<div dir="ltr">Prior discussion: <a href="http://gnusha.org/bitcoin-wizards/2015-11-09.log" target="_blank">http://gnusha.org/bitcoin-wizards/2015-11-09.log</a><br><br>Goal:
Increase transaction throughput without increasing miner centralization
pressure, and without putting undue burden on full validating nodes.<br><br>'Weak Block': a block which meets a target with a lower difficulty, typically some fraction such as 5%.<br><br>'Strong Block': a block that meets the full target. Also called a block.<br> <br>---<br><br>Introduction:<br><br>One
of the key sources of miner centralization is the orphan rate. Miners
with 33% hash power are guaranteed to instantly validate 33% of the
blocks, while miners with only 1% hashrate only get this advantage for
1% of the blocks. If the average orphan rate on the network is high,
miners with significantly more hashpower will have a substantial
advantage over smaller miners.<br><br>One of the strongest reasons to
keep the block size small is to keep the orphan rate low. This is to
protect smaller miners. Numerous pre-consensus schemes have been
discussed which attempt to drive the orphan rate down. When considering
these schemes, the adversarial case must be considered as well as the
average case.<br><br>The circulation of weak blocks has been discussed
as a form of preconsensus. Upon finding a weak block, miners circulate
the block to the other miners, and then when a full block is found, a
diff between the weak block and full block can be circulated instead of
just the full block. This diff is both quicker to validate and quicker
to circulate, resulting in substantially improved block propagation
times and a reduced orphan rate, thus reduced miner centralization
pressure.<br><br>The adversarial case is not addressed by this scheme.
It is still possible to find and circulate a large, difficult-to-verify
block that slowly propagates through the network and drives up the
orphan rate for smaller miners. A new construction (below) introduces a
set of new consensus rules which protect small miners even from the
adversarial case.<br><br>---<br><br>Construction:<br><br>After a block
is found, pre-consensus for the next block begins. Pre-consensus
consists of building a chain of weak blocks which meet a target that has
5% the difficulty of a full block. Each weak block can introduce at
most 200kb of new transactions to the weak-block chain. On average, a
new weak block will appear every 30 seconds. When the next strong block
is found, it must be at the head of a weak block chain and it must
itself introduce a maximum of 200kb in transactions new to the
weak-block chain. The maximum size of a strong block is 16mb, but can be
composed of any number of weak blocks.<br><br>Example:<br><br>[strong block] -> [weak block - 200kb] -> [weak block - 400kb] -> [strong block - 600kb] -> [weak block - 200kb]...<br><br>---<br><br>Analysis:<br><br>On
average, weak blocks will be found every 30 seconds and each block will
build on top of 20 weak blocks. The average block size will be 4mb. 80
weak blocks are required to construct a block of the maximum size of
16mb, which will probably happen 3 out of every 1000 blocks. The
race-size for blocks is kept low, and as explained later, adversarial
mining is inherently decentivized.<br><br>This construction establishes a
'pre-consensus' that allows miners to do faster validation when a new
block is found. Assuming that the miner has seen most of the precursor
weak blocks, only a few hundred kilobytes of validation must be
performed even when the blocks are multiple megabytes in size. In the
usual case, only 100kb of validation needs to be performed.<br><br>More
consistent transaction throughput is achieved. Strong blocks that are
found in rapid succession are likely to each be small, due to having a
small number of weak blocks that they build on top of. Strong blocks
that are found a long time after the previous strong block are likely to
have many weak blocks that they build on top of.<br><br>Better
censorship resistance is achieved. Creating large blocks requires
building on top of weak blocks. A miner actively trying to censor
certain transactions will have to ignore all weak-block chains that
contain the offensive transaction, and thus will be at a disadvantage
due to consistently producing smaller (and less fee-rich) blocks.<br><br>An
attacker that is trying to flood the network with intentionally
slow-validating blocks can no longer easily construct blocks at the
maximum size, and instead must create and hide weak blocks which build
up to a strong block that has many unseen transactions. Hiding weak
blocks has an opportunity cost, because in building a chain of weak
block is exclusive to the attacker, the attacker is missing out on the
opportunity of building on top of the other weak blocks being produced.<br><br>Compared
to Bitcoin-NG, this construction lacks the vulnerability where a
single, more easily-targeted leader is elected and placed in charge of
the next round of consensus.<br><br>Everyone has incentive to build on
top of the most recent weak block. In the event that the next weak block
discovered is also a strong block, the fees reaped by the miner will be
maximized.<br><br>Larger miners appear to have an incentive to withhold
weak blocks in an attempt to drive smaller miners off of the network.
Large miners withholding weak blocks will gain an advantage that amounts
to (% chance of finding a weak block) * (% chance of finding the full
block) * (average fee addition of a weak block) / (average total block
reward). Assuming that fees make up entire block reward, the advantage
for a miner performing a withholding attack is (hashrate^2 * weak block
difficulty). For a 50% miner, that advantage comes to 1.25%. For a 20%
miner, this advantage is just 0.2%. There are probably multiple ways to
decentivize this behavior, the simplest of which involves directly
rewarding miners for announcing weak blocks.<br><br>The orphan rate for
weak blocks is going to be substantially higher for smaller miner, due
to the increased rate of appearance. I do not think that this is going
to create any issues, because small miners are still going to have high
visibility of the longest weak-block chain, and are still going to be
able to create blocks that are nearly as full as the blocks created by
larger miners.<br><br>The more time that passes between mining blocks,
the more a block is worth (because it will have more weak-blocks, and
therefore more transactions). Hashrate is therefore more valuable when a
block has not been found for a while, and may result in hashrate
hopping, where hashrate is disabled or clocked-down immediately after a
block is found and then clocked-up if a block is not found for a while.
This is only a problem while fees from new transactions make up a
significant portion of the block reward.<br><br>---<br><br>Conclusion:<br><br>A
forced-weak-blocks scheme potentially provides a powerful way to reduce
the orphan rate, increasing the safety margins on miner centralization
pressure and allowing the overall transaction throughput to be increased
as a result.<br><br>Additional analysis is needed to be certain that
there are not new attack vectors or mal-aligned incentives that have
been introduced.</div>