Subject: Re: [bitcoin-dev] Taproot activation proposal "Speedy Trial"

From: Ariel Luaces <[email-masked]>
Date: Sat, 6 Mar 2021 12:44:48 -0800

On Sat, Mar 6, 2021 at 10:11 AM Matt Corallo via bitcoin-dev
<[email-masked]> wrote:
>
> I'm really unsure that three months is a short enough time window that there wouldn't be a material effort to split the
> network with divergent consensus rules. Instead, a three month window is certainly long enough to organize and make a
> lot of noise around such an effort, given BIP 148 was organized and reached its peak within a similar such window.
>
> Worse, because the obvious alternative after a three month activation failure is a significant delay prior to
> activation, the vocal UASF minority may be encouraged to pursue such a route to avoid such a delay.
>

I agree with your concern, a three month window motivates a small
group to constantly tell people to upgrade as soon as possible. Which
is mostly fine, but if this group gets near 51% mining support in the
three months it will embolden them to switch the messaging from
"upgrade the client" to "run this new client that has the LOT flag
switched to true" (UASF)
This marketing group will attempt a UASF regardless of the timelines
because there is no cost if they fail a UASF and there is great reward
if they succeed by activating in 3 months. They can run an alt-node,
scare everyone else of being reorged, pretend they have a majority,
and say their chain is the safe one. I say there is no cost because
the leaders of that movement are likely savvy enough to give up the
effort and move back to running core even after the chain split. The
ones who get hurt are the followers of the UASF movement that don't
fully understand the discussion and drink the kool aid.
A short time window doesn't preclude this group from attempting a UASF.

> One alternative may be to reduce the signaling windows involved and start slightly later. Instead of the likelihood of
> failure growing on the horizon, simply have two signaling windows (maybe two weeks, maybe a moth each?). In order to
> ensure success remains likely, begin them somewhat later after software release to give pools and miners a chance to
> configure their mining software in advance.
>
> Matt

The shorter the signaling windows the more unlikely they are to reach
a mining power supermajority.
With a short signaling window the supermajority threshold will
probably be configured to low levels (80% 70% 60%). Which makes the
activation period dangerous because it forces the other miners (20%
30% 40%) to upgrade really suddenly once the threshold is reached.
Unless the LOCKED_IN period is made really long (1 year) but then we
have to wait a really long time.
As long as the mining power running the soft fork is above 50% the
chain will stay together. Anything above that is just a safety margin
to prevent orphan blocks.

So why not encode this 50%+ dynamic into the activation logic.
Spread the deployment window out to a year, make the activation
threshold 95%, and at the end of the window the feature can activate
if there is above 51% signaling.
By definition, the activation will happen if either 95% is reached
early or if at the end of the deployment window mining support is
between 51% and 95%. With this setup the chain is guaranteed to stay
together and no need to rush miners and users.

Cheers
Ariel Lorenzo-Luaces

>
> On 3/5/21 22:43, David A. Harding via bitcoin-dev wrote:
> > On the ##taproot-activation IRC channel, Russell O'Connor recently
> > proposed a modification of the "Let's see what happens" activation
> > proposal.[1] The idea received significant discussion and seemed
> > acceptable to several people who could not previously agree on a
> > proposal (although this doesn't necessarily make it their first
> > choice).  The following is my attempt at a description.
> >
> > 1. Start soon: shortly after the release of software containing this
> >     proposed activation logic, nodes will begin counting blocks towards
> >     the 90% threshold required to lock in taproot.[2]
> >
> > 2. Stop soon: if the lockin threshold isn't reached within approximately
> >     three months, the activation attempt fails.  There is no mandatory
> >     activation and everyone is encouraged to try again using different
> >     activation parameters.
> >
> > 2. Delayed activation: in the happy occasion where the lockin threshold
> >     is reached, taproot is guaranteed to eventually activate---but not
> >     until approximately six months after signal tracking started.
> >
> > ## Example timeline
> >
> > (All dates approximate; see the section below about BIP9 vs BIP8.)
> >
> > - T+0: release of one or more full nodes with activation code
> > - T+14: signal tracking begins
> > - T+28: earliest possible lock in
> > - T+104: locked in by this date or need to try a different activation process
> > - T+194: activation (if lockin occurred)
> >
> > ## Analysis
> >
> > The goal of Speedy Trial is to allow a taproot activation attempt to
> > either quickly succeed or quickly fail---without compromising safety in
> > either case.  Details below:
> >
> > ### Mitigating the problems of early success
> >
> > New rules added in a soft fork need to be enforced by a large part of
> > the economy or there's a risk that a long chain of blocks breaking the
> > rules will be accepted by some users and rejected by others, causing a
> > chain split that can result in large direct losses to transaction
> > receivers and potentially even larger indirect losses to holders due to
> > reduced confidence in the safety of the Bitcoin system.
> >
> > One step developers have taken in the past to ensure widespread adoption
> > of new consensus rules is programming in a delay between the time software
> > with those rules is expected to be released and when the software starts
> > tracking which blocks signal for activation.  For example:
> >
> >      Soft fork        | Release    | Start      | Delta
> >      -----------------+------------+------------+----------
> >      BIP68 (v0.12.1)  | 2016-04-15 | 2016-05-11 | 26 days
> >      BIP141 (v0.13.1) | 2016-10-27 | 2016-11-18 | 24 days
> >
> >      Sources: BitcoinCore.org, https://gist.github.com/ajtowns/1c5e3b8bdead01124c04c45f01c817bc
> >
> > Speedy Trial replaces most of that upfront delay with a backend delay.
> > No matter how fast taproot's activation threshold is reached by miners,
> > there will be six months between the time signal tracking starts and when
> > nodes will begin enforcing taproot's rules.  This gives the userbase even
> > more time to upgrade than if we had used the most recently proposed start
> > date for a BIP8 activation (~July 23rd).[2]
> >
> > ### Succeed, or fail fast
> >
> > The earlier version of this proposal was documented over 200 days ago[3]
> > and taproot's underlying code was merged into Bitcoin Core over 140 days
> > ago.[4]  If we had started Speedy Trial at the time taproot
> > was merged (which is a bit unrealistic), we would've either be less than
> > two months away from having taproot or we would have moved on to the
> > next activation attempt over a month ago.
> >
> > Instead, we've debated at length and don't appear to be any closer to
> > what I think is a widely acceptable solution than when the mailing list
> > began discussing post-segwit activation schemes over a year ago.[5]  I
> > think Speedy Trial is a way to generate fast progress that will either
> > end the debate (for now, if activation is successful) or give us some
> > actual data upon which to base future taproot activation proposals.
> >
> > Of course, for those who enjoy the debate, discussion can continue while
> > waiting for the results of Speedy Trial.
> >
> > ### Base activation protocol
> >
> > The idea can be implemented on top of either Bitcoin Core's existing
> > BIP9 code or its proposed BIP8 patchset.[6]
> >
> > - BIP9 uses two time-based[7] parameters, starttime and timeout.  Using
> >    these values plus a time-based parameter for the minimum activation
> >    delay would give three months for miners to activate taproot, but some
> >    of that time near the start or the end might not be usable due to
> >    signals only being measured in full retarget periods.  However, the
> >    six month time for users to upgrade their node would be not be
> >    affected by either slow or fast block production.
> >
> >      BIP9 is already part of Bitcoin Core and I think the changes being
> >      proposed would be relatively small, resulting in a small patch that
> >      could be easy to review.
> >
> > - BIP8 uses two height-based parameters, startheight and timeoutheight.
> >    Using height values would ensure miners had a certain number of
> >    retarget periods (6) to lock in taproot and that there'd be a certain
> >    number of blocks (about 24,000) until activation, although latest lock
> >    in and expected activation could occur moderately earlier or later
> >    than the estimated three and six months.
> >
> >      BIP8 would likely be used if Speedy Trial fails, so it could be
> >      advantageous to base this proposal on BIP8 so that we gain
> >      experience running that code in production.
> >
> > For additional discussion about using times versus heights, see today's
> > log for ##taproot-activation.[11]
> >
> > ### Additional concerns
> >
> > - Encourages false signaling: false signaling is when miners signal
> >    readiness to enforce rules that their nodes don't actually support.
> >    This was partially responsible for a six-block reorg shortly after the
> >    final BIP66 activation[8] and was found to still be a problem during
> >    the BIP68 lockin period despite BIP9 being designed to avoid it.[9]
> >
> >    Because Speedy Trial only gives miners a maximum of three months to
> >    signal support for taproot, it may encourage such false signaling.  If
> >    taproot locks in as a result of their signaling but most of them fail
> >    to upgrade by the activation date several months later, unprepared
> >    miners could lose large amounts of money and users could see long
> >    reorgs (with unupgraded nodes and SPV lite clients potentially losing
> >    money).
> >
> >    Compared to other activation proposals, I think the only difference is
> >    Speedy Trial's short timeline.  False signaling is possible with any
> >    other proposal and the same problems can occur if miners fail to
> >    upgrade for any mandatory activation.
> >
> > ### Additional advantages
> >
> > - No mandatory signaling: at no time are miners required to signal by
> >    Speedy Trial.  This includes no mandatory signaling during the
> >    locked_in period(s), although such signaling will be encouraged (as it
> >    was with BIP9[10]).
> >
> > - Party time: to a lesser degree, a benefit mentioned for flag day
> >    activation may also apply here: we could get up to six months
> >    advanced notice of taproot activation, allowing users, developers, and
> >    organizations to prepare software, announcements, and celebrations for
> >    that event.
> >
> > ## Implementation details and next steps
> >
> > Initial discussion about implementation may be found in today's
> > ##taproot-activation log.[11] If it appears Speedy Trial may have
> > traction, Russell O'Connor has offered to work on a patch against BIP8
> > implementing it.
> >
> > ## Acknowledgments
> >
> > The original idea for a short-duration attempt was discussed in the
> > ##taproot-activation IRC channel last July and the revised idea saw
> > additional evaluation there this week.  Despite growing frustration,
> > discussion has been overwhelmingly constructive, for which all the
> > contributors should be commended.  Although this should not in any way
> > imply endorsement, I'm grateful for the review and comments on a draft
> > of this email by Adam Gibson, Andrew Chow, Anthony Towns, Chris Belcher,
> > Jeremy Rubin, Jonas Nick, Luke Dashjr, Michael Folkson, Russell
> > O'Connor, and IRC users maybehuman and proofofkeags
> >
> > ## Footnotes
> >
> > [1] https://en.bitcoin.it/wiki/Taproot_activation_proposals#Let.E2.80.99s_see_what_happens.2C_BIP8.28false.2C_3m.29
> >
> > [2] A threshold of 1,815/2,016 blocks (90%) in a single retarget period
> >      seemed to have near-universal support during the 2021-02-16 IRC
> >      meeting.  See: https://en.bitcoin.it/wiki/Taproot_activation_proposal_202102
> >
> > [3] https://en.bitcoin.it/w/index.php?title=Taproot_activation_proposals&oldid=68062
> >
> > [4] https://github.com/bitcoin/bitcoin/pull/19953
> >
> > [5] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-January/017547.html
> >
> > [6] https://github.com/bitcoin/bitcoin/pull/19573
> >
> > [7] BIP9's times are based on the median of the past 11 blocks, which
> >      usually trails UTC by about 90 minutes but which can trail behind
> >      realtime significantly if miners are doing weird things.
> >
> > [8] https://en.bitcoin.it/wiki/July_2015_chain_forks
> >
> > [9] https://buildingbitcoin.org/bitcoin-core-dev/log-2016-06-21.html#l-32
> >
> > [10] https://github.com/bitcoin/bitcoin/blob/ed25cb58f605ba583c735f330482df0bf9348f3a/src/test/versionbits_tests.cpp#L337-L339
> >
> > [11] http://gnusha.org/taproot-activation/2021-03-05.log
> >
> >
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> >
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