I understand that BIP91 needs 269/336 signaled blocks.
My question is is it a rolling window of any 336 blocks or are there activation periods of 336 blocks that start and end at predetermined block numbers?

1 Answer 1


It looks like it is actually periods of 336 blocks instead of a rolling window. It is like what BIP 9 specifies, but uses 336 blocks as the window and 269 blocks as the threshold. The windows begin when a block's height is a multiple of 336.

  • Is there any particular reason why it is implemented in this way instead of rolling fashion?
    – sanket1729
    Commented Jul 19, 2017 at 13:49
  • Yes, this is inherited from BIP9. The reasons there are to be more conservative (there is randomness involved, if you give every block a chance to activate, each of those increases the probability that it activates when in reality the threshold is not reached), giving people less sleepless nights (knowing in advance when something can activate is much nicer than having to check at every block), and to make the behaviour more analysable (each attempt now is a statistically independent sample). Commented Jul 19, 2017 at 16:19

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