That's not really pinning; painning usually refers to pinning something to the bottom of the mempool whereas these mechanisms make it easier to guarantee that progress can be made on confirming the transactions you're interested in.

Often times in these protocols "the call is coming inside the house". It's not a third party adding fees we are scared of, it's a direct party to the protocol!

Sponsors or fee accounts would enable you to ensure the protocol you're working on makes forward progress. For things like Eltoo the internal ratchet makes this work well.

Protocols which depend on in mempool replacements before confirmation already must be happy (should they be secure) with any prior state being mined. If a third party pays the fee you might even be happier since the execution wasn't on your dime.

Cheers,

Jeremy

On Wed, Feb 9, 2022, 10:59 PM Peter Todd via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

On Sat, Jan 01, 2022 at 12:04:00PM -0800, Jeremy via bitcoin-dev wrote:
> Happy new years devs,
>
> I figured I would share some thoughts for conceptual review that have been
> bouncing around my head as an opportunity to clean up the fee paying
> semantics in bitcoin "for good". The design space is very wide on the
> approach I'll share, so below is just a sketch of how it could work which
> I'm sure could be improved greatly.
>
> Transaction fees are an integral part of bitcoin.
>
> However, due to quirks of Bitcoin's transaction design, fees are a part of
> the transactions that they occur in.
>
> While this works in a "Bitcoin 1.0" world, where all transactions are
> simple on-chain transfers, real world use of Bitcoin requires support for
> things like Fee Bumping stuck transactions, DoS resistant Payment Channels,
> and other long lived Smart Contracts that can't predict future fee rates.
> Having the fees paid in band makes writing these contracts much more
> difficult as you can't merely express the logic you want for the
> transaction, but also the fees.
>
> Previously, I proposed a special type of transaction called a "Sponsor"
> which has some special consensus + mempool rules to allow arbitrarily
> appending fees to a transaction to bump it up in the mempool.
>
> As an alternative, we could establish an account system in Bitcoin as an
> "extension block".

<snip>

> This type of design works really well for channels because the addition of
> fees to e.g. a channel state does not require any sort of pre-planning
> (e.g. anchors) or transaction flexibility (SIGHASH flags). This sort of
> design is naturally immune to pinning issues since you could offer to pay a
> fee for any TXID and the number of fee adding offers does not need to be
> restricted in the same way the descendant transactions would need to be.

So it's important to recognize that fee accounts introduce their own kind of
transaction pinning attacks: third parties would be able to attach arbitrary
fees to any transaction without permission. This isn't necessarily a good
thing: I don't want third parties to be able to grief my transaction engines by
getting obsolete transactions confirmed in liu of the replacments I actually
want confirmed. Eg a third party could mess up OpenTimestamps calendars at
relatively low cost by delaying the mining of timestamp txs.

Of course, there's an obvious way to fix this: allow transactions to designate
a pubkey allowed to add further transaction fees if required. Which Bitcoin
already has in two forms: Replace-by-Fee and Child Pays for Parent.

--
https://petertodd.org 'peter'[:-1]@petertodd.org
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