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From: Antoine Riard <antoine.riard@gmail.com>
Date: Wed, 2 Nov 2022 15:50:26 -0400
Message-ID: <CALZpt+HHsqN0tq7LYfR9YFRjWo_dKwDDAwB_HPLacafVj8anWw@mail.gmail.com>
To: Suhas Daftuar <sdaftuar@gmail.com>, 
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] On mempool policy consistency
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Hi Suhas,

From my understanding, the main crux of the reasoning exposed in your post
would be to solidify the transaction-relay paradigm we have been following
during the last years, e.g introducing the carve-out rule specifically for
lightning commitment transactions, or more recently version=3D3 transaction=
s.
I think this paradigm could be described summarly as "to each use-case
belongs a set of transaction-relay policy rules". Some of this set of rules
could aim to non-replacement guarantees to the consumers of transactions
signaling under this regime (e.g zeroconf). Another set of rules could
provide a high-guarantee that a transaction would always get to a miner, no
matter what the state of node's mempools on the network (e.g "maximal rbf"
for contracting protocols).

First, coming out of my mind, we would have to consider isolation between
each set of policy rules, ensuring the signaling mechanism cannot be abused
by an attacker to create a new pinning vector. E.g, a hypothetical concern
could be BIP125 rules interfering with version=3D3 policy to block the
replacement of better ancestor feerate packages. Further, for each set of
policy rules arises the question of internal consistency, again an attacker
could abuse them to pin transactions
propagation. I think an earlier version of version=3D3 was suffering from
this concern of not scoping potential "junk" ancestors. None of those
issues are unsolvable, however we should be well-aware of the
non-negligeable design complexity encumbered by transaction-relay protocol
developers to achieve the correct goal. There is not only a need to ensure
careful policy rules security analysis, but further to communicate well
their usage to second-layers and wallets developers (a task far from easy
with all the confusions contained by current BIP125).

Now, in the evaluation process of a set of policy rules soundness are
proposed a few reasoning heuristics: namely that it shouldn't interfere
sensibly with a anti-DoS mempool acceptance algorithm, or shouldn't
interfere with other protocols on the network, or counter the interests of
miners or node operators. I hold the belief the latest question could be
the one raising the most concerns. Browsing in the history of Bitcoin Core,
I think one of the design goals aimed for has always been to level the
playing field between miners, e.g BIP152 improving block transfer latency
to reduce orphan rate. Following this principle, we might wonder if our
transaction-relay network should guarantee some equal access to transaction
information to all the miners.

In the present case of a non-replacement policy regime, we could see the
following situation to arise, on one side miners deploying private
transaction-relay communication channels or API to capture higher fees
income from non-standard transactions. On the other-side, transaction
issuers or consumers bypass the standard transaction-relay policy rules.
Bypass could be motivated by either a zeroconf service double-spend, or
faster confirmation of a collaborative transaction. E.g, to reuse the
example of unconfirmed transaction chaining, where the sender commit to
non-replacement by opting out from the RBF flag, this commitment could be
reevaluated in the light of changing network mempools congestion, or
liquidity preferences (e.g the quick need to open LN routing channels). The
sender could leverage such hypothetical private transaction-relay
communication channels to revoke its non-replacement commitment. Therefore
discrepancies between a set of policy rules design and miners incentives
sounds to lead to informational asymmetries, harmful for the long-term
decentralization of the mining ecosystem. Of course, miner incomes
asymmetries due to edge in transaction flows access might not be weighted
as serious today, in a world where transaction fees contribute to most of
the block reward, this is far more worrying!

Of course, one position could be to estimate that miner centralization is
beyond the scope of responsibility of the Bitcoin Core project. Or at least
as a result of lightweighted risks.

Such discrepancy between a set of policy rules design and miners incentives
could also lead to hidden security risks for second-layers. As we see more
security assumptions made on policy rules extension, e.g version=3D3, a
lightning channel counterparty could have a competing interest to forge a
raw package suiting better incentives, and as such nullify the security
advantage expected. This could be seen as a loose concern, however the last
time we have seen an actor deliberately providing non-standard transactions
to a miner to break second-layers was yesterday [0]!. From observing other
cryptocurrencies spaces, such "MEV-style" attacks could be more and more
concerning [1]. I don't think we should assume miners to behave as "network
gentlemen", in a world where mining can be anonymous, permissionless and
censorship-resistant (e.g Stratum V2 giving back template construction to
miners operators rather than pools).

According to me, one of the harder problem we're seeing with this fullrbf
discussion is the lack of a consistent, grounded and well-understood miner
incentive model, where not only block template construction but also
transaction collection and replacement strategies are analyzed, and against
which we could simulate the efficiency of a policy. Assuming we would have
such a model, rather than qualify a policy rule as incentive-compatible in
a binary fashion, we could evaluate them on a scale, and agree on when
they're satisfying enough in face of technical complexity, validation
resources, margin of adversarial exploitation, or whatever other relevant
criteria.

Answering a few other points raised in this post, what appears to me
obscure is the qualification that fullrbf doesn't solve the DoS issues for
contracting protocols (e..g coinjoin/dual-funded lightning). If I can
understand, it's on the ground that the imperfections of BIP125 underscore
only the direct conflict feerate. It should be remembered that allowing
replacement without considering the opting flag would be already an
improvement against the DoS attack, as the attack would have to offer a
more compelling feerate to maintain the pin. Improvements of BIP125 can
happen on the top, but solving the opt-out double-spend issue sounds to me
a prerequisite.

Beyond that, I think few questions are laid out on the conceptual soundness
of v3 transaction policy w.r.t concerns raised about fullrbf today. In my
opinion, I'm sadly with most of them, especially that miners might earn
more revenue if we allowed multiple descendant v3 transactions and the
unenforceable promise for the recipient of such package to not add more
high-value children, I've echoed those concerns earlier in the review of
nVersion=3D3 proposal [2]. We might have to swallow the bullet for now, and
be okay as lightning developers and operators that there is only a social
inertia of the miners and lack of reliable communication channels towards
them by an adversarial counterparty to offer security [3]. Additionally, I
think it would be acceptable to have an option to disable v3 transaction
policy, an operator could be willing to reduce the CPU/memory DoS surface
of its node from partaking to any package relay. Even if it comes at the
loss of a better view of blockspace demand and downgrades its
fee-estimation, I think we should give the maximum flexibility to operators
in choosing their risk model.

To put it in a nutshell, if we would like to pursue further in the paradigm
that "to each use-case belongs its set of policy rules" (as long as they
don't introduce any harm for the network stakeholders), I believe we would
be more grounded with a better quantitative understanding of so-called
"miners incentives". I'm still wondering if it's realistic to deploy policy
rules that are not sustainable in face of long-term mining dynamics.

Best,
Antoine


[0] https://github.com/lightningnetwork/lnd/issues/7096

[1] On risks of introducing miner harvesting attacks, especially when you
consider implications on lightning-style constructions
https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-February/002=
569.html
and
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-November/01961=
5.html

[2]
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-September/0209=
39.html

"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...=
"

[3] This wonders if we should look forward in the future to lock in the
CPFP weight of a Lightning commitment with some new consensus semantic, or
leveraging any covenant magic, cf.
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-March/020122.h=
tml
and
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-October/020991=
.html

Le lun. 31 oct. 2022 =C3=A0 11:02, Suhas Daftuar via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> a =C3=A9crit :

> AJ,
>
> Thanks for the thoughtful post. I think your observations about how we
> view mempool policy in the Bitcoin Core project, and how that seems to be
> changing in the discussions around `-mempoolfullrbf`, are on-point and
> provide a helpful baseline for considering future policy changes.
>
> For a long time I viewed fullrbf as an eventuality and I considered mysel=
f
> to be philosophically supportive of the idea.  However, after giving this
> issue some thought in the past few weeks, I am reversing my thinking on
> this.  Concretely, I will argue that we should continue to maintain a rel=
ay
> policy where replacements are rejected for transactions that don't opt-in
> to RBF (as described in BIP 125), and moreover, that we should remove the
> `-mempoolfullrbf` flag from Bitcoin Core=E2=80=99s latest release candida=
te and not
> plan to release software with that flag, unless (or until) circumstances
> change on the network, which I'll discuss below.
>
> This is, of course, a nuanced topic, and among the considerations is a
> philosophy of how to think about the relay policy and configuration optio=
ns
> that we make available in Bitcoin Core (a consideration that is perhaps
> unique to that project, but I think relevant for this mailing list).
>
> I'll start with some technical issues regarding the benefits of enabling
> fullrbf on the network.  In the current BIP 125 regime, every time a
> transaction is created, a choice is made whether to subject the transacti=
on
> to BIP 125=E2=80=99s RBF rules or not (based on the sequence values of th=
e
> inputs).  So given that users can already opt-in to RBF, the benefit of a
> =E2=80=9Cfullrbf=E2=80=9D network policy would be if, somehow, RBF users =
were still denied
> the benefits of RBF due to the existence of other transactions that don=
=E2=80=99t
> opt-in.
>
> Along those lines, Antoine Riard brought up[1] a DoS vector that is
> available to someone who wants to interfere with multi-party funded
> transactions, and suggested that fullrbf would eliminate the problem.
> After exploring that question again in this thread (thanks to Greg Sander=
s
> for clarifying this to me), I understand that the issue is around ensurin=
g
> that a multiparty (coinjoin-type) protocol is able to make eventual
> progress, by having a candidate multiparty transaction either eventually
> confirm or become conflicted with something that has been confirmed, in
> which case the double-spend information could be used to start a new
> coinjoin round with fewer participants.  The concern Antoine and Greg hav=
e
> brought up is that non-rbf transactions can persist in the mempool
> ~indefinitely (at a low feerate and not subject to replacement) and
> interfere with progress being made in a coinjoin protocol.
>
> However, it seems to me that similar problems exist for such a protocol
> even in a fullrbf world, as we understand that term today.  I mentioned t=
he
> ability for rbf =E2=80=9Cpinning=E2=80=9D to interfere with relay of the =
multiparty
> transaction (even if the conflicting transaction signals for RBF =E2=80=
=93 a set of
> large but low feerate conflicting transactions can persist in the mempool
> and make it difficult for the coinjoin transaction from confirming, at
> least without attaching a very large fee); and as Greg mentioned in a
> followup, the BIP 125 rule to only permit 100 transactions to be removed
> from the mempool at a time during a replacement can also be used to pin a
> coinjoin protocol in the same way as a non-rbf transaction today.  It see=
ms
> to me that what these multiparty protocols actually need is some sort of
> "maximal rbf" network policy: a way to guarantee that a transaction which
> should be desirable for a miner to mine would always get to a miner and
> considered for inclusion in a block, no matter what the state of node=E2=
=80=99s
> mempools on the network.
>
> While that sounds like a reasonable thing to want on its face (and worth
> working on), it's not how opt-in RBF works today, nor is it how transacti=
on
> relay has ever conceptually worked.  We have not, thus far, been able to
> come up with a total ordering on transaction desirability.  Moreover, due
> to all the DoS issues that exist with transaction relay, there are plenty
> of seemingly legitimate ways to construct transactions that would not rel=
ay
> well on the network.  Relay has only ever been a best-efforts concept,
> where we carve out a small subset of the entire transaction universe for
> which we try to optimize propagation.  The idea behind this approach is
> that if every use case we can come up with has some way to achieve its
> goals using transactions that should (eventually) be able to relay, then
> users wouldn=E2=80=99t have much demand for transactions that would devia=
te from
> the supported policies, and we therefore shouldn=E2=80=99t need to worry =
too much
> about incentive compatibility concerns when it comes to transaction types
> that wouldn=E2=80=99t relay at all, even if they are high feerate.  (And =
when those
> situations arise where the standard transactions do not accommodate some
> needed use case, developers typically work to define a policy that is
> compatible with our anti-DoS goals to support such use cases, such as wit=
h
> the recent proposal for version=3D3 transactions [2].)
>
> BIP 125's RBF rules themselves were an effort to carve out just a subset
> of situations where a transaction should evict conflicting ones -- it was
> not a design that anyone thought would ensure that all replacements which
> "should" be mined would always propagate.  And I don't believe that we kn=
ow
> how to design policy rules that would achieve the goals of this kind of
> multiparty protocol in a DoS resistant way, today.  Along those lines, I
> would point out that even the BIP 125 design itself is not entirely
> incentive compatible, in that it is possible to construct a replacement
> transaction that would evict transactions which would be preferable to be
> included in a block! [3]  (This has been known for years, but fixing this
> has proven difficult, and the only way to fix it that I=E2=80=99m aware o=
f would be
> to make BIP 125 RBF even more restrictive than it is today. I do think th=
is
> is something that needs to be worked on.)
>
> Given the limitations of RBF as we have it today, it appears to be
> incorrect that a fullrbf network policy would solve the problems Antoine
> raised.  And so absent any other examples, it does not seem to me that
> fullrbf solves any problems for RBF users, who are already free to choose
> to subject their transactions to BIP 125=E2=80=99s RBF policy.  From this
> perspective, "enabling fullrbf" is really just taking away user choice to
> opt a transaction into a non-replacement policy regime.
>
> I think we should ask, then, whether it is reasonable on its face that
> users might want to opt-in to a non-replacement policy?  Or in other word=
s,
> is it reasonable for a user to mark a transaction as non-replaceable and
> have that indication be enforced by the network? Note that these are two
> different questions: you could imagine a world where fullrbf is a dominan=
t
> policy, but users still use the BIP 125 signaling method to indicate, in =
an
> unenforced way, their intention to not replace a transaction.  This might
> give useful information to the network or the recipient for how to intera=
ct
> with such a transaction.
>
> And I think that it's entirely possible that users would continue to use
> the BIP 125 signaling to indicate that they do not intend to replace a
> transaction.  For better or worse, this might be because zeroconf service=
s
> continue to differentiate their behavior based on such a signal (possibly
> in conjunction with other factors), or it could be because there are othe=
r
> behaviors that could be utilized more effectively if the transaction
> originator has made such a signal, such as the recipient chaining an
> unconfirmed transaction as a way to bump the fee (CPFP) [4].
>
> If it were to be the case that users continued to use BIP 125-style
> signaling to indicate that they do not plan to replace a transaction, wou=
ld
> that be harmful to the network?  This is not something we can stop in our
> policy rules (short of censoring such transactions, an obviously bad
> idea).  I think network actors can always do things that we might think a=
re
> harmful for the network, but that doesn=E2=80=99t mean that there are no =
legitimate
> use cases for the tools that such actors might be using.  Just because
> someone might use some policy to adopt a zeroconf model, doesn=E2=80=99t =
mean that
> others aren=E2=80=99t using the same policy to achieve benign ends (such =
as better
> CPFP behavior).
>
> Moreover, while users might attempt to exploit services that offer
> zeroconf or other differentiated behavior to non-replacement signaling
> transactions, they also might not -- I think predicting user behavior in
> this way (and specifically predicting the complexities of what a business
> might do and whether users might try to subvert it) is beyond the scope o=
f
> what we can do as protocol developers.  Instead, I think we can try to
> answer a different question: if a group of users were to want the ability
> to opt-in to a non-replacement policy regime, is that a technically sound
> option for us to have on the network and enforce in software?
> Specifically, does that interfere with having a sensible anti-DoS mempool
> acceptance algorithm, or interfere with other protocols on the network, o=
r
> necessarily run counter to the interests of miners or node operators?
>
> And I think the answer to that question, in looking at the difference
> between opt-in RBF and fullrbf, is no: offering the ability to opt-in to =
a
> non-replacement regime for transactions doesn't introduce any fundamental
> issues with software or network policy or other protocols.  In a world
> where we only had fullrbf, I could imagine at some point down the road
> proposing a non-replacement signal myself, because the complexities aroun=
d
> transaction chains (and pinning) are more complex for the RBF case than f=
or
> the non-RBF case (and BIP 125 is not always incentive compatible to begin
> with!).  Conceptually, this is no different to me than the version=3D3
> transaction policy proposal that has been advancing, if we think of it as=
 a
> special set of restrictions on transactions designed to accommodate a
> particular use case.
>
> Philosophically, I think we should be looking to add non-interfering use
> cases to what the network supports.
>
> To those who argue for making fullrbf a default policy on the network (or
> even just offering a flag for users to enable fullrbf), I pose this
> hypothetical: suppose we deploy the v3 transaction policy proposal (which=
 I
> hope will happen in the near future).  That policy would restrict the way=
s
> that outputs of a v3 transaction can be spent while the transaction is
> unconfirmed, including by limiting the number and size of descendants tha=
t
> such a transaction can have, and limiting the types of unconfirmed
> ancestors that can be included.  Suppose in a few years someone proposes
> that we add a "-disable_v3_transaction_enforcement" flag to our software,
> to let users decide to turn off those policy restrictions and treat v3
> transactions the same as v2, for all the same reasons that could be argue=
d
> today with fullrbf: miners might earn more revenue if we allowed multiple
> descendant v3 transactions; it's illogical for the recipient of a v3
> transaction to believe what is a fundamentally unenforceable promise of a
> sender to not issue more high value children that descend from an
> unconfirmed transaction; it's inappropriate for Bitcoin Core to dictate
> policy on the network and we should honor user choice to turn off that fl=
ag
> if that=E2=80=99s what users want; if users are relying on v3=E2=80=99s p=
olicy restrictions
> for security then that is an unstable model and we should assume it will
> get broken[5].
>
> It=E2=80=99s obvious to me that adding a flag to disable v3 policy would =
be
> subversive to making the lightning use case for v3 transactions work.  An=
d
> so my response to such a hypothetical proposal would be to argue that no,
> we should not enable users to disable this policy, because as long as tha=
t
> policy is just optional and working for those who want it, it shouldn=E2=
=80=99t
> harm anyone that we offer a tighter set of rules for a particular use
> case.  Adding a way to bypass those rules is just trying to break someone
> else=E2=80=99s use case, not trying to add a new one.  We should not wiel=
d
> "incentive compatibility" as a bludgeon for breaking things that appear t=
o
> be working and not causing others harm.
>
> I think this is exactly what is happening with fullrbf.
>
> In comparing v3 transaction policy with opting out of transaction
> replacement, there is of course one significant difference that I have
> ignored thus far: I think the real difference is an opinion about whether
> non-replacement transactions that are being used today are, overall, bad
> for Bitcoin, and whether lightning=E2=80=99s use of v3 transactions in th=
e future
> would be bad for Bitcoin. If you think that zeroconf is unequivocally bad=
,
> and that no one will be able to plausibly construct a case that lightning
> is bad, then that qualitative judgment might sway you to not worrying abo=
ut
> the philosophical issues I've raised above, because these situations can =
be
> distinguished.
>
> However I am not personally willing to say that I think, overall,
> non-rbf-signaling transactions in use on the network today are bad for
> Bitcoin (or that fullrbf is definitely good =E2=80=93 BIP 125=E2=80=99s r=
bf rules are
> something we=E2=80=99ve been trying to improve upon for years, with littl=
e
> success).  Nor am I convinced that someone couldn=E2=80=99t put together =
a cogent
> argument for lightning being bad for Bitcoin, because of its reliance on
> relay policies that are difficult to design and impossible to guarantee a=
s
> part of its security model.  So I choose instead to merely make a judgmen=
t
> that seems more factually verifiable, which is that non-replacement is a
> policy widely in use on the network today, and we largely don't have reas=
on
> to think (as far as I know!) that the network is seeing a lot of
> transactions that would violate that policy.
>
> If it did turn out that users were commonly signaling non-replacement, bu=
t
> then signing and trying to relay doublespends, then I think that would be=
 a
> very good reason for Bitcoin Core to adopt fullrbf to reflect the reality
> of what is happening.  In the meantime, I think it makes more sense to sa=
y
> that because we have BIP 125, there seems to be no need for users to sign=
al
> one way and behave another, and therefore there is no need to offer
> software that might break a policy that is working well for some users.
> Other software projects might choose differently, and it is after all a
> permissionless network, so if this is in fact an unstable equilibrium tha=
t
> will not last, then presumably someday it will be apparent it is not
> working and we=E2=80=99ll abandon it.  But I think the philosophy of tran=
saction
> relay policy in Bitcoin Core should be to support disparate use cases in
> order to try to make everything work better, rather than break things
> prematurely because we guess others will break them eventually anyway.
>
> For those that have read this long email and still favor a fullrbf networ=
k
> policy (or even just the ability for users to be able to turn on fullrbf
> for themselves), I=E2=80=99d ask for thoughts on the following questions,=
 which
> have guided my thinking on this:
>
> Does fullrbf offer any benefits other than breaking zeroconf business
> practices?  If so, what are they?
>
> Is it reasonable to enforce BIP 125's rbf rules on all transactions, if
> those rules themselves are not always incentive compatible?
>
> If someone were to propose a command line option that breaks v3
> transaction relay in the future, is there a logical basis for opposing th=
at
> which is consistent with moving towards fullrbf now?
>
> Cheers,
> Suhas
>
>
> [1]
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-May/003033=
.html
>
> [2]
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-September/02=
0937.html
>
> [3] This is because under the BIP 125 rules, the feerate of the
> replacement transaction is not compared to the individual feerates of all
> transactions being evicted =E2=80=93 we just compare feerates with the tr=
ansactions
> that are directly in conflict (and not their descendants). So it=E2=80=99=
s possible
> for a transaction that would evict 2 or more transactions to have a highe=
r
> feerate than the direct conflicts, and higher total fee than the set bein=
g
> evicted, but have a lower feerate (eg if it is larger) than that of some
> subset of the set of transactions being evicted.
>
> [4]  Chaining unconfirmed transactions when the sender might RBF the
> parent is far riskier than if the sender indicates they don't plan to do =
so
> (chaining onto an RBF transaction creates pinning issues for the sender,
> and risks having the child wiped out if the parent is replaced), so I thi=
nk
> this is a concrete reason why signaling that a transaction won=E2=80=99t =
be
> replaced could be useful.
>
> [5] This is a subtle point. I don=E2=80=99t think v3 transactions create =
an
> unreasonable security assumption for the use case it is being designed fo=
r.
> However, I don=E2=80=99t think anyone could rule out the possibility that=
 someone
> could adopt a usage pattern for v3 transactions that subverts the intent =
of
> this policy.  For example, if users started using v3 transactions for all
> their payments, then the limitations on the number of descendants could
> directly interfere with CPFP by a recipient, and someone could argue that
> we should break the policy in order to allow for this hypothetical
> behavior. I think this is a similar form of argument as saying that
> zeroconf practices + BIP 125 create an incentive to double-spend non-rbf
> signaling transactions.
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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<div dir=3D"ltr">Hi Suhas,<br><br>From my understanding, the main crux of t=
he reasoning exposed in your post would be to solidify the transaction-rela=
y paradigm we have been following during the last years, e.g introducing th=
e carve-out rule specifically for lightning commitment transactions, or mor=
e recently version=3D3 transactions. I think this paradigm could be describ=
ed summarly as &quot;to each use-case belongs a set of transaction-relay po=
licy rules&quot;. Some of this set of rules could aim to non-replacement gu=
arantees to the consumers of transactions signaling under this regime (e.g =
zeroconf). Another set of rules could provide a high-guarantee that a trans=
action would always get to a miner, no matter what the state of node&#39;s =
mempools on the network (e.g &quot;maximal rbf&quot; for contracting protoc=
ols).<br><br>First, coming out of my mind, we would have to consider isolat=
ion between each set of policy rules, ensuring the signaling mechanism cann=
ot be abused by an attacker to create a new pinning vector. E.g, a hypothet=
ical concern could be BIP125 rules interfering with version=3D3 policy to b=
lock the replacement of better ancestor feerate packages. Further, for each=
 set of policy rules arises the question of internal consistency, again an =
attacker could abuse them to pin transactions<br>propagation. I think an ea=
rlier version of version=3D3 was suffering from this concern of not scoping=
 potential &quot;junk&quot; ancestors. None of those issues are unsolvable,=
 however we should be well-aware of the non-negligeable design complexity e=
ncumbered by transaction-relay protocol developers to achieve the correct g=
oal. There is not only a need to ensure careful policy rules security analy=
sis, but further to communicate well their usage to second-layers and walle=
ts developers (a task far from easy with all the confusions contained by cu=
rrent BIP125).<br><br>Now, in the evaluation process of a set of policy rul=
es soundness are proposed a few reasoning heuristics: namely that it should=
n&#39;t interfere sensibly with a anti-DoS mempool acceptance algorithm, or=
 shouldn&#39;t interfere with other protocols on the network, or counter th=
e interests of miners or node operators. I hold the belief the latest quest=
ion could be the one raising the most concerns. Browsing in the history of =
Bitcoin Core, I think one of the design goals aimed for has always been to =
level the playing field between miners, e.g BIP152 improving block transfer=
 latency to reduce orphan rate. Following this principle, we might wonder i=
f our transaction-relay network should guarantee some equal access to trans=
action information to all the miners.<br><br>In the present case of a non-r=
eplacement policy regime, we could see the following situation to arise, on=
 one side miners deploying private transaction-relay communication channels=
 or API to capture higher fees income from non-standard transactions. On th=
e other-side, transaction issuers or consumers bypass the standard transact=
ion-relay policy rules. Bypass could be motivated by either a zeroconf serv=
ice double-spend, or faster confirmation of a collaborative transaction. E.=
g, to reuse the example of unconfirmed transaction chaining, where the send=
er commit to non-replacement by opting out from the RBF flag, this commitme=
nt could be reevaluated in the light of changing network mempools congestio=
n, or liquidity preferences (e.g the quick need to open LN routing channels=
). The sender could leverage such hypothetical private transaction-relay co=
mmunication channels to revoke its non-replacement commitment. Therefore di=
screpancies between a set of policy rules design and miners incentives soun=
ds to lead to informational asymmetries, harmful for the long-term decentra=
lization of the mining ecosystem. Of course, miner incomes asymmetries due =
to edge in transaction flows access might not be weighted as serious today,=
 in a world where transaction fees contribute to most of the block reward, =
this is far more worrying!<br><br>Of course, one position could be to estim=
ate that miner centralization is beyond the scope of responsibility of the =
Bitcoin Core project. Or at least as a result of lightweighted risks.<br><b=
r>Such discrepancy between a set of policy rules design and miners incentiv=
es could also lead to hidden security risks for second-layers. As we see mo=
re security assumptions made on policy rules extension, e.g version=3D3, a =
lightning channel counterparty could have a competing interest to forge a r=
aw package suiting better incentives, and as such nullify the security adva=
ntage expected. This could be seen as a loose concern, however the last tim=
e we have seen an actor deliberately providing non-standard transactions to=
 a miner to break second-layers was yesterday [0]!. From observing other cr=
yptocurrencies spaces, such &quot;MEV-style&quot; attacks could be more and=
 more concerning [1]. I don&#39;t think we should assume miners to behave a=
s &quot;network gentlemen&quot;, in a world where mining can be anonymous, =
permissionless and censorship-resistant (e.g Stratum V2 giving back templat=
e construction to miners operators rather than pools).<br><br>According to =
me, one of the harder problem we&#39;re seeing with this fullrbf discussion=
 is the lack of a consistent, grounded and well-understood miner incentive =
model, where not only block template construction but also transaction coll=
ection and replacement strategies are analyzed, and against which we could =
simulate the efficiency of a policy. Assuming we would have such a model, r=
ather than qualify a policy rule as incentive-compatible in a binary fashio=
n, we could evaluate them on a scale, and agree on when they&#39;re satisfy=
ing enough in face of technical complexity, validation resources, margin of=
 adversarial exploitation, or whatever other relevant criteria.<br><br>Answ=
ering a few other points raised in this post, what appears to me obscure is=
 the qualification that fullrbf doesn&#39;t solve the DoS issues for contra=
cting protocols (e..g coinjoin/dual-funded lightning). If I can understand,=
 it&#39;s on the ground that the imperfections of BIP125 underscore only th=
e direct conflict feerate. It should be remembered that allowing replacemen=
t without considering the opting flag would be already an improvement again=
st the DoS attack, as the attack would have to offer a more compelling feer=
ate to maintain the pin. Improvements of BIP125 can happen on the top, but =
solving the opt-out double-spend issue sounds to me a prerequisite.<br><br>=
Beyond that, I think few questions are laid out on the conceptual soundness=
 of v3 transaction policy w.r.t concerns raised about fullrbf today. In my =
opinion, I&#39;m sadly with most of them, especially that miners might earn=
 more revenue if we allowed multiple descendant v3 transactions and the une=
nforceable promise for the recipient of such package to not add more high-v=
alue children, I&#39;ve echoed those concerns earlier in the review of nVer=
sion=3D3 proposal [2]. We might have to swallow the bullet for now, and be =
okay as lightning developers and operators that there is only a social iner=
tia of the miners and lack of reliable communication channels towards them =
by an adversarial counterparty to offer security [3]. Additionally, I think=
 it would be acceptable to have an option to disable v3 transaction policy,=
 an operator could be willing to reduce the CPU/memory DoS surface of its n=
ode from partaking to any package relay. Even if it comes at the loss of a =
better view of blockspace demand and downgrades its fee-estimation, I think=
 we should give the maximum flexibility to operators in choosing their risk=
 model.<br><br>To put it in a nutshell, if we would like to pursue further =
in the paradigm that &quot;to each use-case belongs its set of policy rules=
&quot; (as long as they don&#39;t introduce any harm for the network stakeh=
olders), I believe we would be more grounded with a better quantitative und=
erstanding of so-called &quot;miners incentives&quot;. I&#39;m still wonder=
ing if it&#39;s realistic to deploy policy rules that are not sustainable i=
n face of long-term mining dynamics.<br><br>Best,<br>Antoine<br><br><br>[0]=
 <a href=3D"https://github.com/lightningnetwork/lnd/issues/7096">https://gi=
thub.com/lightningnetwork/lnd/issues/7096</a><br><br>[1] On risks of introd=
ucing miner harvesting attacks, especially when you consider implications o=
n lightning-style constructions <a href=3D"https://lists.linuxfoundation.or=
g/pipermail/lightning-dev/2020-February/002569.html">https://lists.linuxfou=
ndation.org/pipermail/lightning-dev/2020-February/002569.html</a> and <a hr=
ef=3D"https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-November=
/019615.html">https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-=
November/019615.html</a><br><br>[2] <a href=3D"https://lists.linuxfoundatio=
n.org/pipermail/bitcoin-dev/2022-September/020939.html">https://lists.linux=
foundation.org/pipermail/bitcoin-dev/2022-September/020939.html</a><br><br>=
&quot;If you&#39;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 bac=
klog, I think you would have an interest to accept such a transaction.<br><=
br>So I&#39;m not sure if those two rules are compatible with miners incent=
ives...&quot;<br><br>[3] This wonders if we should look forward in the futu=
re to lock in the CPFP weight of a Lightning commitment with some new conse=
nsus semantic, or leveraging any covenant magic, cf. <a href=3D"https://lis=
ts.linuxfoundation.org/pipermail/bitcoin-dev/2022-March/020122.html">https:=
//lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-March/020122.html</a=
> and <a href=3D"https://lists.linuxfoundation.org/pipermail/bitcoin-dev/20=
22-October/020991.html">https://lists.linuxfoundation.org/pipermail/bitcoin=
-dev/2022-October/020991.html</a><br></div><br><div class=3D"gmail_quote"><=
div dir=3D"ltr" class=3D"gmail_attr">Le=C2=A0lun. 31 oct. 2022 =C3=A0=C2=A0=
11:02, Suhas Daftuar via bitcoin-dev &lt;<a href=3D"mailto:bitcoin-dev@list=
s.linuxfoundation.org">bitcoin-dev@lists.linuxfoundation.org</a>&gt; a =C3=
=A9crit=C2=A0:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0=
px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><=
div dir=3D"ltr"><div dir=3D"ltr">AJ,<br><br>Thanks for the thoughtful post.=
 I think your observations about how we view mempool policy in the Bitcoin =
Core project, and how that seems to be changing in the discussions around `=
-mempoolfullrbf`, are on-point and provide a helpful baseline for consideri=
ng future policy changes.<br><br>For a long time I viewed fullrbf as an eve=
ntuality and I considered myself to be philosophically supportive of the id=
ea.=C2=A0 However, after giving this issue some thought in the past few wee=
ks, I am reversing my thinking on this.=C2=A0 Concretely, I will argue that=
 we should continue to maintain a relay policy where replacements are rejec=
ted for transactions that don&#39;t opt-in to RBF (as described in BIP 125)=
, and moreover, that we should remove the `-mempoolfullrbf` flag from Bitco=
in Core=E2=80=99s latest release candidate and not plan to release software=
 with that flag, unless (or until) circumstances change on the network, whi=
ch I&#39;ll discuss below.<br><br>This is, of course, a nuanced topic, and =
among the considerations is a philosophy of how to think about the relay po=
licy and configuration options that we make available in Bitcoin Core (a co=
nsideration that is perhaps unique to that project, but I think relevant fo=
r this mailing list).<br><br>I&#39;ll start with some technical issues rega=
rding the benefits of enabling fullrbf on the network.=C2=A0 In the current=
 BIP 125 regime, every time a transaction is created, a choice is made whet=
her to subject the transaction to BIP 125=E2=80=99s RBF rules or not (based=
 on the sequence values of the inputs).=C2=A0 So given that users can alrea=
dy opt-in to RBF, the benefit of a =E2=80=9Cfullrbf=E2=80=9D network policy=
 would be if, somehow, RBF users were still denied the benefits of RBF due =
to the existence of other transactions that don=E2=80=99t opt-in.<br><br>Al=
ong those lines, Antoine Riard brought up[1] a DoS vector that is available=
 to someone who wants to interfere with multi-party funded transactions, an=
d suggested that fullrbf would eliminate the problem.=C2=A0 After exploring=
 that question again in this thread (thanks to Greg Sanders for clarifying =
this to me), I understand that the issue is around ensuring that a multipar=
ty (coinjoin-type) protocol is able to make eventual progress, by having a =
candidate multiparty transaction either eventually confirm or become confli=
cted with something that has been confirmed, in which case the double-spend=
 information could be used to start a new coinjoin round with fewer partici=
pants.=C2=A0 The concern Antoine and Greg have brought up is that non-rbf t=
ransactions can persist in the mempool ~indefinitely (at a low feerate and =
not subject to replacement) and interfere with progress being made in a coi=
njoin protocol.<br><br>However, it seems to me that similar problems exist =
for such a protocol even in a fullrbf world, as we understand that term tod=
ay.=C2=A0 I mentioned the ability for rbf =E2=80=9Cpinning=E2=80=9D to inte=
rfere with relay of the multiparty transaction (even if the conflicting tra=
nsaction signals for RBF =E2=80=93 a set of large but low feerate conflicti=
ng transactions can persist in the mempool and make it difficult for the co=
injoin transaction from confirming, at least without attaching a very large=
 fee); and as Greg mentioned in a followup, the BIP 125 rule to only permit=
 100 transactions to be removed from the mempool at a time during a replace=
ment can also be used to pin a coinjoin protocol in the same way as a non-r=
bf transaction today.=C2=A0 It seems to me that what these multiparty proto=
cols actually need is some sort of &quot;maximal rbf&quot; network policy: =
a way to guarantee that a transaction which should be desirable for a miner=
 to mine would always get to a miner and considered for inclusion in a bloc=
k, no matter what the state of node=E2=80=99s mempools on the network.<br><=
br>While that sounds like a reasonable thing to want on its face (and worth=
 working on), it&#39;s not how opt-in RBF works today, nor is it how transa=
ction relay has ever conceptually worked.=C2=A0 We have not, thus far, been=
 able to come up with a total ordering on transaction desirability.=C2=A0 M=
oreover, due to all the DoS issues that exist with transaction relay, there=
 are plenty of seemingly legitimate ways to construct transactions that wou=
ld not relay well on the network.=C2=A0 Relay has only ever been a best-eff=
orts concept, where we carve out a small subset of the entire transaction u=
niverse for which we try to optimize propagation.=C2=A0 The idea behind thi=
s approach is that if every use case we can come up with has some way to ac=
hieve its goals using transactions that should (eventually) be able to rela=
y, then users wouldn=E2=80=99t have much demand for transactions that would=
 deviate from the supported policies, and we therefore shouldn=E2=80=99t ne=
ed to worry too much about incentive compatibility concerns when it comes t=
o transaction types that wouldn=E2=80=99t relay at all, even if they are hi=
gh feerate. =C2=A0(And when those situations arise where the standard trans=
actions do not accommodate some needed use case, developers typically work =
to define a policy that is compatible with our anti-DoS goals to support su=
ch use cases, such as with the recent proposal for version=3D3 transactions=
 [2].)<br><br>BIP 125&#39;s RBF rules themselves were an effort to carve ou=
t just a subset of situations where a transaction should evict conflicting =
ones -- it was not a design that anyone thought would ensure that all repla=
cements which &quot;should&quot; be mined would always propagate.=C2=A0 And=
 I don&#39;t believe that we know how to design policy rules that would ach=
ieve the goals of this kind of multiparty protocol in a DoS resistant way, =
today.=C2=A0 Along those lines, I would point out that even the BIP 125 des=
ign itself is not entirely incentive compatible, in that it is possible to =
construct a replacement transaction that would evict transactions which wou=
ld be preferable to be included in a block! [3] =C2=A0(This has been known =
for years, but fixing this has proven difficult, and the only way to fix it=
 that I=E2=80=99m aware of would be to make BIP 125 RBF even more restricti=
ve than it is today. I do think this is something that needs to be worked o=
n.)<br><br>Given the limitations of RBF as we have it today, it appears to =
be incorrect that a fullrbf network policy would solve the problems Antoine=
 raised.=C2=A0 And so absent any other examples, it does not seem to me tha=
t fullrbf solves any problems for RBF users, who are already free to choose=
 to subject their transactions to BIP 125=E2=80=99s RBF policy.=C2=A0 From =
this perspective, &quot;enabling fullrbf&quot; is really just taking away u=
ser choice to opt a transaction into a non-replacement policy regime.<br><b=
r>I think we should ask, then, whether it is reasonable on its face that us=
ers might want to opt-in to a non-replacement policy?=C2=A0 Or in other wor=
ds, is it reasonable for a user to mark a transaction as non-replaceable an=
d have that indication be enforced by the network? Note that these are two =
different questions: you could imagine a world where fullrbf is a dominant =
policy, but users still use the BIP 125 signaling method to indicate, in an=
 unenforced way, their intention to not replace a transaction.=C2=A0 This m=
ight give useful information to the network or the recipient for how to int=
eract with such a transaction.<br><br>And I think that it&#39;s entirely po=
ssible that users would continue to use the BIP 125 signaling to indicate t=
hat they do not intend to replace a transaction.=C2=A0 For better or worse,=
 this might be because zeroconf services continue to differentiate their be=
havior based on such a signal (possibly in conjunction with other factors),=
 or it could be because there are other behaviors that could be utilized mo=
re effectively if the transaction originator has made such a signal, such a=
s the recipient chaining an unconfirmed transaction as a way to bump the fe=
e (CPFP) [4]. <br><br>If it were to be the case that users continued to use=
 BIP 125-style signaling to indicate that they do not plan to replace a tra=
nsaction, would that be harmful to the network?=C2=A0 This is not something=
 we can stop in our policy rules (short of censoring such transactions, an =
obviously bad idea).=C2=A0 I think network actors can always do things that=
 we might think are harmful for the network, but that doesn=E2=80=99t mean =
that there are no legitimate use cases for the tools that such actors might=
 be using.=C2=A0 Just because someone might use some policy to adopt a zero=
conf model, doesn=E2=80=99t mean that others aren=E2=80=99t using the same =
policy to achieve benign ends (such as better CPFP behavior).<br><br>Moreov=
er, while users might attempt to exploit services that offer zeroconf or ot=
her differentiated behavior to non-replacement signaling transactions, they=
 also might not -- I think predicting user behavior in this way (and specif=
ically predicting the complexities of what a business might do and whether =
users might try to subvert it) is beyond the scope of what we can do as pro=
tocol developers.=C2=A0 Instead, I think we can try to answer a different q=
uestion: if a group of users were to want the ability to opt-in to a non-re=
placement policy regime, is that a technically sound option for us to have =
on the network and enforce in software?=C2=A0 Specifically, does that inter=
fere with having a sensible anti-DoS mempool acceptance algorithm, or inter=
fere with other protocols on the network, or necessarily run counter to the=
 interests of miners or node operators?<br><br>And I think the answer to th=
at question, in looking at the difference between opt-in RBF and fullrbf, i=
s no: offering the ability to opt-in to a non-replacement regime for transa=
ctions doesn&#39;t introduce any fundamental issues with software or networ=
k policy or other protocols.=C2=A0 In a world where we only had fullrbf, I =
could imagine at some point down the road proposing a non-replacement signa=
l myself, because the complexities around transaction chains (and pinning) =
are more complex for the RBF case than for the non-RBF case (and BIP 125 is=
 not always incentive compatible to begin with!).=C2=A0 Conceptually, this =
is no different to me than the version=3D3 transaction policy proposal that=
 has been advancing, if we think of it as a special set of restrictions on =
transactions designed to accommodate a particular use case. =C2=A0<br><br>P=
hilosophically, I think we should be looking to add non-interfering use cas=
es to what the network supports. =C2=A0<br><br>To those who argue for makin=
g fullrbf a default policy on the network (or even just offering a flag for=
 users to enable fullrbf), I pose this hypothetical: suppose we deploy the =
v3 transaction policy proposal (which I hope will happen in the near future=
).=C2=A0 That policy would restrict the ways that outputs of a v3 transacti=
on can be spent while the transaction is unconfirmed, including by limiting=
 the number and size of descendants that such a transaction can have, and l=
imiting the types of unconfirmed ancestors that can be included.=C2=A0 Supp=
ose in a few years someone proposes that we add a &quot;-disable_v3_transac=
tion_enforcement&quot; flag to our software, to let users decide to turn of=
f those policy restrictions and treat v3 transactions the same as v2, for a=
ll the same reasons that could be argued today with fullrbf: miners might e=
arn more revenue if we allowed multiple descendant v3 transactions; it&#39;=
s illogical for the recipient of a v3 transaction to believe what is a fund=
amentally unenforceable promise of a sender to not issue more high value ch=
ildren that descend from an unconfirmed transaction; it&#39;s inappropriate=
 for Bitcoin Core to dictate policy on the network and we should honor user=
 choice to turn off that flag if that=E2=80=99s what users want; if users a=
re relying on v3=E2=80=99s policy restrictions for security then that is an=
 unstable model and we should assume it will get broken[5]. =C2=A0<br><br>I=
t=E2=80=99s obvious to me that adding a flag to disable v3 policy would be =
subversive to making the lightning use case for v3 transactions work.=C2=A0=
 And so my response to such a hypothetical proposal would be to argue that =
no, we should not enable users to disable this policy, because as long as t=
hat policy is just optional and working for those who want it, it shouldn=
=E2=80=99t harm anyone that we offer a tighter set of rules for a particula=
r use case.=C2=A0 Adding a way to bypass those rules is just trying to brea=
k someone else=E2=80=99s use case, not trying to add a new one.=C2=A0 We sh=
ould not wield &quot;incentive compatibility&quot; as a bludgeon for breaki=
ng things that appear to be working and not causing others harm.<br><br>I t=
hink this is exactly what is happening with fullrbf.<br><br>In comparing v3=
 transaction policy with opting out of transaction replacement, there is of=
 course one significant difference that I have ignored thus far: I think th=
e real difference is an opinion about whether non-replacement transactions =
that are being used today are, overall, bad for Bitcoin, and whether lightn=
ing=E2=80=99s use of v3 transactions in the future would be bad for Bitcoin=
. If you think that zeroconf is unequivocally bad, and that no one will be =
able to plausibly construct a case that lightning is bad, then that qualita=
tive judgment might sway you to not worrying about the philosophical issues=
 I&#39;ve raised above, because these situations can be distinguished.<br><=
br>However I am not personally willing to say that I think, overall, non-rb=
f-signaling transactions in use on the network today are bad for Bitcoin (o=
r that fullrbf is definitely good =E2=80=93 BIP 125=E2=80=99s rbf rules are=
 something we=E2=80=99ve been trying to improve upon for years, with little=
 success).=C2=A0 Nor am I convinced that someone couldn=E2=80=99t put toget=
her a cogent argument for lightning being bad for Bitcoin, because of its r=
eliance on relay policies that are difficult to design and impossible to gu=
arantee as part of its security model.=C2=A0 So I choose instead to merely =
make a judgment that seems more factually verifiable, which is that non-rep=
lacement is a policy widely in use on the network today, and we largely don=
&#39;t have reason to think (as far as I know!) that the network is seeing =
a lot of transactions that would violate that policy.<br><br>If it did turn=
 out that users were commonly signaling non-replacement, but then signing a=
nd trying to relay doublespends, then I think that would be a very good rea=
son for Bitcoin Core to adopt fullrbf to reflect the reality of what is hap=
pening.=C2=A0 In the meantime, I think it makes more sense to say that beca=
use we have BIP 125, there seems to be no need for users to signal one way =
and behave another, and therefore there is no need to offer software that m=
ight break a policy that is working well for some users.=C2=A0 Other softwa=
re projects might choose differently, and it is after all a permissionless =
network, so if this is in fact an unstable equilibrium that will not last, =
then presumably someday it will be apparent it is not working and we=E2=80=
=99ll abandon it.=C2=A0 But I think the philosophy of transaction relay pol=
icy in Bitcoin Core should be to support disparate use cases in order to tr=
y to make everything work better, rather than break things prematurely beca=
use we guess others will break them eventually anyway.<br><br>For those tha=
t have read this long email and still favor a fullrbf network policy (or ev=
en just the ability for users to be able to turn on fullrbf for themselves)=
, I=E2=80=99d ask for thoughts on the following questions, which have guide=
d my thinking on this:<br><br>Does fullrbf offer any benefits other than br=
eaking zeroconf business practices?=C2=A0 If so, what are they?<br><br>Is i=
t reasonable to enforce BIP 125&#39;s rbf rules on all transactions, if tho=
se rules themselves are not always incentive compatible?<br><br>If someone =
were to propose a command line option that breaks v3 transaction relay in t=
he future, is there a logical basis for opposing that which is consistent w=
ith moving towards fullrbf now?</div><div dir=3D"ltr"><br></div><div dir=3D=
"ltr">Cheers,<br>Suhas<br><br><br>[1] <a href=3D"https://lists.linuxfoundat=
ion.org/pipermail/lightning-dev/2021-May/003033.html" target=3D"_blank">htt=
ps://lists.linuxfoundation.org/pipermail/lightning-dev/2021-May/003033.html=
</a><div><br>[2] <a href=3D"https://lists.linuxfoundation.org/pipermail/bit=
coin-dev/2022-September/020937.html" target=3D"_blank">https://lists.linuxf=
oundation.org/pipermail/bitcoin-dev/2022-September/020937.html</a><br><br><=
/div><div>[3] This is because under the BIP 125 rules, the feerate of the r=
eplacement transaction is not compared to the individual feerates of all tr=
ansactions being evicted =E2=80=93 we just compare feerates with the transa=
ctions that are directly in conflict (and not their descendants). So it=E2=
=80=99s possible for a transaction that would evict 2 or more transactions =
to have a higher feerate than the direct conflicts, and higher total fee th=
an the set being evicted, but have a lower feerate=C2=A0(eg if it is larger=
) than that of some subset of the set of transactions being evicted.<br><br=
></div><div>[4] =C2=A0Chaining unconfirmed transactions when the sender mig=
ht RBF the parent is far riskier than if the sender indicates they don&#39;=
t plan to do so (chaining onto an RBF transaction creates pinning issues fo=
r the sender, and risks having the child wiped out if the parent is replace=
d), so I think this is a concrete reason why signaling that a transaction w=
on=E2=80=99t be replaced could be useful.<br><br></div><div>[5] This is a s=
ubtle point. I don=E2=80=99t think v3 transactions create an unreasonable s=
ecurity assumption for the use case it is being designed for. However, I do=
n=E2=80=99t think anyone could rule out the possibility that someone could =
adopt a usage pattern for v3 transactions that subverts the intent of this =
policy.=C2=A0 For example, if users started using v3 transactions for all t=
heir payments, then the limitations on the number of descendants could dire=
ctly interfere with CPFP by a recipient, and someone could argue that we sh=
ould break the policy in order to allow for this hypothetical behavior. I t=
hink this is a similar form of argument as saying that zeroconf practices +=
 BIP 125 create an incentive to double-spend non-rbf signaling transactions=
.</div></div></div>
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