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From: Kaz Wesley <keziahw@gmail.com>
Date: Thu, 31 Jul 2014 17:58:23 -0700
Message-ID: <CA+iPb=HkxeVPF0SynxCPgUkq4msrdfayFrVNFjzg29rFwqXv1w@mail.gmail.com>
To: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
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Subject: [Bitcoin-development] deterministic transaction expiration
Precedence: list

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.

I have a proposal for a way to add finite and predictable lifespans to
transactions in mempools: we d=CC=B6e=CC=B6s=CC=B6t=CC=B6r=CC=B6o=CC=B6y=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=B6i=CC=
=B6o=CC=B6n=CC=B6 =CC=B6h=CC=B6u=CC=B6b=CC=B6 use nLockTime and a new stand=
ardness
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)

Transactions would stop being relayed and drop out of mempools a fixed
number of blocks from their creation; once that window had passed, the
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.