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  BIP: 100
  Title: Block v4, floating block size hard limit
  Author: Jeff Garzik <[email protected]>
  Status: Draft
  Type: Standards Track
  Created: 2015-06-11

Table of Contents

Abstract

Replace static 1M block size hard limit with a hard limit that floats between 1M and 32M.

Motivation

  1. Long term, wean the bitcoin protocol away from any block size limit; let the free market establish a natural size limit equilibrium.
  2. Eliminate 1M limit as impediment to adoption.
  3. Execute a hard fork network upgrade within safety rails, gathering data and experience for future upgrades.
  4. Contain checks-and-balances that users must opt into, to enable system activation and growth.
  5. Validated growth: a large majority is required to change the value.

Specification

  1. Replace static 1M block size hard limit with a floating limit ("hardLimit").
  2. hardLimit floats within the range 1-32M, inclusive.
  3. Initial value of hardLimit is 1M, preserving current system.
  4. Changing hardLimit is accomplished by encoding a proposed value within a block's coinbase scriptSig.
    1. Votes refer to a byte value, encoded within the pattern "/BV\d+/" Example: /BV8000000/ votes for 8,000,000 byte hardLimit.
    2. Absent/invalid votes and votes below minimum cap (1M) are counted as 1M votes. Votes above the maximum cap (32M) are counted as 32M votes.
    3. A new hardLimit is calculated at each difficult adjustment period (2016 blocks), and applies to the next 2016 blocks.
    4. Calculate hardLimit by examining the coinbase scriptSig votes of the previous 12,000 blocks, and taking the 20th percentile.
    5. A new hardLimit may not increase or decrease by more than 1.2x beyond the prior hardLimit.
  5. 75% rule: If 9,000 of the last 12,000 blocks are version 4 or greater, reject invalid version 4 blocks. (testnet4: 501 of last 1000)
  6. 95% rule ("Point of no return"): If 11,400 of the last 12,000 blocks are version 4 or greater, reject all version <= 3 blocks. (testnet4: 750 of last 1000)
  7. Block version number is calculated after masking out high 16 bits (final bit count TBD by versionBits outcome).

Backward compatibility

All older clients are not compatible with this change. The first block larger than 1M will create a network partition excluding not-upgraded network nodes and miners.