Subject: Re: [bitcoin-dev] Scaling by Partitioning

From: Dave Scotese <[email-masked]>
Date: Wed, 9 Dec 2015 20:08:04 -0800

If we partition the work using bits from the TxID (once it is no longer
malleable) or even bits from whatever definition we use for "coin," then
every transaction may have to use all the other partitions to verify that
the incoming coin is good.

If all partitions are involved in validating and storing every transaction,
then we may be doing more work in total, but any one node will only have to
do (and store) a fraction of what it is now.  We would want the current
situation to be identical to one in which all the participants are handling
all the partitions.  So how can we break up the work so that any
participant can handle whatever fraction of the work he or she wants?  One
idea is to use the last bits of the address that will receive the subsidy
and fees.  You solve the block for your partition by determining that all
transactions in the block are valid against the subset of blocks whose
hashes end with the same bits.

This solution is broadcast in the hope that others will start attempting to
validate that same block on their own partition. If they are mining the
same partition, they simply change their subsidy address to work on a
different partition.  Each time a new-but-not-last partition is solved,
everyone working on the block adds the new solver's output address to their
generation transaction with the appropriate fraction of the
reward-plus-subsidy.  In this way, several miners contribute to the
solution of a single block and need only store those blocks that match the
partitions they want to work on.

Suppose we use eight bits so that there are 256 partitions and a miner
wishes to do about 1/5 of the work. That would be 51 partitions.  This is
signaled in the generation transaction, where the bit-pattern of the last
byte of the public key identifies the first partition, and the proportion
of the total reward for the block (51/256) indicates how many partitions a
solution will cover.

Suppose that the last byte of the subsidy address is 0xF0.  This means
there are only 16 partitions left, so we define partition selection to wrap
around.  This 51/256 miner must cover partitions 0xF0 - 0xFF and 0x00 -
0x23. In this way, all mining to date has covered all partitions.

The number of bits to be used might be able to be abstracted out to a
certain level.  Perhaps a miner can indicate how many bits B the
partitioning should use in the CoinBase. The blocks for which a partition
miner claims responsibility are all those with a bit pattern of length B at
the end of their hash matching the the bits at the end of the first
output's public key in the generation transaction, as well as those blocks
with hashes for which the last B bits match any of the next N bit patterns
where for the largest integer N for which the claimed output is not less
than (subsidy+fees)*(N/(2^B)).

If you only store and validate against one partition, and that partition
has a solution already, then you would start working on the next block
(once you've validated the current one against your subset of the
blockchain).  You could even broadcast a solution for that next block
before the previous block is fully solved, thus claiming a piece of the
next block reward (assuming the current block is valid on all partitions).

It seems that a miner who covers only one partition will be at a serious
disadvantage, but as the rate of incoming transactions increases, the
fraction of time he must spend validating (being about half of all other
miners who cover just one more partition) makes up for this disadvantage
somewhat.  He is a "spry" miner and therefore wins more rewards during
times of very dense transaction volume.  If we wish to encourage miners to
work on smaller partitions, we can provide a difficulty break for smaller
fractions of the work.  In fact, the difficulty can be adjusted down for
the first solution, and then slowly back up to full for the last
partition(s).

This proposal has the added benefit of encouraging the assembly of blocks
by miners who work on single partitions to get them out there with a
one-partition solution.

On Wed, Dec 9, 2015 at 2:35 PM, Andrew via bitcoin-dev <
[email-masked]> wrote:

> Hi Akiva
>
> I sketched out a similar proposal here:
> https://bitcointalk.org/index.php?topic=1083345.0
>
> It's good to see people talking about this :). I'm not quite convinced
> with segregated witness, as it might mess up some things, but will take a
> closer look.
> On Dec 9, 2015 7:32 AM, "Loi Luu via bitcoin-dev" <
> [email-masked]> wrote:
>
>> Dear Akiva,
>>
>> Its Loi Luu, one of the authors of the SCP protocol (
>> http://eprint.iacr.org/2015/1168.pdf ).
>>
>> Before SCP, we had been thinking hard about how to do sharding
>> efficiently without degrading any security guarantee. A simple solution
>> which splits the coins, or TXs in to several partitions will just not work.
>> You have to answer more questions to have a good solutions. For example, I
>> wonder in your proposal, if a transaction spends a "coin" that ends in "1"
>> and creates a new coin that ends in "1", which partition should process the
>> transaction? What is the prior data needed to validate that kind of TXs?
>>
>> The problem with other proposals, and probably yours as well,  that we
>> see is that the amount of data that you need to broadcast immediately to
>> the network increases linearly with the number of TXs that the network can
>> process. Thus, sharding does not bring any advantage than simply using
>> other techniques to publish more blocks in one epoch (like Bitcoin-NG,
>> Ghost). The whole point of using sharding/ partition is to localize
>> the bandwidth used, and only broadcast only a minimal data to the network.
>>
>> Clearly we are able to localize the bandwidth used with our SCP protocol.
>> The cost is that now recipients need to  themselves verify whether a
>> transaction is double spending. However, we think that it is a reasonable
>> tradeoff, given the potential scalability that SCP can provides.
>>
>> Thanks,
>> Loi Luu.
>>
>> On Wed, Dec 9, 2015 at 12:27 AM, Akiva Lichtner via bitcoin-dev <
>> [email-masked]> wrote:
>>
>>> Hello,
>>>
>>> I am seeking some expert feedback on an idea for scaling Bitcoin. As a
>>> brief introduction: I work in the payment industry and I have twenty years'
>>> experience in development. I have some experience with process groups and
>>> ordering protocols too. I think I understand Satoshi's paper but I admit I
>>> have not read the source code.
>>>
>>> The idea is to run more than one simultaneous chain, each chain
>>> defeating double spending on only part of the coin. The coin would be
>>> partitioned by radix (or modulus, not sure what to call it.) For example in
>>> order to multiply throughput by a factor of ten you could run ten parallel
>>> chains, one would work on coin that ends in "0", one on coin that ends in
>>> "1", and so on up to "9".
>>>
>>> The number of chains could increase automatically over time based on the
>>> moving average of transaction volume.
>>>
>>> Blocks would have to contain the number of the partition they belong to,
>>> and miners would have to round-robin through partitions so that an attacker
>>> would not have an unfair advantage working on just one partition.
>>>
>>> I don't think there is much impact to miners, but clients would have to
>>> send more than one message in order to spend money. Client messages will
>>> need to enumerate coin using some sort of compression, to save space. This
>>> seems okay to me since often in computing client software does have to
>>> break things up in equal parts (e.g. memory pages, file system blocks,) and
>>> the client software could hide the details.
>>>
>>> Best wishes for continued success to the project.
>>>
>>> Regards,
>>> Akiva
>>>
>>> P.S. I found a funny anagram for SATOSHI NAKAMOTO: "NSA IS OOOK AT MATH"
>>>
>>>
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>>>
>>>
>>
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