Note, I think that the tx mutation proposal relies on interactivity in the worst-case scenario where a counterparty wants to increase its fee-bumping output from the contract balance. This interactivity may lure a counterparty to alway lock the worst-case fee-bumping reserve in the output. I believe anchor output enables more "real-time" fee-bumping reserve adjustment ?

Anchor outputs / malleability allow for real-time adjustment of long-lived contracts (for which the today worst case is much larger than the worst case
estimated at the contract creation time). However it's a really interested for vaults, as you have multiple parties with the same goal (getting this Cancel
transaction confirmed). Therefore you have this slight "tragedy of the commons" of whose fee-bumping wallet is going to pay for sponsoring the next
Cancel (and it's exacerbated by / for external redundancy providers). With this approach, fees are taxed from the shared coins, so there is no pernicious
incentive to delay the broadcast of your revocation transaction in the hope that another watchtower will pay the fee instead of you. I think this applies to
multi-party channels too, by having some kind of a shared budget.

You would also have a large UX improvement with regard to the fee-bumping wallet: no need to have one (fb wallet refills are really *really* poor UX)
one and maintain a nice laid-out UTXO pool.
In the end, both approaches seem desirable: the output for paying most of the fees from shared coins, therefore dwarfing the "tragedy of the common"
concerns, and the malleability to still be able to dynamically allocate more funds to feebump in case of a black swan event (but essentially needs a single
refill at startup as it's never spent from).

As a side note, this can "just" be implemented by exchanging N (varying depending on the granularity) signatures with increasing feerates. Again, this
might be reasonable in some usecases but not others (eg if you are already generating tons of sigs, or have longer chain of unconfirmed transactions).


Cheers,
Antoine

[0] Incautious sighash alleability is unsafe. Be careful, otherwise kitties will perish by the thousands :
https://github.com/revault/practical-revault/pull/83

Le dim. 6 juin 2021 à 22:28, Lloyd Fournier <lloyd.fourn@gmail.com> a écrit :
Hi Antione,

Thanks for bringing up this important topic. I think there might be another class of solutions over input based, CPFP and sponsorship. I'll call them tx mutation schemes. The idea is that you can set a key that can increase the fee by lowering a particular output after the tx is signed without invalidating the signature. The premise is that anytime you need to bump the fee of a transaction you must necessarily have funds in an output that are going to you and therefore you can sacrifice some of them to increase the fee. This is obviously destructive to txids so child presigned transactions will have to use ANYPREVOUT as in your proposal. The advantage is that it does not require keeping extra inputs around to bump the fee.

So imagine a new opcode OP_CHECKSIG_MUTATED <output index> <publickey> <value> <signature>.
This would check that <signature> is valid against <publickey> if the current transaction had the output at <output index> reduced by <value>. To make this more efficient, if the public key is one byte: 0x02 it references the taproot *external key* (similar to how ANYPREVOUT uses 0x01 to refer to internal key[1]).
Now for our protocol we want both parties (p1 and p2) to be able to fee bump a commitment transaction. They use MuSig to sign the commitment tx under the external key with a decent fee for the current conditions. But in case it proves insufficient they have added the following two leaves to their key in the funding output as a backup so that p1 and p2 can unilaterally bump the fee of anything they sign spending from the funding output:

1. OP_CHECKSIG_MUTATED(0, 0x02, <fee-bump-value>, <original-signature>) OP_CHECKSIGADD(p1-fee-bump-key, <p1-fee-bump-signature>)  OP_2 OP_NUMEQUALVERIFY
2. OP_CHECKSIG_MUTATED(1, 0x02, <fee-bump-value>, <original-signature>) OP_CHECKSIGADD(p2-fee-bump-key, <p2-fee-bump-signature>) OP_2 OP_NUMEQUALVERIFY

where <...> indicates the thing comes from the witness stack.
So to bump the fee of the commit tx after it has been signed either party takes the <original-signature> and adds a signature under their fee-bump-key for the new tx and reveals their fee bump leaf. <original-signature> is checked against the old transaction while the fee bumped transaction is checked against the fee bump key.

I know I have left out how to change mempool eviction rules to accommodate this kind of fee bumping without DoS or pinning attacks but hopefully I have demonstrated that this class of solutions also exists.


Cheers,

LL



On Fri, 28 May 2021 at 07:13, Antoine Riard via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
Hi,

This post is pursuing a wider discussion around better fee-bumping strategies for second-layer protocols. It draws out a comparison between input-based and CPFP fee-bumping techniques, and their apparent trade-offs in terms of onchain footprint, tx-relay bandwidth rebroadcast, batching opportunity and mempool flexibility.

Thanks to Darosior for reviews, ideas and discussions.

## Child-Pay-For-Parent

CPFP is a mature fee-bumping technique, known and used for a while in the Bitcoin ecosystem. However, its usage in contract protocols with distrusting counterparties raised some security issues. As mempool's chain of unconfirmed transactions are limited in size, if any output is spendable by any contract participant, it can be leveraged as a pinning vector to downgrade odds of transaction confirmation [0].

That said, contract transactions interested to be protected under the carve-out logic require to add a new output for any contract participant, even if ultimately only one of them serves as an anchor to attach a CPFP.

## Input-Based

I think input-based fee-bumping has been less studied as fee-bumping primitive for L2s [1]. One variant of input-based fee-bumping usable today is the leverage of the SIGHASH_ANYONECANPAY/SIGHASH_SINGLE malleability flags. If the transaction is the latest stage of the contract, a bumping input can be attached just-in-time, thus increasing the feerate of the whole package.

However, as of today, input-based fee-bumping doesn't work to bump first stages of contract transactions as it's destructive of the txid, and as such breaks chain of pre-signed transactions. A first improvement would be the deployment of the SIGHASH_ANYPREVOUT softfork proposal. This new malleability flag allows a transaction to be signed without reference to any specific previous output. That way,  spent transactions can be fee-bumped without altering validity of the chain of transactions.

Even assuming SIGHASH_ANYPREVOUT, if the first stage contract transaction includes multiple outputs (e.g the LN's commitment tx has multiple HTLC outputs), SIGHASH_SINGLE can't be used and the fee-bumping input value might be wasted. This edge can be smoothed by broadcasting a preliminary fan-out transaction with a set of outputs providing a range of feerate points for the bumped transaction.

This overhead could be smoothed even further in the future with more advanced sighash malleability flags like SIGHASH_IOMAP, allowing transaction signers to commit to a map of inputs/outputs [2]. In the context of input-based, the overflowed fee value could be redirected to an outgoing output.

## Onchain Footprint

CPFP: One anchor output per participant must be included in the commitment transaction. To this anchor must be attached a child transaction with 2 inputs (one for the commitment, one for the bumping utxo) and 1 output. Onchain footprint: 2 inputs + 3 outputs.

Input-based (today): If the bumping utxo is offering an adequate feerate point in function of network mempools congestion at time of broadcast, only 1 input. If a preliminary fan-out transaction to adjust feerate point must be broadcasted first, 1 input and 2 outputs more must be accounted for. Onchain footprint: 2 inputs + 3 outputs.

Input-based (SIGHASH_ANYPREVOUT+SIGHASH_IOMAP): As long as the bumping utxo's value is wide enough to cover the worst-case of mempools congestion, the bumped transaction can be attached 1 input and 1 output. Onchain footprint: 1 input + 1 output.

## Tx-Relay Bandwidth Rebroadcast

CPFP: In the context of multi-party protocols, we should assume bounded rationality of the participants w.r.t to an unconfirmed spend of the contract utxo across network mempools. Under this assumption, the bumped transaction might have been replaced by a concurrent state. To guarantee efficiency of the CPFP the whole chain of transactions should be rebroadcast, perhaps wasting bandwidth consumption for a still-identical bumped transaction [3]. Rebroadcast footprint: the whole chain of transactions.

Input-based (today): In case of rebroadcast, the fee-bumping input is attached to the root of the chain of transactions and as such breaks the chain validity in itself. Beyond the rebroadcast of the updated root under replacement policy, the remaining transactions must be updated and rebroadcast. Rebroadcast footprint: the whole chain of transactions.

Input-based(SIGHASH_ANYPREVOUT+SIGHASH_IOMAP): In case of rebroadcast, the fee-bumping is attached to the root of the chain of transactions but it doesn't break the chain validity in itself. Assuming a future mempool acceptance logic to authorize in-place substitution, the rest of the chain could be preserved. Rebroadcast footprint: the root of the chain of transactions.

## Fee-Bumping Batching

CPFP: In the context of multi-party protocols, in optimistic scenarios, we can assume aggregation of multiple chains of transactions. For e.g, a LN operator is desirous to non-cooperatively close multiple channels at the same time and would like to combine their fee-bumping. With CPFP, one anchor output and one bumping input must be consumed per aggregated chain, even if the child transaction fields can be shared. Batching perf: 1 input/1 output per aggregated chain.

Input-based (today): Unless the contract allows interactivity, multiple chains of transactions cannot be aggregated. One bumping input must be attached per chain, though if a preliminary fan-out transaction is relied on to offer multiple feerate points, transaction fields can be shared. Batching perf: 1 input/1 output per aggregated chain.

Input-based (SIGHASH_ANYPREVOUT+SIGHASH_IOMAP): Multiple chains of transactions might be aggregated together *non-interactively*. One bumping input and outgoing output can be attached to the aggregated root. Batching perf: 1 input/1 output per aggregation.

## Fee-Bumping Mempool Flexibility

CPFP: In the context of multi-party protocols, one of your counterparties might build a branch of transactions from one of the root outputs thus saturating the in-mempool package limits. To avoid these shenanigans, LN channels are relying on the carve-out mechanism. Though, the carve-out mechanism includes its own limitation and doesn't scale beyond 2 contract participants.

Input-based: The root of the chain of transaction is the package's oldest ancestor, so package limits don't restrain its acceptance and it works whatever the number of contract participants.

To conclude, this post scores 2 fee-bumping primitives for multi-party protocols on a range of factors. It hopes to unravel the ground for a real feerate performance framework of second-layers protocols .

Beyond that, few points can be highlighted a) future soft forks allow significant onchain footprint savings, especially in case of batching, b) future package relay bandwidth efficiency should account for rebroadcast frequency of CPFPing multi-party protocols. On this latter point one follow-up might be to evaluate differing package relay *announcement* schemes in function of odds of non-cooperative protocol broadcast/odds of concurrent broadcast/rebroadcast frequencies.

Thoughts ?

Cheers,
Antoine

[2] Already proposed a while back : https://bitcointalk.org/index.php?topic=252960.0
[3] In theory, an already-relayed transaction shouldn't pass Core's `filterInventoryKnown`. In practice, if the transaction is announced as part of a package_id, the child might have changed, not the parent, leading to a redundant relay of the latter.
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