Date: Tue, 1 Feb 2022 11:56:37 +1000
From: Anthony Towns <aj@erisian.com.au>
To: Bastien TEINTURIER <bastien@acinq.fr>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <20220201015637.GA4302@erisian.com.au>
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Subject: Re: [bitcoin-dev] Improving RBF Policy
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On Mon, Jan 31, 2022 at 04:57:52PM +0100, Bastien TEINTURIER via bitcoin-dev wrote:
> I'd like to propose a different way of looking at descendants that makes
> it easier to design the new rules. The way I understand it, limiting the
> impact on descendant transactions is only important for DoS protection,
> not for incentive compatibility. I would argue that after evictions,
> descendant transactions will be submitted again (because they represent
> transactions that people actually want to make),

I think that's backwards: we're trying to discourage people from wasting
the network's bandwidth, which they would do by publishing transactions
that will never get confirmed -- if they were to eventually get confirmed
it wouldn't be a waste of bandwith, after all. But if the original
descendent txs were that sort of spam, then they may well not be
submitted again if the ancestor tx reaches a fee rate that's actually
likely to confirm.

I wonder sometimes if it could be sufficient to just have a relay rate
limit and prioritise by ancestor feerate though. Maybe something like:

 - instead of adding txs to each peers setInventoryTxToSend immediately,
   set a mempool flag "relayed=false"

 - on a time delay, add the top N (by fee rate) "relayed=false" txs to
   each peer's setInventoryTxToSend and mark them as "relayed=true";
   calculate how much kB those txs were, and do this again after
   SIZE/RATELIMIT seconds

 - don't include "relayed=false" txs when building blocks?

 - keep high-feerate evicted txs around for a while in case they get
   mined by someone else to improve compact block relay, a la the
   orphan pool?

That way if the network is busy, any attempt to do low fee rate tx spam
will just cause those txs to sit as relayed=false until they're replaced
or the network becomes less busy and they're worth relaying. And your
actual mempool accept policy can just be "is this tx a higher fee rate
than the txs it replaces"...

> Even if bitcoin core releases a new version with updated RBF rules, as a
> wallet you'll need to keep using the old rules for a long time if you
> want to be safe.

All you need is for there to be *a* path that follows the new relay rules
and gets from your node/wallet to perhaps 10% of hashpower, which seems
like something wallet providers could construct relatively quickly?

Cheers,
aj