Subject: Re: [bitcoin-dev] Thoughts on fee bumping

From: Billy Tetrud <[email-masked]>
Date: Tue, 15 Feb 2022 11:09:56 -0600

>   If you wish to fee-bump transaction X with sponsor, how can you be sure
that transaction isn't present in the majority of network nodes, and X has
_not_ been dropped since your last broadcast ?

You're right that you can't assume your target transaction hasn't been
dropped. However, I assume when James said "No rebroadcast (wasted
bandwidth) is required for the original txn data" he meant that in the
context of the "diff" he was talking about. It would be easy enough to
specify a sponsorship transaction that points to a transaction with a
specific id without *requiring* that transaction to be rebroadcast. If your
partner node has that transaction, no rebroadcast is necessary. If your
partner node doesn't have it, they can request it. That way rebroadcast is
only done when necessary. Correct me if my understanding of your suggestion
is wrong James.

>> 2. (from Suhas) "once a valid transaction is created, it should not
become invalid later on unless the inputs are double-spent."
> This doesn't seem like a huge concern to me

I agree that this shouldn't be a concern. In fact, I've asked numerous
people in numerous places what practical downside there is to transactions
that become invalid, and I've heard basically radio silence other than one
off hand remark by satoshi at the dawn of time which didn't seem to me to
have good reasoning. I haven't seen any downside whatsoever of transactions
that can become invalid for anyone waiting the standard 6 confirmations -
the reorg risks only exists for people not waiting for standard
finalization. So I don't think we should consider that aspect of a
sponsorship transaction that can only be mined with the transaction it
sponsors to be a problem unless a specific practical problem case can be
identified. Even if a significant such case was identified, an easy
solution would be to simply allow sponsorship transactions to be mined on
or after the sponsored transaction is mined.

On Mon, Feb 14, 2022 at 7:10 PM Antoine Riard via bitcoin-dev <
[email-masked]> wrote:

> > In the context of fee bumping, I don't see how this is a criticism
> > unique to transaction sponsors, since it also applies to CPFP: if you
> > tried to bump fees for transaction A with child txn B, if some mempool
> > hasn't seen parent A, it will reject B.
>
> Agree, it's a comment raising the shenanigans of tx-diff-only propagation,
> afaict affecting equally all fee-bumping primitives. It wasn't a criticism
> specific to transaction sponsors, as at that point of your post, sponsors
> are not introduced yet.
>
> > This still doesn't address the issue I'm talking about, which is if you
> > pre-commit to some "fee-bumping" key in your CPFP outputs and that key
> > ends up being compromised. This isn't a matter of data availability or
> > redundancy.
>
> I'm not sure about the real safety risk of the compromise of the anchor
> output key. Of course, if your anchor output key is compromised and the
> bumped package is already public/known, an attacker can extend your package
> with junk to neutralize your carve-out capability (I think). That said,
> this issue sounds solved to me with package relay, as you can always
> broadcast a new version of the package from the root UTXO, without
> attention to the carve-out limitation.
>
> (Side-note: I think we can slowly deprecate the carve-out once package
> relay is deployed, as the fee-bumping flexibility of the latter is a
> superset of the former).
>
> > As I mentioned in the reply to Matt's message, I'm not quite
> > understanding this idea of wanting to bump the fee for something
> > without knowing what it is; that doesn't make much sense to me.
> > The "bump fee" operation seems contingent on knowing
> > what you want to bump.
>
> From your post : "No rebroadcast (wasted bandwidth) is required for the
> original txn data."
>
> I'm objecting to that supposed benefit of a transaction sponsor. If you
> have transaction X and transaction Y spending the same UTXO, both of them
> can be defined as "the original txn data". If you wish to fee-bump
> transaction X with sponsor, how can you be sure that transaction
> Y isn't present in the majority of network nodes, and X has _not_ been
> dropped since your last broadcast ? Otherwise iirc sponsor design, your
> sponsor transaction is going to be rejected.
>
> I think you can't, and thus preventively you should broadcast as a (new
> type) of package the sponsoring/sponsored transaction.
>
> That said, I'm not sure if that issue is equally affecting vaults than
> payment channels. With vaults, the tree of transactions is  known ahead,
> and there is no competition in the spends. Assuming the first broadcast has
> been efficient (and it could be a reasonable assumption thanks to mempool
> rebroadcast), the sponsor should propagate.
>
> So I think here for the sake of sponsor efficiency analysis, we might have
> to class between the protocol with once-for-all-transaction-negotiation
> (vaults) and the ones with off-chain, dynamic re-negotiation (payment
> channels, factories) ?
>
> > I'm not familiar with the L2 dust-limit issues, and I do think that
> > "fixing" RBF behavior is *probably* worthwhile.
>
> Sadly, it sounds that "fixing" RBF behavior is a requirement to eradicate
> the most advanced pinnings... That fix is independent of the fee-bumping
> primitive considered.
>
> >  Those issues aside, I
> > think the transaction sponsors idea may be closer to a silver bullet
> > than you're giving it credit for, because designing specifically for the
> > fee-management use case has some big benefits.
>
> I don't deny the scheme is interesting, though I would argue SIGHASH_GROUP
> is more efficient, while offering more flexibility. In any case, I think we
> should still pursue further the collections of problems and requirements
> (batching, key management, ...) that new fee-bumping primitives should aim
> to solve, before engaging more on the deployment of one of them [0].
>
> [0] In that sense see
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/019031.html
>
> Le lun. 14 févr. 2022 à 15:29, James O'Beirne <[email-masked]> a
> écrit :
>
>> Thanks for your thoughtful reply Antoine.
>>
>> > In a distributed system such as the Bitcoin p2p network, you might
>> > have transaction A and transaction B  broadcast at the same time and
>> > your peer topology might fluctuate between original send and
>> > broadcast of the diff, you don't know who's seen what... You might
>> > inefficiently announce diff A on top of B and diff B on top A. We
>> > might leverage set reconciliation there a la Erlay, though likely
>> > with increased round-trips.
>>
>> In the context of fee bumping, I don't see how this is a criticism
>> unique to transaction sponsors, since it also applies to CPFP: if you
>> tried to bump fees for transaction A with child txn B, if some mempool
>> hasn't seen parent A, it will reject B.
>>
>> > Have you heard about SIGHASH_GROUP [0] ?
>>
>> I haven't - I'll spend some time reviewing this. Thanks.
>>
>> > > [me complaining CPFP requires lock-in to keys]
>> >
>> > It's true it requires to pre-specify the fee-bumping key. Though note
>> > the fee-bumping key can be fully separated from the
>> > "vaults"/"channels" set of main keys and hosted on replicated
>> > infrastructure such as watchtowers.
>>
>> This still doesn't address the issue I'm talking about, which is if you
>> pre-commit to some "fee-bumping" key in your CPFP outputs and that key
>> ends up being compromised. This isn't a matter of data availability or
>> redundancy.
>>
>> Note that this failure may be unique to vault use cases, when you're
>> pre-generating potentially large numbers of transactions or covenants
>> that cannot be altered after the fact. If you generate vault txns that
>> assume the use of some key for CPFP-based fee bumping and that key
>> winds up being compromised, that puts you in a an uncomfortable
>> situation: you can no longer bump fees on unvaulting transactions,
>> rendering the vaults possibly unretrievable depending on the fee market.
>>
>> > As a L2 transaction issuer you can't be sure the transaction you wish
>> > to point to is already in the mempool, or have not been replaced by
>> > your counterparty spending the same shared-utxo, either competitively
>> > or maliciously. So as a measure of caution, you should broadcast
>> > sponsor + target transactions in the same package, thus cancelling
>> > the bandwidth saving (I think).
>>
>> As I mentioned in the reply to Matt's message, I'm not quite
>> understanding this idea of wanting to bump the fee for something
>> without knowing what it is; that doesn't make much sense to me.
>> The "bump fee" operation seems contingent on knowing
>> what you want to bump.
>>
>> And if you're, say, trying to broadcast a lightning channel close and
>> you know you need to bump the fee right away, before even broadcasting
>> it, either you're going to
>>
>> - reformulate the txn to bring up the fee rate (e.g. add inputs
>>   with some yet-undeployed sighash) as you would have done with RBF, or
>>
>> - you'd have the same "package relay" problem with CPFP that you
>>   would with transaction sponsors.
>>
>> So I don't understand the objection here.
>>
>> Also, I didn't mean to discourage existing work on package relay or
>> fixing RBF, which seem clearly important. Maybe I should have noted
>> that explicitly in the original message
>>
>> > I don't think a sponsor is a silver-bullet to solve all the
>> > L2-related mempool issues. It won't solve the most concerning pinning
>> > attacks, as I think the bottleneck is replace-by-fee. Neither solve
>> > the issues encumbered by the L2s by the dust limit.
>>
>> I'm not familiar with the L2 dust-limit issues, and I do think that
>> "fixing" RBF behavior is *probably* worthwhile. Those issues aside, I
>> think the transaction sponsors idea may be closer to a silver bullet
>> than you're giving it credit for, because designing specifically for the
>> fee-management use case has some big benefits.
>>
>> For one, it makes migration easier. That is to say: there is none,
>> whereas there is existing RBF policy that needs consideration.
>>
>> But maybe more importantly, transaction sponsors' limited use case also
>> allows for specifying much more targeted "replacement" policy since
>> sponsors are special-purpose transactions that only exist to
>> dynamically bump feerate. E.g. my SIGHASH_{NONE,SINGLE}|ANYONECANPAY
>> proposal might make complete sense for the sponsors/fee-management use
>> case, and clarify the replacement problem, but obviously wouldn't work
>> for more general transaction replacement. In other words, RBF's
>> general nature might make it a much harder problem to solve well.
>>
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