[bitcoin-dev] Reduced signalling threshold activation of existing segwit deployment

Mon May 22 22:40:13 UTC 2017

I would like to propose an implementation that accomplishes the first
part of the Barry Silbert proposal independently from the second:

"Activate Segregated Witness at an 80% threshold, signaling at bit 4"
in a way that

The goal here is to minimize chain split risk and network disruption
while maximizing backwards compatibility and still providing for rapid
activation of segwit at the 80% threshold using bit 4.

By activating segwit immediately and separately from any HF we can
scale quickly without risking a rushed combined segwit+HF that would
almost certainly cause widespread issues.

Draft proposal:
https://github.com/jameshilliard/bips/blob/bip-segsignal/bip-segsignal.mediawiki

Proposal text:
<pre>
  BIP: segsignal
  Layer: Consensus (soft fork)
  Title: Reduced signalling threshold activation of existing segwit deployment
  Author: James Hilliard <james.hilliard1 at gmail.com>
  Status: Draft
  Type: Standards Track
  Created: 2017-05-22
  License: BSD-3-Clause
           CC0-1.0
</pre>

==Abstract==

This document specifies a method to activate the existing BIP9 segwit
deployment with a majority hashpower less than 95%.

==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==

Segwit increases the blocksize, fixes transaction malleability, and
makes scripting easier to upgrade as well as bringing many other
[https://bitcoincore.org/en/2016/01/26/segwit-benefits/ benefits].

This BIP provides a way for a simple majority of miners to coordinate
activation of the existing segwit deployment with less than 95%
hashpower. For a number of reasons a complete redeployment of segwit
is difficulty to do until the existing deployment expires. This is due
to 0.13.1+ having many segwit related features active already,
including all the P2P components, the new network service flag, the
witness-tx and block messages, compact blocks v2 and preferential
peering. A redeployment of segwit will need to redefine all these
things and doing so before expiry would greatly complicate testing.

==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 a "version bits" with an 80%(this can be
adjusted if desired) activation threshold BIP9 with the name
"segsignal" and using bit 4.

This BIP will have a start time of midnight June 1st, 2017 (epoch time
1496275200) and timeout on midnight November 15th 2017 (epoch time
1510704000). 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);
}

// SEGSIGNAL mandatory segwit signalling.
if ( VersionBitsState(pindex->pprev, chainparams.GetConsensus(),
Consensus::DEPLOYMENT_SEGSIGNAL, versionbitscache) == THRESHOLD_ACTIVE
&&
     !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:segsignal-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. Miners will need to upgrade their nodes to
support segsignal otherwise they may build on top of an invalid block.
While this bip is active users should either upgrade to segsignal 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.

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.

==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-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.