<p>The problem arises from the uneven propagation of information in the network. While the propagation of small blocks is fast, i.e., every miner gets to know a new block approximately at the same time, this is no longer true for large blocks.</p>

<p>Large blocks take a lot of time to be forwarded to every node in the network, since this includes miners, these miners are unaware that there is a new block and they'll continue mining on top of the old parent, i.e., they will work on a competitor of the newly found block. This means that they are actively wasting their computational resources on an obsolete solution if the propagating block gets accepted into the blockchain.</p>

<p>Big miners are likely to have a better connectivity with the Bitcoin network, resulting in them losing less of their computational time due. This in turn puts small miners at a disadvantage since they are now less competitive.</p>

<p>The extreme case would be one strong miner, with close to a majority of the computational resources making such big blocks that everybody else learns about its blocks very late, while it already has spent considerable time finding the followup block.</p>

<p>The uneven propagation is due to both network latencies and verification times of the block that is being propagated, so this is a compound problem from both slow verifiers and high latency/low bandwidth.</p>

<p>For more detail see <a href="http://www.tik.ee.ethz.ch/file/49318d3f56c1d525aabf7fda78b23fc0/P2P2013_041.pdf">Information Propagation in the Bitcoin Network</a> (disclosure: I am the author of that paper) and <a href="http://arxiv.org/abs/1311.0243">Majority is not Enough: Bitcoin Mining is Vulnerable</a>.</p>

CC BY-SA 3.0
2016-03-06T15:55:14.630Z
1 MB per 10 minutes is about 1.7 KB per sec, 2 MB blocks would require 3.4 KB per sec. Anyone who doesn't have several orders of magnitudes more bandwidth than that, should not be part of any online network in the first place (but instead, get a decent connection). Old fashioned mobile 3G or even the lowest grade budget consumer internet connections offer **thousand** times faster speeds than that.
RocketNuts
CC BY-SA 3.0
2016-03-06T15:57:58.457Z
Oh and I know it's not like that in China yet, but what's preventing them for running their relaying server outside China? The actual mining work (i.e. their factories full of mining hardware) does not need the full block data at all, just the stratum which is independent on block size. If all it takes is a $5/month VPS to mitigate any bandwidth issues for a serious mining operation, you can't seriously claim that it hurts decentralization.
RocketNuts
CC BY-SA 3.0
2016-03-06T16:01:04.783Z
Also, I agree 8 MB or 20 MB blocks might be a bit steep for some people (although even then the effects should be minimal and easily circumvented) but 2 MB instead of 1 MB would be absolutely no problem whatsoever.
RocketNuts
CC BY-SA 3.0
2016-03-06T16:28:04.117Z
Yes, each individual node will trivially have the required bandwidth for 2MB, but since these blocks are relayed over multiple hops the simple bandwidth of a single node is not the bottleneck. Rather each nodes adds to the propagation delay and the longer the distance the longer it takes. For example take the propagation data from last week (http://bitcoinstats.com/network/propagation/2016/03/03), after a minute there still were 10% that had not heard about a block on average.
cdecker
CC BY-SA 3.0
2016-09-12T10:27:57.797Z
@cdecker If propagation delay is such a problem, why don't bitcoin developers, miners, and other stakeholders consider decreasing the block size?
Derek Mahar
CC BY-SA 3.0
2016-09-13T05:22:21.193Z
Well there is a sweet spot between fork-rate and usability. Decreasing the block size further reduces the transaction rate that the network can handle, without increasing consistency much. On the other handle if we increase the block size we get a super-linear decrease in consistency. There certainly is a bit of wiggle room, but we simply cannot use block size to scale beyond a certain point.
cdecker
CC BY-SA 4.0
2023-04-24T00:20:54.803Z
@cdecker Cited paper is dated 2013 - bandwidth has increased circa 30% a year.
13th IEEE International Conference on Peer-to-Peer Computing, IEEE P2P 2013, Trento, Italy, September 9-11, 2013
OneArb
CC BY-SA 4.0
2023-04-24T00:31:26.950Z
@cdecker as per your own paper conclusion: "Our measurements show that a single node implementing these changes reduces the number of blockchain forks in the network
by over 50%".
Would the prominent change be: "[...] connect to every node in the network creating a star sub-graph [...] speeding up the propagation [...]"?
OneArb
CC BY-SA 4.0
2023-04-25T10:27:50.543Z
That is correct, however you need to keep in mind that an average 30% yearly increase in bandwidth on individual connections does not translate into the equivalent network wide bandwidth increase. Also noteworthy is that at some point the network latency (speed of light) starts dominating the overall network propagation.
cdecker
CC BY-SA 3.0
2016-03-06T14:35:49.233Z
2016-03-06T14:35:49.233Z
cdecker
<p>Why does Bitcoin Core contributor Peter Todd think that <a href="https://bitcoinmagazine.com/articles/four-key-disagreements-between-bitcoin-classic-and-bitcoin-core-four-key-disagreements-between-bitcoin-classic-and-bitcoin-core-four-key-disagreements-between-bitcoin-classic-and-bitcoin-core-1457106744">increasing the Bitcoin block size would lead to a more centralised system</a>?</p>

<p>“The system doesn’t scale, and you just have to accept that and do something smarter. Every time you do a [block size limit] increase, you’re making the system more centralized.”</p>

<p>How does Bitcoin block size affect centralisation?</p>

<bitcoin-core><bitcoin-core-development><decentralization><block-size-increase><bitcoin-classic>
Why would increasing the Bitcoin block size lead to a more centralised system?
Derek Mahar
# Why would increasing the Bitcoin block size lead to a more centralised system?

(Asked by: username_0 on 2016-03-05)

Why does Bitcoin Core contributor Peter Todd think that [increasing the Bitcoin block size would lead to a more centralised system](https://bitcoinmagazine.com/articles/four-key-disagreements-between-bitcoin-classic-and-bitcoin-core-four-key-disagreements-between-bitcoin-classic-and-bitcoin-core-four-key-disagreements-between-bitcoin-classic-and-bitcoin-core-1457106744)?

“The system doesn’t scale, and you just have to accept that and do something smarter. Every time you do a [block size limit] increase, you’re making the system more centralized.”

How does Bitcoin block size affect centralisation?

**Tags:** bitcoin-core, bitcoin-core-development, decentralization, block-size-increase, bitcoin-classic


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## 1 answers

### Answer by username_1 on 2016-03-06. Score: 8

The problem arises from the uneven propagation of information in the network. While the propagation of small blocks is fast, i.e., every miner gets to know a new block approximately at the same time, this is no longer true for large blocks.

Large blocks take a lot of time to be forwarded to every node in the network, since this includes miners, these miners are unaware that there is a new block and they'll continue mining on top of the old parent, i.e., they will work on a competitor of the newly found block. This means that they are actively wasting their computational resources on an obsolete solution if the propagating block gets accepted into the blockchain.

Big miners are likely to have a better connectivity with the Bitcoin network, resulting in them losing less of their computational time due. This in turn puts small miners at a disadvantage since they are now less competitive.

The extreme case would be one strong miner, with close to a majority of the computational resources making such big blocks that everybody else learns about its blocks very late, while it already has spent considerable time finding the followup block.

The uneven propagation is due to both network latencies and verification times of the block that is being propagated, so this is a compound problem from both slow verifiers and high latency/low bandwidth.

For more detail see [Information Propagation in the Bitcoin Network](http://www.tik.ee.ethz.ch/file/49318d3f56c1d525aabf7fda78b23fc0/P2P2013_041.pdf) (disclosure: I am the author of that paper) and [Majority is not Enough: Bitcoin Mining is Vulnerable](http://arxiv.org/abs/1311.0243).

> **Comments:**
> **username_2**: 1 MB per 10 minutes is about 1.7 KB per sec, 2 MB blocks would require 3.4 KB per sec. Anyone who doesn't have several orders of magnitudes more bandwidth than that, should not be part of any online network in the first place (but instead, get a decent connection). Old fashioned mobile 3G or even the lowest grade budget consumer internet connections offer **thousand** times faster speeds than that.  
> **username_2**: Oh and I know it's not like that in China yet, but what's preventing them for running their relaying server outside China? The actual mining work (i.e. their factories full of mining hardware) does not need the full block data at all, just the stratum which is independent on block size. If all it takes is a $5/month VPS to mitigate any bandwidth issues for a serious mining operation, you can't seriously claim that it hurts decentralization.  
> **username_2**: Also, I agree 8 MB or 20 MB blocks might be a bit steep for some people (although even then the effects should be minimal and easily circumvented) but 2 MB instead of 1 MB would be absolutely no problem whatsoever.  
> **username_1**: Yes, each individual node will trivially have the required bandwidth for 2MB, but since these blocks are relayed over multiple hops the simple bandwidth of a single node is not the bottleneck. Rather each nodes adds to the propagation delay and the longer the distance the longer it takes. For example take the propagation data from last week (http://bitcoinstats.com/network/propagation/2016/03/03), after a minute there still were 10% that had not heard about a block on average.  
> **username_0**: @username_1 If propagation delay is such a problem, why don't bitcoin developers, miners, and other stakeholders consider decreasing the block size?  
> **username_1**: Well there is a sweet spot between fork-rate and usability. Decreasing the block size further reduces the transaction rate that the network can handle, without increasing consistency much. On the other handle if we increase the block size we get a super-linear decrease in consistency. There certainly is a bit of wiggle room, but we simply cannot use block size to scale beyond a certain point.  
> **username_3**: @username_1 Cited paper is dated 2013 - bandwidth has increased circa 30% a year. 13th IEEE International Conference on Peer-to-Peer Computing, IEEE P2P 2013, Trento, Italy, September 9-11, 2013  
> **username_3**: @username_1 as per your own paper conclusion: "Our measurements show that a single node implementing these changes reduces the number of blockchain forks in the network by over 50%". Would the prominent change be: "[...] connect to every node in the network creating a star sub-graph [...] speeding up the propagation [...]"?  
> **username_1**: That is correct, however you need to keep in mind that an average 30% yearly increase in bandwidth on individual connections does not translate into the equivalent network wide bandwidth increase. Also noteworthy is that at some point the network latency (speed of light) starts dominating the overall network propagation.  

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