[Bitcoin-development] Proposed alternatives to the 20MB step function

Pieter Wuille pieter.wuille at gmail.com
Thu May 28 17:34:32 UTC 2015

> until we have size-independent new block propagation

I don't really believe that is possible. I'll argue why below. To be clear,
this is not an argument against increasing the block size, only against
using the assumption of size-independent propagation.

There are several significant improvements likely possible to various
aspects of block propagation, but I don't believe you can make any part
completely size-independent. Perhaps the remaining aspects result in terms
in the total time that vanish compared to the link latencies for 1 MB
blocks, but there will be some block sizes for which this is no longer the
case, and we need to know where that is the case.

* You can't assume that every transaction is pre-relayed and pre-validated.
This can happen due to non-uniform relay policies (different codebases, and
future things like size-limited mempools), double spend attempts, and
transactions generated before a block had time to propagate. You've
previously argued for a policy of not including too recent transactions,
but that requires a bound on network diameter, and if these late
transactions are profitable, it has exactly the same problem as making
larger blocks non-proportionally more economic for larger pools groups if
propagation time is size dependent).
  * This results in extra bandwidth usage for efficient relay protocols,
and if discrepancy estimation mispredicts the size of IBLT or error
correction data needed, extra roundtrips.
  * Signature validation for unrelayed transactions will be needed at block
relay time.
  * Database lookups for the inputs of unrelayed transactions cannot be
cached in advance.

* Block validation with 100% known and pre-validated transactions is not
constant time, due to updates that need to be made to the UTXO set (and
future ideas like UTXO commitments would make this effect an order of
magnitude worse).

* More efficient relay protocols also have higher CPU cost for

Again, none of this is a reason why the block size can't increase. If
availability of hardware with higher bandwidth, faster disk/ram access
times, and faster CPUs increases, we should be able to have larger blocks
with the same propagation profile as smaller blocks with earlier technology.

But we should know how technology scales with larger blocks, and I don't
believe we do, apart from microbenchmarks in laboratory conditions.

 On Fri, May 8, 2015 at 3:20 AM, Matt Whitlock <bip at mattwhitlock.name>

> Between all the flames on this list, several ideas were raised that did
> not get much attention. I hereby resubmit these ideas for consideration and
> discussion.
> - Perhaps the hard block size limit should be a function of the actual
> block sizes over some trailing sampling period. For example, take the
> median block size among the most recent 2016 blocks and multiply it by 1.5.
> This allows Bitcoin to scale up gradually and organically, rather than
> having human beings guessing at what is an appropriate limit.

A lot of people like this idea, or something like it. It is nice and
simple, which is really important for consensus-critical code.

With this rule in place, I believe there would be more "fee pressure"
(miners would be creating smaller blocks) today. I created a couple of
histograms of block sizes to infer what policy miners are ACTUALLY
following today with respect to block size:

Last 1,000 blocks:

Notice a big spike at 750K -- the default size for Bitcoin Core.
This graph might be misleading, because transaction volume or fees might
not be high enough over the last few days to fill blocks to whatever limit
miners are willing to mine.

So I graphed a time when (according to statoshi.info) there WERE a lot of
transactions waiting to be confirmed:

That might also be misleading, because it is possible there were a lot of
transactions waiting to be confirmed because miners who choose to create
small blocks got lucky and found more blocks than normal.  In fact, it
looks like that is what happened: more smaller-than-normal blocks were
found, and the memory pool backed up.

So: what if we had a dynamic maximum size limit based on recent history?

The average block size is about 400K, so a 1.5x rule would make the max
block size 600K; miners would definitely be squeezing out transactions /
putting pressure to increase transaction fees. Even a 2x rule (implying
800K max blocks) would, today, be squeezing out transactions / putting
pressure to increase fees.

Using a median size instead of an average means the size can increase or
decrease more quickly. For example, imagine the rule is "median of last
2016 blocks" and 49% of miners are producing 0-size blocks and 51% are
producing max-size blocks. The median is max-size, so the 51% have total
control over making blocks bigger.  Swap the roles, and the median is

Because of that, I think using an average is better-- it means the max size
will change (up or down) more slowly.

I also think 2016 blocks is too long, because transaction volumes change
quicker than that. An average over 144 blocks (last 24 hours) would be
better able to handle increased transaction volume around major holidays,
and would also be able to react more quickly if an economically irrational
attacker attempted to flood the network with fee-paying transactions.

So my straw-man proposal would be:  max size 2x average size over last 144
blocks, calculated at every block.

There are a couple of other changes I'd pair with that consensus change:

+ Make the default mining policy for Bitcoin Core neutral-- have its target
block size be the average size, so miners that don't care will "go along
with the people who do care."

+ Use something like Greg's formula for size instead of bytes-on-the-wire,
to discourage bloating the UTXO set.


When I've proposed (privately, to the other core committers) some dynamic
algorithm the objection has been "but that gives miners complete control
over the max block size."

I think that worry is unjustified right now-- certainly, until we have
size-independent new block propagation there is an incentive for miners to
keep their blocks small, and we see miners creating small blocks even when
there are fee-paying transactions waiting to be confirmed.

I don't even think it will be a problem if/when we do have size-independent
new block propagation, because I think the combination of the random timing
of block-finding plus a dynamic limit as described above will create a
healthy system.

If I'm wrong, then it seems to me the miners will have a very strong
incentive to, collectively, impose whatever rules are necessary (maybe a
soft-fork to put a hard cap on block size) to make the system healthy again.

Gavin Andresen


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