[Bitcoin-development] Block Size Increase

Gregory Maxwell gmaxwell at gmail.com
Thu May 7 00:37:54 UTC 2015

On Wed, May 6, 2015 at 10:12 PM, Matt Corallo <bitcoin-list at bluematt.me>
wrote: > Recently there has been a flurry of posts by Gavin at >
http://gavinandresen.svbtle.com/ which advocate strongly for increasing >
the maximum block size. However, there hasnt been any discussion on this >
mailing list in several years as far as I can tell.

Thanks Matt; I was actually really confused by this sudden push with
not a word here or on Github--so much so that I responded on Reddit to
people pointing to commits in Gavin's personal repository saying they
were reading too much into it.

So please forgive me for the more than typical disorganization in this
message; I've been caught a bit flatfooted on this and I'm trying to
catch up. I'm juggling a fair amount of sudden pressure in my mailbox,
and trying to navigate complex discussions in about eight different
forums concurrently.

There have been about a kazillion pages of discussion elsewhere
(e.g. public IRC and Bitcointalk; private discussions in the past),
not all of which is well known, and I can't hope to summarize even a
tiny fraction of it in a single message-- but that's no reason to not
start on it.

> Block size is a question to which there is no answer, but which >
certainly has a LOT of technical tradeoffs to consider.

There are several orthogonal angles from which block size is a concern
(both increases and non-increases). Most of them have subtle implications
and each are worth its own research paper or six, so it can be difficult
to only touch them slightly without creating a gish gallop that is hard
to respond to.

We're talking about tuning one of the fundamental scarcities of the
Bitcoin Economy and cryptosystem--leaving the comfort of "rule by
math" and venturing into the space of political decisions; elsewhere
you'd expect to see really in-depth neutral analysis of the risks and
tradeoffs, technically and economically.  And make no mistake: there
are real tradeoffs here, though we don't know their exact contours.

Fundamentally this question exposes ideological differences between people
interested in Bitcoin.  Is Bitcoin more of a digital gold or is it more
of a competitor to Square?  Is Bitcoin something that should improve
personal and commercial autonomy from central banks?  From commercial
banks? Or from just the existing status-quo commercial banks?   What are
people's fundamental rights with Bitcoin?  Do participants have a
right to mine? How much control should third parties have over their
transactions?  How much security must be provided? Is there a deadline
for world domination or bust?  Is Bitcoin only for the developed world?
Must it be totally limited by the most impoverished parts of the world?

Bitcoin exists at the intersection of many somewhat overlapping belief
systems--and people of many views can find that Bitcoin meets their
needs even when they don't completely agree politically.  When Bitcoin
is changed fundamentally, via a hard fork, to have different properties,
the change can create winners or losers (if nothing else, then in terms
of the kind of ideology supported by it).

There are non-trivial number of people who hold extremes on any of
these general belief patterns; Even among the core developers there is
not a consensus on Bitcoin's optimal role in society and the commercial

To make it clear how broad the views go, even without getting into
monetary policy... some people even argue that Bitcoin should act
as censor-resistant storage system for outlawed content; -- I think
this view is unsound, not achievable with the technology, and largely
incompatible with Bitcoin's use as a money (because it potentially
creates an externalized legal/harassment liability for node operators);
but these are my personal value judgments; the view is earnestly held
by more than a few; and that's a group that certainly wants the largest
possible blocksizes (though even then that won't be enough).

The subject is complicated even more purely on the technical side
by the fact that Bitcoin has a layered security model which is not
completely defined or understood: Bitcoin is secure if a majority of
hashrate is "honest" (where "honesty" is a technical term which means
"follows the right rules" without fail, even at a loss), but why might
it be honest? That sends us into complex economic and social arguments,
and the security thresholds start becoming worse when we assume some
miners are economically rational instead of "honest".

> increase in the near future. Long-term incentive compatibility requires
> that there be some fee pressure, and that blocks be relatively >
consistently full or very nearly full. What we see today are

To elaborate, in my view there is a at least a two fold concern on this
particular ("Long term Mining incentives") front:

One is that the long-held argument is that security of the Bitcoin system
in the long term depends on fee income funding autonomous, anonymous,
decentralized miners profitably applying enough hash-power to make
reorganizations infeasible.

For fees to achieve this purpose, there seemingly must be an effective
scarcity of capacity.  The fact that verifying and transmitting
transactions has a cost isn't enough, because all the funds go to pay
that cost and none to the POW "artificial" cost; e.g., if verification
costs 1 then the market price for fees should converge to 1, and POW
cost will converge towards zero because they adapt to whatever is
being applied. Moreover, the transmission and verification costs can
be perfectly amortized by using large centralized pools (and efficient
differential block transmission like the "O(1)" idea) as you can verify
one time instead of N times, so to the extent that verification/bandwidth
is a non-negligible cost to miners at all, it's a strong pressure to
centralize.  You can understand this intuitively: think for example of
carbon credit cap-and-trade: the trade part doesn't work without an
actual cap; if everyone was born with a 1000 petaton carbon balance,
the market price for credits would be zero and the program couldn't hope
to share behavior. In the case of mining, we're trying to optimize the
social good of POW security. (But the analogy applies in other ways too:
increases to the chain side are largely an externality; miners enjoy the
benefits, everyone else takes the costs--either in reduced security or
higher node operating else.)

This area has been subject to a small amount of academic research
(e.g. http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2400519). But
there is still much that is unclear.

The second is that when subsidy has fallen well below fees, the incentive
to move the blockchain forward goes away.  An optimal rational miner
would be best off forking off the current best block in order to capture
its fees, rather than moving the blockchain forward, until they hit
the maximum. That's where the "backlog" comment comes from, since when
there is a sufficient backlog it's better to go forward.  I'm not aware
of specific research into this subquestion; it's somewhat fuzzy because
of uncertainty about the security model. If we try to say that Bitcoin
should work even in the face of most miners being profit-maximizing
instead of altruistically-honest, we must assume the chain will not
more forward so long as a block isn't full.  In reality there is more
altruism than zero; there are public pressures; there is laziness, etc.

One potential argument is that maybe miners would be _regulated_ to
behave correctly. But this would require undermining the openness of the
system--where anyone can mine anonymously--in order to enforce behavior,
and that same enforcement mechanism would leave a political level to
impose additional rules that violate the extra properties of the system.

So far the mining ecosystem has become incredibly centralized over time.
I believe I am the only remaining committer who mines, and only a few
of the regular contributors to Bitcoin Core do. Many participants
have never mined or only did back in 2010/2011... we've basically
ignored the mining ecosystem, and this has had devastating effects,
causing a latent undermining of the security model: hacking a dozen or
so computers--operated under totally unknown and probably not strong
security policies--could compromise the network at least at the tip...
Rightfully we should be regarding this an an emergency, and probably
should have been have since 2011.  This doesn't bode well for our ability
to respond if a larger blocksize goes poorly. In kicking the can with
the trivial change to just bump the size, are we making an implicit
decision to go down a path that has a conclusion we don't want?

(There are also shorter term mining incentives concerns; which Peter
Todd has written more about, that I'll omit for now)

> pretending these systems scale. Thus, instead of working on technologies
> which bring Bitcoin's trustlessness to systems which scale beyond a

I made a few relevant points back in 2011
after Dan Kaminsky argued that Bitcoin's decentralization was pretext:
that it was patently centralized since scaling directly in the network
would undermine decentralization, that the Bitcoin network necessarily
makes particular tradeoffs which prevent it from concurrently being all
things to all people.  But tools like the Lightning network proposal could
well allow us to hit a greater spectrum of demands at once--including
secure zero-confirmation (something that larger blocksizes reduce if
anything), which is important for many applications.  With the right
technology I believe we can have our cake and eat it too, but there needs
to be a reason to build it; the security and decentralization level of
Bitcoin imposes a _hard_ upper limit on anything that can be based on it.

Another key point here is that the small bumps in blocksize which
wouldn't clearly knock the system into a largely centralized mode--small
constants--are small enough that they don't quantitatively change the
operation of the system; they don't open up new applications that aren't
possible today. Deathandtaxes on the forum argued that Bitcoin needs
a several hundred megabyte blocksize to directly meet the worldwide
transaction needs _without retail_... Why without retail? Retail needs
near instant soft security, which cannot be achieved directly with a
global decentralized blockchain.

I don't think 1MB is magic; it always exists relative to widely-deployed
technology, sociology, and economics. But these factors aren't a simple
function; the procedure I'd prefer would be something like this: if there
is a standing backlog, we-the-community of users look to indicators to
gauge if the network is losing decentralization and then double the
hard limit with proper controls to allow smooth adjustment without
fees going to zero (see the past proposals for automatic block size
controls that let miners increase up to a hard maximum over the median
if they mine at quadratically harder difficulty), and we don't increase
if it appears it would be at a substantial increase in centralization
risk. Hardfork changes should only be made if they're almost completely
uncontroversial--where virtually everyone can look at the available data
and say "yea, that isn't undermining my property rights or future use
of Bitcoin; it's no big deal".  Unfortunately, every indicator I can
think of except fee totals has been going in the wrong direction almost
monotonically along with the blockchain size increase since 2012 when
we started hitting full blocks and responded by increasing the default
soft target.  This is frustrating; from a clean slate analysis of network
health I think my conclusion would be to _decrease_ the limit below the
current 300k/txn/day level.

This is obviously not acceptable, so instead many people--myself
included--have been working feverishly hard behind the scenes on Bitcoin
Core to increase the scalability.  This work isn't small-potatoes
boring software engineering stuff; I mean even my personal contributions
include things like inventing a wholly new generic algebraic optimization
applicable to all EC signature schemes that increases performance by 4%,
and that is before getting into the R&D stuff that hasn't really borne
fruit yet, like fraud proofs.  Today Bitcoin Core is easily >100 times
faster to synchronize and relay than when I first got involved on the
same hardware, but these improvements have been swallowed by the growth.
The ironic thing is that our frantic efforts to keep ahead and not
lose decentralization have both not been enough (by the best measures,
full node usage is the lowest its been since 2011 even though the user
base is huge now) and yet also so much that people could seriously talk
about increasing the block size to something gigantic like 20MB. This
sounds less reasonable when you realize that even at 1MB we'd likely
have a smoking hole in the ground if not for existing enormous efforts
to make scaling not come at a loss of decentralization.

I'm curious as to what discussions people have seen; e.g., are people
even here aware of these concerns? Are you aware of things like the
hashcash mediated dynamic blocksize limiting?  About proposals like
lightning network (instant transactions and massive scale, in exchange
for some short term DOS risk if a counterparty opts out)?   Do people
(other than Mike Hearn; I guess) think a future where everyone depends
on a small number of "Google scale" node operations for the system is
actually okay? (I think not, and if so we're never going to agree--but
it can be helpful to understand when a disagreement is ideological).

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