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 encoding/decoding.

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@mattwhitlock.name> wrote:

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:

http://bitcoincore.org/~gavin/sizes_last1000.html

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:

http://bitcoincore.org/~gavin/sizes_357511.html

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 min-size.

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

------------------------------------------------------------------------------

_______________________________________________
Bitcoin-development mailing list
Bitcoin-development@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/bitcoin-development