May 2024: ABLA, the adaptive block size limit

What changed

Activation came when the median time past reached Unix time 1715774400 (15 May 2024, 12:00 UTC). The first block under the new rules is height 845,891.

One CHIP activated: CHIP-2023-04 ABLA, by bitcoincashautist. See ABLA.

The fixed 32 MB block size limit became an adaptive limit:

• After every block, the limit is recomputed from a weighted moving average of recent block sizes.
• 32 MB is the floor. When blocks are small, as they are today, the limit stays at 32 MB.
• When blocks stay full for a sustained period, the limit grows. An elastic buffer adds extra headroom during fast growth, so short bursts of demand fit.
• When demand falls, the buffer decays. The CHIP proposes a half-life of about half a year for the buffer.

The algorithm responds to sustained load, not to single large blocks. The CHIP models a spam attack by a miner with 50% of hashrate stuffing its own blocks: it would take about 1.5 years to double the limit, and 2.5 more years for another +50%, with costs rising the whole time.

Why

The CHIP puts it in two sentences: “Needing to coordinate manual increases to Bitcoin Cash’s blocksize limit incurs a meta cost on all network participants. The need to regularly come to agreement makes the network vulnerable to social attacks.”

Bitcoin’s scaling fight, and the 2017 split that created BCH, was a fight over one number. ABLA changes the default. The old default was “do nothing until something is decided.” The new default is “adjust according to this algorithm, until something else is decided.”

The CHIP also says the algorithm should have “minimal to no impact on the game theory and incentives” BCH has today. It is a response to demand, not a forecast of it.

What it enables

• No more block size wars. Growth in real usage raises capacity without a social fight or a hard fork.
• Room for spikes. The elastic buffer lets the network “borrow” about a year of growth quickly, for example a 2× rise over a few months, and pay it back over the following year.
• Predictability for businesses. Payment processors and apps can plan for growth knowing the limit follows usage.

Support and dissent

The CHIP’s stakeholder table lists 60 approvals, 8 neutral and 4 disapprovals. Bitcoin Cash Node and Bitcoin Verde approved; BCHD was neutral. Disapprovals included Coin Wallet, Bmap.app, Memo Technology and The Real Bitcoin Club. See the CHIP page for details.

Current blocks use a tiny fraction of 32 MB, so ABLA has not yet raised the limit in practice. The live metrics panel on this page shows recent block sizes.