A hard-fork is a situation where non-upgraded nodes reject a block mined and relayed by upgraded nodes.
As Peter pointed out, that is the case here.
This creates a fork that cannot heal regardless of what follows.
That is not a condition of the hard fork concept.
- Softfork
- A consensus fork wherein everything that was previously invalid remains invalid while blocks that would have previously considered valid become invalid. A hashrate majority of miners can impose the new rules. They have some deployment advantages like backward compatibility.
- Hardfork
- A consensus fork that makes previously invalid blocks valid. Hardforks require all users to upgrade.
The essential element of a hard fork is that the new rule may cause rejection of blocks that are not rejected by old rules (thereby requiring that all users adopt the new rule in order to avoid a split). The reason a hard fork is interesting is that it can create a chain split even if it is enforced by majority hash power.
That is not the case with a soft fork and it is not the case here. A split can occur. The fact that it is possible for the split to also eventually orphan the old nodes does not make it a soft fork. A soft fork requires that a hash power majority can impose the rule. However, under the proposed new rule the hash power majority (according to the new rule) cannot impose the rule on existing nodes.
This proposal is not a hard-fork, because the non-upgraded node *will heal* if the attack has less than 1/2 of the original-POW power in the long term.
Nothing about this proposal implies an attack. From the Motivation section:
- Mitigate centralization pressures by introducing a POW that does not have economies of scale
- Introduce an intermediary confirmation point, reducing the impact of mining power fluctuations
The cost of such an attack is the cost of a normal "51%" attack, multiplied by the fractional weight of the original POW (e.g. 0.75 or 0.5).
So rather than saying this is a hard-fork, I would say that this is a soft-fork with reduced security for non-upgraded nodes.
Presumably this preference exists because it implies the new rule would not cause a chain split, making it more acceptable to a risk-averse economy. This is precisely why it should be described correctly.
I would also say that the reduction in security is proportional to the reduction in weight of the original POW at the time of attack.
As mentioned before, the original-POW weight starts at 1.0 and is reduced over a long period of time. I would set up the transition curve so that all nodes upgrade by the time the weight is, say, 0.75. In reality, nodes protecting high economic value would upgrade early.
In reality you have no way to know if/when people would adopt this rule. What matters in the proposal is that people who do adopt it are well aware of its ability to split them from the existing economy.
e
If a block that would be discarded under previous rules becomes accepted after a rule addition, there is no reason to not simply call the new rule a hard fork. IOW it's perfectly rational to consider a weaker block as "invalid" relative to the strong chain. As such I don't see any reason to qualify the term, it's a hard fork. But Peter's observation (the specific behavior) is ultimately what matters.
+1 to all of Peter Todd's comments
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