt of a malformed "pckginfo1" message or package that
> does not abide by the max_count, max_weight, or other rules
> specified by the version agreed upon in the initial negotiation, the
> sender should be disconnected. If a node receives a "pckginfo1"
> message for which the "pckg_fee" or "pckg_weight" do not reflect the
> true total fees and weight, respectively, or the transactions in the
> package, the message is malformed.
>
> 5. A node MUST NOT send a "pckginfo1" message that has not been
> requested by the recipient. Upon receipt of an unsolicited
> "pckginfo1", a node should disconnect the sender.
>
> 6. A "pckginfo1" message should only be sent if both peers agreed to
> send version 1 packages in the version handshake. If a "pckginfo1"
> message is received from a peer with which package relay was not
> negotiated, the sender should be disconnected.
>
> =====MSG_PCKG1=====
>
> 1. A new inv type (MSG_PCKG1 == 0x6) is added, for use in inv messages
> and getdata requests pertaining to version 1 packages.
>
> 2. As an inv type, it indicates that both transaction data and version
> 1 package information are available for the transaction. The
> transaction is referenced by its wtxid. As a getdata request type, it
> indicates that the sender wants package information for the
> transaction.
>
> 3. Upon receipt of a "getdata" request for "MSG_PCKG1", the node
> should respond with the version 1 package corresponding to the
> requested transaction and its current chain tip, or with NOTFOUND.
> The node should not assume that the sender is requesting the
> transaction data as well.
>
> ====Child With Parent Packages Rules====
>
> A child-with-unconfirmed-parents package sent between nodes must abide
> by the rules below, otherwise the package is malformed and the sender
> should be disconnected.
>
> A version 1 or ''child-with-unconfirmed-parents'' package can be
> defined for any transaction that spends unconfirmed inputs. The child
> can be thought of as the "representative" of the package. This package
> can be uniquely identified by the transaction's wtxid and the current
> chain tip block hash.
>
> A ''child-with-unconfirmed-parents'' package MUST be:
>
> 1. ''Sorted topologically.'' For every transaction t in the package,
> if any of t's parents are present in the package, the parent must
> appear somewhere in the list before t. In other words, the
> transactions must be sorted in ascending order of the number of
> ancestors present in the package.
>
> 2. ''Only 1 child with unconfirmed parents.'' The package must consist
> of one transaction and its unconfirmed parents. There must not be
> any other transactions in the package. Other dependency relationships
> may exist within the package (e.g. one parent may spend the output of
> another parent) provided that topological order is respected.
>
> 3. ''All unconfirmed parents.'' All of the child's unconfirmed parents
> must be present.
>
> 4. ''No conflicts.'' None of the transactions in the package may
> conflict with each other (i.e. spend the same prevout).
>
> 5. ''Total fees and weight.'' The 'total_fee' and 'total_weight'
> fields must accurately represent the sum total of all transactions'
> fees and weights as defined in BIP141, respectively.
>
> Not all of the child's parents must be present; the child transaction
> may also spend confirmed inputs. However, if the child has confirmed
> parents, they must not be in the package.
>
> While a child-with-unconfirmed-parents package is perhaps most
> relevant when the child has a higher feerate than its parents, this
> property is not required to construct a valid package.
>
> ====Clarifications====
>
> ''Q: Under what circumstances should a sender announce a
> child-with-unconfirmed-parents package?''
>
> A child-with-unconfirmed-parents package for a transaction should be
> announced when it meets the peer's fee filter but one or more of its
> parents don't; a "inv(MSG_PCKG1)" instead of "inv(WTX)" should be sent
> for the child. Each of the parents which meet the peer's fee filter
> should still be announced normally.
>
> ''Q: What if a new block arrives in between messages?''
>
> A child-with-unconfirmed-parents package is defined for a transaction
> based on the current chain state. As such, a new block extending the
> tip may decrease the number of transactions in the package (i.e. if
> any of the transaction's parents were included in the block). In a
> reorg, the number of transactions in the package may decrease or
> increase (i.e. if any of the transaction's parents were included in a
> block in the previous chain but not the new one).
>
> If the new block arrives before the "getdata" or "pckginfo1", nothing
> needs to change.
>
> If the new block arrives before "getpckgtxns" or before "pckgtxns",
> the receiver may need to re-request package information if the block
> contained a transaction in the package. If the block doesn't contain
> any transactions in the package, whether it extends the previous tip
> or causes a reorg, nothing needs to change.
>
> ''Q: Can "getpckgtxns" and "pckgtxns" messages contain only one
> transaction?''
>
> Yes.
>
> ===Further Protocol Extensions===
>
> When introducing a new type of package, assign it a version number "n"
> and use an additional "sendpackages" message during version handshake
> to negotiate support for it. An additional package information message
> "pckginfon" and inv type "MSG_PCKGn" should be defined for the type of
> package. However, "getpckgtxns" and "pckgtxns" do not need to be
> changed.
>
> Example proposal for tx-with-unconfirmed-ancestors package relay: [19]
>
> ===Compatibility===
>
> Older clients remain fully compatible and interoperable after this
> change. Clients implementing this protocol will only attempt to send
> and request packages if agreed upon during the version handshake.
>
> ===Package Erlay===
>
> Clients using BIP330 reconciliation-based transaction relay (Erlay)
> are able to use package relay without interference. In fact, a package
> of transactions may be announced using both Erlay and package relay.
> After reconciliation, if the initiator would have announced a
> transaction by wtxid but also has package information for it, they may
> send "inv(MSG_PCKG)" instead of "inv(WTX)".
>
> ===Rationale===
>
> ====P2P Message Design====
>
> These p2p messages are added for communication efficiency and, as
> such, one should measure alternative solutions based on the resources
> used to communicate (not necessarily trustworthy) information: We
> would like to minimize network bandwidth, avoid downloading a
> transaction more than once, avoid downloading transactions that are
> eventually rejected, and minimize storage allocated for
> not-yet-validated transactions.
>
> Consider these (plausible) scenarios in transaction relay:
>
> Alice (the "sender") is relaying transactions to Bob (the "receiver").
> Alice's mempool has a minimum feerate of 1sat/vB and Bob's has a
> minimum feerate of 3sat/vB. For simplicity, all transactions are
> 1600Wu in virtual size and 500 bytes in serialized size. Apart from
> the spending relationships specified, all other inputs are from
> confirmed UTXOs.
>
> 1. Package {A, B} where A pays 0 satoshis and B pays 8000 satoshis in
> fees.
>
> 2. Package {C, D} where C pays 0 satoshis and D pays 1200 satoshis in
> fees.
>
> 3. Package {E, F, G, H, J} that pays 4000, 8000, 0, 2000, and 4000
> satoshis in fees, respectively.
>
> ====Alternative Designs Considered====
>
> ''Package Information Only:'' Just having "pckginfo" gives enough
> information for the receiver to accept the package. Omit the
> "getpckgtxns" and "pckgtxns" messages. While this option is a good
> fallback if batched transaction download fails for some reason, it
> shouldn't be used as the default because it 'always' requires storage
> of unvalidated transactions.
>
> ''No Package Information Round:'' Instead of having a package
> information round, just use the child's wtxid to refer to the package
> and always send the entire package together. This would cause nodes to
> redownload duplicate transactions.
>
> I have also created a slidedeck exploring various alternative designs
> and some examples in which they fall flat [20]. Please feel free to
> suggest other alternatives.
>
> ====Versioning System====
>
> This protocol should be extensible to support multiple types of
> packages based on future desired use cases. Two "flavors" of
> versioning were considered:
>
> 1. When package mempool acceptance is upgraded to support more types
> of packages, increment the version number (similar to Erlay).
> During version handshake, peers negotiate which version of package
> relay they will use by each sending one "sendpackages" message.
>
> 2. When introducing another type of package, assign a version number
> to it and announce it as an additional supported version (similar
> to Compact Block Relay). During version handshake, peers send one
> "sendpackages" message for each version supported.
>
> The second option was favored because it allows different parameters
> for different versions. For example, it should be possible to support
> both "arbitrary topology but maximum 3-transaction" package as well as
> "child-with-unconfirmed-parents with default mempool ancestor limits"
> packages simultaneously.
>
> ==Acknowledgements==
>
> I hope to have made it abundantly clear that this proposal isn’t
> inventing the concept of package relay, and in fact builds upon years
> of work by many others, including Suhas Daftuar and Antoine Riard.
>
> Thank you to John Newbery and Martin Zumsande for input on the design.
>
> Thank you to Matt Corallo, Christian Decker, David Harding, Antoine
> Poinsot, Antoine Riard, Gregory Sanders, Chris Stewart, Bastien
> Teinturier, and others for input on the desired interface for
> contracting protocols.
>
> Looking forward to hearing your thoughts!
>
> Best,
> Gloria
>
> [0]:
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019817.html
> [1]:
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-April/002639.html
> [2]:
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-June/002758.html
> [3]:
> https://github.com/t-bast/lightning-docs/blob/master/pinning-attacks.md
> [4]:
> https://github.com/revault/practical-revault/blob/master/transactions.md#cancel_tx
> [5]:
> https://github.com/discreetlogcontracts/dlcspecs/blob/master/Transactions.md#refund-transaction
> [6]: https://gist.github.com/instagibbs/60264606e181451e977e439a49f69fe1
> [7]:
> https://btctranscripts.com/adopting-bitcoin/2021/2021-11-16-gloria-zhao-transaction-relay-policy/#lightning-attacks
> [8]: https://youtu.be/fbWSQvJjKFs?t=1438
> [9]:
> https://www.reddit.com/r/Bitcoin/comments/unew4e/looks_like_70_mvb_of_transactions_just_got_dumped/
> [10]: https://github.com/bitcoin/bitcoin/pull/7594
> [11]: https://github.com/bitcoin/bitcoin/pull/7600
> [12]: https://github.com/bitcoin/bitcoin/pull/6455#issuecomment-122716820
> [13]: https://gist.github.com/sdaftuar/8756699bfcad4d3806ba9f3396d4e66a
> [14]: https://github.com/bitcoin/bitcoin/issues/14895
> [15]: https://github.com/bitcoin/bitcoin/pull/16401
> [16]: https://github.com/bitcoin/bitcoin/pull/19621
> [17]:
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-September/019464.html
> [18]: https://github.com/users/glozow/projects/5/views/4?layout=board
> [19]: https://gist.github.com/glozow/9b321cd3ef6505135c763112033ff2a7
> [20]:
> https://docs.google.com/presentation/d/1B__KlZO1VzxJGx-0DYChlWawaEmGJ9EGApEzrHqZpQc/edit?usp=sharing
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev at lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
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