[bitcoin-dev] BIP proposal: Reserved nversion bits in blockheader

[Btc Drak <btcdrak@gmail.com>]

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Hi,

The following proposal reduces the number of version-bits that can be used
for parallel soft-fork signalling, reserving 16 bits for non-specific use.
This would reduce the number of parallel soft-fork activations using
versionbits to from 29 to 13 and prevent node software from emitting false
warnings about unknown signalling bits under the versionbits signalling
system (BIP8/9). I chose the upper bits of the nVersion, because looking at
the versionbits implementation in the most widely deployed node software,
it is easier to implement than say annexing the lower 2 bytes of the field.

The scope of the BIP is deliberately limited to reserving bits for general
use without specifying specific uses for each bit, although there have
previously been various discussions of some use-cases of nVersion bits
including version-rolling AsicBoost[1], and nonce rolling to reduce CPU
load on mining controllers because ntime-rolling can only be done for short
periods otherwise it could have negative side effects distorting time.
However, specific use cases are not important for this BIP.

I am reviving discussion on this topic now, specifically, because the new
DragonMint miner uses version-rolling AsicBoost on mainnet[2]. It is
important to bring up so node software can adapt the versionbits warning
system to prevent false positives. This BIP has the added advantage that
when a new use for bits is found, mining manufacturers can play in the
designated area without causing disruption or inconvenience (as
unfortuntely, the use of version-rolling will cause until BIP8/9 warning
systems are adapted). I appologise for the inconvenience in advance, but
this is the unfortunate result of restraints while negotiating to get the
patent opened[3] and licensed defensively[4] in the first place.

I believe there was a similar proposal[5] made some years ago, before the
advent of BIP9. This proposal differs in that it's primary purpose is to
remove bits from the versionbits soft-fork activation system and earmark 16
bits for general use without allocating fixed uses for each bit. The BIP
cites a couple of usecases for good measure, but they are just
informational examples, not part of a specification laid down. For this
reason, there no is mention of the version-rolling Stratum extension[6]
specifics within the BIP text other than a reference to the specification
itself.

Refs:

[1] https://arxiv.org/pdf/1604.00575.pdf
[2]
https://halongmining.com/blog/2018/03/07/dragonmint-btc-miner-uses-version-rolling-asicboost/
[3]
https://www.asicboost.com/single-post/2018/03/01/opening-asicboost-for-defensive-use/
[4] https://blockchaindpl.org/
[5] https://github.com/BlockheaderNonce2/bitcoin/wiki
[6] http://stratumprotocol.org/stratum-extensions

<pre>
  BIP: ?
  Title: Reserved nversion bits in blockheader
  Author: BtcDrak <btcdrak@gmail.com>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-????
  Status: Draft
  Type: Informational
  Created: 2018-03-01
  License: BSD-3-Clause
           CC0-1.0
</pre>

==Abstract==

This BIP reserves 16 bits of the block header nVersion field for
general purpose use and removes their meaning for the purpose of
version bits soft-fork signalling.

==Motivation==

There are a variety of things that miners may desire to use some of
the nVersion field bits for. However, due to their use to coordinate
miner activated soft-forks, full node software will generate false
warnings about unknown soft forks if those bits are used for non soft
fork signalling purposes. By reserving bits from the nVersion field
for general use, node software can be updated to ignore those bits and
therefore will not emit false warnings. Reserving 16 bits for general
use leaves enough for 13 parallel soft-forks using version bits.

==Example Uses==

The following are example cases that would benefit from using some of
the bits from the nVersion field. This list is not exhaustive.

Bitcoin mining hardware currently can exhaust the 32 bit nonce field
in less than 200ms requiring the controller to distribute new jobs
very frequently to each mining chip consuming a lot of bandwidth and
CPU time. This can be greatly reduced by rolling more bits. Rolling
too many bits from nTime is not ideal because it may distort the
timestamps over a longer period.

Version-rolling AsicBoost requires two bits from the nVersion field to
calculate 4-way collisions. Any two bits can be used and mining
equipment can negotiate which bits are to be used with mining pools
via the Stratum "version-rolling" extension.

==Specification==

Sixteen bits from the block header nVersion field, starting from 13
and ending at 28 inclusive (0x1fffe000), are reserved for general use
and removed from BIP8 and BIP9 specifications. A mask of 0xe0001fff
should be applied to nVersion bits so bits 13-28 inclusive will be
ignored for soft-fork signalling and unknown soft-fork warnings.

This specification does not reserve specific bits for specific purposes.

==Backwards Compatibility==

This proposal is backwards compatible, and does not require a soft
fork to implement.

==References==

[[bip-0008.mediawiki|BIP8]]
[[bip-0009.mediawiki|BIP9]]
[https://arxiv.org/pdf/1604.00575.pdf AsicBoost white paper]
[https://github.com/BlockheaderNonce2/bitcoin/wiki nNonce2 proposal]
[http://stratumprotocol.org/ Stratum protocol extension for version-rolling]

==Copyright==

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

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