[bitcoin-dev] CLTV/CSV/etc. deployment considerations due to XT/Not-BitcoinXT miners

[Peter Todd <pete@petertodd.org>]

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Deployment of the proposed CLTV, CSV, etc. soft-forks has been recently
complicated by the existence of XT(1) and Not-Bitcoin-XT(2) miners. Both
mine blocks with nVersion=0x20000007, which would falsely trigger the
previously suggested implementation using the IsSuperMajority()
mechanism and nVersion=4 blocks. Additionally while the
XT/Not-Bitcoin-XT software claims to support Wuille/Todd/Maxwell's
nVersion soft-fork mechanism(3) a key component of it - fork
deadlines(3) - is not implemented.


XT/Not-Bitcoin-XT behavior
--------------------------

Both implementations produce blocks with nVersion=0x20000007,
or in binary: 0b001...111

Neither implementation supports a fork deadline; both Not-Bitcoin-XT and
XT will produce blocks with those bits set indefinitely under any
circumstance, with the proviso that while XT has a hashing power
majority, blocks it produces might not be part of the Bitcoin blockchain
after Jan 11th 2016. (though this can flap back and forth if reorgs
happen)

Curiously the BIP101 draft was changed(4) at the last minute from using
the nVersion bits compliant 0x20000004 block nVersion, to using two more
bits unnecessarily. The rational for doing this is unknown; the git
commit message associated with the change suggested "compatibility
concerns", but what the concerns actually were isn't specified. Equally
even though implementing the fork deadline would be very each in the XT
implementation, this was not done. (the XT codebase has had almost no
peer review)


Options for CLTV/CSV/etc. deployment
------------------------------------

1) Plain IsSuperMajority() with nVersion=4

This option can be ruled out immediately due to the high risk of
premature triggering, without genuine 95% miner support.


2) nVersion mask, with IsSuperMajority()

In this option the nVersion bits set by XT/Not-Bitcoin-XT miners would
be masked away, prior to applying standard IsSuperMajority() logic:

    block.nVersion & ~0x20000007

This means that CLTV/CSV/etc. miners running Bitcoin Core would create
blocks with nVersion=8, 0b1000. From the perspective of the
CLTV/CSV/etc.  IsSuperMajority() test, XT/Not-Bitcoin-XT miners would be
advertising blocks that do not trigger the soft-fork.

For the perpose of soft-fork warnings, the highest known version can
remain nVersion=8, which is triggered by both XT/Not-Bitcoin-XT blocks
as well as a future nVersion bits implementation. Equally,
XT/Not-Bitcoin-XT soft-fork warnings will be triggered, by having an
unknown bit set.

When nVersion bits is implemented by the Bitcoin protocol, the plan of
setting the high bits to 0b001 still works. The three lowest bits will
be unusable for some time, but will be eventually recoverable as
XT/Not-Bitcoin-XT mining ceases.

Equally, further IsSuperMajority() softforks can be accomplished with
the same masking technique.

This option does complicate the XT-coin protocol implementation in the
future. But that's their problem, and anyway, the maintainers
(Hearn/Andresen) has strenuously argued(5) against the use of soft-forks
and/or appear to be in favor of a more centralized mandatory update
schedule.(6)


3) Full nVersion bits implementation

The most complex option would be to deploy via full nVersion bits
implementation using flag bit #4 to trigger the fork. Compliant miners
would advertise 0x20000008 initially, followed by 0x20000000 once the
fork had triggered. The lowest three bits would be unusable for forks
for some time, although they could be eventually recovered as
XT/Not-Bitcoin-XT mining ceases.

The main disadvantage of this option is high initial complexity - the
reason why IsSuperMajority() was suggested for CLTV/CSV in the first
place. That said, much of the code required has been implemented in XT
for the BIP101 hard-fork logic, although as mentioned above, the code
has had very little peer review.


References
----------

1) https://github.com/bitcoinxt/bitcoinxt

2) https://github.com/xtbit/notbitcoinxt

3) "Version bits proposal",
    Pieter Wuille, May 26th 2015, Bitcoin-development mailing list,
    http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-May/008282.html,
    https://gist.github.com/sipa/bf69659f43e763540550

4) https://github.com/bitcoin/bips/commit/3248c9f67bd7fcd1d05b8db7c5c56e4788deebfe

5) "On consensus and forks - What is the difference between a hard and soft fork?",
   Mike Hearn, Aug 12th 2015,
   https://medium.com/@octskyward/on-consensus-and-forks-c6a050c792e7

6) 2013 San Jose Bitcoin conference developer round-table

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