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

Jared Lee Richardson jaredr26 at gmail.com
Sat Apr 1 06:55:42 UTC 2017


> 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
>>(DEPLOYMENT_SEGWIT_AND_2MB_BLOCKS).
>>
>>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
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