The problem with >100kb transactions isn't that there are a lot of already-signed transactions out there, or even that there are good use cases for them, but that such transactions and use cases could exist, and there is no way of disallowing them without possibly costing someone a lot of money. Slowly reducing the limit doesn't really solve this problem.

I propose to make them hard enough to confirm that no-one will use them without a very good reason. Just require that any block containing an outsized transaction be mined at a greater difficulty – quadratically greater. Such a block is more expensive *for the block's miner*, not just for the other miners, or for other nodes. Anyone who really, really needs to use a 400kb transaction can pay a miner to mine it.

Quadratic hashing isn't risky when it is inherently limited by a corresponding reduction in the rate at which the "bad" blocks can be generated. That said, there's nothing I can see which is actually good about large, expensive to validate transactions, and so >1MB transactions should remain invalid, as they are today.


Nathan Cook

On 2 June 2017 at 22:40, Jared Lee Richardson via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
> Maybe there's some hole in Jorge's logic and scrapping blockmaxsize has quadratic hashing risks, and maybe James' 10KB is too ambitious; but even if so, a simple 1MB tx size limit would clearly do the trick.  The broader point is that quadratic hashing is not a compelling reason to keep blockmaxsize post-HF: does someone have a better one?

I think this is exactly the right direction to head.  There are
arguments to be made for various maximum sizes... Maybe the limit
could be set to 1mb initially, and at a distant future block
height(years?) automatically drop to 500kb or 100kb?  That would give
anyone with existing systems or pre-signed transactions several years
to adjust to the change.  Notification could (?possibly?) be done with
a non-default parameter that must be changed to continue to use 100kb
- <1mb transactions, so no one running modern software could claim
they were not informed when that future date hits.

I don't see any real advantages to continuing to support transactions
larger than 100kb excepting the need to update legacy use cases /
already signed transactions.

On Tue, May 30, 2017 at 8:07 PM, Jacob Eliosoff via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> Maybe there's some hole in Jorge's logic and scrapping blockmaxsize has
> quadratic hashing risks, and maybe James' 10KB is too ambitious; but even if
> so, a simple 1MB tx size limit would clearly do the trick.  The broader
> point is that quadratic hashing is not a compelling reason to keep
> blockmaxsize post-HF: does someone have a better one?
>
>
>
> On May 30, 2017 9:46 PM, "Jean-Paul Kogelman via bitcoin-dev"
> <bitcoin-dev@lists.linuxfoundation.org> wrote:
>>
>> That would invalidate any pre-signed transactions that are currently out
>> there. You can't just change the rules out from under people.
>>
>>
>> On May 30, 2017, at 4:50 PM, James MacWhyte via bitcoin-dev
>> <bitcoin-dev@lists.linuxfoundation.org> wrote:
>>
>>
>>>
>>>  The 1MB classic block size prevents quadratic hashing
>>> problems from being any worse than they are today.
>>>
>>
>> Add a transaction-size limit of, say, 10kb and the quadratic hashing
>> problem is a non-issue. Donezo.
>>
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