BIP 91 (BTC) Definition

What is BIP 91?

BIP91 is a Bitcoin Improvement Protocol set forth by James Hilliard. It comprises part one of the SegWit2x Bitcoin scaling plan. BIP 91 attempts to prevent a coin split by making SegWit2x and BIP 148, which was developed to push through BIP 141 via a user activated soft fork, compatible. Once activated, all BIP91 nodes will reject all blocks that do not support SegWit.

BIP 91 Accomplishes two Primary Objectives.

• First, it makes it much easier for the Bitcoin network to adopt SegWit, providing a fix for transaction malleability.
• It paves the way for off-chain implementations like the Lightning Network. Second, BIP 91 will supplant BIP 148, preventing a possible network split.

BIP 91 History

BIP 91 was originally offered as an alternative solution to redeploying BIP 141, which was the original protocol activating SegWit. This would require miners to deploy bits in the version field of blocks they mine, thereby indicating that they are ready to implement the new rules. The problem was that SegWit2x wanted BIP 141 to require a bit 4 signaling, but BIP 141 was previously implemented with a bit 1 signaling.

In order to circumvent this problem, BIP 91 uses a secondary bit to cause mandatory enforcement of the original bit. As a result, miners begin signaling with bit1, and any blocks that don’t signal with bit 1 are blocked from the network.

The blockchain will not split as long as a majority of the miners by hash power enforce the bit 1 signaling. If the majority of miners have already supported BIP 91 activation, then maintaining that hash power will not be a problem.

BIP 91 was implemented with nearly 90 percent of total hash power supporting the soft fork. BIP 148 is therefore rendered obsolete, and BIP 91 is currently live on the Bitcoin network.