Re: [bitcoin-dev] Draft BIP : fixed-schedule block size increase

From: Roy Badami <roy@gnomon•org.uk>
To: Mark Friedenbach <mark@friedenbach•org>
Cc: bitcoin-dev@lists•linuxfoundation.org
Subject: Re: [bitcoin-dev] Draft BIP : fixed-schedule block size increase
Date: Wed, 24 Jun 2015 18:24:45 +0100	[thread overview]
Message-ID: <20150624172444.GC3192@giles.gnomon.org.uk> (raw)
In-Reply-To: <CAOG=w-uf3K+vW_zHXUm1utK7tR7osm_1hLMxw8gqkwm0Hn7d_A@mail.gmail.com>

Or put another way, lowering the block size limit (or cancelling an
increase) is a soft fork.  Like all soft forks, a majority of the hash
power can force the soft fork to take place.

On Wed, Jun 24, 2015 at 10:05:42AM -0700, Mark Friedenbach wrote:
> They do so by not building on larger blocks
> On Jun 23, 2015 9:31 PM, "Raystonn" <raystonn@hotmail•com> wrote:
> 
> > No, they can lower their own block sizes.  But they cannot currently lower
> > the sizes of blocks mined by others.  That is not the same thing by any
> > stretch of the imagination.
> > On 23 Jun 2015 8:50 pm, Jeff Garzik <jgarzik@gmail•com> wrote:
> >
> > Miners can collude today to lower the block size limit.
> >
> > In fact, this largely happens already out of laziness - miners often
> > follow the "soft" default limit set by Bitcoin Core, to the point where you
> > can chart when miners upgrade to new software:
> > http://hashingit.com/analysis/39-the-myth-of-the-megabyte-bitcoin-block
> >
> >
> >
> > On Tue, Jun 23, 2015 at 8:05 PM, William Madden <will.madden@novauri•com>
> > wrote:
> >
> > Here are refutations of the approach in BIP-100 here:
> > http://gtf.org/garzik/bitcoin/BIP100-blocksizechangeproposal.pdf
> >
> > To recap BIP-100:
> >
> > 1) Hard form to remove static 1MB block size limit
> > 2) Add new floating block size limit set to 1MB
> > 3) Historical 32MB message limit remains
> > 4) Hard form on testnet 9/1/2015
> > 5) Hard form on main 1/11/2016
> > 6) 1MB limit changed via one-way lock in upgrade with a 12,000 block
> > threshold by 90% of blocks
> > 7) Limit increase or decrease may not exceed 2x in any one step
> > 8) Miners vote by encoding 'BV'+BlockSizeRequestValue into coinbase
> > scriptSig, e.g. "/BV8000000/" to vote for 8M.
> > 9) Votes are evaluated by dropping bottom 20% and top 20%, and then the
> > most common floor (minimum) is chosen.
> >
> > 8MB limits doubling just under every 2 years makes a static value grow
> > in a predictable manner.
> >
> > BIP-100 makes a static value grow (or more importantly potentially
> > shrink) in an unpredictable manner based on voting mechanics that are
> > untested in this capacity in the bitcoin network.  Introducing a highly
> > variable and untested dynamic into an already complex system is
> > unnecessarily risky.
> >
> > For example, the largely arbitrary voting rules listed in 9 above can be
> > gamed.  If I control pools or have affiliates involved in pools that
> > mine slightly more than 20% of blocks, I could wait until block sizes
> > are 10MB, and then suddenly vote "/BV5000000/" for 20% of blocks and
> > "/BV5000001/" for the remaining 10%.  If others don't consistently vote
> > for the same "/BV#/" value, vote too consistently and have their value
> > thrown out as the top 20%, I could win the resize to half capacity
> > "/BV5000001/" because it was the lowest repeated value not in the bottom
> > 20%.
> >
> > I could use this to force an exodus to my sidechain/alt coin, or to
> > choke out the bitcoin network.  A first improvement would be to only let
> > BIP-100 raise the cap and not lower it, but if I can think of a
> > vulnerability off the top of my head, there will be others on the other
> > side of the equation that have not been thought of.  Why bother
> > introducing a rube goldberg machine like voting when a simple 8mb cap
> > with predictable growth gets the job done, potentially permanently?
> >
> >
> > On 6/23/2015 9:43 PM, odinn wrote:
> > > -----BEGIN PGP SIGNED MESSAGE-----
> > > Hash: SHA1
> > >
> > > 1) Hard fork not (necessarily) needed
> > > 2) See Garzik's BIP 100, better (this is not meant to say "superior to
> > > your stuff," but rather simply to say, "Better you should work with
> > > Garzik to implement BIP-100, that would be good")
> > > 3) See points 1 and 2 above
> > > 4) If still reading... changes should be (as you seem to have been
> > > trying to lean towards)... lean towards gradual change; hence, changes
> > > that would flow from this BIP would be better off oriented in a
> > > process that dies not require the "way you have done it."
> > >
> > > You did address that, to be fair - in your TODO, this link:
> > > http://gavinandresen.ninja/time-to-roll-out-bigger-blocks
> > >
> > > contained the following link:
> > >
> > > http://gavinandresen.ninja/bigger-blocks-another-way
> > >
> > > However, in reading that, I didn't see any meaningful statements that
> > > would refute the approach in Garzik's BIP-100.
> > >
> > > Maybe a better way to say this is,
> > >
> > > Work with Jeff Garzik (which I am sure you are already having such
> > > discussions in private) as well as the list discussions,
> > > Move forward on BIP-100 with Garzik and other developers (not such a
> > > bad plan really) and don't get caught up in XT.  (If you feel you can
> > > develop XT further, that is your thing but it would perhaps make you
> > > lose focus, work together with other developers.)
> > >
> > > Relax into the process.  Things will be ok.
> > >
> > > Respectfully,
> > >
> > > - -O
> > >
> > > On 06/22/2015 11:18 AM, Gavin Andresen wrote:
> > >> I promised to write a BIP after I'd implemented
> > >> increase-the-maximum-block-size code, so here it is. It also lives
> > >> at:
> > >> https://github.com/gavinandresen/bips/blob/blocksize/bip-8MB.mediawiki
> > >>
> > >>  I don't expect any proposal to please everybody; there are
> > >> unavoidable tradeoffs to increasing the maximum block size. I
> > >> prioritize implementation simplicity -- it is hard to write
> > >> consensus-critical code, so simpler is better.
> > >>
> > >>
> > >>
> > >>
> > >> BIP: ?? Title: Increase Maximum Block Size Author: Gavin Andresen
> > >> <gavinandresen@gmail•com <mailto:gavinandresen@gmail•com>> Status:
> > >> Draft Type: Standards Track Created: 2015-06-22
> > >>
> > >> ==Abstract==
> > >>
> > >> This BIP proposes replacing the fixed one megabyte maximum block
> > >> size with a maximum size that grows over time at a predictable
> > >> rate.
> > >>
> > >> ==Motivation==
> > >>
> > >> Transaction volume on the Bitcoin network has been growing, and
> > >> will soon reach the one-megabyte-every-ten-minutes limit imposed by
> > >> the one megabyte maximum block size. Increasing the maximum size
> > >> reduces the impact of that limit on Bitcoin adoption and growth.
> > >>
> > >> ==Specification==
> > >>
> > >> After deployment on the network (see the Deployment section for
> > >> details), the maximum allowed size of a block on the main network
> > >> shall be calculated based on the timestamp in the block header.
> > >>
> > >> The maximum size shall be 8,000,000 bytes at a timestamp of
> > >> 2016-01-11 00:00:00 UTC (timestamp 1452470400), and shall double
> > >> every 63,072,000 seconds (two years, ignoring leap years), until
> > >> 2036-01-06 00:00:00 UTC (timestamp 2083190400). The maximum size of
> > >> blocks in between doublings will increase linearly based on the
> > >> block's timestamp. The maximum size of blocks after 2036-01-06
> > >> 00:00:00 UTC shall be 8,192,000,000 bytes.
> > >>
> > >> Expressed in pseudo-code, using integer math:
> > >>
> > >> function max_block_size(block_timestamp):
> > >>
> > >> time_start = 1452470400 time_double = 60*60*24*365*2 size_start =
> > >> 8000000 if block_timestamp >= time_start+time_double*10 return
> > >> size_start * 2^10
> > >>
> > >> // Piecewise-linear-between-doublings growth: time_delta =
> > >> block_timestamp - t_start doublings = time_delta / time_double
> > >> remainder = time_delta % time_double interpolate = (size_start *
> > >> 2^doublings * remainder) / time_double max_size = size_start *
> > >> 2^doublings + interpolate
> > >>
> > >> return max_size
> > >>
> > >> ==Deployment==
> > >>
> > >> Deployment shall be controlled by hash-power supermajority vote
> > >> (similar to the technique used in BIP34), but the earliest possible
> > >> activation time is 2016-01-11 00:00:00 UTC.
> > >>
> > >> Activation is achieved when 750 of 1,000 consecutive blocks in the
> > >> best chain have a version number with bits 3 and 14 set (0x20000004
> > >> in hex). The activation time will be the timestamp of the 750'th
> > >> block plus a two week (1,209,600 second) grace period to give any
> > >> remaining miners or services time to upgrade to support larger
> > >> blocks. If a supermajority is achieved more than two weeks before
> > >> 2016-01-11 00:00:00 UTC, the activation time will be 2016-01-11
> > >> 00:00:00 UTC.
> > >>
> > >> Block version numbers are used only for activation; once activation
> > >> is achieved, the maximum block size shall be as described in the
> > >> specification section, regardless of the version number of the
> > >> block.
> > >>
> > >>
> > >> ==Rationale==
> > >>
> > >> The initial size of 8,000,000 bytes was chosen after testing the
> > >> current reference implementation code with larger block sizes and
> > >> receiving feedback from miners stuck behind bandwidth-constrained
> > >> networks (in particular, Chinese miners behind the Great Firewall
> > >> of China).
> > >>
> > >> The doubling interval was chosen based on long-term growth trends
> > >> for CPU power, storage, and Internet bandwidth. The 20-year limit
> > >> was chosen because exponential growth cannot continue forever.
> > >>
> > >> Calculations are based on timestamps and not blockchain height
> > >> because a timestamp is part of every block's header. This allows
> > >> implementations to know a block's maximum size after they have
> > >> downloaded it's header, but before downloading any transactions.
> > >>
> > >> The deployment plan is taken from Jeff Garzik's proposed BIP100
> > >> block size increase, and is designed to give miners, merchants,
> > >> and full-node-running-end-users sufficient time to upgrade to
> > >> software that supports bigger blocks. A 75% supermajority was
> > >> chosen so that one large mining pool does not have effective veto
> > >> power over a blocksize increase. The version number scheme is
> > >> designed to be compatible with Pieter's Wuille's proposed "Version
> > >> bits" BIP.
> > >>
> > >> TODO: summarize objections/arguments from
> > >> http://gavinandresen.ninja/time-to-roll-out-bigger-blocks.
> > >>
> > >> TODO: describe other proposals and their advantages/disadvantages
> > >> over this proposal.
> > >>
> > >>
> > >> ==Compatibility==
> > >>
> > >> This is a hard-forking change to the Bitcoin protocol; anybody
> > >> running code that fully validates blocks must upgrade before the
> > >> activation time or they will risk rejecting a chain containing
> > >> larger-than-one-megabyte blocks.
> > >>
> > >> Simplified Payment Verification software is not affected, unless
> > >> it makes assumptions about the maximum depth of a transaction's
> > >> merkle branch based on the minimum size of a transaction and the
> > >> maximum block size.
> > >>
> > >> ==Implementation==
> > >>
> > >> https://github.com/gavinandresen/bitcoinxt/tree/blocksize_fork
> > >>
> > >>
> > >>
> > >> _______________________________________________ bitcoin-dev mailing
> > >> list bitcoin-dev@lists•linuxfoundation.org
> > >> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
> > >>
> > >
> > > - --
> > > http://abis.io ~
> > > "a protocol concept to enable decentralization
> > > and expansion of a giving economy, and a new social good"
> > > https://keybase.io/odinn
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