[bitcoin-dev] Segwit2Mb - combined soft/hard fork - Request For Comments

Sergio Demian Lerner sergio.d.lerner at gmail.com
Sat Apr 1 11:44:11 UTC 2017

Some people have asked me for the current implementation of this patch to
review. I remind you that the current patch does not implement the
hard-fork signaling, as requested by Matt.

The Segwit2Mb patch can be found here:

For now, the segwit2mb repo has a single test file using the old internal
blockchain building method (test/block_size_tests.cpp). This must be
replaced soon with a better external test using the bitcoin/qa/rpc-tests
tests, which I will begin to work on now after I collect all comments from
the community.


On Sat, Apr 1, 2017 at 3:55 AM, Jared Lee Richardson <jaredr26 at gmail.com>

> > Remember that the "hashpower required to secure bitcoin" is determined
> > as a percentage of total Bitcoins transacted on-chain in each block
> Can you explain this statement a little better?  What do you mean by
> that?  What does the total bitcoins transacted have to do with
> hashpower required?
> On Fri, Mar 31, 2017 at 2:22 PM, Matt Corallo via bitcoin-dev
> <bitcoin-dev at lists.linuxfoundation.org> wrote:
> > Hey Sergio,
> >
> > You appear to have ignored the last two years of Bitcoin hardfork
> > research and understanding, recycling instead BIP 102 from 2015. There
> > are many proposals which have pushed the state of hard fork research
> > much further since then, and you may wish to read some of the posts on
> > this mailing list listed at https://bitcoinhardforkresearch.github.io/
> > and make further edits based on what you learn. Your goal of "avoid
> > technical changes" appears to not have any basis outside of perceived
> > compromise for compromise sake, only making such a hardfork riskier
> > instead.
> >
> > At a minimum, in terms of pure technical changes, you should probably
> > consider (probably among others):
> >
> > a) Utilizing the "hard fork signaling bit" in the nVersion of the block.
> > b) Either limiting non-SegWit transactions in some way to fix the n**2
> > sighash and FindAndDelete runtime and memory usage issues or fix them by
> > utilizing the new sighash type which many wallets and projects have
> > already implemented for SegWit in the spending of non-SegWit outputs.
> > c) Your really should have replay protection in any HF. The clever fix
> from
> > Spoonnet for poor scaling of optionally allowing non-SegWit outputs to
> > be spent with SegWit's sighash provides this all in one go.
> > d) You may wish to consider the possibility of tweaking the witness
> > discount and possibly discounting other parts of the input - SegWit went
> > a long ways towards making removal of elements from the UTXO set cheaper
> > than adding them, but didn't quite get there, you should probably finish
> > that job. This also provides additional tuneable parameters to allow you
> > to increase the block size while not having a blowup in the worst-case
> > block size.
> > e) Additional commitments at the top of the merkle root - both for
> > SegWit transactions and as additional space for merged mining and other
> > commitments which we may wish to add in the future, this should likely
> > be implemented an "additional header" ala Johnson Lau's Spoonnet
> proposal.
> >
> > Additionally, I think your parameters here pose very significant risk to
> > the Bitcoin ecosystem broadly.
> >
> > a) Activating a hard fork with less than 18/24 months (and even then...)
> > from a fully-audited and supported release of full node software to
> > activation date poses significant risks to many large software projects
> > and users. I've repeatedly received feedback from various folks that a
> > year or more is likely required in any hard fork to limit this risk, and
> > limited pushback on that given the large increase which SegWit provides
> > itself buying a ton of time.
> >
> > b) Having a significant discontinuity in block size increase only serves
> > to confuse and mislead users and businesses, forcing them to rapidly
> > adapt to a Bitcoin which changed overnight both by hardforking, and by
> > fees changing suddenly. Instead, having the hard fork activate technical
> > changes, and then slowly increasing the block size over the following
> > several years keeps things nice and continuous and also keeps us from
> > having to revisit ye old blocksize debate again six months after
> activation.
> >
> > c) You should likely consider the effect of the many technological
> > innovations coming down the pipe in the coming months. Technologies like
> > Lightning, TumbleBit, and even your own RootStock could significantly
> > reduce fee pressure as transactions move to much faster and more
> > featureful systems.
> >
> > Commitments to aggressive hard fork parameters now may leave miners
> > without much revenue as far out as the next halving (which current
> > transaction growth trends are indicating we'd just only barely reach 2MB
> > of transaction volume, let alone if you consider the effects of users
> > moving to systems which provide more features for Bitcoin transactions).
> > This could lead to a precipitous drop in hashrate as miners are no
> > longer sufficiently compensated.
> >
> > Remember that the "hashpower required to secure bitcoin" is determined
> > as a percentage of total Bitcoins transacted on-chain in each block, so
> > as subsidy goes down, miners need to be paid with fees, not just price
> > increases. Even if we were OK with hashpower going down compared to the
> > value it is securing, betting the security of Bitcoin on its price
> > rising exponentially to match decreasing subsidy does not strike me as a
> > particularly inspiring tradeoff.
> >
> > There aren't many great technical solutions to some of these issues, as
> > far as I'm aware, but it's something that needs to be incredibly
> > carefully considered before betting the continued security of Bitcoin on
> > exponential on-chain growth, something which we have historically never
> > seen.
> >
> > Matt
> >
> >
> > On March 31, 2017 5:09:18 PM EDT, Sergio Demian Lerner via bitcoin-dev <
> bitcoin-dev at lists.linuxfoundation.org> wrote:
> >>Hi everyone,
> >>
> >>Segwit2Mb is the project to merge into Bitcoin a minimal patch that
> >>aims to
> >>untangle the current conflict between different political positions
> >>regarding segwit activation vs. an increase of the on-chain blockchain
> >>space through a standard block size increase. It is not a new solution,
> >>but
> >>it should be seen more as a least common denominator.
> >>
> >>Segwit2Mb combines segwit as it is today in Bitcoin 0.14+ with a 2MB
> >>block
> >>size hard-fork activated ONLY if segwit activates (95% of miners
> >>signaling), but at a fixed future date.
> >>
> >>The sole objective of this proposal is to re-unite the Bitcoin
> >>community
> >>and avoid a cryptocurrency split. Segwit2Mb does not aim to be best
> >>possible technical solution to solve Bitcoin technical limitations.
> >>However, this proposal does not imply a compromise to the future
> >>scalability or decentralization of Bitcoin, as a small increase in
> >>block
> >>size has been proven by several core and non-core developers not to
> >>affect
> >>Bitcoin value propositions.
> >>
> >>In the worst case, a 2X block size increase has much lower economic
> >>impact
> >>than the last bitcoin halving (<10%), which succeeded without problem.
> >>
> >>On the other side, Segwit2Mb primary goal is to be minimalistic: in
> >>this
> >>patch some choices have been made to reduce the number of lines
> >>modified in
> >>the current Bitcoin Core state (master branch), instead of implementing
> >>the
> >>most elegant solution. This is because I want to reduce the time it
> >>takes
> >>for core programmers and reviewers to check the correctness of the
> >>code,
> >>and to report and correct bugs.
> >>
> >>The patch was built by forking the master branch of Bitcoin Core,
> >>mixing a
> >>few lines of code from Jeff Garzik's BIP102,  and defining a second
> >>versionbits activation bit (bit 2) for the combined activation.
> >>
> >>The combined activation of segwit and 2Mb hard-fork nVersion bit is 2
> >>
> >>This means that segwit can still be activated without the 2MB hard-fork
> >>by
> >>signaling bit 1 in nVersion  (DEPLOYMENT_SEGWIT).
> >>
> >>The tentative lock-in and hard-fork dates are the following:
> >>
> >>Bit 2 signaling StartTime = 1493424000; // April 29th, 2017
> >>
> >>Bit 2 signaling Timeout = 1503964800; // August 29th, 2017
> >>
> >>HardForkTime = 1513209600; // Thu, 14 Dec 2017 00:00:00 GMT
> >>
> >>
> >>The hard-fork is conditional to 95% of the hashing power has approved
> >>the
> >>segwit2mb soft-fork and the segwit soft-fork has been activated (which
> >>should occur 2016 blocks after its lock-in time)
> >>
> >>For more information on how soft-forks are signaled and activated, see
> >>https://github.com/bitcoin/bips/blob/master/bip-0009.mediawiki
> >>
> >>This means that segwit would be activated before 2Mb: this is
> >>inevitable,
> >>as versionbits have been designed to have fixed activation periods and
> >>thresholds for all bits. Making segwit and 2Mb fork activate together
> >>at a
> >>delayed date would have required a major re-write of this code, which
> >>would
> >>contradict the premise of creating a minimalistic patch. However, once
> >>segwit is activated, the hard-fork is unavoidable.
> >>
> >>Although I have coded a first version of the segwit2mb patch (which
> >>modifies 120 lines of code, and adds 220 lines of testing code), I
> >>would
> >>prefer to wait to publish the source code until more comments have been
> >>received from the community.
> >>
> >>To prevent worsening block verification time because of the O(N^2)
> >>hashing
> >>problem, the simple restriction that transactions cannot be larger than
> >>1Mb
> >>has been kept. Therefore the worse-case of block verification time has
> >>only
> >>doubled.
> >>
> >>Regarding the hard-fork activation date, I want to give enough time to
> >>all
> >>active economic nodes to upgrade. As of Fri Mar 31 2017,
> >>https://bitnodes.21.co/nodes/ reports that 6332 out of 6955 nodes (91%)
> >>have upgraded to post 0.12 versions. Upgrade to post 0.12 versions can
> >>be
> >>used to identify economic active nodes, because in the 0.12 release
> >>dynamic
> >>fees were introduced, and currently no Bitcoin automatic payment system
> >>can
> >>operate without automatic discovery of the current fee rate. A pre-0.12
> >>would require constant manual intervention.
> >>Therefore I conclude that no more than 91% of the network nodes
> >>reported by
> >>bitnodes are active economic nodes.
> >>
> >>As Bitcoin Core 0.12 was released on February 2016, the time for this
> >>91%
> >>to upgrade has been around one year (under a moderate pressure of
> >>operational problems with unconfirmed transactions).
> >>Therefore we can expect a similar or lower time to upgrade for a
> >>hard-fork,
> >>after developers have discussed and approved the patch, and it has been
> >>reviewed and merged and 95% of the hashing power has signaled for it
> >>(the
> >>pressure not to upgrade being a complete halt of the operations).
> >>However I
> >>suggest that we discuss the hard-fork date and delay it if there is a
> >>real
> >>need to.
> >>
> >>Currently time works against the Bitcoin community, and so is delaying
> >>a
> >>compromise solution. Most of the community agree that halting the
> >>innovation for several years is a very bad option.
> >>
> >>After the comments collected by the community, a BIP will be written
> >>describing the resulting proposal details.
> >>
> >>If segwit2mb locks-in, before hard-fork occurs all bitcoin nodes should
> >>be
> >>updated to a Segwit2Mb enabled node to prevent them to be forked-away
> >>in a
> >>chain with almost no hashing-power.
> >>
> >>The proof of concept patch was made for Bitcoin Core but should be
> >>easily
> >>ported to other Bitcoin protocol implementations that already support
> >>versionbits. Lightweight (SPV) wallets should not be affected as they
> >>generally do not check the block size.
> >>
> >>I personally want to see the Lightning Network in action this year, use
> >>the
> >>non-malleability features in segwit, see the community discussing other
> >>exciting soft-forks in the scaling roadmap, Schnorr sigs, drivechains
> >>and
> >>MAST.
> >>
> >>I want to see miners, developers and industry side-by-side pushing
> >>Bitcoin
> >>forward, to increase the value of Bitcoin and prevent high transaction
> >>fees
> >>to put out of business use-cases that could have high positive social
> >>impact.
> >>
> >>I believe in the strength of a unified Bitcoin community. If you're a
> >>developer, please give your opinion, suggest changes, audit it, and
> >>take a
> >>stand with me to unlock the current Bitcoin deadlock.
> >>
> >>Contributions to the segwit2mb project are welcomed and awaited. The
> >>only
> >>limitation is to stick to the principle that the patch should be as
> >>simple
> >>to audit as possible. As an example, I wouldn't feel confident if the
> >>patch
> >>modified more than ~150 lines of code.
> >>
> >>Improvements unrelated to a 2 Mb increase or segwit, as beneficial as
> >>it
> >>may be to Bitcoin, should not be part of segwit2Mb.
> >>
> >>This proposal should not prevent other consensus proposals to be
> >>simultaneously merged: segwit2mb is a last resort solution in case we
> >>can
> >>not reach consensus on anything better.
> >>
> >>Again, the proposal is only a starting point: community feedback is
> >>expected and welcomed.
> >>
> >>Regards,
> >>Sergio Demian Lerner
> > _______________________________________________
> > bitcoin-dev mailing list
> > bitcoin-dev at lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
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