Re: [bitcoin-dev] Ephemeral Anchors: Fixing V3 Package RBF against package limit pinning

From: Antoine Riard <antoine.riard@gmail•com>
To: Greg Sanders <gsanders87@gmail•com>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Ephemeral Anchors: Fixing V3 Package RBF against package limit pinning
Date: Tue, 18 Oct 2022 20:33:13 -0400	[thread overview]
Message-ID: <CALZpt+Hr1wCT8VFLWN5iqLG04FqSkdGSQuawCRKab2_uj6OoZQ@mail.gmail.com> (raw)
In-Reply-To: <CAB3F3Dukoz3P3Ne7tCxMiwwAGm3Fv8r_fUkNbGAtGhAZDYDgCQ@mail.gmail.com>

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Hi Greg,

Thanks for proposing forward the "ephemeral anchors" policy change.

> In Gloria's proposal for ln-penalty, this is worked
> 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.

IIRC, here I think we also need _package relay_ in strict addition of
_package RBF_, otherwise if your Lightning transactions are still relayed
and accepted one by one, your version of the commitment transaction won't
succeed to replace the other counterparties's commitments sleeping in
network mempools. The presence of a remote anchor output on the
counterparty commitment still offers an ability to fee-bump, albeit in
practice more a lucky shot as you might have partitioned network mempools
between your local commitment and the remote commitment disputing the spend
of the same funding UTXO.

> 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 sibling
> out of the mempool and takes the one child slot. This would solve it, but
> is a new eviction paradigm that would need to be carefully worked through.

Note, I wonder about the robustness of such a "sibling eviction" mechanism
in the context of multi-party construction. E.g, a batching payout, where
the participants are competing to each other in a blind way, as they do
want their CPFPs paying back to them to confirm first, enforcing their
individual liquidity preferences. I would think it might artificially lead
the participants to overbid far beyond the top mempool block fees.

>  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 funds. They
just
>  confirmed your parent transaction anyways, so do we care?

If we allow non-zero value in ephemeral outputs, I think we're slightly
modifying the incentives games of the channels counterparties, in the sense
if you have a link Alice-Bob, Bob could circular loop a bunch of dust
offered HTLC deduced from Alice balance and committed as fees in the
ephemeral output value, then break the channel on-chain to pocket in the
trimmed value sum (in the limit of your Lightning implementation dust
exposure). Note, this is already possible today if your counterparty is a
miner however iiuc the proposal, here we're lowering the bar.

>  SIGHASH_GROUP like constructs would allow uncommitted ephemeral anchors
>  to be added at spend time, depending on spending requirements.
>  SIGHASH_SINGLE already allows this.

Note, with SIGHASH_GROUP, you're still allowed to aggregate in a single
bundle multiple ln-penalty commitments or eltoo settlement transactions,
with only one fee-bumping output. It's a cool space performance trick, but
a) I think this is still more a whiteboard idea than a sound proposal and
b) sounds more a long-term, low-hanging fruit optimization of blockspace
consumption.

Best,
Antoine

Le mar. 18 oct. 2022 à 09:53, Greg Sanders via bitcoin-dev <
bitcoin-dev@lists•linuxfoundation.org> a écrit :

> Hello Everyone,
>
> Following up on the "V3 Transaction" discussion here
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-September/020937.html
> , I would like to elaborate a bit further on some potential follow-on work
> 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 and
> propagated, it costs the expected amount of fees to do so. This is a great
> 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 the
> 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 worked
> 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 an
> existing transaction in the mempool, the parent will be detected and
> removed from the package proposal. 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 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 sibling
> out of the mempool and takes the one child slot. This would solve it, but
> is a new eviction paradigm that would need to be carefully worked through.
>
> 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 well.
>
> 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 this
> policy for maximum smart contract flexibility and specification simplicity..
>
> 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 smart
> 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 miniscript
> 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/018168.html)
> proposal without consensus changes. As long as someone gets a fully signed
> parent, they can execute a bump with minimal wallet tooling. If a
> transaction author doesn’t want a “sponsor”, do not include the output.
>
> d) Lightning Carve-out(
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-October/002240.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 “pinning” in the
> first place(
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-February/015717.html),
> even if LN/L2 discussion has largely overtaken it due to HTLC theft risks.
>
> 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 funds.
>    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 broken. We
> need a more complex solution, which I’m punting for the sake of progress.
>
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