Bitcoin Unlimited (BU) is a full node software client for the bitcoin network. Compared to the Bitcoin Core client hard-coding the block size limit to 1 megabyte, from which it is forked, Bitcoin Unlimited does not hard-code the limit, allowing the users to signal which block size limit they prefer, find the limit having a majority consensus and set their block size limit to that value.
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 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. 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.
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.'
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. Mining pools including Antpool, bitcoin.com,
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." Furthermore, critics are worried about the small amount of BU developers and a lack of peer review of new code. A bug in BU caused Bitcoin.com to mine an invalid block on 2 February. 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 software's lead developer or maintainer is to be elected annually. Elected positions exist for a president in charge of high level management and an elected secretary to deal with administrative issues.
Bitcoin Unlimited has a number of developers including Peter Tschipper, Andrea Suisani, Andrew Stone and Amaury Sèchet.<ref name="BUabout" />