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From: Greg Sanders <gsanders87@gmail.com>
Date: Fri, 30 Sep 2022 08:17:38 -0400
Message-ID: <CAB3F3Dtx58n1U0sK4ev6UwSqLJ8nn=wgJJE8V6YrWt4yeZ_4Yw@mail.gmail.com>
To: Bastien TEINTURIER <bastien@acinq.fr>
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Subject: Re: [bitcoin-dev] New transaction policies (nVersion=3) for
 contracting protocols
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It's likely better if the ephemeral output can be any value, including
dust. This lets contract designers put "trimmed output" value indirectly
towards CPFP fees without making the parent tx have fees itself.

On Fri, Sep 30, 2022, 8:08 AM Bastien TEINTURIER <bastien@acinq.fr> wrote:

> Hey Ruben,
>
> I discussed this further over IRC, and I now agree that this particular
> change would be very desirable and can likely fit in the initial release
> (even though I'm not the one writing that code, but I'd be happy to
> review it and test it).
>
> Greg already has a draft design that addresses your concerns: if there is
> an "ephemeral output" (0-value, OP_TRUE) in an unconfirmed v3 transaction=
,
> it MUST be spent by any child v3 transaction. This way, you ensure that
> any child transaction spending the unconfirmed parent spends the ephemera=
l
> output(s). @Greg, correct me if I misunderstood something here. Note that
> we will need to precisely define the criteria for those "ephemeral output=
s"
> (it can probably simply be "outputs that are 0 sats").
>
> Coupled with transactions that pay no fees (and thus require a child to
> CPFP in order to be included in a block), this ensures those outputs can
> never leak into the utxo set. How does that sound?
>
> I'm curious why you would need more than one such output, can you detail?
> I believe we only ever need one, spendable by anyone.
>
> Cheers,
> Bastien
>
> Le ven. 30 sept. 2022 =C3=A0 02:14, Ruben Somsen <rsomsen@gmail.com> a =
=C3=A9crit :
>
>> Hi Bastien,
>>
>> >The other change mentioned (making OP_TRUE standard and allowing output=
s
>> that are below dust) can be added later, as those won't be standard unti=
l
>> we start allowing them, so there shouldn't be any backwards-compatibilit=
y
>> issue with postponing this change. But maybe it's still worth having fro=
m
>> the get-go, even though it may take a bit more time? Again, I'm curious =
to
>> have other people's opinion here
>>
>> I'm sensitive to not wanting to overload the current discussion but this
>> also interests me, provided it can be done in a way that is acceptable
>> (i.e. minimizing the potential UTXO set impact). It would solve a big co=
st
>> issue in my spacechains design if transactions could be 0 fees and have =
a 0
>> sat output that could be used in order to pay all the fees with CPFP.
>>
>> My current view is that a tx containing a single 0 sat OP_TRUE output
>> should only get relayed if it is a package where the OP_TRUE output is
>> currently being spent in a way that increases the overall fee rate. But
>> even then, one theoretical edge case remains:
>> - Another CPFP tx can feebump the package on a different (non-OP_TRUE)
>> output with an even higher fee rate
>> - Subsequently, the tx that is spending the OP_TRUE might fall out of th=
e
>> mempool if the mempool fee rate rises
>> - This could cause the 0 sat output to enter the UTXO set (specifically,
>> rational miners wouldn't refuse to mine such a tx)
>>
>> It doesn't seem like this would happen much in practice (nor is there an
>> incentive to do it on purpose), but the chance isn't 0.
>>
>> Cheers,
>> Ruben
>>
>>
>>
>> On Thu, Sep 29, 2022 at 4:50 PM Greg Sanders via bitcoin-dev <
>> bitcoin-dev@lists.linuxfoundation.org> wrote:
>>
>>> > 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@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 woul=
d
>>>> 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 anne=
x
>>>> (also note that it's probably not just the v3 package's size, it shoul=
d
>>>> 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 th=
is
>>>> 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 ann=
ex
>>>> 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 outpu=
ts
>>>> 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 curiou=
s
>>>> 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 =C3=A0 18:48, Gloria Zhao <gloriajzhao@gmail.com=
> a
>>>> =C3=A9crit :
>>>>
>>>>> 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 criti=
cisms
>>>>> 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 clo=
se all
>>>>> pinning attacks possible (sorry for confusing anyone). I also agree t=
hat
>>>>> 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-broadc=
ast. I
>>>>> was hoping to get some ideas with the "RBF Improvements" post in Janu=
ary,
>>>>> 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 s=
mall
>>>>> 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 sin=
gle
>>>>> 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 a=
nd not
>>>>> perfect).
>>>>>
>>>>> So in the scenario above with the "inflated" parent that was signed
>>>>> ACP, the replacement would be rejected because the package ancestor f=
eerate
>>>>> is lower than the feerate of what is being replaced. But it is imperf=
ect
>>>>> (explained below) and thus I wouldn't recommend it for single transac=
tion
>>>>> 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 L=
N
>>>>> Penalty is not affected by this since it doesn't use ACP, though obvi=
ously
>>>>> 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 ot=
hers.
>>>>> 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-rela=
ted
>>>>> 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 ho=
w 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 individ=
ual
>>>>> feerate, unless we run the mining algorithm, it cannot tell us exactl=
y 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 l=
etting
>>>>> a transaction replace something that would have been more incentive
>>>>> compatible to mine. As such, we don't want to overestimate how good t=
he
>>>>> replacement is, and we don't want to underestimate how good the origi=
nal
>>>>> transactions are. This rule "The minimum between package feerate and
>>>>> ancestor feerate of the child is not lower than the individual feerat=
es of
>>>>> all directly conflicting transactions and the ancestor feerates of al=
l
>>>>> 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 replac=
ement
>>>>> rules? Getting rid of the requirement to 1 block csv lock every outpu=
t
>>>>> would be quite nice from a smart contracting composability point of v=
iew.
>>>>>
>>>>> 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 bet=
ter
>>>>> 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 ance=
stor
>>>>> 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 transact=
ion.
>>>>>
>>>>> > 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 Do=
S
>>>>> 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 woul=
d
>>>>> be to constraint the pinning counterparty to attach a high fee to a c=
hild
>>>>> due to the limited size, if they would like this transaction to be st=
uck in
>>>>> the network mempools. By doing so  this child has high odds to confir=
m.
>>>>>
>>>>> 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 *do=
n'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 replacem=
ent
>>>>> cost without also fee-bumping the transaction to make it confirm fast=
er.
>>>>>
>>>>> > As of today, I think yes you can already fingerprint LN transaction=
s
>>>>> on the  spec-defined amount value of the anchor outputs, 330 sats. Th=
ere is
>>>>> always one of them on post-anchor commitment transactions. And sadly =
I
>>>>> would say we'll always have tricky fingerprints leaking from unilater=
al LN
>>>>> closures such as HTLC/PTLC timelocks...
>>>>>
>>>>> > I agree with you, this isn't worse than today, unilateral closes wi=
ll
>>>>> 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@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@lists.linuxfoundation.org> wrote:
>>>>>>
>>>>>>> Thanks Gloria for this great post.
>>>>>>>
>>>>>>> This is very valuable work for L2 contracts, and will greatly impro=
ve
>>>>>>> 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 t=
o
>>>>>>> > 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 ha=
s
>>>>>>> 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 wi=
ll
>>>>>>> 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 th=
at
>>>>>>> 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 b=
y
>>>>>>> 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 wou=
ld
>>>>>>> probably be *better* than what we're doing today.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Bastien
>>>>>>>
>>>>>>> Le lun. 26 sept. 2022 =C3=A0 03:22, Antoine Riard via bitcoin-dev <
>>>>>>> bitcoin-dev@lists.linuxfoundation.org> a =C3=A9crit :
>>>>>>>
>>>>>>>> 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 a=
n
>>>>>>>> unconfirmed V3 transaction, if the offered fee is in the top mempo=
ol
>>>>>>>> backlog, I think you would have an interest to accept such a trans=
action.
>>>>>>>>
>>>>>>>> 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 b=
e
>>>>>>>> >    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 f=
ee 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 od=
ds 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 transa=
ction of
>>>>>>>> size 100Kvb offering 2 sat/vb, it's more interesting than a V3 rep=
lacement
>>>>>>>> candidate of size 1000 vb offering 10 sat/vb. It could be argued t=
he 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-feera=
te 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 ma=
y
>>>>>>>> use
>>>>>>>> > to fund this fee-bump. For example, only allowing the V3 child t=
o
>>>>>>>> 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 counter=
party
>>>>>>>> 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 t=
he 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 reser=
ve pool,
>>>>>>>> as the cost of acquiring UTXO is non-null and in the optimistic ca=
se, 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 remainin=
g
>>>>>>>> in-flight channels.
>>>>>>>>
>>>>>>>> > 1. The rule around unconfirmed inputs was
>>>>>>>> > originally "A package may include new unconfirmed inputs, but th=
e
>>>>>>>> > 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, wh=
ere 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 anc=
estor
>>>>>>>> feerate" HTLC-timeout transaction. With `option_anchor_outputs" th=
ere is no
>>>>>>>> unconfirmed ancestor to replace, as the commitment transaction, wh=
atever
>>>>>>>> the party it is originating from, should already be confirmed.
>>>>>>>>
>>>>>>>> > "Is this a privacy issue, i.e. doesn't it allow fingerprinting L=
N
>>>>>>>> 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 outpu=
ts, 330
>>>>>>>> sats. There is always one of them on post-anchor commitment transa=
ctions.
>>>>>>>> And sadly I would say we'll always have tricky fingerprints leakin=
g 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 signifi=
cant
>>>>>>>> advantage for LN, as for the current ~85k of opened channels, the =
old V2
>>>>>>>> states shouldn't be pinning vectors. Currently, commitment transac=
tions
>>>>>>>> signal replaceability.
>>>>>>>>
>>>>>>>> Le ven. 23 sept. 2022 =C3=A0 11:26, Gloria Zhao via bitcoin-dev <
>>>>>>>> bitcoin-dev@lists.linuxfoundation.org> a =C3=A9crit :
>>>>>>>>
>>>>>>>>> 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 mor=
e
>>>>>>>>> [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 P=
R
>>>>>>>>> - thanks Suhas Daftuar, Gregory Sanders, Bastien Teinturier, Anth=
ony Towns,
>>>>>>>>> and others.)
>>>>>>>>>
>>>>>>>>> If you are interested in using package RBF/relay to bump presigne=
d
>>>>>>>>> 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 wou=
ld
>>>>>>>>> really really like to know.
>>>>>>>>>
>>>>>>>>> Note that transactions with nVersion=3D3 ("V3 transactions") are
>>>>>>>>> currently non-standard in Bitcoin Core. That means **anything tha=
t
>>>>>>>>> was
>>>>>>>>> standard before this policy change would still be standard
>>>>>>>>> afterwards.** If you don't want your transactions to be subject t=
o
>>>>>>>>> these rules, just continue whatever you're doing and don't use
>>>>>>>>> nVersion=3D3. AFAICT this shouldn't break anything, but let me kn=
ow
>>>>>>>>> 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 additiona=
l
>>>>>>>>> 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 V=
3.
>>>>>>>>>
>>>>>>>>> *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 traversa=
l,
>>>>>>>>> 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 pinni=
ng
>>>>>>>>> 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 sufficien=
t.
>>>>>>>>>
>>>>>>>>> 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 virtua=
l
>>>>>>>>> bytes as the descendant limit, the cost to replace a V3 transacti=
on
>>>>>>>>> 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 o=
n
>>>>>>>>> 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 feera=
te
>>>>>>>>> and ancestor feerate of the child is not lower than the individua=
l
>>>>>>>>> feerates of all directly conflicting transactions and the ancesto=
r
>>>>>>>>> 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 b=
e
>>>>>>>>> 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 difficul=
t
>>>>>>>>> to account for all of the possibilities. For example, it gets muc=
h
>>>>>>>>> harder to see that we're applying the descendant limits correctly
>>>>>>>>> if
>>>>>>>>> the package has a gnarly, many-generation, non-tree shape. I'm al=
so
>>>>>>>>> not sure if this policy is 100% incentive-compatible if the spons=
or
>>>>>>>>> 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. Th=
ey
>>>>>>>>> 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 mus=
t
>>>>>>>>> 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 tha=
t
>>>>>>>>> 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 t=
o
>>>>>>>>> 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 wors=
e.
>>>>>>>>>
>>>>>>>>> "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-Sept=
ember/019464.html
>>>>>>>>> [2]:
>>>>>>>>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-Janu=
ary/019817.html
>>>>>>>>>
>>>>>>>>> Best,
>>>>>>>>> Gloria
>>>>>>>>> _______________________________________________
>>>>>>>>> bitcoin-dev mailing list
>>>>>>>>> bitcoin-dev@lists.linuxfoundation.org
>>>>>>>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> bitcoin-dev mailing list
>>>>>>>> bitcoin-dev@lists.linuxfoundation.org
>>>>>>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> bitcoin-dev mailing list
>>>>>>> bitcoin-dev@lists.linuxfoundation.org
>>>>>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>>>>>
>>>>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev@lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>

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Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

<div dir=3D"auto">It&#39;s likely better if the ephemeral output can be any=
 value, including dust. This lets contract designers put &quot;trimmed outp=
ut&quot; value indirectly towards CPFP fees without making the parent tx ha=
ve fees itself.=C2=A0</div><br><div class=3D"gmail_quote"><div dir=3D"ltr" =
class=3D"gmail_attr">On Fri, Sep 30, 2022, 8:08 AM Bastien TEINTURIER &lt;<=
a href=3D"mailto:bastien@acinq.fr">bastien@acinq.fr</a>&gt; wrote:<br></div=
><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1=
px #ccc solid;padding-left:1ex"><div dir=3D"ltr">Hey Ruben,<br><br>I discus=
sed this further over IRC, and I now agree that this particular<br>change w=
ould be very desirable and can likely fit in the initial release<div>(even =
though I&#39;m not the one writing that code, but I&#39;d be happy to<br>re=
view it and test it).</div><div><br>Greg already has a draft design that ad=
dresses your concerns: if there is<br>an &quot;ephemeral output&quot; (0-va=
lue, OP_TRUE) in an unconfirmed v3 transaction,<br>it MUST be spent by any =
child v3 transaction. This way, you ensure that<br>any child transaction sp=
ending the unconfirmed parent spends the ephemeral<br>output(s). @Greg, cor=
rect me if I misunderstood something here. Note that<br>we will need to pre=
cisely define the criteria for those &quot;ephemeral outputs&quot;<br>(it c=
an probably simply be &quot;outputs that are 0 sats&quot;).<br><br>Coupled =
with transactions that pay no fees (and thus require a child to<br>CPFP in =
order to be included in a block), this ensures those outputs can<br>never l=
eak into the utxo set. How does that sound?<br><br>I&#39;m curious why you =
would need more than one such output, can you detail?<br>I believe we only =
ever need one, spendable by anyone.<br><br>Cheers,<br>Bastien</div></div><b=
r><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">Le=C2=A0=
ven. 30 sept. 2022 =C3=A0=C2=A002:14, Ruben Somsen &lt;<a href=3D"mailto:rs=
omsen@gmail.com" target=3D"_blank" rel=3D"noreferrer">rsomsen@gmail.com</a>=
&gt; a =C3=A9crit=C2=A0:<br></div><blockquote class=3D"gmail_quote" style=
=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding=
-left:1ex"><div dir=3D"ltr"><div>Hi Bastien,</div><div><br></div>&gt;The ot=
her change mentioned (making OP_TRUE standard and allowing outputs<br>that =
are below dust) can be added later, as those won&#39;t be standard until<br=
>we start allowing them, so there shouldn&#39;t be any backwards-compatibil=
ity<br>issue with postponing this change. But maybe it&#39;s still worth ha=
ving from<br>the get-go, even though it may take a bit more time? Again, I&=
#39;m curious to<br>have other people&#39;s opinion here<div><br></div><div=
>I&#39;m sensitive to not wanting to overload the current discussion but th=
is also interests me, provided it can be done in a way that is acceptable (=
i.e. minimizing the potential UTXO set impact). It would solve a big cost i=
ssue in my spacechains design if transactions could be 0 fees and have a 0 =
sat output that could be used in order to pay all the fees with CPFP.</div>=
<div><br></div><div>My current view is that a tx containing a single 0 sat =
OP_TRUE output should only get relayed if it is a package where the OP_TRUE=
 output is currently being spent in a way that increases the overall fee ra=
te. But even then, one theoretical edge case remains:<br></div><div>- Anoth=
er CPFP tx can feebump the package on a different (non-OP_TRUE) output with=
 an even higher fee rate</div><div>- Subsequently, the tx that is spending =
the OP_TRUE might fall out of the mempool if the mempool fee rate rises</di=
v><div>- This could cause the 0 sat output to enter the UTXO set (specifica=
lly, rational miners wouldn&#39;t refuse to mine such a tx)</div><div><br><=
/div><div>It doesn&#39;t seem like this would happen much in practice (nor =
is there an incentive to do it on purpose), but the chance isn&#39;t 0.</di=
v><div><br></div><div>Cheers,</div><div>Ruben</div><div><br></div><div><br>=
</div><div></div></div><br><div class=3D"gmail_quote"><div dir=3D"ltr" clas=
s=3D"gmail_attr">On Thu, Sep 29, 2022 at 4:50 PM Greg Sanders via bitcoin-d=
ev &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_=
blank" rel=3D"noreferrer">bitcoin-dev@lists.linuxfoundation.org</a>&gt; wro=
te:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px =
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir=3D"=
ltr">&gt; Right, good catch, this does require new logic to handle this cas=
e.<br>As Gloria points out, this should be doable, and is definitely worth<=
br>adding (those CSV 1 on every other output are really hacky, glad to<br>f=
ind a way to get rid of them).<div><br></div><div>For the record, it turns =
out ephemeral anchors=C2=A0+ v3 solves this already, as the anchor must be =
spent, and the parent tx may only have one child. Somehow I missed this imp=
lication for a few months. It&#39;s great news if we can directly source fe=
es from any output claimable, including HTLCs!</div></div><br><div class=3D=
"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Thu, Sep 29, 2022 at=
 5:15 AM Bastien TEINTURIER &lt;<a href=3D"mailto:bastien@acinq.fr" target=
=3D"_blank" rel=3D"noreferrer">bastien@acinq.fr</a>&gt; wrote:<br></div><bl=
ockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-lef=
t:1px solid rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr">Hi Gloria, =
Greg,<br><br>&gt; I interpret most of the discussion around limitations as =
ideas for<br>&gt; future improvements rather than criticisms of the proposa=
l<br><br>As far as I&#39;m concerned, definitely!<br><br>My current underst=
anding is that the main change/improvement that would<br>make sense here is=
 restricting the whole v3 package&#39;s size (instead of<br>just the child)=
 via committing to a specific value in the taproot annex<br>(also note that=
 it&#39;s probably not just the v3 package&#39;s size, it should<br>be the =
whole unconfirmed package including potential v2 unconfirmed<br>ancestors).=
<br><br>While I think this would be very valuable and would like to see thi=
s<br>happen, I believe that can be done in a second, separate step since th=
is<br>would make relay policy stricter (some v3 transactions that previousl=
y<br>propagated wouldn&#39;t propagate under this new rule). As long as you=
 are<br>able to find a path to miners through upgraded peers that use this =
annex<br>approach, you should be able to resolve ACP pinning issues?<br><br=
>I&#39;m curious to know how other people feel about that: is it ok to do<b=
r>later or should we try to implement this for the first release of v3<br>t=
ransactions?<br><br>The other change mentioned (making OP_TRUE standard and=
 allowing outputs<br>that are below dust) can be added later, as those won&=
#39;t be standard until<br>we start allowing them, so there shouldn&#39;t b=
e any backwards-compatibility<br>issue with postponing this change. But may=
be it&#39;s still worth having from<br>the get-go, even though it may take =
a bit more time? Again, I&#39;m curious to<br>have other people&#39;s opini=
on here, I&#39;d be happy to get all of those directly<br>in the first rele=
ase of v3 transactions, but I don&#39;t know how much<br>implementation wil=
l have to go into that.<br><br>&gt; For clarification, package RBF is Paren=
tTx*s*(plural), and ChildTx(singular),<br>&gt; so it might be a bit more co=
mplicated than we&#39;re thinking<br><br>Right, good catch, this does requi=
re new logic to handle this case.<br>As Gloria points out, this should be d=
oable, and is definitely worth<br>adding (those CSV 1 on every other output=
 are really hacky, glad to<br>find a way to get rid of them).<br><br>Thanks=
,<br>Bastien</div><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"=
gmail_attr">Le=C2=A0lun. 26 sept. 2022 =C3=A0=C2=A018:48, Gloria Zhao &lt;<=
a href=3D"mailto:gloriajzhao@gmail.com" target=3D"_blank" rel=3D"noreferrer=
">gloriajzhao@gmail.com</a>&gt; a =C3=A9crit=C2=A0:<br></div><blockquote cl=
ass=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid=
 rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr">Hi Greg, Antoine, Bast=
ien, =C2=A0<br>=C2=A0 <br>Thanks very much for the feedback! I interpret mo=
st of the discussion around limitations as ideas for future improvements ra=
ther than criticisms of the proposal (please correct me if I&#39;m wrong). =
I&#39;ll try to respond to as much as possible.<br><br>Also I realize that =
I didn&#39;t contextualize this proposal clearly enough; it is very tailore=
d for LN Penalty and definitely doesn&#39;t close all pinning attacks possi=
ble (sorry for confusing anyone). I also agree that some bits can be a litt=
le 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 &quot;RBF Improvements&quot; post in Janu=
ary, but it doesn&#39;t seem like we&#39;re much closer to a workable propo=
sal. I think this is a minimally-invasive step that works for Lightning tod=
ay, a small fix similar to CPFP carve out.<br><br>&gt; As you likely know f=
rom previous discussions the biggest scenario this does not fix in my estim=
ation is ANYONECANPAY situations. If the parent transaction can be &quot;in=
flated&quot; by tacking on additional inputs, this means the total weight o=
f the parent tx lowers the effective feerate of the package. =C2=A0<br>=C2=
=A0 <br>(For more context to other readers I wrote an explanation for this =
in &quot;SIGHASH_ANYONECANPAY Pinning&quot; section of RBF ML post).=C2=A0 =
Yes, this unfortunately doesn&#39;t fix any of the existing pinning attacks=
 for single transaction RBF but also doesn&#39;t make them worse. This boil=
s down to adding an incentive compatibility rule that ensures you can&#39;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).<br><br>So in the scenario above with the &quot;inflated&=
quot; parent that was signed ACP, the replacement would be rejected because=
 the package ancestor feerate is lower than the feerate of what is being re=
placed. But it is imperfect (explained below) and thus I wouldn&#39;t recom=
mend it for single transaction replacement. So that attack still exists for=
 single transactions, yes. =C2=A0<br><br>The strategy of using ACP to bring=
-your-own-fees has its own challenges but hopefully has no current use case=
s as you say. AFAIK LN Penalty is not affected by this since it doesn&#39;t=
 use ACP, though obviously I agree we should fix it for the future.<br><br>=
So when I said &quot;this is intended for fee-bumping presigned txns in con=
tracting protocols,&quot; I should have said &quot;this is intended for fee=
-bumping presigned txns specifically using CPFP and anchor outputs.&quot; A=
pologies for forgetting to contextualize, I&#39;ve been sitting on this for=
 too long.<br>=C2=A0 <br>&gt; The other scenario it doesn&#39;t really fix =
is where HTLC/commitment-like transactions are being resolved in a batch, b=
ut due to relative time constraints, you may want to accelerate some and no=
t others. Now you must pay higher rates to replace all of the transaction b=
umps. This is a &quot;self-pin&quot; and &quot;get good at utxos noob&quot;=
 type problem, but it&#39;s something that axing rule#3 in favor of a Repla=
ce-by-ancestor-feerate system would get us. =C2=A0<br>=C2=A0 <br>I understa=
nd you to mean &quot;if you don&#39;t have enough UTXOs and you&#39;re forc=
ed to batch-bump, you over-pay because you need to bring them all to the hi=
ghest target feerate.&quot; Isn&#39;t this kind of separate, wallet-related=
 problem? Contracting or not, surely every wallet needs to have enough UTXO=
s to not batch transactions that shouldn&#39;t be batched... I don&#39;t se=
e how a replace-by-ancestor-feerate policy would make any difference for th=
is?<br><br>Also in general I&#39;d like to reiterate that ancestor feerate =
is not a panacea to all our RBF incentive compatibility concerns. Like indi=
vidual feerate, unless we run the mining algorithm, it cannot tell us exact=
ly how quickly this transaction would be mined.<br><br>We&#39;re estimating=
 the incentive compatibility of the original transaction(s) and replacement=
 transaction(s), with the goal of not letting a transaction replace somethi=
ng that would have been more incentive compatible to mine. As such, we don&=
#39;t want to overestimate how good the replacement is, and we don&#39;t wa=
nt to underestimate how good the original transactions are. This rule &quot=
;The minimum between package feerate and ancestor feerate of the child is n=
ot lower than the individual feerates of all directly conflicting transacti=
ons and the ancestor feerates of all original transactions&quot; is a conse=
rvative estimate.<br><br>&gt; Would kind of be nice if package RBF would de=
tect a &quot;sibling output spend&quot; conflict, and knock it out of the m=
empool 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 c=
omposability point of view.<br><br>Interesting, so when a transaction hits =
a mempool tx&#39;s descendant limit, we consider evicting one of its descen=
dants in favor of this transaction, based on the RBF rules.<br>Cool idea! A=
fter 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. A=
s in, if we added this policy and it can make us evict the sibling and acce=
pt a transaction with a bunch of low-feerate ancestor junk, it would be a n=
ew pinning vector. <br>=C2=A0 <br>&gt; If you&#39;re a miner and you receiv=
e a non-V3, second descendant of an unconfirmed V3 transaction, if the offe=
red fee is in the top mempool backlog, I think you would have an interest t=
o accept such a transaction. =C2=A0 =C2=A0 =C2=A0<br>&gt; So I&#39;m not su=
re if those two rules are compatible with miners incentives... =C2=A0<br>=
=C2=A0 <br>The same argument can be made for the 26th descendant of a mempo=
ol transaction; it&#39;s also not entirely incentive-compatible to reject i=
t, 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 pro=
tection, while this 1-descendant limit rule is more of a &quot;help the Bit=
coin ecosystem&quot; policy, just like CPFP carve-out, dust limit, etc. I c=
an of course understand why not everyone would be in favor of this, but I d=
o think it&#39;s worth it.<br>=C2=A0 <br>&gt; &gt; 4. A V3 transaction that=
 has an unconfirmed V3 ancestor cannot be =C2=A0 =C2=A0<br>&gt; &gt; =C2=A0=
 =C2=A0larger than 1000 virtual bytes. =C2=A0 =C2=A0<br>=C2=A0 =C2=A0<br>&g=
t; 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=C2=A0 this child has high odds to confirm. =
=C2=A0<br>=C2=A0 <br><div>Yeah exactly, the &quot;Rule 3 pin&quot; is done =
by adding a child that&#39;s high-fee (so you have to pay that much to evic=
t it). Because they *don&#39;t* want this tx to confirm, normally, this chi=
ld would be really large. If they only have 1000vB for the child, they can&=
#39;t increase the replacement cost without also fee-bumping the transactio=
n to make it confirm faster.=C2=A0 </div><br>&gt; As of today, I think yes =
you can already fingerprint LN transactions on the=C2=A0 spec-defined amoun=
t value of the anchor outputs, 330 sats. There is always one of them on pos=
t-anchor commitment transactions. And sadly I would say we&#39;ll always ha=
ve tricky fingerprints leaking from unilateral LN closures such as HTLC/PTL=
C timelocks... =C2=A0<br><div><br></div><div>&gt; I agree with you, this is=
n&#39;t worse than today, unilateral closes will</div><div>probably always =
be identifiable on-chain.</div><br>Great to hear that there is no privacy w=
orsening!<br><br>Best, =C2=A0<br>Gloria<br></div><br><div class=3D"gmail_qu=
ote"><div dir=3D"ltr" class=3D"gmail_attr">On Mon, Sep 26, 2022 at 5:02 PM =
Greg Sanders &lt;<a href=3D"mailto:gsanders87@gmail.com" target=3D"_blank" =
rel=3D"noreferrer">gsanders87@gmail.com</a>&gt; wrote:<br></div><blockquote=
 class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px so=
lid rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr">Bastien,<div><br></=
div><div>&gt; This may be already covered by the current package RBF logic,=
 in that</div>scenario we are simply replacing [ParentTx, ChildTx1] with<br=
>[ParentTx, ChildTx2] that pays more fees, right?<div><br></div><div>For cl=
arification, package RBF is ParentTx*s*(plural), and ChildTx(singular), so =
it might be a bit more complicated than we&#39;re thinking, and currently t=
he V3 proposal would first de-duplicate=C2=A0the ParentTx based on what is =
in the mempool, then look at the &quot;rest&quot; of the transactions as a =
package, then individually. Not the same, not sure how different. I&#39;ll =
defer to experts.</div><div><br></div><div>Best,</div><div>Greg</div></div>=
<br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Mon=
, Sep 26, 2022 at 11:48 AM Bastien TEINTURIER via bitcoin-dev &lt;<a href=
=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank" rel=3D"=
noreferrer">bitcoin-dev@lists.linuxfoundation.org</a>&gt; wrote:<br></div><=
blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-l=
eft:1px solid rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr">Thanks Gl=
oria for this great post.<br><br>This is very valuable work for L2 contract=
s, and will greatly improve<br>their security model.<br><br>&gt; &quot;Only=
 1 anchor output? What if I need to bump counterparty&#39;s commitment tx i=
n mempool?&quot;<br>&gt; You won&#39;t need to fee-bump a counterparty&#39;=
s commitment tx using CPFP.<br>&gt; You would just package RBF it by attach=
ing a high-feerate child to<br>&gt; your commitment tx.<br><br>Note that we=
 can also very easily make that single anchor spendable by<br>both particip=
ants (or even anyone), so if you see your counterparty&#39;s<br>commitment =
in your mempool, you can bump it without publishing your<br>own commitment,=
 which is quite desirable (your own commitment tx has<br>CSV delays on your=
 outputs, whereas your counterparty&#39;s commitment tx<br>doesn&#39;t).<br=
><br>&gt; &quot;Is this a privacy issue, i.e. doesn&#39;t it allow fingerpr=
inting LN<br>transactions based on nVersion?&quot;<br><br>I agree with you,=
 this isn&#39;t worse than today, unilateral closes will<br>probably always=
 be identifiable on-chain.<br><br>&gt; Would kind of be nice if package RBF=
 would detect a &quot;sibling output spend&quot;<br>&gt; conflict, and knoc=
k it out of the mempool via the other replacement rules?<br>&gt; Getting ri=
d of the requirement to 1 block csv lock every output would be<br>&gt; quit=
e nice from a smart contracting composability point of view.<br><br>+1, tha=
t would be very neat!<br><br>This may be already covered by the current pac=
kage RBF logic, in that<br>scenario we are simply replacing [ParentTx, Chil=
dTx1] with<br>[ParentTx, ChildTx2] that pays more fees, right?<br><br>&gt; =
1) I do think that we should seriously consider allowing OP_TRUE to become<=
br>&gt; a standard script type as part of this policy update. If pinning is=
 solved,<br>&gt; then there&#39;s no reason to require all those extra byte=
s for &quot;binding&quot; an<br>&gt; anchor to a specific wallet/user. We c=
an save quite a few bytes by having<br>&gt; the input be empty of witness d=
ata.<br>&gt; 2) If we allow for a single dust-value(0 on up) output which i=
s immediately<br>&gt; spent by the package, anchors become even easier to t=
o design. No value has<br>&gt; to be &quot;sapped&quot; from contract parti=
cipants to make an anchor output. There&#39;s<br>&gt; more complications fo=
r this, such as making sure the parent transaction is<br>&gt; dropped if th=
e child spend is dropped, but maybe it&#39;s worth the squeeze.<br><br>I al=
so think both of these could be quite useful. This would probably always<br=
>be used in combination with a parent transaction that pays 0 fees, so the<=
br>0-value output would always be spent in the same block.<br><br>But this =
means we could end up with 0-value outputs in the utxo set, if for<br>some =
reason the parent tx is CPFP-ed via another output than the 0-value one,<br=
>which would be a utxo set bloat issue. But I&#39;d argue that we&#39;re pr=
obably<br>already creating utxo set bloat with the 330 sat anchor outputs (=
especially<br>since we use two of them, but only one is usually spent), so =
it would<br>probably be *better* than what we&#39;re doing today.<br><br>Th=
anks,<br>Bastien</div><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=
=3D"gmail_attr">Le=C2=A0lun. 26 sept. 2022 =C3=A0=C2=A003:22, Antoine Riard=
 via bitcoin-dev &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.or=
g" target=3D"_blank" rel=3D"noreferrer">bitcoin-dev@lists.linuxfoundation.o=
rg</a>&gt; a =C3=A9crit=C2=A0:<br></div><blockquote class=3D"gmail_quote" s=
tyle=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);pad=
ding-left:1ex"><div dir=3D"ltr">Hi Gloria,<br><br>Thanks for the progress o=
n package RBF, few early questions.<br><br>&gt; 2. Any descendant of an unc=
onfirmed V3 transaction must also be V3.<br><br>&gt; 3. An unconfirmed V3 t=
ransaction cannot have more than 1 descendant.<br><br>If you&#39;re a miner=
 and you receive a non-V3, second descendant of an unconfirmed V3 transacti=
on, if the offered fee is in the top mempool backlog, I think you would hav=
e an interest to accept such a transaction.<br><br>So I&#39;m not sure if t=
hose two rules are compatible with miners incentives...<br><br>&gt; 4. A V3=
 transaction that has an unconfirmed V3 ancestor cannot be<br>&gt; =C2=A0 =
=C2=A0larger than 1000 virtual bytes.<br><br>If I understand correctly the =
1000 vb upper bound rational, it would be to constraint the pinning counter=
party to attach a high fee to a child due to the limited size, if they woul=
d like this transaction to be stuck in the network mempools. By doing so=C2=
=A0 this child has high odds to confirm.<br><br>I still wonder if this comp=
atible with miner incentives in period of empty mempools, in the sense that=
 if you&#39;ve already a V3 transaction of size 100Kvb offering 2 sat/vb, i=
t&#39;s more interesting than a V3 replacement candidate of size 1000 vb of=
fering 10 sat/vb. It could be argued the former should be conserved.<br><br=
>(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...)<br><br>&gt; (Lower =
bound) the smaller this limit, the fewer UTXOs a child may use<br>&gt; to f=
und this fee-bump. For example, only allowing the V3 child to have<br>&gt; =
2 inputs would require L2 protocols to manage a wallet with high-value<br>&=
gt; UTXOs and make batched fee-bumping impossible. However, as the<br>&gt; =
fee-bumping child only needs to fund fees (as opposed to payments),<br>&gt;=
 just a few UTXOs should suffice.<br><br>Reminder for L2 devs, batched fee-=
bumping of time-sensitive confirmations of commitment transactions is unsaf=
e, as the counterparty could enter in a &quot;cat-and-mouse&quot; game to r=
eplace one of the batch element at each block to delay confirmation of the =
remaining elements in the batch, I think.<br><br>On the other hand, I wonde=
r if we wouldn&#39;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&#39;t need to do unilateral=
 closure. Let&#39;s say you close dozens of channels at the same time, a UT=
XO 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.<br><br>&gt=
; 1. The rule around unconfirmed inputs was<br>&gt; originally &quot;A pack=
age may include new unconfirmed inputs, but the<br>&gt; ancestor feerate of=
 the child must be at least as high as the ancestor<br>&gt; feerates of eve=
ry transaction being replaced.&quot;<br><br>Note, I think we would like thi=
s new RBF rule to also apply to single transaction package, e.g second-stag=
e HTLC transactions, where a counterparty pins a HTLC-preimage by abusing r=
ule 3. In that case, the honest LN node should be able to broadcast a &quot=
;at least as high ancestor feerate&quot; HTLC-timeout transaction. With `op=
tion_anchor_outputs&quot; there is no unconfirmed ancestor to replace, as t=
he commitment transaction, whatever the party it is originating from, shoul=
d already be confirmed.<br><br>&gt; &quot;Is this a privacy issue, i.e. doe=
sn&#39;t it allow fingerprinting LN<br>transactions based on nVersion?&quot=
;<br><br>As of today, I think yes you can already fingerprint LN transactio=
ns on the=C2=A0 spec-defined amount value of the anchor outputs, 330 sats. =
There is always one of them on post-anchor commitment transactions. And sad=
ly I would say we&#39;ll always have tricky fingerprints leaking from unila=
teral LN closures such as HTLC/PTLC timelocks...<br><br>&gt; &quot;Can a V2=
 transaction replace a V3 transaction and vice versa?&quot;<br><br>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&#39;t b=
e pinning vectors. Currently, commitment transactions signal replaceability=
.<br></div><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_a=
ttr">Le=C2=A0ven. 23 sept. 2022 =C3=A0=C2=A011:26, Gloria Zhao via bitcoin-=
dev &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"=
_blank" rel=3D"noreferrer">bitcoin-dev@lists.linuxfoundation.org</a>&gt; a =
=C3=A9crit=C2=A0:<br></div><blockquote class=3D"gmail_quote" style=3D"margi=
n:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex=
"><div dir=3D"ltr">Hi everyone,<br><br>I&#39;m writing to propose a very si=
mple set of mempool/transaction relay<br>policies intended to aid L2/contra=
ct protocols. I realized that<br>the previously proposed Package Mempool Ac=
cept package RBF [1]<br><div>had a few remaining problems after digging int=
o the RBF logic more [2].</div><div>This additional set of policies solves =
them without requiring a huge RBF overhaul.<br></div><br>I&#39;ve written a=
n implementation (and docs) for Bitcoin Core:<br><a href=3D"https://github.=
com/bitcoin/bitcoin/pull/25038" target=3D"_blank" rel=3D"noreferrer">https:=
//github.com/bitcoin/bitcoin/pull/25038</a><br><br>(You may notice that thi=
s proposal incorporates feedback on the PR - thanks Suhas Daftuar, Gregory =
Sanders, Bastien Teinturier, Anthony Towns, and others.)<br><br>If you are =
interested in using package RBF/relay to bump presigned<br>transactions, I =
think you may be interested in reviewing this proposal.<br>This should solv=
e Rule 3 pinning and perhaps allow us<br>to get rid of CPFP carve-out (yay!=
). I&#39;m keen to hear if people find<br>the 1-anchor-output, 1000vB child=
 limit too restrictive. Also, if you find a<br>pinning attack or something =
that makes it unusable for you, I would<br>really really like to know.<br><=
br>Note that transactions with nVersion=3D3 (&quot;V3 transactions&quot;) a=
re<br>currently non-standard in Bitcoin Core. That means **anything that wa=
s<br>standard before this policy change would still be standard<br>afterwar=
ds.** If you don&#39;t want your transactions to be subject to<br>these rul=
es, just continue whatever you&#39;re doing and don&#39;t use<br>nVersion=
=3D3. AFAICT this shouldn&#39;t break anything, but let me know if<br>this =
would be disruptive for you?<br><br>**New Policies:**<br><br>This includes:=
<br>- a set of additional policy rules applying to V3 transactions<br>- mod=
ifications to package RBF rules<br><br>**V3 transactions:**<br><br>Existing=
 standardness rules apply to V3 (e.g. min/max tx weight,<br>standard output=
 types, cleanstack, etc.). The following additional<br>rules apply to V3:<b=
r><br>1. A V3 transaction can be replaced, even if it does not signal BIP12=
5<br>=C2=A0 =C2=A0replaceability. (It must also meet the other RBF rules ar=
ound fees,<br>etc. for replacement to happen).<br><br>2. Any descendant of =
an unconfirmed V3 transaction must also be V3.<br><br>*Rationale*: Combined=
 with Rule 1, this gives us the property of<br>&quot;inherited&quot; replac=
eability signaling when descendants of unconfirmed<br>transactions are crea=
ted. Additionally, checking whether a transaction<br>signals replaceability=
 this way does not require mempool traversal,<br>and does not change based =
on what transactions are mined. It also<br>makes subsequent rules about des=
cendant limits much easier to check.<br><br>*Note*: The descendant of a *co=
nfirmed* V3 transaction does not need to be V3.<br><br>3. An unconfirmed V3=
 transaction cannot have more than 1 descendant.<br><br>*Rationale*: (Upper=
 bound) the larger the descendant limit, the more<br>transactions may need =
to be replaced. This is a problematic pinning<br>attack, i.e., a malicious =
counterparty prevents the transaction from<br>being replaced by adding many=
 descendant transactions that aren&#39;t<br>fee-bumping.<br><br>(Lower boun=
d) at least 1 descendant is required to allow CPFP of the<br>presigned tran=
saction. The contract protocol can create presigned<br>transactions paying =
0 fees and 1 output for attaching a CPFP at<br>broadcast time (&quot;anchor=
 output&quot;). Without package RBF, multiple anchor<br>outputs would be re=
quired to allow each counterparty to fee-bump any<br>presigned transaction.=
 With package RBF, since the presigned<br>transactions can replace each oth=
er, 1 anchor output is sufficient.<br><br>4. A V3 transaction that has an u=
nconfirmed V3 ancestor cannot be<br>=C2=A0 =C2=A0larger than 1000 virtual b=
ytes.<br><br>*Rationale*: (Upper bound) the larger the descendant size limi=
t, the<br>more vbytes may need to be replaced. With default limits, if the =
child<br>is e.g. 100,000vB, that might be an additional 100,000sats (at<br>=
1sat/vbyte) or more, depending on the feerate.<br><br>(Lower bound) the sma=
ller this limit, the fewer UTXOs a child may use<br>to fund this fee-bump. =
For example, only allowing the V3 child to have<br>2 inputs would require L=
2 protocols to manage a wallet with high-value<br>UTXOs and make batched fe=
e-bumping impossible. However, as the<br>fee-bumping child only needs to fu=
nd fees (as opposed to payments),<br>just a few UTXOs should suffice.<br><b=
r>With a limit of 1000 virtual bytes, depending on the output types, the<br=
>child can have 6-15 UTXOs, which should be enough to fund a fee-bump<br>wi=
thout requiring a carefully-managed UTXO pool. With 1000 virtual<br>bytes a=
s the descendant limit, the cost to replace a V3 transaction<br>has much lo=
wer variance.<br><br>*Rationale*: This makes the rule very easily &quot;tac=
ked on&quot; to existing<br>logic for policy and wallets. A transaction may=
 be up to 100KvB on its<br>own (`MAX_STANDARD_TX_WEIGHT`) and 101KvB with d=
escendants<br>(`DEFAULT_DESCENDANT_SIZE_LIMIT_KVB`). If an existing V3 tran=
saction<br>in the mempool is 100KvB, its descendant can only be 1000vB, eve=
n if<br>the policy is 10KvB.<br><br>**Package RBF modifications:**<br><br>1=
. The rule around unconfirmed inputs was<br>originally &quot;A package may =
include new unconfirmed inputs, but the<br>ancestor feerate of the child mu=
st be at least as high as the ancestor<br>feerates of every transaction bei=
ng replaced.&quot;<br><br>The package may still include new unconfirmed inp=
uts. However,<br>the new rule is modified to be &quot;The minimum between p=
ackage feerate<br>and ancestor feerate of the child is not lower than the i=
ndividual<br>feerates of all directly conflicting transactions and the ance=
stor<br>feerates of all original transactions.&quot;<br><br>*Rationale*: We=
 are attempting to ensure that the replacement<br>transactions are not less=
 incentive-compatible to mine. However, a<br>package/transaction&#39;s ance=
stor feerate is not perfectly representative<br>of its incentive compatibil=
ity; it may overestimate (some subset of<br>the ancestors could be included=
 by itself if it has other high-feerate<br>descendants or are themselves hi=
gher feerate than this<br>package/transaction). Instead, we use the minimum=
 between the package<br>feerate and ancestor feerate of the child as a more=
 conservative value<br>than what was proposed originally.<br><br>2. A new r=
ule is added, requiring that all package transactions with<br>mempool confl=
icts to be V3. This also means the &quot;sponsoring&quot;<br>child transact=
ion must be V3.<br><br>*Note*: Combined with the V3 rules, this means the p=
ackage must be<br>a child-with-parents package. Since package validation is=
 only<br>attempted if the transactions do not pay sufficient fees to be<br>=
accepted on their own, this effectively means that only V3<br>transactions =
can pay to replace their ancestors&#39; conflicts, and only<br>V3 transacti=
ons&#39; replacements may be paid for by a descendant.<br><br>*Rationale*: =
The fee-related rules are economically rational for<br>ancestor packages, b=
ut not necessarily other types of packages.<br>A child-with-parents package=
 is a type of ancestor package. It<br>may be fine to allow any ancestor pac=
kage, but it&#39;s more difficult<br>to account for all of the possibilitie=
s. For example, it gets much<br>harder to see that we&#39;re applying the d=
escendant limits correctly if<br>the package has a gnarly, many-generation,=
 non-tree shape. I&#39;m also<br>not sure if this policy is 100% incentive-=
compatible if the sponsor<br>is not a direct descendant of the sponsee.<br>=
<br>Please see doc/policy/version3_transactions.md and<br>doc/policy/packag=
es.md in the PR for the full set of rules.<br><br>**Intended usage for LN:*=
*<br><br>Commitment transactions should be V3 and have 1 anchor output. The=
y<br>can be signed with 0 fees (or 1sat/vbyte) once package relay is deploy=
ed<br>on a significant portion of the network. If the commitment tx must<br=
>be broadcast, determine the desired feerate at broadcast time and<br>spend=
 the anchor output in a high feerate transaction. I&#39;m going to<br>call =
the broadcasted commitment tx &quot;the parent&quot; and the attached<br>fe=
e-bumping tx &quot;the child.&quot;<br><br>- This child must be V3.<br>- Th=
is child must be at most 1000vB. Note this restricts the<br>=C2=A0 number o=
f inputs you can use to fund the fee bump. Depending<br>on the output types=
, this is around 6-15.<br>- One child may fund fees for multiple commitment=
 tx (&quot;batched<br>=C2=A0 fee-bumping&quot;).<br>- To do a second fee-bu=
mp to add more fees, replace the<br>=C2=A0 *child* with a higher-feerate tx=
. Do not try to attach a grandchild.<br><br>Otherwise, never try to spend f=
rom an unconfirmed V3 transaction. The<br>descendant limits for V3 transact=
ions are very restrictive.<br><br>**Expected Questions:**<br><br>&quot;Does=
 this fix Rule 3 Pinning?&quot;<br>Yes. The V3 descendant limit restricts b=
oth you and your counterparty.<br>Assuming nodes adopted this policy, you m=
ay reasonably assume that you<br>only need to replace the commitment transa=
ction + up to 1000vB.<br><br>&quot;Only 1 anchor output? What if I need to =
bump counterparty&#39;s commitment tx in mempool?&quot;<br><div>You won&#39=
;t need to fee-bump a counterparty&#39;s commitment tx using CPFP.</div><di=
v>You would just package RBF it by attaching a high-feerate child to</div>y=
our commitment tx.<br><br>&quot;Is this a privacy issue, i.e. doesn&#39;t i=
t allow fingerprinting LN<br>transactions based on nVersion?&quot;<br>Indee=
d it may be unrealistic to assume V3 transactions will be in<br>widespread =
use outside of L2. IIUC, unilateral closes are already<br>obvious LN transa=
ctions because of the HTLC inputs. For e.g.<br>cooperative closes and opens=
, I think it makes sense to continue using<br>V2. So, unless I&#39;m missin=
g something, this shouldn&#39;t make it worse.<br><br>&quot;So a V3 transac=
tion that doesn&#39;t signal BIP125 replaceability is<br>replaceable? Is th=
at a backward compatibility issue?&quot;<br>Yes it&#39;s replaceable. It&#3=
9;s not an issue AFAICT because,<br>under previous policy, the V3 transacti=
on wouldn&#39;t have been<br>in the mempool in the first place.<br><br>&quo=
t;Can a V2 transaction replace a V3 transaction and vice versa?&quot;<br>Ye=
s, otherwise someone can use V3 transactions to censor V2<br>transactions s=
pending shared inputs. Note if the<br>original V3 transaction has an unconf=
irmed V3 parent, this would<br>violate the &quot;inherited V3&quot; rule an=
d would be rejected.<br><br>Thanks for reading! Feedback and review would b=
e much appreciated.<br><br>[1]: <a href=3D"https://lists.linuxfoundation.or=
g/pipermail/bitcoin-dev/2021-September/019464.html" target=3D"_blank" rel=
=3D"noreferrer">https://lists.linuxfoundation.org/pipermail/bitcoin-dev/202=
1-September/019464.html</a><br><div>[2]: <a href=3D"https://lists.linuxfoun=
dation.org/pipermail/bitcoin-dev/2022-January/019817.html" target=3D"_blank=
" rel=3D"noreferrer">https://lists.linuxfoundation.org/pipermail/bitcoin-de=
v/2022-January/019817.html</a></div><div><br></div>Best,<br>Gloria</div>
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