[bitcoin-dev] Managing block size the same way we do difficulty (aka Block75)

Andrew Johnson andrew.johnson83 at gmail.com
Sun Dec 11 20:38:27 UTC 2016


"You miss something obvious that makes this attack actually free of cost.
Nothing will "cost them more in transaction fees". A miner can create
thousands of transactions paying to himself, and not broadcast them to
the network, but hold them and include them in the blocks he mines. The
fees are collected by him because transactions are included in a block
that he mined and the left amount is in another wallet of the same
person. Repeat this continuously to fill blocks."

This is easily detectable as long as the network isn't heavily
partitioned(which is an assumption we make today in order for transaction
propagation to work reliably as well as for xThin and CompactBlocks to work
effectively to reduce block transmission time).  Other miners would have an
incentive to intentionally orphan blocks that contained a large number of
transactions that their nodes were unaware of.

I don't think this sort of attack would last long.  Even later when
subsidies are drastically reduced, you would still lose out on significant
genuine fee revenue if your orphan rate increased even 10%(one out of ten
of your poison blocks intentionally orphaned by another miner).

On Dec 11, 2016 11:12 AM, "s7r via bitcoin-dev" <
bitcoin-dev at lists.linuxfoundation.org> wrote:

 t. khan wrote:
> Miners 'gaming' the Block75 system -
> There is no financial incentive for miners to attempt to game the
> Block75 system. Even if it were attempted and assuming the goal was to
> create bigger blocks, the maximum possible increase would be 25% over
> the previous block size. And, that size would only last for two weeks
> before readjusting down. It would cost them more in transaction fees to
> stuff the network than they could ever make up. To game the system,
> they'd have to game it forever with no possibility of profit.
>

This is an incentive, if few miners agree to create a large conglomerate
that will ultimately control the network.

You miss something obvious that makes this attack actually free of cost.
Nothing will "cost them more in transaction fees". A miner can create
thousands of transactions paying to himself, and not broadcast them to
the network, but hold them and include them in the blocks he mines. The
fees are collected by him because transactions are included in a block
that he mined and the left amount is in another wallet of the same
person. Repeat this continuously to fill blocks.


> Blocks would get too big -
> Eventually, blocks would get too big, but only if bandwidth stopped
> increasing and the cost of disk space stopped decreasing. Otherwise, the
> incremental adjustments made by Block75 (especially in combination with
> SegWit) wouldn't break anyone's connection or result in significantly
> more orphaned blocks.
>

Topology and bandwidth speed / hash rate of the network cannot be
controlled - if we make assumptions about these it might have terrible
consequences.

Even if we take in consideration that bandwidth will only grow and disk
space will only cost less (which is not something we can safely assume,
by the way) the hard limit max. block size cannot grow to unlimited
value (even if the growth happens over time). There is also a validation
cost in time for each block, for the health of the network any node
should be able to download _and_ validate a block, before next block
gets mined.

You said in another post that a permanent solution is preferred, rather
than kicking the can down the road. I fully agree, as well as many
others reading this list, but the permanent solution doesn't necessarily
have to be increasing the max block size dynamically.

If you think about it the other way around, dynamically growing the max
block size is also kicking the can down the road ... just without having
to touch it and get dust on the boot ;)


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