Gavin Andresen

Bitcoin developer. All-around geek.

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A Guided Tour of the 2mb Fork

Increasing the block size limit from 1 million bytes to 2 million bytes sounds so simple: just change the “1” to a “2” in the source code and we’re done, right?

If we didn’t care about a smooth upgrade, then it could be that simple. Just change this line of code (in src/consensus/consensus.h):

... MAX_BLOCK_SIZE=1000000


... MAX_BLOCK_SIZE=2000000

If you make that one-byte change then recompile and run Bitcoin Core, it will work. You computer will download the blockchain and will interoperate with the other computers on the network with no issues.

If your computer is assembling transactions into blocks (you are a solo miner or a mining pool operator), then things get more complicated. I’ll walk through that complexity in the rest of this blog post, hopefully giving you a flavor for the time and care put into making sure consensus-level changes are safe.

Github has a handy

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Minority Branches

What would happen if some minority of mining hash power and maybe a merchant or exchange decided to stay with, or move to, different consensus rules than everybody else?

Would there be two different flavors of Bitcoin? Would it cause massive disruptions to the Bitcoin economy? Would your coins be safe?

(spoiler alert if you’re in a hurry: no, no, and yes)

I’ll start with the assumption that there is a supermajority (two-thirds or more– comfortably over 50%) that wants one set of consensus rules, and a minority that wants another set of consensus rules. This analysis doesn’t work if there is an even split in opinion about the rules. I’m also assuming that there is a supermajority of both hash power and transaction creators (the ‘economic majority’) on the same side; the analysis is different if miners and exchanges/merchants/users disagree about what the rules should be.

So, if there

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Classic? Unlimited? XT? Core?

Almost two years ago, when I stepped down as lead maintainer for Bitcoin Core, I wrote:

I’m pleased to be able to focus more on protocol-level, cross-implementation issues and less on issues specific to the Bitcoin Core software.

I’d still like to focus on protocol-level, cross-implementation issues but lately I’ve been distracted and have generated a lot of controversy (and hurt feelings) by helping out with some other implementations (first XT, lately Bitcoin Classic, maybe Bitcoin Unlimited soon.

Madness! Chaos! ANARCHY! … I hear some people say, but there is a method to my madness. When I was lead maintainer of Core I had the following top-three priorities:

1) Keep the system secure.
2) Keep the network reliably processing transactions.
3) Eliminate single points of failure.

Those are still my top priorities, but I try to take a higher-level view, looking at the entire Bitcoin

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Segregated Witness is cool

Pieter Wuille gave a fantastic presentation on “Segregated Witness” in Hong Kong. It’s a great idea, and should be rolled into Bitcoin as soon as safely possible. It is the kind of fundamental idea that will have huge benefits in the future. It also needs a better name (“segregated” has all sorts of negative connotations…).

You should watch Pieter’s presentation, but I’ll give a different spin on explaining what it is (I know I often need something explained to me a couple different ways before I really understand it).

So… sending bitcoin into a segregate witness-locked output will look like a weird little beastie in today’s blockchain explorers– it will look like an “anyone can spend” transaction, with a scriptPubKey of:

PUSHDATA [version_byte + validation_script]

Spends of segregated witness-locked outputs will have a completely empty scriptSig.

The reason that is not insane is

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Designing for success

I just listened to Emin Gün Sirer and Ittay Eyal from Cornell University on the Epicenter Bitcoin podcast.

They’re doing great work; full-scale emulation of the Bitcoin network is a fantastic idea, and I plan on doing a lot of testing and optimizations using the tools they’ve developed. I also plan on writing about their Bitcoin NG idea… but not right now.

Listening to the podcast, and listening to complaints about Bitcoin XT from one of the other Core committers, I realized there’s a fundamental disagreement about protocol design.

The most successful protocols were forward-looking. When the IP protocol was designed in 1970’s, the idea of 4 billion computers connected to a single network was ludicrous. But the designers were forward-looking and used 32-bits for IP addresses, and the protocol grew from a little research project to the global internet that is just now, 40 years later

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Big-O scaling

Computer science has this thing called “big-O notation” (that’s O as in “oh”, not zero). It is a way of describing how algorithms behave as they’re given bigger problems to solve.

During the Great Maximum Blocksize Debate, there have been repeated claims that “Bitcoin is not scalable, because it is O(n2).” N-squared scaling is not sustainable; double N, and you need four times the resources (memory or CPU).

At the Montreal Scaling Bitcoin conference, I had a chance to talk to a couple of those people and ask them what the heck they’re talking about. Turns out they’re talking about a few different things.

Some of them are taking Metcalfe’s Law (“the value of a telecommunications network is proportional to the square of the number of connected users of the system (n2)” and applying it to Bitcoin transactions, assuming that if there are N people using Bitcoin they will collectively

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Bigger blocks another way?

Get ten engineers on a mailing list, ask them to solve a big problem, and you’ll probably end up with eleven different solutions.

Even if people agree that the one megabyte block size limit should be raised (and almost everybody does agree that it should be raised at some point), agreeing how is difficult.

I’m not going to try to list all of the proposals for how to increase the size; there are too many of them, and I’d just manage to miss somebody’s favorite (and end up with a wall-of-text blog post that nobody would read). But I will write about one popular family of ideas, and will explain the reasoning behind the twenty-megabyte proposal.

 Dynamic limits

One very popular idea is to implement a dynamic limit, based on historical block sizes.

The details vary: how often should the maximum size be adjusted? Every block? Every difficulty adjustment? How much of an increase should be

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When the block reward goes away…

Discussions of raising the maximum block size often turn into discussions of Bitcoin’s very-long-term future – “but what about when the block reward goes to zero” or “but how, exactly, will it all work if Bitcoin is used by billions of people per day…”

I’m going to upset a lot of engineers, but I think it is a mistake to try to predict what is going to happen that far in the future, and an even bigger mistake to spend a lot of time now worrying about what might happen ten or twenty years from now.

I’m inspired by F.A. Hayek’s “The Fatal Conceit”:

The curious task of economics is to demonstrate to men how little they really know about what they imagine they can design.

To the naive mind that can conceive of order only as the product of deliberate arrangement, it may seem absurd that in complex conditions order, and adaptation to the unknown, can be achieved more effectively by

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Analysis Paralysis

I’ve been accused of being too flippant about increasing the block size limit. This series of blog posts is meant to show that I’m not, that I have carefully thought about risks and benefits. I stepped back from the role of lead committer exactly so I would have the time to think about bigger-picture issues like this one. Today I’d like to address, head-on, this argument against changing the one-megabyte blocksize limit:

Larger-than-one-megabyte blocks have had insufficient testing and/or insufficient research into economic implications and/or insufficient security review of the risks versus benefits.

This is tough to respond to– there can always be more testing or research, especially for a security-critical project like Bitcoin. It is easy to suffer from “analysis paralysis,” and I think the Core Bitcoin project has been suffering from analysis paralysis over the block size issue for

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Are bigger blocks better for bigger miners?

After taking a break to help review some pull requests and take a trip to New York, I’m ready to continue tackling objections to increasing the block size:

Bigger blocks give bigger miners an economic advantage

This is a hard blog post to write; the arguments for why bigger blocks give bigger miners an economic advantage over smaller miners are highly technical (whenever I use the term “miner” in this post I really mean “solo miner or mining pool”).

To start: think about what happens when a miner gets lucky and finds a new block. They send it to their peers and immediately start working on finding another block on top of that new block.

Their peers will receive and validate the block, and, assuming validation passes, they then relay the block to their peers and start mining on top of the new block. The original miner is busy working during this whole validate-then-relay process; they

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