The bitcoin scalability problem is the limited rate at which the bitcoin network can process transactions. It is related to the fact that records in the bitcoin blockchain are limited in size and frequency. Bitcoin's blocks contain the transactions on the bitcoin network. The on-chain transaction processing capacity of the bitcoin network is limited by the average block creation time of 10 minutes and the block size limit of 1 megabyte. These jointly constrain the network's throughput. The transaction processing capacity maximum estimated using an average or median transaction size is between 3.3 and 7 transactions per second. As opposed to that, when minimal-size transactions both in complexity and byte size are used for the estimate, the bitcoin's theoretical transaction throughput is 27 transactions/sec. There are various proposed and activated solutions to address this issue.
Background
The block size limit, in concert with the proof-of-work difficulty adjustment settings of bitcoin's consensus protocol, constitutes a bottleneck in bitcoin's transaction processing capacity. This can result in increasing transaction fees and delayed processing of transactions that cannot be fit into a block. Various proposals have come forth on how to scale bitcoin, and a contentious debate has resulted. Business Insider in 2017 characterized this debate as an "ideological battle over bitcoin's future."
Forks
Increasing the network's transaction processing limit requires making changes to the technical workings of bitcoin, in a process known as a fork. Forks can be grouped into two types:
Hard fork
is a hard fork of bitcoin increasing the maximum block size. Bitcoin XT, Bitcoin Classic and Bitcoin Unlimited each supported an increase to the maximum block size. Bitcoin Cash is a separate cryptocurrency from Bitcoin ; and both trade at entirely independent valuations relative to each other, fiat currencies, and other assets.
Soft fork
is an example of a soft fork. In case of a soft fork, all mining nodes meant to work in accordance with the new rules need to upgrade their software.
Efficiency improvements
Technical optimizations may decrease the amount of computing resources required to receive, process and record bitcoin transactions, allowing increased throughput without placing extra demand on the bitcoin network. These modifications can be to either the network, in which case a fork is required, or to individual node software.
Schnorr signatures have been proposed as a scaling solution by long-time developer and Blockstream co-founder Pieter Wuille.
A 2006 paper by Mihir Bellare enables signature aggregation in O size, which means that it will not take more space to have multiple signers. Bellare-Neven reduces to Schnorr for a single key. Bellare-Neven has been implemented.
"Layer 2" systems
The Lightning Network is a protocol that aims to improve bitcoin's scalability and speed without sacrificing trustless operation. The Lightning Network requires putting a funding transaction on the blockchain to open a payment channel. Once a channel is opened, connected participants are able to make rapid payments within the channel or may route payments by "hopping" between channels at intermediate nodes for little to no fee. In January 2018 Blockstream launched a payment processing system for web retailers called "Lightning Charge", noted that lightning was live on mainnet with 200 nodes operating as of 27 January 2018 and advised it should still be considered "in testing". On 15 March 2018, Lightning Labs released the beta version of its lnd Lightning Network implementation for bitcoin mainnet, and on 28 March 2018, ACINQ released a mainnet beta of its eclair implementation and desktop application. In January 2019 the online retailer Bitrefill announced that it receives more payments in Bitcoin via the lightning network than any of the altcoins they accept.
Block size increases
Transaction throughput is limited practically by a parameter known as the block size limit. Various increases to this limit, and proposals to remove it completely, have been proposed over bitcoin's history.
Proposed
In 2015, by Jeff Garzik and by Gavin Andresen were introduced.
Bitcoin XT was proposed in 2015 to increase the transaction processing capacity of bitcoin by increasing the block size limit.
Bitcoin Classic was proposed in 2016 to increase the transaction processing capacity of bitcoin by increasing the block size limit.
"The Hong Kong Agreement" was a 2016 agreement of some miners and developers that contained a timetable that would see both the activation of the Segregated Witness proposal established in December 2015 by Bitcoin Core developers, and the development of a block size limit increased to 2 MB. However, both timelines were missed.
SegWit2x was a proposed hard fork of the cryptocurrency bitcoin. The implementation of Segregated Witness in August 2017 was only the first half of the so-called "New York Agreement" by which those who wanted to increase effective block size by SegWit compromised with those who wanted to increase block size by a hard fork to a larger block size. The second half of SegWit2x involved a hard fork in November 2017 to increase the blocksize to 2 megabytes. On 8 November 2017 the developers of SegWit2x announced that the hard fork planned for around 16 November 2017 was canceled for the time being due to a lack of consensus.
Bitcoin Unlimited advocated for miner flexibility to increase the block size limit and is supported by mining pools ViaBTC, AntPool and investor Roger Ver.
Bitcoin Unlimited's proposal is different from Bitcoin Core in that the block size parameter is not hard-coded, and rather the nodes and miners flag support for the size that they want, using an idea they refer to as 'emergent consensus.' Those behind Bitcoin Unlimited proposal argue that from an ideological standpoint the miners should decide about the scaling solution since they are the ones whose hardware secure the network.
