From: Rusty Russell <rusty@rustcorp.com.au>
To: Matt Corallo <lf-lists@mattcorallo.com>,
	bitcoin-dev@lists.linuxfoundation.org
In-Reply-To: <c3f68b73-84c6-7428-4bf6-b47802141392@mattcorallo.com>
References: <c3f68b73-84c6-7428-4bf6-b47802141392@mattcorallo.com>
Date: Tue, 04 Dec 2018 14:03:53 +1030
Message-ID: <878t163qzi.fsf@rustcorp.com.au>
MIME-Version: 1.0
Content-Type: text/plain
Cc: lightning-dev@lists.linuxfoundation.org
Subject: Re: [bitcoin-dev] [Lightning-dev] CPFP Carve-Out for Fee-Prediction
	Issues in Contracting Applications (eg Lightning)
Precedence: list

Matt Corallo <lf-lists@mattcorallo.com> writes:
> As an alternative proposal, at various points there have been 
> discussions around solving the "RBF-pinning" problem by allowing 
> transactors to mark their transactions as "likely-to-be-RBF'ed", which 
> could enable a relay policy where children of such transactions would be 
> rejected unless the resulting package would be "near the top of the 
> mempool". This would theoretically imply such attacks are not possible 
> to pull off consistently, as any "transaction-delaying" channel 
> participant will have to place the package containing A at an effective 
> feerate which makes confirmation to occur soon with some likelihood. It 
> is, however, possible to pull off this attack with low probability in 
> case of feerate spikes right after broadcast.

I like this idea.

Firstly, it's incentive-compatible[1]: assuming blocks are full, miners
should always take a higher feerate tx if that tx would be in the
current block and the replaced txs would not.[2]

Secondly, it reduces the problem that the current lightning proposal
adds to the UTXO set with two anyone-can-spend txs for 1000 satoshis,
which might be too small to cleanup later.  This rule would allow a
simple single P2WSH(OP_TRUE) output, or, with IsStandard changed,
a literal OP_TRUE.

> Note that this clearly relies on some form of package relay, which comes 
> with its own challenges, but I'll start a separate thread on that.

Could be done client-side, right?  Do a quick check if this is above 250
satoshi per kweight but below minrelayfee, put it in a side-cache with a
60 second timeout sweep.  If something comes in which depends on it
which is above minrelayfee, then process them as a pair[3].

Cheers,
Rusty.
[1] Miners have generally been happy with Defaults Which Are Good For The
    Network, but I feel a long term development aim should to be reduce
    such cases to smaller and smaller corners.
[2] The actual condition is subtler, but this is a clear subset AFAICT.
[3] For Lightning, we don't care about child-pays-for-grandparent etc.