Feedback-ID: i525146e8:Fastmail
Date: Mon, 07 Nov 2022 10:32:17 -0400
From: Peter Todd <pete@petertodd.org>
To: email@yancy.lol,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
 yancy via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>,
 Anthony Towns <aj@erisian.com.au>
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References: <16eb6a50691ccc661310051de6b8e2c0@yancy.lol>
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Subject: Re: [bitcoin-dev] On mempool policy consistency
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On November 3, 2022 5:06:52 PM AST, yancy via bitcoin-dev <bitcoin-dev@lis=
ts=2Elinuxfoundation=2Eorg> wrote:
>
>AJ/Antoine et al
>
>> What should folks wanting to do coinjoins/dualfunding/dlcs/etc do to
>> solve that problem if they have only opt-in RBF available?
>
>Assuming Alice is a well funded advisory, with enough resources to spam t=
he network so that enough nodes see her malicious transaction first, how do=
es full-rbf solve this vs=2E opt-in rbf?

First of all, to make things clear, remember that the attacks were talking=
 about are aimed at _preventing_ a transaction from getting mined=2E Alice =
wants to cheaply broadcast something with low fees that won't get mined soo=
n (if ever), that prevents a protocol from making forward progress=2E

With full-rbf, who saw what transaction first doesn't matter: the higher f=
ee paying transaction will always(*) replace the lower fee one=2E With opt-=
in RBF, spamming the network can beat out the alternative=2E

*) So what's the catch? Well, due to limitations in today's mempool implem=
entation, sometimes we can't fully evaluate which tx pays the higher fee=2E=
 For example, if Alice spams the network with very _large_ numbers transact=
ions spending that input, the current mempool code doesn't even try to figu=
re out if a replacement is better=2E

But those limitations are likely to be fixable=2E And even right now, with=
out fixing them, Alice still has to use a lot more money to pull off these =
attacks with full-rbf=2E So full-rbf definitely improves the situation even=
 if it doesn't solve the problem completely=2E