Subject: Re: [bitcoindev] Over Half of Replace-by-Fee-Rate Replacements Are Mined

From: Peter Todd <[email-masked]>
Date: Sat, 24 Feb 2024 18:45:41 +0000

On Sat, Feb 24, 2024 at 02:55:26PM -0300, Nagaev Boris wrote:
> > I recently released a [prototype Libre Relay fork](https://github.com/petertodd/bitcoin/tree/libre-relay-v26.0) of Bitcoin Core v26.0, that among
> > other things, implements [Pure Replace-By-Fee-Rate](/2024/one-shot-replace-by-fee-rate#pure-replace-by-fee-rate) (RBFR)
> > with a 2x ratio. This means that transactions will be replaced if the fee-rate
> > of the new transaction is at least 2x higher than the old transaction(s), even
> > if the absolute fee is lower.
> 
> I like the idea of pure replacement by fee rate, but I'm not sure
> about the 2x coefficient. I'm afraid it can result in overpaying.
> 
> What is the purpose of the 2x coefficient? Is it needed to prevent DoS
> by continuously increasing feerate by a fraction of a percent? Isn't
> 1.1x enough to prevent this DoS vector? A rise from 100 sats/byte to
> 110 sats/byte is easier to bear, then a rise from 100 to 200,
> especially if the next block feerate is 105.

The coefficient is a trade-off between overpaying and DoS attack cost. The
smaller the coefficient, the more replacements that can be done:

2^10    = 1024
1.5^10  = 58
1.25^10 = 9.3
1.1^10  = 2.6

I picked 2x for the prototype because it's:

1) A safe default that makes DoS attacks _very_ expensive.
2) Low enough that the primary purpose of preventing pinning attacks still
   works.
3) High enough to be clearly incentive compatible for miners.
4) A ratio that's easy to implement with addition.

The last reason is kinda funny... CFeeRate in Bitcoin Core v26.0 doesn't
support multiplication or division operations. So I implemented the 2x ratio by
just adding a fee-rate to itself.

Personally, I think 1.25 would be a reasonable ratio. But I wanted to release a
conservative version first to minimize the impact of DoS attacks.

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

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