[bitcoin-dev] Thoughts on soft-fork activation

[Anthony Towns <aj@erisian.com.au>]

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