[bitcoin-dev] User Activated Soft Fork Split Protection

[James Hilliard <james.hilliard1@gmail.com>]

Due to the proposed calendar(https://segwit2x.github.io/) for the
SegWit2x agreement being too slow to activate SegWit mandatory
signalling ahead of BIP148 using BIP91 I would like to propose another
option that miners can use to prevent a chain split ahead of the Aug
1st BIP148 activation date.

The splitprotection soft fork is essentially BIP91 but using BIP8
instead of BIP9 with a lower activation threshold and immediate
mandatory signalling lock-in. This allows for a majority of miners to
activate mandatory SegWit signalling and prevent a potential chain
split ahead of BIP148 activation.

This BIP allows for miners to respond to market forces quickly ahead
of BIP148 activation by signalling for splitprotection. Any miners
already running BIP148 should be encouraged to use splitprotection.

<pre>
  BIP: splitprotection
  Layer: Consensus (soft fork)
  Title: User Activated Soft Fork Split Protection
  Author: James Hilliard <james.hilliard1@gmail.com>
  Comments-Summary: No comments yet.
  Comments-URI:
  Status: Draft
  Type: Standards Track
  Created: 2017-05-22
  License: BSD-3-Clause
           CC0-1.0
</pre>

==Abstract==

This document specifies a coordination mechanism for a simple majority
of miners to prevent a chain split ahead of BIP148 activation.

==Definitions==

"existing segwit deployment" refer to the BIP9 "segwit" deployment
using bit 1, between November 15th 2016 and November 15th 2017 to
activate BIP141, BIP143 and BIP147.

==Motivation==

The biggest risk of BIP148 is an extended chain split, this BIP
provides a way for a simple majority of miners to eliminate that risk.

This BIP provides a way for a simple majority of miners to coordinate
activation of the existing segwit deployment with less than 95%
hashpower before BIP148 activation. Due to time constraints unless
immediately deployed BIP91 will likely not be able to enforce
mandatory signalling of segwit before the Aug 1st activation of
BIP148. This BIP provides a method for rapid miner activation of
SegWit mandatory signalling ahead of the BIP148 activation date. Since
the primary goal of this BIP is to reduce the chance of an extended
chain split as much as possible we activate using a simple miner
majority of 65% over a 504 block interval rather than a higher
percentage. This BIP also allows miners to signal their intention to
run BIP148 in order to prevent a chain split.

==Specification==

While this BIP is active, all blocks must set the nVersion header top
3 bits to 001 together with bit field (1<<1) (according to the
existing segwit deployment). Blocks that do not signal as required
will be rejected.

==Deployment==

This BIP will be deployed by "version bits" with a 65%(this can be
adjusted if desired) activation threshold BIP9 with the name
"splitprotecion" and using bit 2.

This BIP starts immediately and is a BIP8 style soft fork since
mandatory signalling will start on midnight August 1st 2017 (epoch
time 1501545600) regardless of whether or not this BIP has reached its
own signalling threshold. This BIP will cease to be active when segwit
is locked-in.

=== Reference implementation ===

<pre>
// Check if Segregated Witness is Locked In
bool IsWitnessLockedIn(const CBlockIndex* pindexPrev, const
Consensus::Params& params)
{
    LOCK(cs_main);
    return (VersionBitsState(pindexPrev, params,
Consensus::DEPLOYMENT_SEGWIT, versionbitscache) ==
THRESHOLD_LOCKED_IN);
}

// SPLITPROTECTION mandatory segwit signalling.
if ( VersionBitsState(pindex->pprev, chainparams.GetConsensus(),
Consensus::DEPLOYMENT_SPLITPROTECTION, versionbitscache) ==
THRESHOLD_LOCKED_IN &&
     !IsWitnessLockedIn(pindex->pprev, chainparams.GetConsensus()) &&
// Segwit is not locked in
     !IsWitnessEnabled(pindex->pprev, chainparams.GetConsensus()) ) //
and is not active.
{
    bool fVersionBits = (pindex->nVersion & VERSIONBITS_TOP_MASK) ==
VERSIONBITS_TOP_BITS;
    bool fSegbit = (pindex->nVersion &
VersionBitsMask(chainparams.GetConsensus(),
Consensus::DEPLOYMENT_SEGWIT)) != 0;
    if (!(fVersionBits && fSegbit)) {
        return state.DoS(0, error("ConnectBlock(): relayed block must
signal for segwit, please upgrade"), REJECT_INVALID, "bad-no-segwit");
    }
}

// BIP148 mandatory segwit signalling.
int64_t nMedianTimePast = pindex->GetMedianTimePast();
if ( (nMedianTimePast >= 1501545600) &&  // Tue 01 Aug 2017 00:00:00 UTC
     (nMedianTimePast <= 1510704000) &&  // Wed 15 Nov 2017 00:00:00 UTC
     (!IsWitnessLockedIn(pindex->pprev, chainparams.GetConsensus()) &&
 // Segwit is not locked in
      !IsWitnessEnabled(pindex->pprev, chainparams.GetConsensus())) )
 // and is not active.
{
    bool fVersionBits = (pindex->nVersion & VERSIONBITS_TOP_MASK) ==
VERSIONBITS_TOP_BITS;
    bool fSegbit = (pindex->nVersion &
VersionBitsMask(chainparams.GetConsensus(),
Consensus::DEPLOYMENT_SEGWIT)) != 0;
    if (!(fVersionBits && fSegbit)) {
        return state.DoS(0, error("ConnectBlock(): relayed block must
signal for segwit, please upgrade"), REJECT_INVALID, "bad-no-segwit");
    }
}
</pre>

https://github.com/bitcoin/bitcoin/compare/0.14...jameshilliard:splitprotection-v0.14.1

==Backwards Compatibility==

This deployment is compatible with the existing "segwit" bit 1
deployment scheduled between midnight November 15th, 2016 and midnight
November 15th, 2017. This deployment is also compatible with the
existing BIP148 deployment. This BIP is compatible with BIP91 only if
BIP91 activates before it and before BIP148. Miners will need to
upgrade their nodes to support splitprotection otherwise they may
build on top of an invalid block. While this bip is active users
should either upgrade to splitprotection or wait for additional
confirmations when accepting payments.

==Rationale==

Historically we have used IsSuperMajority() to activate soft forks
such as BIP66 which has a mandatory signalling requirement for miners
once activated, this ensures that miners are aware of new rules being
enforced. This technique can be leveraged to lower the signalling
threshold of a soft fork while it is in the process of being deployed
in a backwards compatible way. We also use a BIP8 style timeout to
ensure that this BIP is compatible with BIP148 and that BIP148
compatible mandatory signalling activates regardless of miner
signalling levels.

By orphaning non-signalling blocks during the BIP9 bit 1 "segwit"
deployment, this BIP can cause the existing "segwit" deployment to
activate without needing to release a new deployment. As we approach
BIP148 activation it may be desirable for a majority of miners to have
a method that will ensure that there is no chain split.

==References==

*[https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-March/013714.html
Mailing list discussion]
*[https://github.com/bitcoin/bitcoin/blob/v0.6.0/src/main.cpp#L1281-L1283
P2SH flag day activation]
*[[bip-0009.mediawiki|BIP9 Version bits with timeout and delay]]
*[[bip-0016.mediawiki|BIP16 Pay to Script Hash]]
*[[bip-0091.mediawiki|BIP91 Reduced threshold Segwit MASF]]
*[[bip-0141.mediawiki|BIP141 Segregated Witness (Consensus layer)]]
*[[bip-0143.mediawiki|BIP143 Transaction Signature Verification for
Version 0 Witness Program]]
*[[bip-0147.mediawiki|BIP147 Dealing with dummy stack element malleability]]
*[[bip-0148.mediawiki|BIP148 Mandatory activation of segwit deployment]]
*[[bip-0149.mediawiki|BIP149 Segregated Witness (second deployment)]]
*[https://bitcoincore.org/en/2016/01/26/segwit-benefits/ Segwit benefits]

==Copyright==

This document is dual licensed as BSD 3-clause, and Creative Commons
CC0 1.0 Universal.