Bitcoin Unlimited is a fork of the Bitcoin Core client. Compared to Core client the Unlimited client removes the limit of the block size. The size is determined by the users based on a consensus mechanism. The protocol follows the releases of Bitcoin XT and Bitcoin Classic, serving the increase of Bitcoin’s transaction capacity to 3 transactions per second.
Bitcoin Unlimited aims to avoid workflow bottlenecks due to the further adoption of Bitcoin and therefore the number of transactions. The approach stems from the Bitcoin Unlimited Improvement Proposal drafted by lead developer Andrew Stone.
The scalability is addressed within Bitcoin Unlimited by giving rights to the miners. Miners are independently able to configure the block size. The Maximum Generation (MG) size is by default set to 1MB. Users can select via the MG the size of the blocks they mine. Excessive Block (EB) size allows nodes to choose the size of the block they access which by default is set to 16MB. The consensus is retroactive, larger blocks can be accepted if the majority of others miners have done so. The Excessive Acceptance Depth (AD) enables the consensus mechanism.
The miners use Bitcoin Unlimited to process the ‘standard’ Bitcoin Core block size of 1 MB until a threshold in the number of nodes willing to accept larger blocks is signaled. As from this point, the first larger block which has been mined will be added to the Blockchain. The limit of the block size will emerge from the preferences of the node operators and miners. As Satoshi Nakamoto stated: “The eventual solution will be to not care how big it gets”.
Increasing block size is not new, it also happened when miners increased the size from 250KB to 1MB in March 2013. It then increased to 750KB to the 1MB size. With Bitcoin Unlimited the size can be increased likewise. 2MB or 4MB blocks are likely and depending on the state of technology larger blocks can be foreseen.
The scalability of Bitcoin is a hot topic addressing the next phase of Bitcoin. To groups have different opinions about the scalability and therefore the use of Bitcoin. Supporter of Bitcoin Unlimited is Roger Ver. He propagates the further use of Bitcoin on a micro transactional level. Therefore the need of scalability in order to increase transaction capacity.
Miners which support Bitcoin Unlimited include Antpool, bitcoin.com, BTC.top, GBMiners and ViaBTC. They use the Bitcoin Unlimited protocol. Around 11% of the nodes run Bitcoin Unlimited. (March 2017)