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From: Andrew Chow <achow101-lists@achow101.com>
Message-ID: <c14039f3-637e-e56d-786a-2354b0f354e0@achow101.com>
Date: Tue, 23 May 2017 16:39:19 -0400
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Subject: Re: [bitcoin-dev] Reduced signalling threshold activation of
 existing segwit deployment
Precedence: list

Hi James,

From what I understand, this proposal is incompatible with the current
segwit implementation with regards to the NODE_WITNESS service bit. I
believe it could cause network partitioning if the service bit is not
changed.

Andrew


On 5/22/2017 6:40 PM, James Hilliard via bitcoin-dev wrote:
> 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@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.
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