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Apr 2017
Re: Segwit2Mb - combined soft/hard fork

Sergio Demian Lerner
  • Mar 31, 2017
Segwit2Mb is the project to merge into Bitcoin a minimal patch that aims to untangle the current conflict between different political positions regarding segwit activation vs. an increase of the on-chain blockchain space through a standard block size increase. It is not a new solution, but it should be seen more as a least common denominator.

Segwit2Mb combines segwit as it is today in Bitcoin 0.14+ with a 2MB block size hard-fork activated ONLY if segwit activates (95% of miners signaling), but at a fixed future date.

The sole objective of this proposal is to re-unite the Bitcoin community and avoid a cryptocurrency split. Segwit2Mb does not aim to be best possible technical solution to solve Bitcoin technical limitations. However, this proposal does not imply a compromise to the future scalability or decentralization of Bitcoin, as a small increase in block size has been proven by several core and non-core developers not to affect Bitcoin value propositions.

In the worst case, a 2X block size increase has much lower economic impact than the last bitcoin halving (<10%), which succeeded without problem.

On the other side, Segwit2Mb primary goal is to be minimalistic: in this patch some choices have been made to reduce the number of lines modified in the current Bitcoin Core state (master branch), instead of implementing the most elegant solution. This is because I want to reduce the time it takes for core programmers and reviewers to check the correctness of the code, and to report and correct bugs.

The patch was built by forking the master branch of Bitcoin Core, mixing a few lines of code from Jeff Garzik's BIP102, and defining a second versionbits activation bit (bit 2) for the combined activation.

The combined activation of segwit and 2Mb hard-fork nVersion bit is 2 (DEPLOYMENT_SEGWIT_AND_2MB_BLOCKS).

This means that segwit can still be activated without the 2MB hard-fork by signaling bit 1 in nVersion (DEPLOYMENT_SEGWIT).

The tentative lock-in and hard-fork dates are the following:
  • Bit 2 signaling StartTime = 1493424000; // April 29th, 2017
  • Bit 2 signaling Timeout = 1503964800; // August 29th, 2017
  • HardForkTime = 1513209600; // Thu, 14 Dec 2017 00:00:00 GMT
The hard-fork is conditional to 95% of the hashing power has approved the segwit2mb soft-fork and the segwit soft-fork has been activated (which should occur 2016 blocks after its lock-in time)

For more information on how soft-forks are signaled and activated, see

This means that segwit would be activated before 2Mb: this is inevitable, as versionbits have been designed to have fixed activation periods and thresholds for all bits. Making segwit and 2Mb fork activate together at a delayed date would have required a major re-write of this code, which would contradict the premise of creating a minimalistic patch. However, once segwit is activated, the hard-fork is unavoidable.

Although I have coded a first version of the segwit2mb patch (which modifies 120 lines of code, and adds 220 lines of testing code), I would prefer to wait to publish the source code until more comments have been received from the community.

To prevent worsening block verification time because of the O(N^2) hashing problem, the simple restriction that transactions cannot be larger than 1Mb has been kept. Therefore the worse-case of block verification time has only doubled.

Regarding the hard-fork activation date, I want to give enough time to all active economic nodes to upgrade. As of Fri Mar 31 2017, reports that 6332 out of 6955 nodes (91%) have upgraded to post 0.12 versions. Upgrade to post 0.12 versions can be used to identify economic active nodes, because in the 0.12 release dynamic fees were introduced, and currently no Bitcoin automatic payment system can operate without automatic discovery of the current fee rate. A pre-0.12 would require constant manual intervention. Therefore I conclude that no more than 91% of the network nodes reported by bitnodes are active economic nodes.

As Bitcoin Core 0.12 was released on February 2016, the time for this 91% to upgrade has been around one year (under a moderate pressure of operational problems with unconfirmed transactions). Therefore we can expect a similar or lower time to upgrade for a hard-fork, after developers have discussed and approved the patch, and it has been reviewed and merged and 95% of the hashing power has signaled for it (the pressure not to upgrade being a complete halt of the operations). However I suggest that we discuss the hard-fork date and delay it if there is a real need to.

Currently time works against the Bitcoin community, and so is delaying a compromise solution. Most of the community agree that halting the innovation for several years is a very bad option.

After the comments collected by the community, a BIP will be written describing the resulting proposal details.

If segwit2mb locks-in, before hard-fork occurs all bitcoin nodes should be updated to a Segwit2Mb enabled node to prevent them to be forked-away in a chain with almost no hashing-power.

The proof of concept patch was made for Bitcoin Core but should be easily ported to other Bitcoin protocol implementations that already support versionbits. Lightweight (SPV) wallets should not be affected as they generally do not check the block size.

I personally want to see the Lightning Network in action this year, use the non-malleability features in segwit, see the community discussing other exciting soft-forks in the scaling roadmap, Schnorr sigs, drivechains and MAST.

I want to see miners, developers and industry side-by-side pushing Bitcoin forward, to increase the value of Bitcoin and prevent high transaction fees to put out of business use-cases that could have high positive social impact.

I believe in the strength of a unified Bitcoin community. If you're a developer, please give your opinion, suggest changes, audit it, and take a stand with me to unlock the current Bitcoin deadlock.

Contributions to the segwit2mb project are welcomed and awaited. The only limitation is to stick to the principle that the patch should be as simple to audit as possible. As an example, I wouldn't feel confident if the patch modified more than ~150 lines of code.

Improvements unrelated to a 2 Mb increase or segwit, as beneficial as it may be to Bitcoin, should not be part of segwit2Mb.

This proposal should not prevent other consensus proposals to be simultaneously merged: segwit2mb is a last resort solution in case we can not reach consensus on anything better.

Again, the proposal is only a starting point: community feedback is expected and welcomed.

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