[Bitcoin-development] deterministic transaction expiration

[Kaz Wesley <keziahw@gmail.com>]

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 d̶e̶s̶t̶r̶o̶y̶ ̶t̶h̶e̶
̶r̶e̶s̶u̶r̶r̶e̶c̶t̶i̶o̶n̶ ̶h̶u̶b̶ use nLockTime and a new 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)

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.