Segwit has proven more contentious to activate than anticipated

(although my read has long been that the technical consensus is clear,

despite noisy objections). No matter which method is used to

eventually activate segwit, or on what timeline, it would be

beneficial if validating nodes already capable of supporting segwit

could, without further upgrades, eventually participate to their

fullest capacity.

BIP9 assignments should reserve a backward compatibility bit which all

yet-unknown segwit-compatible proposals may utilize. These future

proposals must be consensus compatible with BIPs 141, 143, & 147,

except that they may use different deployment logic.

The motivation is so that any validating node software released after

this BIP9 assignment can eventually understand if segwit is activated

by alternate means, even when the node is itself a legacy version.

This is important because the realities of system administration on

the Bitcoin network are that upgrades occur slowly (which is inherent

in the security choice of not presenting an auto-upgrade feature).

Even though segwit in particular is backwards compatible with old

validating nodes, there are still distinct advantages to validating

and generating segregated witness transactions.

For example, future BIP9-compatible deployment attempts might

additionally include a date-dependent UASF fallback. If, either

during or after activation, deployment rules also require signaling

for segwit using the backwards-compatible bit here proposed, then

(after 95% of recent blocks signal for the alternate segwit

deployment) more legacy nodes would understand and validate

transactions using segregated witnesses.

An expiration time of five years seems conservative:

// Alternate Deployment 1 of SegWit (BIP141, BIP143, and BIP147)

consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT_ALT1].bit = 2;

consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT_ALT1].nStartTime

= 1510704000; // November 15th, 2017.

consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT_ALT1].nTimeout =

1668470400; // November 15th, 2022.

Segwit deployment logic would then look like:

bool IsWitnessEnabled(const CBlockIndex* pindexPrev,

const Consensus::Params& params)

{

LOCK(cs_main);

return (VersionBitsState(pindexPrev,

params,

Consensus::DEPLOYMENT_SEGWIT,

versionbitscache)

== THRESHOLD_ACTIVE)

|| (VersionBitsState(pindexPrev,

params,

Consensus::DEPLOYMENT_SEGWIT_ALT1,

versionbitscache)

== THRESHOLD_ACTIVE);

}

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