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From: Gregorio Guidi <greg_g@posteo.net>
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Subject: [bitcoin-dev] Exploring alternative activation mechanisms:
decreasing threshold
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Hello,
I followed the debate on LOT=false / LOT=true trying to get a grasp of
the balance of risks and advantages. The summary by Aaron van Wirdum [1]
explains well the difficulties to find a good equilibrium... it
concludes that "perhaps, a new possibility will present itself".
Thinking about such a "new possibility" that overcomes the
LOT=true/false dichotomy, I would like to offer the following proposal.
It could be called "decreasing threshold activation".
Decreasing threshold activation works similarly to BIP8, with the
difference that the threshold that triggers the STARTED -> LOCKED_IN
transition starts at 100% for the first retargeting period, and then is
gradually reduced on each period in steps of 24 blocks (~1,2%). More
precisely:
On the 1st period (starting on start_height): if 2016 out of 2016 blocks
signal, the state is changed to LOCKED_IN on the next period (otherwise
stays STARTED)
On the 2nd period: if 1992 out of 2016 blocks signal (~98.8%), the state
transitions to LOCKED_IN on the next period
On the 3rd period: if 1968 out of 2016 blocks signal (~97.6%), the state
transitions to LOCKED_IN on the next period
...
On the 14th period (~6 months): if 1704 out of 2016 blocks signal
(~84.5%), the state transitions to LOCKED_IN on the next period
...
On the 27th period (~12 months): if 1392 out of 2016 blocks signal
(~69.0%), the state transitions to LOCKED_IN on the next period
...
On the 40th period (~18 months): if 1080 out of 2016 blocks signal
(~53.6%), the state transitions to LOCKED_IN on the next period
...
On the 53th period (~24 months): if 768 out of 2016 blocks signal
(~38.1%), the state transitions to LOCKED_IN on the next period
...
On the 66th period (~30 months): if 456 out of 2016 blocks signal
(~22.6%), the state transitions to LOCKED_IN on the next period
...
On the 79th period (~36 months): if 144 out of 2016 blocks signal
(~7.1%), the state transitions to LOCKED_IN on the next period
...
On the 84th and final period (~39 months): if 24 out of 2016 blocks
signal (~1.2%), the state transitions to LOCKED_IN on the next period,
otherwise goes to FAILED
(For reference, I include below a snippet of pseudocode for the
decreasing thresholds in the style of BIP8 and BIP9.)
Here are the main features and advantages of this approach:
1. It is relatively conservative at the beginning: for activation to
happen in the first year, it requires a clear majority of signaling
hashrate, indicating that the activation is relatively safe. Only later
the threshold starts to move towards "unsafe" territory, accepting the
tradeoff of less support from existing hashrate in exchange for ensuring
that the activation eventually happens.
2. Like LOT=true, the activation will always occur in the end (except in
the negligible case where less than 1.2% of hashrate supports it).
3. This approach is quite easy to implement, in particular it avoids the
extra code to deal with the MUST_SIGNAL period.
4. There are no parameters to set (except startheight). I am a KISS fan,
so this is a plus for me, making the activation mechanism robust and
predictable with less chance for users to shoot themselves in the foot.
It is also a plus for me that - if adopted as the default mechanism - it
would require very little discussion on how to activate future
soft-forks. In fact I think it would be a winning move for Core to
commit to such a scheme, to avoid getting lost in game-theoretic rabbit
holes.
5. Since there is no MUST_SIGNAL period, no automatic chain split occurs
around activation when not all miners have upgraded (so activation is
generally as benign as a MASF). A chain split will occur only when/if an
invalid block is created (and this requires dedicated effort! it can
only happen by circumventing the normal policy rules [2]). This
mitigates the risk of reorgs and involuntary forks around activation,
even with low miner signaling.
6. It removes motivation to create UASF clients that force activation.
While individual nodes could still try to force a quicker activation,
the motivation to do so is reduced since the same result is obtained
just by waiting a little more.
7. Compared to LOT=true, activation is cleaner and quicker when it is
relatively safe to do so (when the signaling hashrate is - let's say -
in the 70%-80% range). On the other hand, activation is pushed further
and further in time when it is less safe (when signaling hashrate is
<50%, meaning that there is a serious risk that users/miners that did
not upgrade start following an alternative chain). This gives everyone
time to prepare properly for such a potentially disruptive event.
8. If a significant number of users and miners consciously decide (for
whatever reasons) that they don't want to upgrade and want to fork
themselves off from the chain followed by Core (as is their
prerogative), they will have time to do so safely.
9. Compared to the strategy of doing LOT=false and then LOT=true if it
fails, using the decreasing threshold approach may not seem very
different. But it completely removes the need to fiddle with different
client releases and with the issues associated with deployed nodes with
different consensus parameters.
All in all, reading the various perspectives on this mailing list and
outside I have the feeling that the strongest arguments against LOT=true
have at their core a certain uneasiness with the MUST_SIGNAL mechanism
and the related automatic chain split on activation, which is something
that greatly complicates the analysis (but please tell me if I am
wrong...). In this sense, this proposal achieves the big objective of
always ending in activation (like LOT=true) without resorting to
MUST_SIGNAL and chain splits.
A final note: this proposal should be seen as somewhat independent from
the discussion on taproot activation. Personally I would be happy with a
LOT=false activation for taproot that succeeds quickly, while the
decreasing threshold approach could be evaluated as potential default
activation mechanism for the future.
I would be happy to hear what you think about this. What are the
possible issues/drawbacks of using this mechanism?
Thanks,
Gregorio
[1]
https://bitcoinmagazine.com/articles/lottrue-or-lotfalse-this-is-the-last-hurdle-before-taproot-activation
[2] This was not the case in the past for upgrades such as BIP16 (P2SH),
which generated frequent reorgs due to a combination of low activation
threshold (55%) and no policy protection. But for upgrades such as
taproot the normal policy rules prevent the creation of invalid blocks
by non-upgraded miners. See
https://blog.bitmex.com/the-arts-of-making-softforks-protection-by-policy-rule/
Pseudocode:
case STARTED:
int elapsed_periods = (block.height - startheight) / 2016;
if (elapsed_periods > 2016 / 24) {
return FAILED;
}
int threshold = 2016 - 24 * (elapsed_periods - 1);
int count = 0;
walk = block;
for (i = 0; i < 2016; i++) {
walk = walk.parent;
if (walk.nVersion & 0xE0000000 == 0x20000000 && (walk.nVersion >> bit) & 1 == 1) {
++count;
}
}
if (count >= threshold) {
return LOCKED_IN;
}
return STARTED;
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