BlueMatt's Blog On Building a Bitcoin for Everyone

Modern Soft Fork Activation

This was originally posted on the bitcoin-dev mailing list targeting a technical audience bbut is preserved here as it is of more general interest. The requirements and goals of soft fork activation are of particular interest.

There are a series of soft-fork designs which have recently been making good progress towards implementation and future adoption. However, for various reasons, activation methods therefor have gotten limited discussion. I’d like to reopen that discussion here.

It is likely worth revisiting the goals both for soft forks and their activation methods to start. I’m probably missing some, but some basic requirements:

1) Avoid activating in the face of significant, reasonable, and directed objection. Period. If someone has a well-accepted, reasonable use of Bitcoin that is working today, have no reason to believe wouldn’t work long into the future without a change, and which would be made impossible or significantly more difficult by a change, that change must not happen. I certainly hope there is no objection on this point (see the last point for an important caveat that I’m sure everyone will jump to point out).

2) Avoid activating within a timeframe which does not make high node-level-adoption likely. As with all “node” arguments, I’ll note that I mean “economically-used” nodes, not the thousand or so spy nodes on Google Cloud and AWS. Rule changes don’t make sense without nodes enforcing them, whether they happen to be a soft fork, hard fork, or a blue fork, so activating in a reduced timeframe that doesn’t allow for large-scale node adoption doesn’t have any value, and may cause other unintended side effects.

3) Don’t (needlessly) lose hashpower to un-upgraded miners. As a part of Bitcoin’s security comes from miners, reducing the hashpower of the network as a side effect of a rule change is a needless reduction in a key security parameter of the network. This is why, in recent history, soft forks required 95% of hashpower to indicate that they have upgraded and are capable of enforcing the new rules. Further, this is why recent soft forks have not included changes which would result in a standard Bitcoin Core instance mining invalid-by-new-rules changes (by relying on the standardness behavior of Bitcoin Core).

4) Use hashpower enforcement to de-risk the upgrade process, wherever possible. As a corollary of the above, one of the primary reasons we use soft forks is that hashpower-based enforcement of rules is an elegant way to prevent network splits during the node upgrade process. While it does not make sense to invest material value in systems protected by new rules until a significant majority of “economic nodes” is enforcing said rules, hashpower lets us neatly bridge the gap in time between activation and then. By having a supermajority of miners enforce the new rules, attempts at violating the new rules does not result in a significant network split, disrupting existing users of the system. If we aren’t going to take advantage of this, we should do a hard fork instead, with the necessarily slow timescale that entails.

5) Follow the will of the community, irrespective of individuals or unreasoned objection, but without ever overruling any reasonable objection. Recent history also includes “objection” to soft forks in the form of “this is bad because it doesn’t fix a different problem I want fixed ASAP”. I don’t think anyone would argue this qualifies as a reasonable objection to a change, and we should be in a place, as a community (never as developers or purely one group), to ignore such objections and make forward progress in spite of them. We don’t make good engineering decisions by “bundling” unrelated features together to enable political football and compromise.

I think BIP 9 (plus a well-crafted softfork) pretty effectively checks the boxes for #2-4 here, and when done carefully with lots of community engagement and measurement, can effectively fulfill #1 as well. #5 is, as I’m sure everyone is aware, where it starts to fall down pretty hard.

BIP 8 has been proposed as an alternative, largely in response to issues with #5. However, a naive deployment of it, rather obviously, completely fails #1, #3, and #4, and, in my view, fails #5 as well by both giving an impression of, setting a precedent of, and possibly even in practice increasing the ability of developers to decide the consensus rules of the system. A BIP 8 deployment that more accurately measures community support as a prerequisite could arguably fulfill #1 and #5, though I’m unaware of any concrete proposals on how to accomplish that. Arguably, a significantly longer activation window could also allow BIP 8 to fulfill #3 and #4, but only by exploiting the “needlessly” and “wherever possible” loopholes.

You may note that, from the point of view of achieving the critical goals here, BIP 8 is only different from a flag-day activation in that, if it takes the “happy-path” of activating before the flag day, it looks like BIP 9, but isn’t guaranteed to. It additionally has the “nice-to-have” property that activation can occur before the flag-day in the case of faster miner adoption, though there is a limit of how fast is useful due to node adoption.

Thus, to strike a balance between the drawbacks of BIP 8 and BIP 9, the Great Consensus Cleanup softfork proposal included this text in the discussion section (with the spec describing a BIP 9 deployment):

In spite of some suggestion that other activation methods be used, BIP 9 is proposed as ensuring miners have upgraded to enforce new rules is an important part of minimizing disruption. While previous BIP 9 soft- forks have resulted in political contention, this comparatively- unimportant soft-fork provides a good opportunity to attempt to return to utilizing BIP 9 to ensure miner upgrade prior to activation, which the authors believe is a critical goal. However, if there is broad agreement to activate these rules when the BIP 9 expiry time is reached, and miners have not yet signaled sufficient level of readiness, a later flag-day activation may be merited. For this reason, implementations may wish to provide a compatibility option which allows flag-day enforcement of these rules without an update.

Ultimately, through admittedly rather limited discussion, I still like this model (though I cannot claim it as my own, the original proposal came from Greg Maxwell). BIP 9 only falls apart in case of unreasonable objection, which, naturally, should carry a high bar to ignore, given we have to have some level of agreement that it is, in fact, unreasonable (or untargeted). While I admit this is a possibility, I both find it less likely than in previous soft-forks, and even if it is the case, it only slows down the process, it doesn’t necessarily stop it. In the case that it does fail, BIP 9 process, in fact, provides a good learning opportunity as to the level of community readiness and desire for a given change. While we can (and should, and are) learning a lot about community readiness for, and acceptability of a change through outreach and discussion, there is something about a change with a timeframe that forces people to more carefully consider it.

Thus, as something a bit more concrete, I think an activation method which sets the right precedent and appropriately considers the above goals, would be:

1) a standard BIP 9 deployment with a one-year time horizon for activation with 95% miner readiness, 2) in the case that no activation occurs within a year, a six month quieting period during which the community can analyze and discussion the reasons for no activation and, 3) in the case that it makes sense, a simple command-line/bitcoin.conf parameter which was supported since the original deployment release would enable users to opt into a BIP 8 deployment with a 24-month time-horizon for flag-day activation (as well as a new Bitcoin Core release enabling the flag universally).

This provides a very long time horizon for more standard activation, while still ensuring the goals in #5 are met, even if, in those cases, the time horizon needs to be significantly extended to meet the goals of #3. Developing Bitcoin is not a race. If we have to, waiting 42 months ensures we’re not setting a negative precedent that we’ll come to regret as Bitcoin continues to grow.