From: Matt Whitlock <bip@mattwhitlock.name>
To: Kaz Wesley <keziahw@gmail.com>
Date: Thu, 31 Jul 2014 21:38:56 -0400
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Cc: bitcoin-development@lists.sourceforge.net
Subject: Re: [Bitcoin-development] deterministic transaction expiration
Precedence: list

It would make more sense to introduce a new script opcode that pushes t=
he current block height onto the operand stack. Then you could implemen=
t arbitrary logic about which blocks the transaction can be valid in. T=
his would require that the client revalidate all transactions in its me=
mpool (really, only those making use of this opcode) whenever the chain=
 tip changes.


On Thursday, 31 July 2014, at 5:58 pm, Kaz Wesley wrote:
> There is currently little in place for managing transaction lifetime
> in the network's mempools (see discussion in github in #3722 "mempool=

> transaction expiration", and it seems to be a major factor blocking
> some mempool exchange, see #1833/1918, #3721). Expiry per-node a
> certain amount of wall time after receipt has been proposed, but
> that's a fragile mechanism -- a single node could keep all relayable
> transactions alive forever by remembering transactions until most
> nodes have dropped them and then releasing them back into the wild.
>=20
> I have a proposal for a way to add finite and predictable lifespans t=
o
> transactions in mempools: we d=CC=B6e=CC=B6s=CC=B6t=CC=B6r=CC=B6o=CC=B6=
y=CC=B6 =CC=B6t=CC=B6h=CC=B6e=CC=B6
> =CC=B6r=CC=B6e=CC=B6s=CC=B6u=CC=B6r=CC=B6r=CC=B6e=CC=B6c=CC=B6t=CC=B6=
i=CC=B6o=CC=B6n=CC=B6 =CC=B6h=CC=B6u=CC=B6b=CC=B6 use nLockTime and a n=
ew standardness
> rule. It could be done in stages, would not necessarily require even =
a
> soft fork, and does not cause problems with reorgs like the proposal
> in #3509:
> 1. start setting nLockTime to the current height by default in newly
> created transactions (or slightly below the current height, for
> reorg-friendliness)
> 2. once users have had some time to upgrade to clients that set
> nLockTime, start discouraging transactions without nLockTime --
> possibly with a slightly higher fee required for relay
> 3. start rate-limiting relay of transactions without an nLockTime
> (maybe this alone could be used to achieve [2])
> 4. add a new IsStandard rule rejecting transactions with an nLockTime=

> more than N blocks behind the current tip (for some fixed value N, to=

> be determined)
>=20
> Transactions would stop being relayed and drop out of mempools a fixe=
d
> number of blocks from their creation; once that window had passed, th=
e
> sender's wallet could begin to expect the transaction would not be
> confirmed. In case a reorg displaces a transaction until after its
> expiry height, a miner can still put it back in the blockchain; the
> expiry height is just a relay rule. Also, a user who needed to get
> their original "expired" transaction confirmed could still do so by
> submitting it directly to a miner with suitable policies.