BIP9 (version bits)

The 2015 mechanism that lets miners signal for several soft forks in parallel using bits of the block version, with a 95% threshold and a timeout.

Mechanism

BIP-9 treats the block version field “as a bit vector, where each bit can be used to track an independent change”, tallied every retarget period, so several soft forks can be deployed at once^[1]. Each deployment has a bit, a start time and a timeout, and moves through the states DEFINED, STARTED, LOCKED_IN, ACTIVE and FAILED^[1]. Lock-in requires at least 1,916 of 2,016 blocks (95%) to set the bit, and signalling blocks must have 001 as their top three version bits^[1].

Why it replaced version numbers

BIP-9 explains that earlier upgrades such as BIP-34, BIP-66 and BIP-65 raised the version number, which “permanently restricts the set of allowed nVersion field values”^[1]. Pieter Wuille, Peter Todd, Greg Maxwell and Rusty Russell are its authors^[1].

SegWit and Taproot

Bitcoin Takeover's soft-fork table lists SegWit on BIP9 bit 1, “pushed over the line by BIP91 and BIP148”^[2]. BIP-91 had miners reject blocks that did not signal, with an 80% threshold over 336 blocks^[3]. Taproot used Speedy Trial, a BIP9 variant with a 90% threshold and a minimum activation height^[4]. BIP-8 proposed heights instead of times and an optional forced lock-in^[5]. See UASF / BIP148 and Taproot activation.

References

1. primaryBIP-9 (Version bits with timeout and delay) [archived]
2. btkBitcoin: All the Hard Forks and Soft Forks (BTCTKVR: Forks!) [archived]
3. primaryBIP-91 (Reduced threshold Segwit MASF) [archived]
4. primaryBitcoin Core 0.21.1 release notes (Taproot activation parameters) [archived]
5. primaryBIP-8 (Version bits with lock-in by height), Shaolin Fry and Luke Dashjr [archived]
6. primaryBIP-148 (Mandatory activation of segwit deployment), Shaolin Fry [archived]

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