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Date: Fri, 31 Jan 2025 12:02:56 +0000
To: bitcoindev@googlegroups.com
From: "'ArmchairCryptologist' via Bitcoin Development Mailing List" <bitcoindev@googlegroups.com>
Subject: Re: [bitcoindev] Transaction expiration should be based on the most
recent transaction in a group, not the first
Message-ID: <rqLkuDekAhYC8mO9BMrqXGQjv-Oq1a1jA2EDLdiohq9kHIQpE-MpzqFjqY15Cc5flU_GUrE7NoSxB_4wgkQg8Mv8IQdtzlXLtuxK76n7YRQ=@protonmail.com>
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X-Spam-Score: -1.0 (-)
Is expiration-based mempool eviction necessary or even desirable anymore? I=
'm consistently seeing unconfirmed transactions from months ago being rebro=
adcast and (now that the mempool is draining) eventually confirming, withou=
t anyone even trying to exploit anything. So from what I can tell, the only=
thing this really accomplishes is wasting CPU cycles and bandwidth evictin=
g and later re-accepting the transactions in question.
You were never able to rely on unconfirmed transactions ever going away wit=
hout double-spending one of the inputs in the first place, and full-RBF is =
even a thing now, so this will always be possible.
The mempool is capped by size anyway, so while I may be missing something, =
I cannot honestly see any good reasons to keep this mechanism at all, espec=
ially if it can be used as a vector for attacks.
The only drawback I can think of is that abandontransaction currently does =
not work if a transaction is in the mempool, but it would probably be bette=
r to improve it so it actually evicts the transaction from the mempool of t=
he local node if necessary.
--
Best,
ArmchairCryptologist
On Tuesday, January 28th, 2025 at 11:25 PM, Peter Todd <pete@petertodd.org>=
wrote:
> Disclaimer: I haven't actually tested this. So there is a chance I'm
> understanding the code entirely wrong. If so, feel free to make fun of
> me for being too lazy to actually test this.
>=20
>=20
> In Bitcoin Core, mempool expiration is done by:
>=20
> int CTxMemPool::Expire(std::chrono::seconds time)
> {
> AssertLockHeld(cs);
> indexed_transaction_set::index<entry_time>::type::iterator it =3D mapTx.g=
et<entry_time>().begin();
>=20
> setEntries toremove;
> while (it !=3D mapTx.get<entry_time>().end() && it->GetTime() < time) {
>=20
> toremove.insert(mapTx.project<0>(it));
>=20
> it++;
> }
> setEntries stage;
> for (txiter removeit : toremove) {
> CalculateDescendants(removeit, stage);
> }
> RemoveStaged(stage, false, MemPoolRemovalReason::EXPIRY);
> return stage.size();
> }
>=20
> https://github.com/bitcoin/bitcoin/blob/b432e367427f1f9fe0f0a5800e31e496f=
00cd38d/src/txmempool.cpp#L1086
>=20
> This function is expiring transactions based on their entry time into
> the mempool, a value that is set once and never changed. Transactions
> are removed unconditionally on expiration, whether or not they have
> descendents. That means that if you broadcast A, wait just prior to A's
> expiration, and broadcast B, a transaction spending an output of A, B
> will be evicted immediately when A's expiration time is reached.
>=20
> There's at least three problems with this:
>=20
> 1) It's dumb. If I do a CPFP on an old transaction, I want that
> transaction to get mined and am willing to pay money. It's silly to make
> me jump through the hoop of rebroadcasting it again when it expires.
>=20
> 2) It's a free-relay DoS attack: just prior to A expiring, I could
> broadcast B, a very large transaction, and use up bandwidth for "free".
> Frankly, I'm not very concerned about this. But if you care, you
> should fix this.
>=20
> 3) Expiration could maybe be leveraged in transaction cycling attacks:
> https://stacker.news/items/866680
>=20
> Personally, I'm not convinced that transaction expiration is actually a
> good idea. The best argument for it IMO is in the case of some
> soft-fork-style screwup where you're allowing stuff into your mempool
> that will never get mined. But that means something is seriously wrong
> to begin with - you probably should fix that. Otherwise, it's not
> uncommon for transactions that are months old to eventually get mined.
> Do we really need to waste bandwidth re-relaying them in the meantime?
>=20
> --
> https://petertodd.org 'peter'[:-1]@petertodd.org
>=20
> --
> You received this message because you are subscribed to the Google Groups=
"Bitcoin Development Mailing List" group.
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> To view this discussion visit https://groups.google.com/d/msgid/bitcoinde=
v/Z5lZc28t9-tCxdHN%40petertodd.org.
--=20
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