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From: Jeremy Rubin <j@rubin.io>
Date: Tue, 18 Oct 2022 12:41:19 -0400
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To: Greg Sanders <gsanders87@gmail.com>, 
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Ephemeral Anchors: Fixing V3 Package RBF
 againstpackage limit pinning
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Excellent proposal and I agree it does capture much of the spirit of
sponsors w.r.t. how they might be used for V3 protocols.

The only drawbacks I see is they don't work for lower tx version contracts,
so there's still something to be desired there, and that the requirement to
sweep the output must be incentive compatible for the miner, or else they
won't enforce it (pass the buck onto the future bitcoiners). The Ephemeral
UTXO concept can be a consensus rule (see
https://rubin.io/public/pdfs/multi-txn-contracts.pdf "Intermediate UTXO")
we add later on in lieu of managing them by incentive, so maybe it's a
cleanup one can punt.

One question I have is if V3 is designed for lightning, and this is
designed for lightning, is there any sense in requiring these outputs for
v3? That might help with e.g. anonymity set, as well as potentially keep
the v3 surface smaller.

On Tue, Oct 18, 2022 at 11:51 AM Greg Sanders via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> > does that effectively mark output B as unspendable once the child gets
> confirmed?
>
> Not at all. It's a normal spend like before, since the parent has been
> confirmed. It's completely unrestricted, not being bound to any
> V3/ephemeral anchor restrictions on size, version, etc.
>
> On Tue, Oct 18, 2022 at 11:47 AM Arik Sosman via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> Hi Greg,
>>
>> Thank you very much for sharing your proposal!
>>
>> I think there's one thing about the second part of your proposal that I'=
m
>> missing. Specifically, assuming the scenario of a v3 transaction with th=
ree
>> outputs, A, B, and the ephemeral anchor OP_TRUE. If a child transaction
>> spends A and OP_TRUE, does that effectively mark output B as unspendable
>> once the child gets confirmed? If so, isn't the implication therefore th=
at
>> to safely spend a transaction with an ephemeral anchor, all outputs must=
 be
>> spent? Thanks!
>>
>> Best,
>> Arik
>>
>> On Tue, Oct 18, 2022, at 6:52 AM, Greg Sanders via bitcoin-dev wrote:
>>
>> Hello Everyone,
>>
>> Following up on the "V3 Transaction" discussion here
>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-September/0=
20937.html
>> , I would like to elaborate a bit further on some potential follow-on wo=
rk
>> that would make pinning severely constrained in many setups].
>>
>> V3 transactions may solve bip125 rule#3 and rule#5 pinning attacks under
>> some constraints[0]. This means that when a replacement is to be made an=
d
>> propagated, it costs the expected amount of fees to do so. This is a gre=
at
>> start. What's left in this subset of pinning is *package limit* pinning.=
 In
>> other words, a fee-paying transaction cannot enter the mempool due to th=
e
>> existing mempool package it is being added to already being too large in
>> count or vsize.
>>
>> Zooming into the V3 simplified scenario for sake of discussion, though
>> this problem exists in general today:
>>
>> V3 transactions restrict the package limit of a V3 package to one parent
>> and one child. If the parent transaction includes two outputs which can =
be
>> immediately spent by separate parties, this allows one party to disallow=
 a
>> spend from the other. In Gloria's proposal for ln-penalty, this is worke=
d
>> around by reducing the number of anchors per commitment transaction to 1=
,
>> and each version of the commitment transaction has a unique party's key =
on
>> it. The honest participant can spend their version with their anchor and
>> package RBF the other commitment transaction safely.
>>
>> What if there's only one version of the commitment transaction, such as
>> in other protocols like duplex payment channels, eltoo? What about multi
>> party payments?
>>
>> In the package RBF proposal, if the parent transaction is identical to a=
n
>> existing transaction in the mempool, the parent will be detected and
>> removed from the package proposal. You are then left with a single V3 ch=
ild
>> transaction, which is then proposed for entry into the mempool. In the c=
ase
>> of another parent output already being spent, this is simply rejected,
>> regardless of feerate of the new child.
>>
>> I have two proposed solutions, of which I strongly prefer the latter:
>>
>> 1) Expand a carveout for "sibling eviction", where if the new child is
>> paying "enough" to bump spends from the same parent, it knocks its sibli=
ng
>> out of the mempool and takes the one child slot. This would solve it, bu=
t
>> is a new eviction paradigm that would need to be carefully worked throug=
h.
>>
>> 2) Ephemeral Anchors (my real policy-only proposal)
>>
>> Ephemeral Anchors is a term which means an output is watermarked as an
>> output that MUST be spent in a V3 package. We mark this anchor by being =
the
>> bare script `OP_TRUE` and of course make these outputs standard to relay
>> and spend with empty witness data.
>>
>> Also as a simplifying assumption, we require the parent transaction with
>> such an output to be 0-fee. This makes mempool reasoning simpler in case
>> the child-spend is somehow evicted, guaranteeing the parent will be as w=
ell.
>>
>> Implications:
>>
>> a) If the ephemeral anchor MUST be spent, we can allow *any* value, even
>> dust, even 0, without worrying about bloating the utxo set. We relax thi=
s
>> policy for maximum smart contract flexibility and specification simplici=
ty..
>>
>> b) Since this anchor MUST be spent, any spending of other outputs in the
>> same parent transaction MUST directly double-spend prior spends of the
>> ephemeral anchor. This causes the 1 block CSV timelock on outputs to be
>> removed in these situations. This greatly magnifies composability of sma=
rt
>> contracts, as now we can do things like safely splice directly into new
>> channels, into statechains, your custodial wallet account, your cold
>> wallet, wherever, without requiring other wallets to support arbitrary
>> scripts. Also it hurts that 1 CSV time locked scripts may not be miniscr=
ipt
>> compatible to begin with...
>>
>> c) *Anyone* can bump the transaction, without any transaction key
>> material. This is essentially achieving Jeremy's Transaction Sponsors (
>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-September/0=
18168.html)
>> proposal without consensus changes. As long as someone gets a fully sign=
ed
>> parent, they can execute a bump with minimal wallet tooling. If a
>> transaction author doesn=E2=80=99t want a =E2=80=9Csponsor=E2=80=9D, do =
not include the output.
>>
>> d) Lightning Carve-out(
>> https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-October/0=
02240.html)
>> is superseded by this logic, as we are not restricted to two immediately
>> spendable output scenarios. In its place, robust multi-party fee bumping=
 is
>> possible.
>>
>> e) This also benefits more traditional wallet scenarios, as change
>> outputs can no longer be pinned, and RBF/CPFP becomes robust. Payees in
>> simple spends cannot pin you. Batched payouts become a lot less painful.
>> This was one of the motivating use cases that created the term =E2=80=9C=
pinning=E2=80=9D in
>> the first place(
>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-February/01=
5717.html),
>> even if LN/L2 discussion has largely overtaken it due to HTLC theft risk=
s.
>>
>> Open Question(s):
>>
>>
>>    1.
>>
>>    If we allow non-zero value in ephemeral outputs, does this open up a
>>    MEV we are worried about? Wallets should toss all the value directly =
to
>>    fees, and add their own additional fees on top, otherwise miners have
>>    incentive to make the smallest utxo burn transaction to claim those f=
unds.
>>    They just confirmed your parent transaction anyways, so do we care?
>>    2.
>>
>>    SIGHASH_GROUP like constructs would allow uncommitted ephemeral
>>    anchors to be added at spend time, depending on spending requirements=
.
>>    SIGHASH_SINGLE already allows this.
>>
>>
>>
>>
>> Hopefully this gives people something to consider as we move forward in
>> thinking about mempool design within the constraints we have today.
>>
>>
>> Greg
>>
>> 0: With V3 transactions where you have "veto power" over all the inputs
>> in that transaction. Therefore something like ANYONECANPAY is still brok=
en.
>> We need a more complex solution, which I=E2=80=99m punting for the sake =
of progress.
>> _______________________________________________
>> 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"
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<div dir=3D"ltr"><div class=3D"gmail_default" style=3D"font-family:arial,he=
lvetica,sans-serif;font-size:small;color:#000000">Excellent proposal and I =
agree it does capture much of the spirit of sponsors w.r.t. how they might =
be used for V3 protocols.</div><div class=3D"gmail_default" style=3D"font-f=
amily:arial,helvetica,sans-serif;font-size:small;color:#000000"><br></div><=
div class=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif=
;font-size:small;color:#000000">The only drawbacks I see=C2=A0is they don&#=
39;t work for lower tx version contracts, so there&#39;s still something to=
 be desired there, and that the requirement to sweep the output must be inc=
entive compatible for the miner, or else they won&#39;t enforce it (pass th=
e buck onto the future bitcoiners). The Ephemeral UTXO concept can be a con=
sensus rule (see=C2=A0<a href=3D"https://rubin.io/public/pdfs/multi-txn-con=
tracts.pdf">https://rubin.io/public/pdfs/multi-txn-contracts.pdf</a> &quot;=
Intermediate UTXO&quot;) we add later on in lieu of managing them by incent=
ive, so maybe it&#39;s a cleanup one can punt.</div><div class=3D"gmail_def=
ault" style=3D"font-family:arial,helvetica,sans-serif;font-size:small;color=
:#000000"><br></div><div class=3D"gmail_default" style=3D"font-family:arial=
,helvetica,sans-serif;font-size:small;color:#000000">One question I have is=
 if V3 is designed for lightning, and this is designed for lightning, is th=
ere any sense in requiring these outputs for v3? That might help with e.g. =
anonymity set, as well as potentially keep the v3 surface smaller.</div></d=
iv><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On =
Tue, Oct 18, 2022 at 11:51 AM Greg Sanders via bitcoin-dev &lt;<a href=3D"m=
ailto:bitcoin-dev@lists.linuxfoundation.org">bitcoin-dev@lists.linuxfoundat=
ion.org</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"=
margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;bord=
er-left-color:rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr">&gt; does=
 that effectively mark output B as unspendable once the child gets confirme=
d?<div><br></div><div>Not at all. It&#39;s a normal spend like before, sinc=
e the parent has been confirmed. It&#39;s completely unrestricted, not bein=
g bound to any V3/ephemeral anchor restrictions on size, version, etc.</div=
></div><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr"=
>On Tue, Oct 18, 2022 at 11:47 AM Arik Sosman via bitcoin-dev &lt;<a href=
=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank">bitcoin=
-dev@lists.linuxfoundation.org</a>&gt; wrote:<br></div><blockquote class=3D=
"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left-width:1px;borde=
r-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><di=
v><u></u><div><div>Hi Greg,<br></div><div><br></div><div>Thank you very muc=
h for sharing your proposal!</div><div><br></div><div>I think there&#39;s o=
ne thing about the second part of your proposal that I&#39;m missing. Speci=
fically, assuming the scenario of a v3 transaction with three outputs, A, B=
, and the ephemeral anchor OP_TRUE. If a child transaction spends A and OP_=
TRUE, does that effectively mark output B as unspendable once the child get=
s confirmed? If so, isn&#39;t the implication therefore that to safely spen=
d a transaction with an ephemeral anchor, all outputs must be spent? Thanks=
!<br></div><div><br></div><div>Best,<br></div><div>Arik</div><div><br></div=
><div>On Tue, Oct 18, 2022, at 6:52 AM, Greg Sanders via bitcoin-dev wrote:=
<br></div><blockquote type=3D"cite" id=3D"m_8177486709224577946m_-702405504=
3742391057m_4368086065316228638qt"><div dir=3D"ltr"><span id=3D"m_817748670=
9224577946m_-7024055043742391057m_4368086065316228638qt-gmail-docs-internal=
-guid-2d3e64aa-7fff-66f1-ed3d-c94d5a1f62c6"><p dir=3D"ltr" style=3D"line-he=
ight:1.38;margin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0)=
;background-color:transparent;font-variant-numeric:normal;font-variant-east=
-asian:normal;vertical-align:baseline;white-space:pre-wrap"><span style=3D"=
font-family:Arial"><span style=3D"font-size:11pt">Hello Everyone,</span></s=
pan></span><br></p><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;=
margin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);backgroun=
d-color:transparent;font-variant-numeric:normal;font-variant-east-asian:nor=
mal;vertical-align:baseline;white-space:pre-wrap"><span style=3D"font-famil=
y:Arial"><span style=3D"font-size:11pt">Following up on the &quot;V3 Transa=
ction&quot; discussion here <a href=3D"https://lists.linuxfoundation.org/pi=
permail/bitcoin-dev/2022-September/020937.html" target=3D"_blank">https://l=
ists.linuxfoundation.org/pipermail/bitcoin-dev/2022-September/020937.html</=
a> , I would like to elaborate a bit further on some potential follow-on wo=
rk that would make pinning severely constrained in many setups].</span></sp=
an></span><br></p><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;m=
argin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);background=
-color:transparent;font-variant-numeric:normal;font-variant-east-asian:norm=
al;vertical-align:baseline;white-space:pre-wrap"><span style=3D"font-family=
:Arial"><span style=3D"font-size:11pt">V3 transactions may solve bip125 rul=
e#3 and rule#5 pinning attacks under some constraints[0]. This means that w=
hen a replacement is to be made and propagated, it costs the expected amoun=
t of fees to do so. This is a great start. What&#39;s left in this subset o=
f pinning is *package limit* pinning. In other words, a fee-paying transact=
ion cannot enter the mempool due to the existing mempool package it is bein=
g added to already being too large in count or vsize.</span></span></span><=
br></p><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;margin-top:0=
pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);background-color:tran=
sparent;font-variant-numeric:normal;font-variant-east-asian:normal;vertical=
-align:baseline;white-space:pre-wrap"><span style=3D"font-family:Arial"><sp=
an style=3D"font-size:11pt">Zooming into the V3 simplified scenario for sak=
e of discussion, though this problem exists in general today:</span></span>=
</span><br></p><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;marg=
in-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);background-co=
lor:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;=
vertical-align:baseline;white-space:pre-wrap"><span style=3D"font-family:Ar=
ial"><span style=3D"font-size:11pt">V3 transactions restrict the package li=
mit of a V3 package to one parent and one child. If the parent transaction =
includes two outputs which can be immediately spent by separate parties, th=
is allows one party to disallow a spend from the other. In Gloria&#39;s pro=
posal for ln-penalty, this is worked around by reducing the number of ancho=
rs per commitment transaction to 1, and each version of the commitment tran=
saction has a unique party&#39;s key on it. The honest participant can spen=
d their version with their anchor and package RBF the other commitment tran=
saction safely.</span></span></span><br></p><div><br></div><p dir=3D"ltr" s=
tyle=3D"line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style=3D"c=
olor:rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;fo=
nt-variant-east-asian:normal;vertical-align:baseline;white-space:pre-wrap">=
<span style=3D"font-family:Arial"><span style=3D"font-size:11pt">What if th=
ere&#39;s only one version of the commitment transaction, such as in other =
protocols like duplex payment channels, eltoo? What about multi party payme=
nts?</span></span></span><br></p><div><br></div><p dir=3D"ltr" style=3D"lin=
e-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,=
0,0);background-color:transparent;font-variant-numeric:normal;font-variant-=
east-asian:normal;vertical-align:baseline;white-space:pre-wrap"><span style=
=3D"font-family:Arial"><span style=3D"font-size:11pt">In the package RBF pr=
oposal, if the parent transaction is identical to an existing transaction i=
n the mempool, the parent will be detected and removed from the package pro=
posal. You are then left with a single V3 child transaction, which is then =
proposed for entry into the mempool. In the case of another parent output a=
lready being spent, this is simply rejected, regardless of feerate of the n=
ew child.</span></span></span><br></p><div><br></div><p dir=3D"ltr" style=
=3D"line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style=3D"color=
:rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;font-v=
ariant-east-asian:normal;vertical-align:baseline;white-space:pre-wrap"><spa=
n style=3D"font-family:Arial"><span style=3D"font-size:11pt">I have two pro=
posed solutions, of which I strongly prefer the latter:</span></span></span=
><br></p><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;margin-top=
:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);background-color:tr=
ansparent;font-variant-numeric:normal;font-variant-east-asian:normal;vertic=
al-align:baseline;white-space:pre-wrap"><span style=3D"font-family:Arial"><=
span style=3D"font-size:11pt">1) Expand a carveout for &quot;sibling evicti=
on&quot;, where if the new child is paying &quot;enough&quot; to bump spend=
s from the same parent, it knocks its sibling out of the mempool and takes =
the one child slot. This would solve it, but is a new eviction paradigm tha=
t would need to be carefully worked through.</span></span></span><br></p><d=
iv><br></div><p dir=3D"ltr" style=3D"line-height:1.38;margin-top:0pt;margin=
-bottom:0pt"><span style=3D"color:rgb(0,0,0);background-color:transparent;f=
ont-variant-numeric:normal;font-variant-east-asian:normal;vertical-align:ba=
seline;white-space:pre-wrap"><span style=3D"font-family:Arial"><span style=
=3D"font-size:11pt">2) Ephemeral Anchors (my real policy-only proposal)</sp=
an></span></span><br></p><div><br></div><p dir=3D"ltr" style=3D"line-height=
:1.38;margin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);bac=
kground-color:transparent;font-variant-numeric:normal;font-variant-east-asi=
an:normal;vertical-align:baseline;white-space:pre-wrap"><span style=3D"font=
-family:Arial"><span style=3D"font-size:11pt">Ephemeral Anchors is a term w=
hich means an output is watermarked as an output that MUST be spent in a V3=
 package. We mark this anchor by being the bare script `OP_TRUE` and of cou=
rse make these outputs standard to relay and spend with empty witness data.=
</span></span></span><br></p><div><br></div><p dir=3D"ltr" style=3D"line-he=
ight:1.38;margin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0)=
;background-color:transparent;font-variant-numeric:normal;font-variant-east=
-asian:normal;vertical-align:baseline;white-space:pre-wrap"><span style=3D"=
font-family:Arial"><span style=3D"font-size:11pt">Also as a simplifying ass=
umption, we require the parent transaction with such an output to be 0-fee.=
 This makes mempool reasoning simpler in case the child-spend is somehow ev=
icted, guaranteeing the parent will be as well.</span></span></span><br></p=
><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;margin-top:0pt;mar=
gin-bottom:0pt"><span style=3D"color:rgb(0,0,0);background-color:transparen=
t;font-variant-numeric:normal;font-variant-east-asian:normal;vertical-align=
:baseline;white-space:pre-wrap"><span style=3D"font-family:Arial"><span sty=
le=3D"font-size:11pt">Implications:</span></span></span><br></p><div><br></=
div><p dir=3D"ltr" style=3D"line-height:1.38;margin-top:0pt;margin-bottom:0=
pt"><span style=3D"color:rgb(0,0,0);background-color:transparent;font-varia=
nt-numeric:normal;font-variant-east-asian:normal;vertical-align:baseline;wh=
ite-space:pre-wrap"><span style=3D"font-family:Arial"><span style=3D"font-s=
ize:11pt">a) If the ephemeral anchor MUST be spent, we can allow *any* valu=
e, even dust, even 0, without worrying about bloating the utxo set. We rela=
x this policy for maximum smart contract flexibility and specification simp=
licity..</span></span></span><br></p><div><br></div><p dir=3D"ltr" style=3D=
"line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style=3D"color:rg=
b(0,0,0);background-color:transparent;font-variant-numeric:normal;font-vari=
ant-east-asian:normal;vertical-align:baseline;white-space:pre-wrap"><span s=
tyle=3D"font-family:Arial"><span style=3D"font-size:11pt">b) Since this anc=
hor MUST be spent, any spending of other outputs in the same parent transac=
tion MUST directly double-spend prior spends of the ephemeral anchor. This =
causes the 1 block CSV timelock on outputs to be removed in these situation=
s. This greatly magnifies composability of smart contracts, as now we can d=
o things like safely splice directly into new channels, into statechains, y=
our custodial wallet account, your cold wallet, wherever, without requiring=
 other wallets to support arbitrary scripts. Also it hurts that 1 CSV time =
locked scripts may not be miniscript compatible to begin with...</span></sp=
an></span><br></p><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;m=
argin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);background=
-color:transparent;font-variant-numeric:normal;font-variant-east-asian:norm=
al;vertical-align:baseline;white-space:pre-wrap"><span style=3D"font-family=
:Arial"><span style=3D"font-size:11pt">c) *Anyone* can bump the transaction=
, without any transaction key material. This is essentially achieving Jerem=
y&#39;s Transaction Sponsors (</span></span></span><a href=3D"https://lists=
.linuxfoundation.org/pipermail/bitcoin-dev/2020-September/018168.html" styl=
e=3D"text-decoration-line:none" target=3D"_blank"><span style=3D"background=
-color:transparent;font-variant-numeric:normal;font-variant-east-asian:norm=
al;text-decoration-line:underline;vertical-align:baseline;white-space:pre-w=
rap"><span style=3D"font-family:Arial"><span style=3D"font-size:11pt">https=
://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-September/018168.ht=
ml</span></span></span></a><span style=3D"color:rgb(0,0,0);background-color=
:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;ver=
tical-align:baseline;white-space:pre-wrap"><span style=3D"font-family:Arial=
"><span style=3D"font-size:11pt">) proposal without consensus changes. As l=
ong as someone gets a fully signed parent, they can execute a bump with min=
imal wallet tooling. If a transaction author doesn=E2=80=99t want a =E2=80=
=9Csponsor=E2=80=9D, do not include the output.</span></span></span><br></p=
><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;margin-top:0pt;mar=
gin-bottom:0pt"><span style=3D"color:rgb(0,0,0);background-color:transparen=
t;font-variant-numeric:normal;font-variant-east-asian:normal;vertical-align=
:baseline;white-space:pre-wrap"><span style=3D"font-family:Arial"><span sty=
le=3D"font-size:11pt">d) Lightning Carve-out(</span></span></span><a href=
=3D"https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-October/=
002240.html" style=3D"text-decoration-line:none" target=3D"_blank"><span st=
yle=3D"background-color:transparent;font-variant-numeric:normal;font-varian=
t-east-asian:normal;text-decoration-line:underline;vertical-align:baseline;=
white-space:pre-wrap"><span style=3D"font-family:Arial"><span style=3D"font=
-size:11pt">https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-=
October/002240.html</span></span></span></a><span style=3D"color:rgb(0,0,0)=
;background-color:transparent;font-variant-numeric:normal;font-variant-east=
-asian:normal;vertical-align:baseline;white-space:pre-wrap"><span style=3D"=
font-family:Arial"><span style=3D"font-size:11pt">)=C2=A0 is superseded by =
this logic, as we are not restricted to two immediately spendable output sc=
enarios. In its place, robust multi-party fee bumping is possible.</span></=
span></span><br></p><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38=
;margin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);backgrou=
nd-color:transparent;font-variant-numeric:normal;font-variant-east-asian:no=
rmal;vertical-align:baseline;white-space:pre-wrap"><span style=3D"font-fami=
ly:Arial"><span style=3D"font-size:11pt">e) This also benefits more traditi=
onal wallet scenarios, as change outputs can no longer be pinned, and RBF/C=
PFP becomes robust. Payees in simple spends cannot pin you. Batched payouts=
 become a lot less painful. This was one of the motivating use cases that c=
reated the term =E2=80=9Cpinning=E2=80=9D in the first place(</span></span>=
</span><a href=3D"https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2=
018-February/015717.html" style=3D"text-decoration-line:none" target=3D"_bl=
ank"><span style=3D"background-color:transparent;font-variant-numeric:norma=
l;font-variant-east-asian:normal;text-decoration-line:underline;vertical-al=
ign:baseline;white-space:pre-wrap"><span style=3D"font-family:Arial"><span =
style=3D"font-size:11pt">https://lists.linuxfoundation.org/pipermail/bitcoi=
n-dev/2018-February/015717.html</span></span></span></a><span style=3D"colo=
r:rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;font-=
variant-east-asian:normal;vertical-align:baseline;white-space:pre-wrap"><sp=
an style=3D"font-family:Arial"><span style=3D"font-size:11pt">), even if LN=
/L2 discussion has largely overtaken it due to HTLC theft risks.</span></sp=
an></span><br></p><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;m=
argin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);background=
-color:transparent;font-variant-numeric:normal;font-variant-east-asian:norm=
al;vertical-align:baseline;white-space:pre-wrap"><span style=3D"font-family=
:Arial"><span style=3D"font-size:11pt">Open Question(s):</span></span></spa=
n><br></p><div><br></div><ol style=3D"margin-top:0px;margin-bottom:0px"><li=
 dir=3D"ltr" style=3D"list-style-type:decimal;font-size:11pt;font-family:Ar=
ial;color:rgb(0,0,0);background-color:transparent;font-variant-numeric:norm=
al;font-variant-east-asian:normal;vertical-align:baseline;white-space:pre-w=
rap"><p dir=3D"ltr" style=3D"line-height:1.38;margin-top:0pt;margin-bottom:=
0pt" role=3D"presentation"><span style=3D"background-color:transparent;font=
-variant-numeric:normal;font-variant-east-asian:normal;vertical-align:basel=
ine;white-space:pre-wrap"><span style=3D"font-size:11pt">If we allow non-ze=
ro value in ephemeral outputs, does this open up a MEV we are worried about=
? Wallets should toss all the value directly to fees, and add their own add=
itional fees on top, otherwise miners have incentive to make the smallest u=
txo burn transaction to claim those funds. They just confirmed your parent =
transaction anyways, so do we care?</span></span><br></p></li><li dir=3D"lt=
r" style=3D"list-style-type:decimal;font-size:11pt;font-family:Arial;color:=
rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;font-va=
riant-east-asian:normal;vertical-align:baseline;white-space:pre-wrap"><p di=
r=3D"ltr" style=3D"line-height:1.38;margin-top:0pt;margin-bottom:0pt" role=
=3D"presentation"><span style=3D"background-color:transparent;font-variant-=
numeric:normal;font-variant-east-asian:normal;vertical-align:baseline;white=
-space:pre-wrap"><span style=3D"font-size:11pt">SIGHASH_GROUP like construc=
ts would allow uncommitted ephemeral anchors to be added at spend time, dep=
ending on spending requirements. SIGHASH_SINGLE already allows this.</span>=
</span><br></p></li></ol><div><br></div><div><br></div><div><br></div><p di=
r=3D"ltr" style=3D"line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span=
 style=3D"color:rgb(0,0,0);background-color:transparent;font-variant-numeri=
c:normal;font-variant-east-asian:normal;vertical-align:baseline;white-space=
:pre-wrap"><span style=3D"font-family:Arial"><span style=3D"font-size:11pt"=
>Hopefully this gives people something to consider as we move forward in th=
inking about mempool design within the constraints we have today.</span></s=
pan></span><br></p><div><br></div><div><br></div><p dir=3D"ltr" style=3D"li=
ne-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0=
,0,0);background-color:transparent;font-variant-numeric:normal;font-variant=
-east-asian:normal;vertical-align:baseline;white-space:pre-wrap"><span styl=
e=3D"font-family:Arial"><span style=3D"font-size:11pt">Greg</span></span></=
span><br></p><div><br></div><p dir=3D"ltr" style=3D"line-height:1.38;margin=
-top:0pt;margin-bottom:0pt"><span style=3D"color:rgb(0,0,0);background-colo=
r:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;ve=
rtical-align:baseline;white-space:pre-wrap"><span style=3D"font-family:Aria=
l"><span style=3D"font-size:11pt">0: With V3 transactions where you have &q=
uot;veto power&quot; over all the inputs in that transaction. Therefore som=
ething like ANYONECANPAY is still broken. We need a more complex solution, =
which I=E2=80=99m punting for the sake of progress.</span></span></span><br=
></p></span></div><div>_______________________________________________<br><=
/div><div>bitcoin-dev mailing list<br></div><div><a href=3D"mailto:bitcoin-=
dev@lists.linuxfoundation.org" target=3D"_blank">bitcoin-dev@lists.linuxfou=
ndation.org</a><br></div><div><a href=3D"https://lists.linuxfoundation.org/=
mailman/listinfo/bitcoin-dev" target=3D"_blank">https://lists.linuxfoundati=
on.org/mailman/listinfo/bitcoin-dev</a><br></div><div><br></div></blockquot=
e><div><br></div></div>_______________________________________________<br>
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rel=3D"noreferrer" target=3D"_blank">https://lists.linuxfoundation.org/mail=
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</div></blockquote></div>
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