Bitcoin Unlimited (BU) is a full node implementation for the bitcoin and Bitcoin Cash networks. Compared to the Bitcoin Core client hard-coding the block size limit to one megabyte, from which it is forked, Bitcoin Unlimited allows users to signal which block size limit they prefer, find the limit having a majority consensus and automatically track the largest proof-of-work, regardless of block size. However, if a block greater than one megabyte in size is accepted by Bitcoin Unlimited and rejected by nodes with a block size limit, a fork of the network will occur, resulting in two separate blockchains with Bitcoin Unlimited nodes following the chain with the largest proof-of-work.
|Initial release||December 2015|
22.214.171.124 / 5 May 2018
Mac OS X, ARM
The release of Bitcoin Unlimited follows the release of Bitcoin XT and Bitcoin Classic, alternative proposals which aimed to increase bitcoin's transaction capacity of around 2.5-3 transactions per second by increasing the hard-coded block size limit. Bitcoin Unlimited developers maintain an edition of their software that works on the Bitcoin Cash network, known as "BU Cash".
Bitcoin Unlimited is an attempt to upgrade Bitcoin Core into a client that processes bitcoin transactions into blocks with a potential maximum size greater than the Core's hard-coded limit of one megabyte. The one megabyte block size limit was added in 2010 by Satoshi Nakamoto as a temporary anti-DoS measure. This limited the maximum network capacity to about three transactions per second. Per the advocates of the change, a block size increase is needed in order to avoid a workflow bottleneck due to the number of transactions made as bitcoin adoption increases. BUIP001 documented the proposal and was drafted by lead developer Andrew Stone.
With Bitcoin Unlimited, miners are independently able to configure the size of the blocks they will validate. Maximum Generation Size, also referred to as MG is a new parameter which by default is set to one megabyte. The software allows the users to adjust it and select the size of blocks they produce. Excessive Block Size, or EB, parameter allows nodes to choose the size of the block they accept. By default this is set at 16 megabytes. The third new parameter allows a user to select the Excessive Acceptance Depth, or AD. This implements a consensus strategy by retroactively accepting larger blocks if a majority of other miners have done so.
Miners using Bitcoin Unlimited continue to process regular-sized blocks but as soon as a block larger than one megabyte is mined, they will follow the chain containing the most work.
Per the Bitcoin Unlimited website, the scalability solution will be found at a focal point. That is, the size limit of a block is expected to naturally emerge from the cumulative effect of thousands of node operators and miners expressing their preferences.
Per proponents, Bitcoin Unlimited continues the transaction capacity increase method bitcoin used for much of its existence. Business analyst and cryptoanalyst Eli Afram claimed that 'The "1000000" (1MB), number that Satoshi input into the code follows no real decision point, but rather, a simple round numbered limit, that was never supposed to be reached. Except that we did reach that number, and as a result, many users are now paying well over 1USD per transaction.' Satoshi Nakamoto said of the blockchain size, "The eventual solution will be to not care how big it gets."
Bitcoin Unlimited follows the release of Bitcoin XT and Bitcoin Classic, alternative proposals on how to increase bitcoin's transaction capacity. Bitcoin Unlimited is actively supported by Roger Ver. Bitcoin Unlimited development is primarily funded by donations. Mining pools including Antpool, bitcoin.com, BTC.TOP, GBMiners and ViaBTC use BU. As of March 2017 around 11% of the nodes run BU.
Developers of Bitcoin Core have been reluctant to increase the block size limit. Core developer Luke-Jr even claimed that the current limit is too large and that all legitimate uses of bitcoin "amount to approximately 750k/block average." Per David A Johnston, the emerging consensus mechanism could lead to a network split. BU nodes were attacked after developers brought a bug to light on 14 March 2017. The numbers of nodes hosting Unlimited fell to about 370 from 780 following the attacks, the lowest level since October, and returned to about 780 within 24 hours according to website coin.dance which tracks network data. On 24 April 70% of all Bitcoin Unlimited nodes crashed due to memory leaks. On 8 May roughly 70% of all Bitcoin Unlimited nodes went offline again. The exact reason is unknown but BU developer Andrea Suisani suggested that it was related to the Xthin protocol, a feature of BU.
Bitcoin Unlimited seeks to democratize the software development process. The protocol used by Bitcoin Unlimited is administered by a formal process described in the Articles of Federation. Elected positions exist for a developer in charge of software maintenance, a president in charge of high level management and an elected secretary to deal with communication and administrative issues. The software's Developer, President and Secretary is elected every two years. Currently (as of Mar 2018), the president of Bitcoin Unlimited is Andrew Clifford, Peter Rizun is the Secretary, and Andrew Stone is the Developer.
Bitcoin Unlimited has a large number members and developers including Peter Tschipper, Andrea Suisani, and Amaury Séchet.
- 18 Nov 2015 Bitcoin Unlimited formed
- 19 Dec 2015 First Bitcoin Unlimited software release, containing the Emergent Consensus block size determination algorithm (BUIP001).
- 30 May 2016 Xthin block propagation technology released. Xthin block propagation minimizes the size of transmitted blocks by omitting transactions that the destination node already contains.
- 18 Feb 2017 Bitcoin Unlimited Emergent Consensus votes exceeds Segregated Witness.
- 21 Apr 2017 Bitcoin Unlimited mining peaks at 50% of Bitcoin blocks, and is mining about 45% when averaged over 1000 blocks.
- 10 May 2017 Bitcoin Unlimited proposes the Bitcoin Cash hard fork 
- 01 Aug 2017 Bitcoin Unlimited, Bitcoin ABC and Bitcoin XT, create the Bitcoin Cash fork of the Bitcoin blockchain 
- 10 Nov 2017 Bitcoin Unlimited mines a 1 billion byte block in a worldwide test network
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- "Bitcoin Unlimited Financial Report: 1 June 2016 - 31 December 2016". Retrieved 17 June 2018.
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- "About". bitcoinunlimited.info. Retrieved 3 May 2017.
- Peter Rizun (30 May 2016). Towards Massive On-Chain Scaling: Presenting Our Block Propagation Results With Xthin.
- Joël Valenzuela (18 February 2017). Bitcoin Fork Soon? Bitcoin Unlimited Surges Past SegWit, Core Blocks Drop Below 75%. CoinTelegraph. Retrieved 27 March 2018.
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