Summary
-------

Opt-In Full-RBF allows senders to opt-into full-RBF semantics for their
transactions in a way that allows receivers to detect if the sender has
done so. Existing "first-seen" mempool semantics are left unchanged for
transactions that do not opt-in.

At last week's IRC meeting(1) we decided to merge the opt-in Full-RBF
pull-req(2), pending code review and this post, so this feature will
likely make it into Bitcoin Core v0.12.0


Specification
-------------

A transaction is considered to have opted into full-RBF semantics if
nSequence < 0xFFFFFFFF-1 on at least one input. Nodes that respect the
opt-in will allow such opt-in transactions (and their descendents) to be
replaced in the mempool if they meet the economic replacement criteria.
Transactions in blocks are of course unaffected.

To detect if a transaction may be replaced check if it or any
unconfirmed ancestors have set nSequence < 0xFFFFFFFF-1 on any inputs.


Rational
--------

nSequence is used for opting in as it is the only "free-form" field
available for that purpose. Opt-in per output was proposed as well by
Luke-Jr, however the CTxOut data structure simply doesn't contain any
extra fields to use for that purpose. nSequence-based opt-in is also
compatible with the consensus-enforced transaction replacement semantics
in BIP68.

Allowing replacement if any input opts in vs. all inputs opting in is
chosen to ensure that transactions authored by multiple parties aren't
held up by the actions of a single party. Additionally, in the
multi-party scenario the value of any zeroconf guarantees are especially
dubious.

Replacement is allowed even if unconfirmed children did not opt-in to
ensure receivers can't maliciously prevent a replacement by spending the
funds. Additionally, any reasonable attempt at determining if a
transaction can be double-spent has to look at all unconfirmed parents
anyway.

Feedback from wallet authors indicates that first-seen-safe RBF isn't
very useful in practice due to the limitations inherent in FSS rules;
opt-in full-RBF doesn't preclude FSS-RBF from also being implemented.


Compatibility
-------------

Opt-in RBF transactions are currently mined by 100% of the hashing
power. Bitcoin Core has been producing transactions with non-maxint
nSequence since v0.11.0 to discourage fee sniping(3), and currently no
wallets are known that display such transactions yet do not display
opt-in RBF transactions.


Demonstrations
--------------

https://github.com/petertodd/replace-by-fee-tools#incremental-send-many


1) http://lists.linuxfoundation.org/pipermail/bitcoin-discuss/2015-November/000010.html

2) https://github.com/bitcoin/bitcoin/pull/6871

3) https://github.com/bitcoin/bitcoin/pull/2340

-- 
'peter'[:-1]@petertodd.org
00000000000000000f30567c63f8f4f079a8ecc2ab3d380bc7dc370e792b0a3a