[bitcoin-dev] Making a 2MB blocksize hardfork safer

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

Hello world,

The core roadmap calls for having patches at the ready for
implementing hardforking blocksize increases [0]. However, at least
to my understanding, is that the deployment of segregated witness has
a significant impact on what a hardforking blocksize increase should
look like -- with segwit, the increase in the blocksize may have to
be traded off against decreasing the witness discount; without segwit,
alternative changes might need to be made to provide some of the other
benefits of segwit without segwit (in particular, additional limits to
prevent hashing massive amounts of data when checking sigs or to reduce
worst-case UTXO growth).

I don't personally have any real concerns that segregated witness will be
too complicated to implement and release by April, and given how quickly
CLTV rolled out, my guess is it will be usable prior to the block reward
halving. I'm also not terribly worried about fees rising significantly,
or that there will be a "fee event" [1] or "market disruption" -- fees
don't seem to be rising even with the spam attacks we've seen, and all
the problems with transactions not confirming that I've been able to see
so far seem to be due either to people trying to do free transactions,
fees not being calculated based on transaction size, or not checking
for dust outputs, all of which are things that can be dealt with by
individual wallets. [2]

But those are guesses and opinions, and I think it makes sense to have
a backup plan if everything goes horribly wrong -- someone discovers
a problem with segwit that requires major rearchitecturing to fix and
won't happen until 2017, eg.

To me, Gavin's BIP [3] and the Bitcoin Classic approach don't seem like
a good backup plan; but I don't see why they couldn't be *made* into a
good plan. In particular, if segwit turns out too hard to actually deploy
safely, I think Gavin's patchset -- an increase to ~2MB, coupled with
accurate counting and limiting of sighash bytes, and pretty much nothing
else -- is about the right set of *technical* things to do as a backup plan.

So the following are my suggestions for making Gavin's BIP workable
procedurally/politically as a backup plan. But that said, I don't know
if this is even remotely acceptable politically; I'm just following
bitcoin as a hobby and I don't have any backchannel contacts in mining
or bitcoin startups or anything.

1. Level of supermajority
=========================

First, it was reported that the Chinese miners came up with a 2MB
blocksize plan in late January [4], with the following summarised plan:

]  If:
]    1: Blocks are full
]    2: Core proposal is <2MB
]    3: Classic proposal have not gained consensus
]  Then:
]    Under the 90% hash power condition, switch from a 1MB limit to a
]    2MB limit to deal with the block size problem.

The summary also expresses concerns about segwit deployment; that it
makes significant changes, and that any issues with reliability may have
major impact. Those seem like valid concerns to me; though if they are
not addressed directly, then I expect miners will simply not enable the
segwit soft-fork until they are.

I think the only change to make this match Gavin's code for Bitcoin
Classic then is to require 90% hashpower support rather than 75%. I think
that can be easily done by a soft-forking change where miners reject any
block with a Classic vote (ie a version of 0x10000000) if the block height
is cleanly divisible by 6 [5]. As this is a soft-forking change, and one
that's only relevant until either Classic activates or the 2MB hardfork
attempt is permanently aborted on 2018-01-01, it seems like it could
easily be deployed prior to either segwit or Classic voting beginning.

2. Activation Time
==================

The activation time for Gavin's BIP is very short -- 1000 blocks for
voting could be as short as 6 days, followed by 28 days grace period.
I haven't seen any indication that there is an immediate crisis, or
that there will be one in the next few months; and the fact that the
BIP does not expire for two years seems to indicate it's not a short
term issue. Allowing three to six months before attempting to activate
the hardfork seems like it would still provide plenty of opportunity to
address the issue quickly, and would also mean there was time to see if
the segwit rollout worked as planned.

That also could be enforced by a soft-fork: eg having a rule that until
the median time past is 2015-05-27, any block voting for the 2MB hardfork
will be rejected, would ensure the hard fork was not activated until
1st of July. A slightly more complicated rule, eg only rejecting the
blocks if the last three decimal digits of its height was 500 or greater,
would allow support to be measured in the leadup to possible activation,
without any risk of activation happening early.

3. Upgrade encouragement
========================

I think there's three ways the 2MB hardfork could go: (a) not ever being
activated at all, similar to XT; (b) being activated with effective
consensus, where everyone switches to the hard-fork, whether happily
or not; or (c) being activated, but with the old chain being actively
mined and used on an ongoing, long-term basis.

If the 2MB blocksize hardfork is deployed as a fallback after segwit
deployment has failed, or determined to be much more complicated than
currently believed, then it seems like (c) would be a pretty undesirable
outcome.

The only way I can see of avoiding/discouraging (c) is to have the new
hardfork be merge-minable with the existing chain, and having every
block in the new chain also commit to a merged-mined empty block on the
old chain, so that as long as the new chain has more hashpower than the
old chain, the longest valid old chain will have no outgoing payments
after the hardfork activates. (That requirement could probably be safely
dropped after some number of blocks, perhaps 25000 or 6 months?)

Alternatively, if the old blockchain has 10% or less hashpower remaining
(due to the 90% activation above), then the new chain has 9x the
hashpower. Perhaps a rule such that every 8th block in the hard-forked
chain must include an OP_RETURN in the coinbase that provides a valid,
empty block for the old chain. With a 90%/10% split, this would ensure
that the empty chain had more work than any other attempt at extending
it. However at the next difficulty change for the old chain (decreasing
by a factor of 4, presumably), I think they'd have to be mined every
second block rather than every 8th, and by the second difficulty change,
would need to be mined every block; otherwise I think 10% of hashpower
could catch up in chain-work. (Again, the requirement could probably be
dropped entirely after 6 months, or similar)

I believe this latter approach could be implemented as a soft-fork on
top of Gavin's BIP / Bitcoin Classic, provided activation was at 90% [7].

In this scenario, it would be possible for miners to simply sell empty
blocks on the old chain once they find them, so finding an empty block
for the old chain could plausibly be independent of finding the new
block for the new chain.


Conclusion
==========

I think those three changes, which all should be implementable as
soft-forks compatible with Gavin's current code (the first two only
relevant prior to activation; the last only relevant after activation),
would mitigate what I see as the biggest risks of classic:

 - low-consensus/controversial activation
 - short preparation time, and resulting uncertainty and pressure
 - non-trivial chance of old chain remaining active after activation
 - miners' and core's plans being ignored [8]

And I think that would make this BIP (for me) a workable backup plan in
the event segwit doesn't work as planned. And for a multi-billion dollar
service, backup plans seem like a worthwhile thing to have, even if it's
highly unlikely it will actually get used.

However, these are all ideas where the benefits are basically "political"
rather than "technical", and I have no idea if the above *actually* makes
sense... And I guess trying to establish that is probably off-topic for
bitcoin-dev anyway? Anyway, as a consequence I've no idea if a write up
as a BIP and/or patches to implement any/all of the above as soft-forks
for classic/core that could be activated would be interesting for anyone,
and beyond posting about the ideas here, no idea how to find out. It
seemed like an interesting thought experiment to me, anyway. Apologies
in advance if it turns out I'm alone in that :)

Cheers,
aj

[0] "Finally--at some point the capacity increases from the above may not
    be enough.  Delivery on [various improvements], and other advances
    in technology will reduce the risk and therefore controversy around
    moderate block size increase proposals (such as 2/4/8 rescaled to
    respect segwit's increase). Bitcoin will be able to move forward
    with these increases when improvements and understanding render
    their risks widely acceptable relative to the risks of not deploying
    them. In Bitcoin Core we should keep patches ready to implement them
    as the need and the will arises, ..."

    https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-December/011865.html
    via https://bitcoincore.org/en/2015/12/23/capacity-increases-faq/

[1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-December/011973.html

[2] I do think that, without segwit or a blocksize increase, there will be
    a discontinuity for venture funded bitcoin companies, because the
    transactions per second metric will become capped by the end of
    2016. I've argued that at:

    http://lists.linuxfoundation.org/pipermail/bitcoin-discuss/2016-January/000042.html

    but I have not seen anyone from the a VC-backed bitcoin company
    actually confirm that's a concern, so perhaps it isn't something
    worth worrying about even there.

[3] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-February/012358.html
    https://github.com/gavinandresen/bips/blob/bump2mb/bip-bump2mb.mediawiki

[4] https://blog.bitmex.com/translation-of-chinese-miner-consensus-meeting/

[5] In that case, if 90% of miners by hashpower actually support the BIP,
    that would imply that 1/6th of blocks artificially don't vote for
    it, but 90% of the remaining 5/6th of blocks do, and 90% of 5/6th
    gives the 75% activation threshold specified in Gavin's BIP.

[6] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-December/012069.html

[7] With activation at 75%, you'd need to dedicate 1/3rd of hashpower
    to mining empty old blocks to stay in the lead, which would then mean
    the hashpower for the new proof-of-work would only be half what it
    had previously been, and you'd end up with blocks taking 20 minutes
    on the new chain, and at least every second block including an empty
    block on the old chain. You could probably fix this by having the
    difficulty artificially halve when the hardfork activates though.

[8] Miners agree to 90% majority, code comes out with 75% majority. In
    December, core announces plans to deploy segwit with 1.6x capacity
    increase by April; Classic appears in January planning to do a hard
    fork with 2x capacity increase in/around March.