BA-08 · LESSON 12

Version bits and soft-fork activation

KEY DEFINITIONS

Terms used in this lesson

BIP9 deployment states: Bitcoin Improvement Proposal 9 deployment states: DEFINED means configured but not started, STARTED counts miner signals, LOCKED_IN means the threshold was reached, ACTIVE means the new rules apply, and FAILED means the configured timeout passed without activation.

Decode the block-version marker and deployment bits, then follow period thresholds through DEFINED, STARTED, LOCKED_IN, ACTIVE, or FAILED.

65 min Difficulty 3/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · versionbits activationVersion bits move only at defined period boundariesSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
VERSION0x20000002
Only the 001 top marker makes low deployment bits count under BIP9.
02
BIT 1Mask 0x00000002
The deployment definition selects one bit and time window.
03
PERIOD1815 of 2016
Signals are counted across a complete deployment period.
04
STATELOCKED_IN
Threshold changes the state only at the period boundary.
05
NEXT PERIODThen ACTIVE
Installed node rules, not the marker alone, enforce the deployment.

Rule shown: The top marker and assigned bit identify signals; period counts move STARTED to LOCKED_IN, then a later completed period moves LOCKED_IN to ACTIVE.

STAGE 1 OF 5VERSION: 0x20000002

Only the 001 top marker makes low deployment bits count under BIP9.

Governing relationship: The top marker and assigned bit identify signals; period counts move STARTED to LOCKED_IN, then a later completed period moves LOCKED_IN to ACTIVE. Complete path: 1. VERSION, 0x20000002. Only the 001 top marker makes low deployment bits count under BIP9. 2. BIT 1, Mask 0x00000002. The deployment definition selects one bit and time window. 3. PERIOD, 1815 of 2016. Signals are counted across a complete deployment period. 4. STATE, LOCKED_IN. Threshold changes the state only at the period boundary. 5. NEXT PERIOD, Then ACTIVE. Installed node rules, not the marker alone, enforce the deployment.

1. State transitionWhat exists, what changes, and what results

BEFORE

A teaching deployment uses bit one and a threshold of 1,815 signals in a 2,016-block period.

MECHANISM

A BIP9 signal requires the block version's top three bits to match 001. A low deployment bit under a different marker is not counted as that deployment's signal.

AFTER

Version bits coordinate readiness while node software and chain-derived state determine enforcement.

Text equivalent: Starting from A teaching deployment uses bit one and a threshold of 1,815 signals in a 2,016-block period. The mechanism begins when A BIP9 signal requires the block version's top three bits to match 001. A low deployment bit under a different marker is not counted as that deployment's signal. The resulting lesson is Version bits coordinate readiness while node software and chain-derived state determine enforcement.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Check the top marker

A BIP9 signal requires the block version's top three bits to match 001. A low deployment bit under a different marker is not counted as that deployment's signal.

Complete path: 1. Check the top marker: A BIP9 signal requires the block version's top three bits to match 001. A low deployment bit under a different marker is not counted as that deployment's signal. 2. Inspect the assigned bit: Each deployment selects one bit from zero through twenty-eight for its defined window. The same bit can be reused only across non-overlapping deployments understood from node configuration. 3. Count one complete period: Nodes count matching blocks in the deployment period, commonly 2,016 blocks. A threshold decision occurs at a period boundary, not after every individual signal. 4. Lock in before activation: Reaching threshold while STARTED moves the next period to LOCKED_IN. After that complete period, the deployment becomes ACTIVE, subject to any deployment-specific minimum activation height. 5. Keep signaling separate from enforcement: Signals communicate miner readiness. Upgraded node software defines the state machine and enforces the tightened validity rules after activation.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

Each node derives state from its deployment parameters, chain history, period boundaries, top marker, bit and signal count.

CHANGED CONDITION

Counting a wrong top marker, changing state mid-period, skipping LOCKED_IN, or treating hash-power signals as user consent produces the wrong activation model.

Important distinction: Miners activate any rule by voting for it in the block version. The node's installed deployment logic interprets predefined bits and enforces predefined rules. Miner signaling is one coordination input, not permission to invent consensus rules.

Text equivalent: The accepted path is enforced by Each node derives state from its deployment parameters, chain history, period boundaries, top marker, bit and signal count. If the required condition changes, Counting a wrong top marker, changing state mid-period, skipping LOCKED_IN, or treating hash-power signals as user consent produces the wrong activation model.

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Learn the idea

BIP9 version bits let miners signal readiness for a defined soft-fork deployment without giving miners final authority over the rules. A matching top-bit marker makes selected low-order bits meaningful. Nodes evaluate signals across fixed periods and move a deployment through DEFINED, STARTED, LOCKED_IN, ACTIVE, or FAILED according to the deployment parameters they run.

GUIDED EXPLANATION1/5 ideas inspected

Check the top marker

A BIP9 signal requires the block version's top three bits to match 001. A low deployment bit under a different marker is not counted as that deployment's signal.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Miners activate any rule by voting for it in the block version.”

The node's installed deployment logic interprets predefined bits and enforces predefined rules. Miner signaling is one coordination input, not permission to invent consensus rules.

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