Subject: Re: [bitcoin-dev] Variable Block Size Proposal

From: Jameson Lopp <[email-masked]>
Date: Sat, 29 Aug 2015 10:22:01 -0400

I don't think you'll find much support for introducing a mandatory minimum
block size. It's quite wasteful to "pad" blocks with transactions that the
miner is just sending back to themself. If you want to solve the block
propagation issue, I'd recommend instead working on O(1) block propagation.

The Bitcoin Relay Network already allows miners to relay blocks much
faster: http://bitcoinrelaynetwork.org/

The next step would be getting O(1) block propagation into the Bitcoin
protocol. Check out Gavin's proposal:
https://gist.github.com/gavinandresen/e20c3b5a1d4b97f79ac2

- Jameson

On Sat, Aug 29, 2015 at 1:41 AM, Justin M. Wray via bitcoin-dev <
[email-masked]> wrote:

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> Hey Bitcoiners!
>
> While I am an avid Bitcoin supporter, long-term user, and have done
> development work on tools and platforms surrounding Bitcoin, I have
> been very busy these past few weeks and haven't had a chance to fully
> (or closely) monitor the Block Size debate.
>
> I'm familiar with the basics, and have read abstracts about the
> front-running proposals (BIP 100, 101, and 102). Though I've honestly
> not read those in depth either. With that said, I was driving
> the other day and thought of a potential idea. I'll be clear, this is
> just an idea, and I haven't fully fleshed it out. But I thought I'd
> throw it out there and see what people thought.
>
> My Goal:
>
> Provide a variable block size that provides for sustainable, long-term
> growth, and balances the block propagation, while also being mindful
> of potential spam attacks.
>
> The Proposal:
>
> Every 2016 blocks (approximately every two weeks, at the same time the
> difficulty is adjusted), the new block size parameters are calculated.
>
> The calculation determines the average (mean) size of the past 2016
> blocks. This "average" size is then doubled (200%) and used as the
> maximum block size for the subsequent 2016 blocks. At any point, if
> the new maximum size is calculated to be below 1MB, 1MB is used
> instead (which prevents regression from our current state).
>
> Introduce a block minimum, the minimum will be 25% of the current
> maximum, calculated at the same time (that is, every 2016 blocks, at
> the same time the maximum is calculated). All blocks must be at least
> this size in order to be valid, for blocks that do not have enough
> transactions to meet the 25%, padding will be used. This devalues the
> incentive to mine empty blocks in either an attempt to deflate the
> block size, or to obtain a propagation advantage. Miners will be
> incentivized to include transactions, as the block must meet the
> minimum. This should ensure that even miners wishing to always mine
> the minimum are still confirming Bitcoin transactions.
>
> At the block in which this is introduced the maximum would stay at 1MB
> for the subsequent 2016 blocks. With the minimum being enforced of 256KB
> .
>
> Example:
>
>     * Average Block Size for the last 2016 blocks: 724KB
>     * New Maximum: 1448KB
>     * New Minimum: 362KB
>
> Example: (Regression Prevention)
>
>     * Average Block Size for the last 2016 blocks: 250KB
>     * New Maximum: 1MB
>     * New Minimum: 256KB
>
> The Future:
>
> I believe that the 1MB regression prevention might need to be changed
> in the future, to prevent a large mining population from continually
> deflating the block size (and keeping us at the 1MB limit).
>
> For this, the hard limit could be changed in the future manually,
> through a process similar to the current one, though hopefully with
> far less urgency and hysteria.
>
> Another option is to add an additional calculation, preventing the new
> maximum from being lower than 75% of the current maximum. This would
> substantially slow down a block-size deflation attack.
>
>  Example of Block-Size Deflation Attack Prevention:
>
>  * Average Block Size for the last 2016 blocks:  4MB
>  * New Maximum:  8MB
>  * New Minimum:  2MB
>
>  * Average Block Size for the last 2016 blocks:  2MB
>  * New Maximum:  6MB  (2 * 200% = 4, 4< 75% of 8, So use 8 * .75 = 6)
>  * New Minimum:  1.5MB
>
> This would provide a maximum growth of 200% per recalculation, but a
> maximum shrinkage of 75%.
>
> Request For Comments:
>
> I'd love to hear your thoughts. Why wouldn't this work? What portion
> is flawed? Will the miners support such a proposal? Would this even
> solve the block size issue?
>
> I will note that I don't find the 100% and 25% to be hard and fast in
> my idea. Those we're just the values that initially jumped out at me.
> I could easily see the minimum being anything below 50% (above 50% and
> the network can never adjust to smaller block sizes). I could also see
> the maximum being anything over 100%.  Lastly, if a inflation attack
> is a valid concern, a hard upper limit could be set (or the historical
> 32MB limit could remain).
>
> I think the great part about this variable approach is that the
> network can adjust to address spikes in volume and readjust once those
> spikes dissipate.
>
> - --
> Thanks!
>
> - -----
> Justin M. Wray
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