CoinDesk Explainer: How BIP 91 Implements SegWit While Avoiding...

CoinDesk Explainer: How BIP 91 Implements SegWit While Avoiding a Split


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Many thought miners would not begin signaling for bitcoin’s controversial scaling proposal SegWit2x until July 21. Yet, that is exactly what began happening yesterday morning by way of a piece of code called BIP 91.

At press time, nearly 60% of the last 144 blocks mined have signaled support for the measure. But, what are miners signaling for, and what does bitcoin improvement proposal (BIP) 91 mean for the network?

As the first part of the SegWit2x scaling plan, BIP 91 does two things:

  • It makes it significantly easier for the network to adopt Segregated Witness (SegWit), a backward compatible upgrade that fixes transaction malleability and clears the path for off-chain solutions like Lightning Network.
  • If activated by July 31, BIP 91 supersedes BIP 148, a proposal that poses a risk of causing the network to split.

The basics

SegWit2x was introduced during CoinDesk’s Consensus 2017 conference in May. Based on a fork of the Bitcoin Core software client called BTC1, SegWit2x seeks to both implement SegWit and raise the block size.

About a month later, as a response to that, BitmainWarranty engineer James Hilliard introduced BIP 91 as a way to implement SegWit quickly and safely, without causing a fork in the network.

He specifically developed the proposal with two other active proposals for scaling bitcoin in mind.

These include:

  • BIP 141: Introduced in November 2016, BIP 141 is the original plan for activating SegWit.
  • BIP 148: Released in March, BIP 148 was developed as a way to push through BIP 141 via a user-activated soft fork (UASF).

But, more importantly, BIP 91 was proposed as an alternative to having to completely redeploy BIP 141, a task that would have been technically infeasible, given that BIP 141 does not expire until mid-November.

So, to get around that, BIP 91 employs a clever trick. Rather than change the SegWit activation code path, it simply adds an additional rule to enforce BIP 141 mandatory signaling.

As such, BIP 91 starts off resembling BIP 141, but with a few key differences:

  • Miners signal with “bit 4,” as opposed to “bit 1”
  • Activation only requires 80% as opposed to 95% of hash power support
  • The activation window is 336 blocks, as opposed to 2,016.

So, once that 80% threshold is reached, BIP 91 locks in, and another 336 blocks later, it activates.

At that point, BIP 141 is enforced using the same technique as BIP 148:

  • Miners begin signaling with “bit 1”
  • Any blocks that do not signal with “bit 1” will be blocked from the network.

As long as 51% of miners (by hash power) enforce the mandatory signalling, the chain will not split. And since 80% already supported the activation, chances are good that that will not be a problem. As an additional incentive, miners who do not signal run the risk of losing their 12.5 bitcoin block reward.

Opposition and support

BIP 91 was also a recognition of the realities of the scaling debate.

Namely, the fact that, nearly a year down the road, BIP 141 still has not gained traction with miners. While BIP 141 requires a 95% miner support (by hash power), the figure has remained stuck at around 30%, though recently it increased to 45%.

But if BIP 91 is almost identical to BIP 141, why didn’t miners signal support for the latter?

The reason is two-fold:

  • First was the high bar set to achieve activation. BIP 141 requires a super majority of miners to signal their readiness within a two-week (2,016 block) activation period.
  • Second, some miners were possibly holding out for a block-size increase, a measure that has been embraced by the SegWit2x proposal.

A fast lane to SegWit

But, the proposal that has had the biggest impact on BIP 91’s design is BIP 148, the so-called UASF discussed above.

In many ways, BIP 91 can be read as an effort to “deactivate” the BIP 148 proposal, thus removing the potential to create two rival bitcoin blockchains, each with competing assets.

To resolve that issue, Hilliard proposed that BIP 91 should activate before the end of July and BIP 148’s August 1 deadline, which is looking like a strong possibility, given current support.

Miners today are signaling their support for BIP 91 early because of the perceived need to avoid the split BIP 148 could create or, others speculate, because some miners think that a successful UASF would reduce their control over network changes.

For now, your best bet is to watch the next 336-block periodBeginning tonight at block 476,448, this is the next period during which miners can signal for BIP 91, prior to BIP 148.

Should 80% of blocks signal for BIP 91 within a 56-hour period, SegWit will lock-in, thus setting the stage for the next phase of SegWit2x this autumn.

And, with bitcoin’s three largest mining pools throwing their computing power behind the effort, it is possible that the threshold will be reached before the end of the week.

Disclosure: CoinDesk is a subsidiary of Digital Currency Group, which helped organize the Segwit2x agreement.

Computer code image via Shutterstock

The leader in blockchain news, CoinDesk is an independent media outlet that strives for the highest journalistic standards and abides by a strict set of editorial policies. Interested in offering your expertise or insights to our reporting? Contact us at [email protected].

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