Why do you have to mine 432 blocks to activate segwit on regtest?

Question

According to Rusty Russel, you need to mine 432 blocks to activate segwit on regtest. Why this specific blockheight?

Sources:

• https://rusty-lightning.medium.com/segregated-witness-for-developers-8fa9042f43ed#.6ur6kr5nf
• https://github.com/ElementsProject/lightning/blob/f4e94147a7281ec05e078ff9f3e6d65893417bad/test-cli/scripts/generate-block.sh#L11

Answer 1

Note that since 0.16, Segwit is always active on regtest from the genesis block.

Prior to 0.16, Segwit on regtest is defined to have a start time of 0 and can be activated through the BIP 9 versionbits mechanism. In BIP 9, the genesis block always has the state of DEFINED, which means that the signaling period beginning with the genesis block is DEFINED and any signaling in that period does not count for segwit.

Segwit would become STARTED in the second signaling period and only then would signaling blocks be counted. If the threshold is reached, then it will become LOCKED_IN during the third signaling period. Once the third signaling period concludes, Segwit will be active. So under ideal circumstances where all blocks signal for Segwit, 3 signaling periods are needed for Segwit to become active.

In Regtest, each signaling period is 144 blocks. 144 * 3 = 432 blocks need to be mined for Segwit to become active.