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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
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Subject: Re: [bitcoin-dev] [Lightning-dev] CPFP Carve-Out for Fee-Prediction
Issues in Contracting Applications (eg Lightning)
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I don=E2=80=99te see how? Let=E2=80=99s imagine Party A has two spendable ou=
tputs, now they stuff the package size on one of their spendable outlets unt=
il it is right at the limit, add one more on their other output (to meet the=
Carve-Out), and now Party B can=E2=80=99t do anything.
> On Oct 24, 2019, at 21:05, Johan Tor=C3=A5s Halseth <johanth@gmail.com> wr=
ote:
>=20
> =EF=BB=BF
> It essentially changes the rule to always allow CPFP-ing the commitment as=
long as there is an output available without any descendants. It changes th=
e commitment from "you always need at least, and exactly, one non-CSV output=
per party. " to "you always need at least one non-CSV output per party. "
>=20
> I realize these limits are there for a reason though, but I'm wondering if=
could relax them. Also now that jeremyrubin has expressed problems with the=
current mempool limits.
>=20
>> On Thu, Oct 24, 2019 at 11:25 PM Matt Corallo <lf-lists@mattcorallo.com> w=
rote:
>> I may be missing something, but I'm not sure how this changes anything?
>>=20
>> If you have a commitment transaction, you always need at least, and
>> exactly, one non-CSV output per party. The fact that there is a size
>> limitation on the transaction that spends for carve-out purposes only
>> effects how many other inputs/outputs you can add, but somehow I doubt
>> its ever going to be a large enough number to matter.
>>=20
>> Matt
>>=20
>> On 10/24/19 1:49 PM, Johan Tor=C3=A5s Halseth wrote:
>> > Reviving this old thread now that the recently released RC for bitcoind=
>> > 0.19 includes the above mentioned carve-out rule.
>> >=20
>> > In an attempt to pave the way for more robust CPFP of on-chain contract=
s
>> > (Lightning commitment transactions), the carve-out rule was added in
>> > https://github.com/bitcoin/bitcoin/pull/15681. However, having worked o=
n
>> > an implementation of a new commitment format for utilizing the Bring
>> > Your Own Fees strategy using CPFP, I=E2=80=99m wondering if the special=
case
>> > rule should have been relaxed a bit, to avoid the need for adding a 1
>> > CSV to all outputs (in case of Lightning this means HTLC scripts would
>> > need to be changed to add the CSV delay).
>> >=20
>> > Instead, what about letting the rule be
>> >=20
>> > The last transaction which is added to a package of dependent
>> > transactions in the mempool must:
>> > * Have no more than one unconfirmed parent.
>> >=20
>> > This would of course allow adding a large transaction to each output of=
>> > the unconfirmed parent, which in effect would allow an attacker to
>> > exceed the MAX_PACKAGE_VIRTUAL_SIZE limit in some cases. However, is
>> > this a problem with the current mempool acceptance code in bitcoind? I
>> > would imagine evicting transactions based on feerate when the max
>> > mempool size is met handles this, but I=E2=80=99m asking since it seems=
like
>> > there has been several changes to the acceptance code and eviction
>> > policy since the limit was first introduced.
>> >=20
>> > - Johan
>> >=20
>> >=20
>> > On Wed, Feb 13, 2019 at 6:57 AM Rusty Russell <rusty@rustcorp.com.au
>> > <mailto:rusty@rustcorp.com.au>> wrote:
>> >=20
>> > Matt Corallo <lf-lists@mattcorallo.com
>> > <mailto:lf-lists@mattcorallo.com>> writes:
>> > >>> Thus, even if you imagine a steady-state mempool growth, unless=
the
>> > >>> "near the top of the mempool" criteria is "near the top of the n=
ext
>> > >>> block" (which is obviously *not* incentive-compatible)
>> > >>
>> > >> I was defining "top of mempool" as "in the first 4 MSipa", ie. n=
ext
>> > >> block, and assumed you'd only allow RBF if the old package wasn'=
t
>> > in the
>> > >> top and the replacement would be. That seems incentive
>> > compatible; more
>> > >> than the current scheme?
>> > >
>> > > My point was, because of block time variance, even that criteria
>> > doesn't hold up. If you assume a steady flow of new transactions an=
d
>> > one or two blocks come in "late", suddenly "top 4MWeight" isn't
>> > likely to get confirmed until a few blocks come in "early". Given
>> > block variance within a 12 block window, this is a relatively likel=
y
>> > scenario.
>> >=20
>> > [ Digging through old mail. ]
>> >=20
>> > Doesn't really matter. Lightning close algorithm would be:
>> >=20
>> > 1. Give bitcoind unileratal close.
>> > 2. Ask bitcoind what current expidited fee is (or survey your memp=
ool).
>> > 3. Give bitcoind child "push" tx at that total feerate.
>> > 4. If next block doesn't contain unilateral close tx, goto 2.
>> >=20
>> > In this case, if you allow a simpified RBF where 'you can replace i=
f
>> > 1. feerate is higher, 2. new tx is in first 4Msipa of mempool, 3.
>> > old tx isnt',
>> > it works.
>> >=20
>> > It allows someone 100k of free tx spam, sure. But it's simple.
>> >=20
>> > We could further restrict it by marking the unilateral close someho=
w to
>> > say "gonna be pushed" and further limiting the child tx weight (say=
,
>> > 5kSipa?) in that case.
>> >=20
>> > Cheers,
>> > Rusty.
>> > _______________________________________________
>> > Lightning-dev mailing list
>> > Lightning-dev@lists.linuxfoundation.org
>> > <mailto:Lightning-dev@lists.linuxfoundation.org>
>> > https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
>> >=20
--Apple-Mail-A6774A8C-632A-4A79-AD73-EF0AE036EE83
Content-Type: text/html;
charset=utf-8
Content-Transfer-Encoding: quoted-printable
<html><head><meta http-equiv=3D"content-type" content=3D"text/html; charset=3D=
utf-8"></head><body dir=3D"auto"><div dir=3D"ltr">I don=E2=80=99te see how? L=
et=E2=80=99s imagine Party A has two spendable outputs, now they stuff the p=
ackage size on one of their spendable outlets until it is right at the limit=
, add one more on their other output (to meet the Carve-Out), and now Party B=
can=E2=80=99t do anything.</div><div dir=3D"ltr"><br><blockquote type=3D"ci=
te">On Oct 24, 2019, at 21:05, Johan Tor=C3=A5s Halseth <johanth@gmail.co=
m> wrote:<br><br></blockquote></div><blockquote type=3D"cite"><div dir=3D=
"ltr">=EF=BB=BF<div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr">It essenti=
ally changes the rule to always allow CPFP-ing the commitment as long as the=
re is an output available without any descendants. It changes the commitment=
from "you always need at least, and exactly, one non-CSV output per party. "=
to "you always need at least one non-CSV output per party. "</div><div dir=3D=
"ltr"><br></div><div>I realize these limits are there for a reason though, b=
ut I'm wondering if could relax them. Also now that jeremyrubin has expresse=
d problems with the current mempool limits.</div></div></div><br><div class=3D=
"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Thu, Oct 24, 2019 at 1=
1:25 PM Matt Corallo <<a href=3D"mailto:lf-lists@mattcorallo.com">lf-list=
s@mattcorallo.com</a>> wrote:<br></div><blockquote class=3D"gmail_quote" s=
tyle=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padd=
ing-left:1ex">I may be missing something, but I'm not sure how this changes a=
nything?<br>
<br>
If you have a commitment transaction, you always need at least, and<br>
exactly, one non-CSV output per party. The fact that there is a size<br>
limitation on the transaction that spends for carve-out purposes only<br>
effects how many other inputs/outputs you can add, but somehow I doubt<br>
its ever going to be a large enough number to matter.<br>
<br>
Matt<br>
<br>
On 10/24/19 1:49 PM, Johan Tor=C3=A5s Halseth wrote:<br>
> Reviving this old thread now that the recently released RC for bitcoind=
<br>
> 0.19 includes the above mentioned carve-out rule.<br>
> <br>
> In an attempt to pave the way for more robust CPFP of on-chain contract=
s<br>
> (Lightning commitment transactions), the carve-out rule was added in<br=
>
> <a href=3D"https://github.com/bitcoin/bitcoin/pull/15681" rel=3D"norefe=
rrer" target=3D"_blank">https://github.com/bitcoin/bitcoin/pull/15681</a>. H=
owever, having worked on<br>
> an implementation of a new commitment format for utilizing the Bring<br=
>
> Your Own Fees strategy using CPFP, I=E2=80=99m wondering if the special=
case<br>
> rule should have been relaxed a bit, to avoid the need for adding a 1<b=
r>
> CSV to all outputs (in case of Lightning this means HTLC scripts would<=
br>
> need to be changed to add the CSV delay).<br>
> <br>
> Instead, what about letting the rule be<br>
> <br>
> The last transaction which is added to a package of dependent<br>
> transactions in the mempool must:<br>
> * Have no more than one unconfirmed parent.<br>
> <br>
> This would of course allow adding a large transaction to each output of=
<br>
> the unconfirmed parent, which in effect would allow an attacker to<br>
> exceed the MAX_PACKAGE_VIRTUAL_SIZE limit in some cases. However, is<br=
>
> this a problem with the current mempool acceptance code in bitcoind? I<=
br>
> would imagine evicting transactions based on feerate when the max<br>
> mempool size is met handles this, but I=E2=80=99m asking since it seems=
like<br>
> there has been several changes to the acceptance code and eviction<br>
> policy since the limit was first introduced.<br>
> <br>
> - Johan<br>
> <br>
> <br>
> On Wed, Feb 13, 2019 at 6:57 AM Rusty Russell <<a href=3D"mailto:rus=
ty@rustcorp.com.au" target=3D"_blank">rusty@rustcorp.com.au</a><br>
> <mailto:<a href=3D"mailto:rusty@rustcorp.com.au" target=3D"_blank">r=
usty@rustcorp.com.au</a>>> wrote:<br>
> <br>
> Matt Corallo <<a href=3D"mailto:lf-lists@mattcora=
llo.com" target=3D"_blank">lf-lists@mattcorallo.com</a><br>
> <mailto:<a href=3D"mailto:lf-lists@mattcorallo.co=
m" target=3D"_blank">lf-lists@mattcorallo.com</a>>> writes:<br>
> >>> Thus, even if you imagine a steady-stat=
e mempool growth, unless the<br>
> >>> "near the top of the mempool" criteria i=
s "near the top of the next<br>
> >>> block" (which is obviously *not* incent=
ive-compatible)<br>
> >><br>
> >> I was defining "top of mempool" as "in the f=
irst 4 MSipa", ie. next<br>
> >> block, and assumed you'd only allow RBF if t=
he old package wasn't<br>
> in the<br>
> >> top and the replacement would be. Tha=
t seems incentive<br>
> compatible; more<br>
> >> than the current scheme?<br>
> ><br>
> > My point was, because of block time variance, e=
ven that criteria<br>
> doesn't hold up. If you assume a steady flow of new t=
ransactions and<br>
> one or two blocks come in "late", suddenly "top 4MWe=
ight" isn't<br>
> likely to get confirmed until a few blocks come in "=
early". Given<br>
> block variance within a 12 block window, this is a r=
elatively likely<br>
> scenario.<br>
> <br>
> [ Digging through old mail. ]<br>
> <br>
> Doesn't really matter. Lightning close algorit=
hm would be:<br>
> <br>
> 1. Give bitcoind unileratal close.<br>
> 2. Ask bitcoind what current expidited fee is (=
or survey your mempool).<br>
> 3. Give bitcoind child "push" tx at that total=
feerate.<br>
> 4. If next block doesn't contain unilateral cl=
ose tx, goto 2.<br>
> <br>
> In this case, if you allow a simpified RBF where 'yo=
u can replace if<br>
> 1. feerate is higher, 2. new tx is in first 4Msipa o=
f mempool, 3.<br>
> old tx isnt',<br>
> it works.<br>
> <br>
> It allows someone 100k of free tx spam, sure. B=
ut it's simple.<br>
> <br>
> We could further restrict it by marking the unilater=
al close somehow to<br>
> say "gonna be pushed" and further limiting the child=
tx weight (say,<br>
> 5kSipa?) in that case.<br>
> <br>
> Cheers,<br>
> Rusty.<br>
> _______________________________________________<br>
> Lightning-dev mailing list<br>
> <a href=3D"mailto:Lightning-dev@lists.linuxfoundatio=
n.org" target=3D"_blank">Lightning-dev@lists.linuxfoundation.org</a><br>
> <mailto:<a href=3D"mailto:Lightning-dev@lists.lin=
uxfoundation.org" target=3D"_blank">Lightning-dev@lists.linuxfoundation.org<=
/a>><br>
> <a href=3D"https://lists.linuxfoundation.org/mailman=
/listinfo/lightning-dev" rel=3D"noreferrer" target=3D"_blank">https://lists.=
linuxfoundation.org/mailman/listinfo/lightning-dev</a><br>
> <br>
</blockquote></div>
</div></blockquote></body></html>=
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|
