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What is the connection between Terminal Total Difficulty (TTD), Difficulty, and Total Difficulty?

Why can Total Difficulty increase if Difficulty decreases?


I know that:

difficulty: QUANTITY - integer of the difficulty for this block.

totalDifficulty: QUANTITY - integer of the total difficulty of the chain until this block.

And I understand what Terminal Total Difficulty (TTD) is in this link

I know that the Hash Rate is proportional to difficulty of the blocks, so if Hash Rate increase then difficulty of the blocks also increase. And because the definition of totalDifficulty is

integer of the total difficulty of the chain until this block.

So it makes me understand that the more difficulty, the more total difficulty. And therefore, when total difficulty increases, it will reach the TTD, and that time The Merge will be successful.

But when I do statistic on Ropsten testnet. I saw the difficulty was decreasing, while the totalDifficulty was increasingenter image description here

Can anybody help me to understand why the Difficulty (and hash rate) is decreasing, while Total Difficulty is increasing to reach the TTD? I think the Difficulty (and hash rate) need to be increasing for the increase of Total Difficulty because of its definition: Total Difficulty is integer of the total difficulty of the chain until this block.

1 Answer 1

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Each block has a difficulty.

totalDifficulty is the sum of all the difficulty of the block and all previous blocks. Another way to describe totalDifficulty is "cumulative difficulty", but cumulative is a longer and not necessarily simpler word.

Even if hashrate decreases, difficulty is still larger than 0. So totalDifficulty will keep increasing and eventually exceed TTD.

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  • Hi, thanks for the comment. As you said totalDifficulty is the sum of all the difficulty of the block and all previous blocks. So it means if totalDifficulty want to increase, so the difficulty need to be increased. However, in the Ropsten testnet example, the totalDifficulty is increasing, while the difficulty is decreasing. I don't get it when "totalDifficulty is the sum of all the difficulty of the block and all previous block". That is my concern Aug 24 at 4:11
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    Difficulty does not need to increase: if difficulty drops to 1, totalDifficulty will still increase by 1 each time...
    – eth
    Aug 24 at 4:14
  • ohh, so do you know what are the factors can impact the totalDifficulty ? and do you know how to calculate the totalDifficulty ( the calculation ignore the difficulty factor)? Aug 24 at 4:17
  • Let's just have three blocks in order, with difficulty 10, 11, 12. The totalDifficulty at each block is 10, 21, 33. Do you see that? If the next block has difficulty 1, then totalDifficulty will be 34.
    – eth
    Aug 24 at 4:26
  • ohhh, I am very appreciate your explanation. It helps me a lots Aug 24 at 4:29

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