Bitcoin Unlimited – Open Source BTC Network Protocol Software?

Bitcoin Unlimited is a software platform similar to the Bitcoin Core client that aims to solve some of the biggest problems with bitcoin today – for node operators, miners, investors, and users.

What Is Bitcoin Unlimited?

Bitcoin became the world’s first electronic cash system back in 2009. However, a number of issues have cropped up over time. Now, Bitcoin Unlimited promises to solve these issues to make bitcoin more efficient than ever before.

The platform is similar to the Bitcoin Core client. However, it claims to have a number of advantages over that core client – including increased transaction capacity for bitcoin.

Bitcoin Unlimited was originally developed by Andrew Stone. Today, the development team includes dozens of members around the world. The first release occurred in January 2016.

The release occurred after the release of similar projects – including Bitcoin XT and Bitcoin Classic, both of which were designed to increase the transaction capacity of bitcoin by around 2.5 to 3 transactions per second.

The reason it’s called Bitcoin Unlimited is because there’s no hard-coded limit, which means users can signal which block size limit they prefer. They can find the limit that has a majority consensus, for example, and set their block size limit to that value.

Overall, Bitcoin Unlimited is an attempt to upgrade Bitcoin Core. Let’s take a closer look at how it plans to improve bitcoin and Bitcoin Core.

What Does Bitcoin Unlimited Hope To Accomplish?

Bitcoin Unlimited claims that it “provides a voice to all stakeholders in the bitcoin ecosystem”, including users, node operators, miners, and investors. Here’s a brief overview of how Bitcoin Unlimited provides advantages to each of those parties:


With bitcoin, users enjoy low, predictable fees along with reliable confirmation times and usable instant transactions. However, many network nodes enforce a block size limit below today’s demand, which means rising fees, backlogged transactions, and erratic confirmation times.

The current limit is approximately 3 transactions per second. Some nodes even use replace-by-fee policies to allow double-spending of transactions stuck due to the backlogs. This obviously reduces one of the key advantages of bitcoin: security.

Bitcoin needs a better solution, and Bitcoin Unlimited wants to be that solution. Bitcoin Unlimited wants to provide a number of benefits for users, including reduced transaction fees and confirmation times caused by enabling node operators to lift their block size limits. Bitcoin Unlimited also plans to reverse the damage done by the replace-by-fee initiative.

Node Operators

When bigger blocks inevitably arrive, node operators will need to be ready. Whether your node powers a bitcoin business or your own wallet, Bitcoin Unlimited claims you can enjoy powerful benefits.

The Bitcoin Unlimited client lets you securely track consensus as defined by the most proof-of-work chain composed of valid transactions, for example, regardless of the outcome of the “block size limit debate”.

Overall, node operators should enjoy a more free market-based process with Bitcoin Unlimited. Operators will be able to use Bitcoin Unlimited’s UI to easily adjust node block size limit. That tells miners you’re ready for bigger blocks today.

Another benefit for node operators is the fact that Bitcoin Unlimited wants to remove bottlenecks. Bottlenecks are a big problem as the required resources for a node continues to expand.

In fact, Bitcoin Unlimited was the first client to solve the infamous “block propagation bottleneck” first spotted by researchers at Cornell in 2016 as the primary bottleneck to raising network throughput.

That bottleneck solution is called Xtreme Thin Blocks, and it reduced block propagation times by a factor of 5.6x across the normal P2P network. Even across China’s Great Firewall, it reduced block propagation times by 8.7x while using 24x fewer bytes.

One final advantage of Bitcoin Unlimited for node operators is that it’s easy to make the switch: switching to Bitcoin Unlimited from another Satoshi client doesn’t require you to re-download the blockchain. Instead, you just download and install the Bitcoin Unlimited software.


Miners can throw production quotas and central market planning out the window while enjoying the benefits of a free market. Miners are free to meet the demand for block space, if they choose to do so, which makes the market more efficient as a whole.

Bitcoin Unlimited has another feature that will appeal to miners called Xpedited block relay. It’s based on Xthin technology and is designed to ensure you’re always mining on the latest block, and that the blocks you solve propagate across the network quickly regardless of their size.

All of this adds up to one big benefit for Bitcoin Unlimited miners: better profitability.


The developers of Bitcoin Unlimited claim their work could have a significant impact on the value of bitcoin as an investment. Bitcoin Unlimited’s over-arching goal is to return control over bitcoin’s evolution back to the market. Right now, the block size limits are restricting transaction volumes – an inefficiency that could scare off investors.

As the Bitcoin Unlimited website explains, “By removing this limit, bitcoin’s user base will be free to grow, and with it its market capitalization.”

How Does Bitcoin Unlimited Work?

Bitcoin Unlimited is written in C++. The platform is available on Windows, Linux, Mac OS X, and ARM. It’s forked off from the Bitcoin Core client.

Obviously, Bitcoin Unlimited has some lofty goals. To achieve those goals, Bitcoin Unlimited uses three core technologies, including adjustable block-size cap (ABC), Xtreme thin blocks (Xthin), and parallel validation.

Adjustable Block-size Cap (ABC)

Bitcoin Unlimited is unique because it allows node operators and miners to remove the 1MB block size limit. This limit currently inhibits transactions throughout the bitcoin network – an inefficiency that is reducing the appeal of bitcoin and deterring investors.

With the Bitcoin Unlimited client, node operators can easily adjust the size of blocks their node accepts without having to restart the node or recompile new executables from source code.

On the mining side of things, Bitcoin Unlimited lets miners begin to produce larger blocks without needing to restart or recompile.

Plus, both parties can signal their block size preferences across the network.

Xtreme Thin Blocks (Xthin)

Xtreme Tin Blocks fixes the problem where transactions are received twice by each node – once when the transaction is first broadcast by the user to the P2P network, and again when a solved block containing the confirmed transaction is found by a miner.

Here’s how the official Bitcoin Unlimited website explains the Xthin technology:

“Rather than requesting the block verbatim, an Xthin-equipped node images its mempool onto a Bloom filter that its sends with its “get data” request; the transmitting node sends the block contents by hash for all the transactions imaged onto the Bloom filter and in full otherwise. In the unlikely event that the receiving node is unable to reconstruct the block, it requests the still-missing transactions, resulting in a second round trip between the nodes.”

The end result of the Xthin technology is improved block propagation times. The developers claim to have improved block propagation times by 5.6x across normal P2P networks.

Parallel Validation

Parallel validation solves a big problem with bitcoin: before parallel validation, nodes would pretty much freeze when they received a new block. They would stay frozen until the node had either accepted or rejected the block.

While frozen throughout this validation period, nodes cannot relay new transactions or attempt to validate competing blocks. It’s a big inefficiency in the bitcoin system.

It’s not just an inefficiency, however: it’s also a security risk. The frozen node creates an attack opportunity that allows a malicious entity to mine a slow-to-verify block, which would temporarily freeze the network.

That’s why Bitcoin Unlimited introduced parallel validation. Parallel validation neutralizes the line of attack by allowing nodes to validate more than one block – along with new incoming transactions – simultaneously.

That means improved bitcoin performance as a payment platform and shorter transaction processing times. It also means an added cost of 1.25 BTC per validation-minute because slow-to-verify blocks will be “orphaned” after they’re left behind by fast-to-verify blocks.

Who Made Bitcoin Unlimited?

Bitcoin Unlimited’s Lead Developer is the Zerg, also known as Andrew Stone. Other key members of the team include President Andrew Clifford (“solex”) and Chief scientist Dr. Peter Rizun.

You can get in touch with the developers at [email protected]

Bitcoin Unlimited Summary

Ultimately, Bitcoin Unlimited has ambitious goals to solve crucial problems in the bitcoin world. The platform is available to download today, and lets miners and node operators change block size limits for a faster, more efficient, more free market bitcoin ecosystem that’s better prepared for increased block sizes in the future.

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