Updating Consensus

Soft Fork Signaling with Block Version

Since soft forks allow non-upgraded clients to continue to operate within consensus, the mechanism for "activating" a soft fork is through miners signaling readiness: a majority of miners must agree that they are ready and willing to enforce the new consensus rules. To coordinate their actions, there is a signaling mechanism that allows them to show their support for a consensus rule change. This mechanism was introduced with the activation of BIP-34 in March 2013 and replaced by the activation of BIP-9 in July 2016.

BIP-34 Signaling and Activation

The first implementation, in BIP-34, used the block version field to allow miners to signal readiness for a specific consensus rule change. Prior to BIP-34, the block version was set to "1" by convention not enforced by consensus.

BIP-34 defined a consensus rule change that required the coinbase data field of a coinbase transaction input to contain the block height. Prior to BIP-34, the coinbase data could contain any arbitrary data the miners chose to include. After activation of BIP-34, valid blocks had to contain a specific block-height at the beginning of the coinbase data and be identified with a version number greater than or equal to "2".

To signal the change and activation of BIP-34, miners set the block version to "2," instead of "1". This did not immediately make version "1" blocks invalid. Once activated, version "1" blocks would become invalid and all version "2" blocks would be required to contain the block height in the coinbase to be valid.

BIP-34 defined a two-step activation mechanism, based on a rolling window of 1000 blocks. A miner would signal his or her individual readiness for BIP-34 by constructing blocks with "2" as the version number. Strictly speaking, these blocks did not yet have to comply with the new consensus rule of including the block-height in the coinbase transaction because the consensus rule had not yet been activated. The consensus rules activated in two steps:

  • If 75% (750 of the most recent 1000 blocks) are marked with version "2," then version "2" blocks must contain block height in the coinbase transaction or they are rejected as invalid. Version "1" blocks are still accepted by the network and do not need to contain block-height. The old and new consensus rules coexist during this period.
  • When 95% (950 of the most recent 1000 blocks) are version "2," version "1" blocks are no longer considered valid. Version "2" blocks are valid only if they contain the block-height in the coinbase (as per the previous threshold). Thereafter, all blocks must comply with the new consensus rules, and all valid blocks must contain block-height in the coinbase transaction.

After successful signaling and activation under the BIP-34 rules, this mechanism was used twice more to activate soft forks:

  • BIP-66 Strict DER Encoding of Signatures was activated by BIP-34 style signaling with a block version "3" and invalidating version "2" blocks.
  • BIP-65 CHECKLOCKTIMEVERIFY was activated by BIP-34 style signaling with a block version "4" and invalidating version "3" blocks.

After the activation of BIP-65, the signaling and activation mechanism of BIP-34 was retired and replaced with the BIP-9 signaling mechanism described next.

The standard is defined in BIP-34 (Block v2, Height in Coinbase).

BIP-9 Signaling and Activation

The mechanism used by BIP-34, BIP-66, and BIP-65 was successful in activating three soft forks. However, it was replaced because it had several limitations:

  • By using the integer value of the block version, only one soft fork could be activated at a time, so it required coordination between soft fork proposals and agreement on their prioritization and sequencing.
  • Furthermore, because the block version was incremented, the mechanism didn't offer a straightforward way to reject a change and then propose a different one. If old clients were still running, they could mistake signaling for a new change as signaling for the previously rejected change.
  • Each new change irrevocably reduced the available block versions for future changes.

BIP-9 was proposed to overcome these challenges and improve the rate and ease of implementing future changes.

BIP-9 interprets the block version as a bit field instead of an integer. Because the block version was originally used as an integer, versions 1 through 4, only 29 bits remain available to be used as a bit field. This leaves 29 bits that can be used to independently and simultaneously signal readiness on 29 different proposals.

BIP-9 also sets a maximum time for signaling and activation. This way miners don't need to signal forever. If a proposal is not activated within the TIMEOUT period (defined in the proposal), the proposal is considered rejected. The proposal may be resubmitted for signaling with a different bit, renewing the activation period.

Furthermore, after the TIMEOUT has passed and a feature has been activated or rejected, the signaling bit can be reused for another feature without confusion. Therefore, up to 29 changes can be signaled in parallel and after TIMEOUT the bits can be "recycled" to propose new changes.

While signaling bits can be reused or recycled, as long as the voting period does not overlap, the authors of BIP-9 recommend that bits are reused only when necessary; unexpected behavior could occur due to bugs in older software. In short, we should not expect to see reuse until all 29 bits have been used once.

Proposed changes are identified by a data structure that contains the following fields:


A short description used to distinguish between proposals. Most often the BIP describing the proposal, as "bipN," where N is the BIP number.


0 through 28, the bit in the block version that miners use to signal approval for this proposal.


The time (based on Median Time Past, or MTP) that signaling starts after which the bit's value is interpreted as signaling readiness for the proposal.


The time (based on MTP) after which the change is considered rejected if it has not reached the activation threshold.

Unlike BIP-34, BIP-9 counts activation signaling in whole intervals based on the difficulty retarget period of 2016 blocks. For every retarget period, if the sum of blocks signaling for a proposal exceeds 95% (1916 of 2016), the proposal will be activated one retarget period later.

BIP-9 offers a proposal state diagram to illustrate the various stages and transitions for a proposal, as shown in BIP-9 state transition diagram.

Proposals start in the DEFINED state, once their parameters are known (defined) in the bitcoin software. For blocks with MTP after the start time, the proposal state transitions to STARTED. If the voting threshold is exceeded within a retarget period and the timeout has not been exceeded, the proposal state transitions to LOCKED_IN. One retarget period later, the proposal becomes ACTIVE. Proposals remain in the ACTIVE state perpetually once they reach that state. If the timeout elapses before the voting threshold has been reached, the proposal state changes to FAILED, indicating a rejected proposal. FAILED proposals remain in that state perpetually.

Figure 10. BIP-9 state transition diagram

BIP-9 was first implemented for the activation of CHECKSEQUENCEVERIFY and associated BIPs (68, 112, 113). The proposal named "csv" was activated successfully in July of 2016.

The standard is defined in BIP-9 (Version bits with timeout and delay).