[bitcoin-dev] Thoughts on soft-fork activation

[bitcoin-dev] Thoughts on soft-fork activation

[Anthony Towns]

Hi,

I've been trying to figure out a good way to activate soft forks in
future. I'd like to post some thoughts on that. So:

I think there's two proposals that are roughly plausible. The first is
Luke's recent update to BIP 8:

    https://github.com/bitcoin/bips/blob/master/bip-0008.mediawiki

It has the advantage of being about as simple as possible, and (in my
opinion) is an incremental improvement on how segwit was activated. Its
main properties are:

   - signalling via a version bit
   - state tansitions based on height rather than median time
   - 1 year time frame
   - optional mandatory activation at the end of the year
   - mandatory signalling if mandatory activation occurs
   - if the soft fork activates on the most work chain, nodes don't
     risk falling out of consensus depending on whether they've opted in
     to mandatory activation or not

I think there's some fixable problems with that proposal as it stands
(mostly already mentioned in the comments in the recently merged PR,
https://github.com/bitcoin/bips/pull/550 )

The approach I've been working on is based on the more complicated and
slower method described by Matt on this list back in January. I've got a
BIP drafted at:

    https://github.com/ajtowns/bips/blob/202007-activation-dec-thresh/bip-decthresh.mediawiki

The main difference with the mechanism described in January is that the
threshold gradually decreases during the secondary period -- it starts at
95%, gradually decreases until 50%, then mandatorily activates. The idea
here is to provide at least some potential reward for miners signalling
in the secondary phase: if 8% of hashpower had refused to signal for
a soft-fork, then there would have been no chance of activating until
the very end of the period. This way, every additional percentage of
hashpower signalling brings the activation deadline forward.

The main differences between the two proposals is that the BIP 8 approach
has a relatively short time frame for users to upgrade if we want
mandatory activation without a supermajority of hashpower enforcing the
rules; while the "decreasing threshold" approach linked above provides
quite a long timeline.

In addition, there is always the potential to introduce a BIP 91/148
style soft-fork after the fact where either miners or users coordinate to
have mandatory signalling which then activates a pre-existing deployment
attempt.

I think the design constraints we want are:

 * if everyone cooperates and no one objects, we activate pretty quickly

 * there's no obvious exploits, and we have plausible contingency plans
   in place to discourage people from try to use the attempt to deploy
   a new soft fork as a way of attacking bitcoin, either via social
   disruption or by preventing bitcoin from improving

 * we don't want to ship code that causes people to fall out of
   consensus in the (hopefully unlikely) event that things don't go
   smoothly [0]

In light of that, I think I'm leaning towards:

 * use BIP 8 with mandatory activation disabled in bitcoin core -- if
   you want to participate in enforcing mandatory activation, you'll
   need to recompile, or use a fork like bitcoin knots; however if
   mandatory activation occurs on the longest chain, you'll still follow
   that chain and enforce the rules.

 * be prepared to update the BIP 8 parameters to allow mandatory
   activation in bitcoin core if, after 9 months say, it's apparent that
   there aren't reasonable objections, there's strong support for
   activation, the vast majority of nodes have already updated to
   enforce the rules upon activation, and there's strong support for
   mandatory activation

 * change the dec-threshold proposal to be compatible with BIP 8, and
   keep it maintained so that it can be used if there seems to be
   widespread consensus for activation, but BIP 8 activation does
   not seem certain -- ie, as an extra contingency plan.

 * be prepared to support miners coordinating via BIP 91 either to
   bring activation forward in either BIP 8 or "decreasing threshold" or
   de-risk BIP 8 mandatory activation -- ie, an alternative contingency
   plan. This is more appropriate if we've found that users/miners have
   upgraded so that activation is safe; so it's a decision we can make
   later when we have more data, rather than having to make the decision
   early when we don't have enough information to judge whether it's
   safe or not.

 * (also, improve BIP 8 a bit more before deploying it -- I'm hoping for
   some modest changes, which is why "decreasing threshold" isn't already
   compatible with BIP 8)

 * (continue to ensure the underlying soft fork makes sense and is
   well implemented on its own merits)

 * (continue to talk to as many people as we can about the underlying
   changes and make sure people understand what's going on and that
   we've addressed any reasonable objections)

I'm hopeful activating taproot will go smoothly, but I'm not 100% sure
of it, and there are potentially many different ways in which things go
wrong; so starting with something simple and being ready to adapt if/when
we see things starting to go weird seems like a good approach to me.

Cheers,
aj

[0] At least, that's how I'm phrasing some of the concerns that were
    expressed in, eg,

    https://github.com/bitcoin/bitcoin/pull/10428#issuecomment-303098925
    https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-April/014152.html

Re: [bitcoin-dev] Thoughts on soft-fork activation

[Matt Corallo]

Thanks Anthony for this writeup!

I find it incredibly disappointing that the idea of naive flag day fork activation is being seriously discussed in the
form of BIP 9. Activation of forks is not only about the included changes but also around the culture of how changes to
Bitcoin should be and are made. Whether we like it or not, how Taproot activates will set a community understanding and
future norms around how many changes are made.

Members of this list lost sleep and years off their life from stress fighting to ensure that the process by which
Bitcoin changes is not only principled in its rejection of unilateral changes, but also that that idea was broadly
understood, and broadly *enforced* by community members - the only way in which it has any impact. That fight is far
from over - Bitcoin's community grows and changes daily, and the history around what changed and how has been rewritten
time and time again. Worse still, the principled nature of Bitcoin's change process is targeted constantly as untrue in
an attempt by various alternative systems to pretend that their change process of "developers ship new code, users run
it blindly" is identical to Bitcoin.

While members of this list may be aware of significant outreach efforts and design work to ensure that Taproot is not
only broadly acceptable to Bitcoin users, but also has effectively no impact on users who wish not to use it, it is
certainly not the case that all Bitcoin users are aware of that work, nor seen the results directly communicated to them.

Worse still, it is hard to argue that a new version of Bitcoin Core containing a fixed future activation of a new
consensus rule is anything other than "developers have decided on new rules" (even if it is, based on our own knowledge,
not the case). Indeed, even the proposal by Anthony, which makes reference to my previous work has this issue, and it
may not be avoidable - there is very legitimate concern over miners blocking changes to Bitcoin which do not harm them
which users objectively desire, potentially purely through apathy. But to dismiss the concerns over the optics which set
the stage for how future changes are made to Bitcoin purely because miners may be too busy with other things to upgrade
their nodes seems naive at best.

I appreciate the concern over activation timeline given miner apathy, and to some extend Anthony's work here addresses
that with decreasing activation thresholds during the second signaling period, but bikeshedding on timeline may be merited.

To not make every attempt to distance the activation method from the public perception unilateral activation strikes me
as the worst of all possible outcomes for Bitcoin's longevity. Having a quieting period after BIP 9 activation failure
may not be the best way to do that, but it seems like a reasonable attempt.

Matt

On 7/14/20 5:37 AM, Anthony Towns via bitcoin-dev wrote:
> Hi,
> 
> I've been trying to figure out a good way to activate soft forks in
> future. I'd like to post some thoughts on that. So:
> 
> I think there's two proposals that are roughly plausible. The first is
> Luke's recent update to BIP 8:
> 
>     https://github.com/bitcoin/bips/blob/master/bip-0008.mediawiki
> 
> It has the advantage of being about as simple as possible, and (in my
> opinion) is an incremental improvement on how segwit was activated. Its
> main properties are:
> 
>    - signalling via a version bit
>    - state tansitions based on height rather than median time
>    - 1 year time frame
>    - optional mandatory activation at the end of the year
>    - mandatory signalling if mandatory activation occurs
>    - if the soft fork activates on the most work chain, nodes don't
>      risk falling out of consensus depending on whether they've opted in
>      to mandatory activation or not
> 
> I think there's some fixable problems with that proposal as it stands
> (mostly already mentioned in the comments in the recently merged PR,
> https://github.com/bitcoin/bips/pull/550 )
> 
> The approach I've been working on is based on the more complicated and
> slower method described by Matt on this list back in January. I've got a
> BIP drafted at:
> 
>     https://github.com/ajtowns/bips/blob/202007-activation-dec-thresh/bip-decthresh.mediawiki
> 
> The main difference with the mechanism described in January is that the
> threshold gradually decreases during the secondary period -- it starts at
> 95%, gradually decreases until 50%, then mandatorily activates. The idea
> here is to provide at least some potential reward for miners signalling
> in the secondary phase: if 8% of hashpower had refused to signal for
> a soft-fork, then there would have been no chance of activating until
> the very end of the period. This way, every additional percentage of
> hashpower signalling brings the activation deadline forward.
> 
> The main differences between the two proposals is that the BIP 8 approach
> has a relatively short time frame for users to upgrade if we want
> mandatory activation without a supermajority of hashpower enforcing the
> rules; while the "decreasing threshold" approach linked above provides
> quite a long timeline.
> 
> In addition, there is always the potential to introduce a BIP 91/148
> style soft-fork after the fact where either miners or users coordinate to
> have mandatory signalling which then activates a pre-existing deployment
> attempt.
> 
> I think the design constraints we want are:
> 
>  * if everyone cooperates and no one objects, we activate pretty quickly
> 
>  * there's no obvious exploits, and we have plausible contingency plans
>    in place to discourage people from try to use the attempt to deploy
>    a new soft fork as a way of attacking bitcoin, either via social
>    disruption or by preventing bitcoin from improving
> 
>  * we don't want to ship code that causes people to fall out of
>    consensus in the (hopefully unlikely) event that things don't go
>    smoothly [0]
> 
> In light of that, I think I'm leaning towards:
> 
>  * use BIP 8 with mandatory activation disabled in bitcoin core -- if
>    you want to participate in enforcing mandatory activation, you'll
>    need to recompile, or use a fork like bitcoin knots; however if
>    mandatory activation occurs on the longest chain, you'll still follow
>    that chain and enforce the rules.
> 
>  * be prepared to update the BIP 8 parameters to allow mandatory
>    activation in bitcoin core if, after 9 months say, it's apparent that
>    there aren't reasonable objections, there's strong support for
>    activation, the vast majority of nodes have already updated to
>    enforce the rules upon activation, and there's strong support for
>    mandatory activation
> 
>  * change the dec-threshold proposal to be compatible with BIP 8, and
>    keep it maintained so that it can be used if there seems to be
>    widespread consensus for activation, but BIP 8 activation does
>    not seem certain -- ie, as an extra contingency plan.
> 
>  * be prepared to support miners coordinating via BIP 91 either to
>    bring activation forward in either BIP 8 or "decreasing threshold" or
>    de-risk BIP 8 mandatory activation -- ie, an alternative contingency
>    plan. This is more appropriate if we've found that users/miners have
>    upgraded so that activation is safe; so it's a decision we can make
>    later when we have more data, rather than having to make the decision
>    early when we don't have enough information to judge whether it's
>    safe or not.
> 
>  * (also, improve BIP 8 a bit more before deploying it -- I'm hoping for
>    some modest changes, which is why "decreasing threshold" isn't already
>    compatible with BIP 8)
> 
>  * (continue to ensure the underlying soft fork makes sense and is
>    well implemented on its own merits)
> 
>  * (continue to talk to as many people as we can about the underlying
>    changes and make sure people understand what's going on and that
>    we've addressed any reasonable objections)
> 
> I'm hopeful activating taproot will go smoothly, but I'm not 100% sure
> of it, and there are potentially many different ways in which things go
> wrong; so starting with something simple and being ready to adapt if/when
> we see things starting to go weird seems like a good approach to me.
> 
> Cheers,
> aj
> 
> [0] At least, that's how I'm phrasing some of the concerns that were
>     expressed in, eg,
> 
>     https://github.com/bitcoin/bitcoin/pull/10428#issuecomment-303098925
>     https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-April/014152.html
> 
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
> 

Re: [bitcoin-dev] Thoughts on soft-fork activation

[ZmnSCPxj]

Good morning list, BlueMatt and aj,

There is an idea circulating on IRC and elsewhere, which seems to be at least mildly supported by gmax and roconnor, which I will try to explain here.

(These are my words, so if there is some mistake, I apologize)

Basically:

* Deploy a BIP8 `lockinontimeout=true` `lockin=+42 months` (or 36 months, or 24 months) at next release.
  * Pedanty note: BIP8 uses blockheights, not actual times.
* Then 1 year after `starttime`, ***if*** it is not activated yet:
  * Discuss.
  * If we think it is only because of miner apathy and user support seems good regardless, deploy a BIP91 reduced-threshold 80% that enforces the BIP8 bit.
    * We hope that this will stave off independent attempts at a UASF with a faster timeout.
  * If we think there are real reasons not to continue with Taproot as-is, deploy an abort: a softfork that disallows transaction outputs with `OP_1 <32-bytes>` `scriptPubKey` (other lengths and other versions are allowed).

This approximates what aj is proposing:

* Ultimately, we expect to deploy a BIP8 `lockinontimeout=true` that will have a timeout that ends +42 months after the first countdown, even with Modern Softfork Activation.
* The abort is roughly equivalent to the Modern Softfork Activation case where during the 6 month discussion period we decide not to deploy Taproot after all.
* The deployment of a BIP91 reduced-threshold 80% approximates what aj proposes, to reduce the threshold for activation later.

As I understand it, an advantage of this proposal is that we can deploy very quickly a relatively simple BIP8 `locktimeontimeout=true`, then continue debate on various details (is 80% too low? too high? are users actually deploying? are mining pools updating? etc) in parallel.
This lets the code into the hands of users where they can start deploying it and we can start getting better gauges on how well Taproot is supported.


Regards,
ZmnSCPxj