[Bitcoin-development] Mechanics of a hard fork

Tier Nolan tier.nolan at gmail.com
Thu May 7 21:24:45 UTC 2015

In terms of miners, a strong supermajority is arguably sufficient, even 75%
would be enough.

The near total consensus required is merchants and users.  If (almost) all
merchants and users updated and only 75% of the miners updated, then that
would give a successful hard-fork.

On the other hand, if 99.99% of the miners updated and only 75% of
merchants and 75% of users updated, then that would be a serioud split of
the network.

The advantage of strong miner support is that it effectively kills the fork
that follows the old rules.  The 25% of merchants and users sees a
blockchain stall.

Miners are likely to switch to the fork that is worth the most.  A mining
pool could even give 2 different sub-domains.  A hasher can pick which
rule-set to follow.  Most likely, they would converge on the fork which
paid the most, but the old ruleset would likely still have some hashing
power and would eventually re-target.

On Thu, May 7, 2015 at 9:00 PM, Roy Badami <roy at gnomon.org.uk> wrote:

> I'd love to have more discussion of exactly how a hard fork should be
> implemented.  I think it might actually be of some value to have rough
> consensus on that before we get too bogged down with exactly what the
> proposed hard fork should do.  After all, how can we debate whether a
> particular hard fork proposal has consensus if we haven't even decided
> what level of supermajority is needed to establish consensus?
> For instance, back in 2012 Gavin was proposing, effectively, that a
> hard fork should require a supermajority of 99% of miners in order to
> succeed:
> https://gist.github.com/gavinandresen/2355445
> More recently, Gavin has proposed that a supermoajority of only 80% of
> miners should be needed in order to trigger the hard fork.
> http://www.gavintech.blogspot.co.uk/2015/01/twenty-megabytes-testing-results.html
> Just now, on this list (see attached message) Gavin seems to be
> aluding to some mechanism for a hard fork which involves consensus of
> full nodes, and then a soft fork preceeding the hard fork, which I'd
> love to see a full explanation of.
> FWIW, I think 80% is far too low to establish consensus for a hard
> fork.  I think the supermajority of miners should be sufficiently
> large that the rump doesn't constitute a viable coin.  If you don't
> have that very strong level of consensus then you risk forking Bitcoin
> into two competing coins (and I believe we already have one exchange
> promissing to trade both forks as long as the blockchains are alive).
> As a starting point, I think 35/36th of miners (approximately 97.2%)
> is the minimum I would be comfortable with.  It means that the rump
> coin will initially have an average confirmation time of 6 hours
> (until difficulty, very slowly, adjusts) which is probably far enough
> from viable that the majority of holdouts will quickly desert it too.
> Thoughs?
> roy
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