The bitcoin network: block propagation

Earlier we discussed the progress of the bitcoin network, and in particular the spread of blocks. But what does block propagation, also known as block propagation, actually mean?

To make clear the importance of block propagation, we repeat once again the most important points of a well-functioning bitcoin network:

One of the points of interest is the forwarding of blocks over the network. It is important that this happens quickly and that a newly found block is made known to all participants in the network as quickly as possible to prevent the network from operating incorrectly or inconsistently. If this seemingly simple part of the network does not function properly, transactions may receive a false confirmation that will later be reversed. This would compromise the reliability of a confirmation; something that must of course be avoided. A confirmation must be final, across the entire network.

In addition, it would be disadvantageous for some miners if blocks are sent slowly over the network. For example, it could be that the miners who receive a block quickly have a consistent head start in finding the next block, even though the computational power they have is equal to the miners who receive the block later. This makes the ‘race’ to find the next block unfair. Consistency across the network and efficiency in forwarding information is therefore important.

block propagation

The goal of block propagation is therefore to spread blocks over the network as quickly as possible, towards all nodes. In addition, it is also important that the nodes become aware of the newly found blocks as soon as possible.

 

In a fictional future scenario, 75 percent of the hashrate is on Mars. The remaining hashrate – 25 percent – has been left on Earth. Due to the distance between Earth and Mars, sending block data takes an hour. The block propagation is shown in the illustrations below.

 

It indicates per time slot (T) of 40 minutes where new blocks are found, starting with the current green block. The blocks are numbered with the block height, so that the order of blocks can be determined.

 

Because a block should be found every ten minutes on average, the blocks can be divided three to one between Mars and Earth. In T(2), 40 minutes after the last block, Mars has mined three blocks, where Earth has one. Now it is important to forward these found blocks to the rest of the nodes. These are on both Earth and Mars, so propagation takes over an hour between the two globes.

 

During T(3) blocks are of course found again. Three more on Mars and one on Earth, which will also be distributed. The blocks that were on their way have also arrived on the other side.

 

In T(4) the problem becomes clear. Because the block height of the block 1 arriving from Earth has already been found by Mars, and it has already mined blocks 2 and 3, the terrestrial block will be rejected. In Mars’ opinion, this block was too late, and therefore not the valid winner. Bitcoin always maintains the longest chain of blocks based on block height.

 

Blocks seven, eight and nine have now also been found and sent to Earth. Earth itself is now at block three. Due to the delay in propagation, blocks four, five and six are now also arriving on Earth. Because the Mars blocks form a longer chain, and Bitcoin therefore assumes the longest chain, it invalidates the chain that Earth itself was working on.

 

It is therefore important that the distribution proceeds as quickly as possible. In addition to that requirement for a well-functioning network, it is also important for the fair remuneration of miners. In the example above, the chance that Earth ever finds a block in time is very small, and therefore also receiving the block reward. As a result, despite a quarter of the hashrate, soil does not get paid for the work done.

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Selfish mining

As mentioned earlier, the miners who receive a block quickly may have a consistent head start in finding the next block. Miners can also deliberately take advantage of degraded block propagation. This is called selfish mining. In this case, a miner keeps a block that has already been found for the others, so that it can secretly work on the next block. As a result, it creates an advantage for itself.

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