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Author Topic: What (if any) mechanism is there to protect against a massive hash rate drop?  (Read 1733 times)
kaerf
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April 22, 2013, 10:25:51 PM
 #1

If you're unaware, Terracoin had a problem a couple weeks ago with ASICs shooting up their diff then leaving the network (thus leaving the network at a very high difficulty relative to remaining hashing power). I think NMC had the same problem and it was resolved with merged mining, but that effectively killed NMC(??).

I wanted to ask what mechanisms in BTC there are (or can be) to prevent a massive hash rate drop off. Right now, we have some massive amounts of hashing power being generated by Avalon and ASICMINER. If there was a bug in their firmware  (e.g. the recent large coinbase bug that dropped all avalons on eligius to 0 MHS) that caused all Avalons or AMs to drop off the network temporarily or permanently, it could take days for a new block to be found. This would wreak havoc on the entire bitcoin ecosystem by bringing transactions to a standstill.

Of course, as more manufacturers produce ASICs this problem is reduced, but let's say BFL delivers and their customers end up being 30% of the network...we'd still have the same problem. (We're all very confident in BFL's ability to produce a quality product  Grin)

Is this a legit concern? Shoot me down. I'll feel better.
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April 22, 2013, 10:55:30 PM
 #2

By the time there is a massive increase in difficulty, there will most likely be different ASIC manufacturers, so one mass failure would affect a smaller portion of the hashing power. This portion could still be on the order of 80%, though.



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qxzn
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April 23, 2013, 01:31:13 AM
 #3

If you're unaware, Terracoin had a problem a couple weeks ago with ASICs shooting up their diff then leaving the network (thus leaving the network at a very high difficulty relative to remaining hashing power). I think NMC had the same problem and it was resolved with merged mining, but that effectively killed NMC(??).

I wanted to ask what mechanisms in BTC there are (or can be) to prevent a massive hash rate drop off. Right now, we have some massive amounts of hashing power being generated by Avalon and ASICMINER. If there was a bug in their firmware  (e.g. the recent large coinbase bug that dropped all avalons on eligius to 0 MHS) that caused all Avalons or AMs to drop off the network temporarily or permanently, it could take days for a new block to be found. This would wreak havoc on the entire bitcoin ecosystem by bringing transactions to a standstill.

Of course, as more manufacturers produce ASICs this problem is reduced, but let's say BFL delivers and their customers end up being 30% of the network...we'd still have the same problem. (We're all very confident in BFL's ability to produce a quality product  Grin)

Is this a legit concern? Shoot me down. I'll feel better.

I believe there is no such mechanism, and it is a legit concern.

Remember people, bitcoin is an experiment Smiley. Don't risk your life savings!
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April 23, 2013, 03:49:18 AM
 #4

There is no such mechanism.  Furthermore, any possible mechanism would be worse than the (alleged) problem it hopes to solve.

Feel free to search out any of the many, many threads on this topic if you'd like more details.

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April 23, 2013, 05:05:16 AM
 #5

A simple decrease in the number of blocks between difficulty changes would go a long was to address this as an issue.  In BTC it is 2016 which is 2 weeks at target rate.  This is far longer then necessary to smooth the variance in finding times and makes difficulty changes unnecessarily sluggish and vulnerable to rate crashes.

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April 23, 2013, 05:08:59 AM
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Greed. People were still mining when it dropped from $33 to $2.
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April 23, 2013, 05:12:06 AM
 #7

A simple decrease in the number of blocks between difficulty changes would go a long was to address this as an issue.  In BTC it is 2016 which is 2 weeks at target rate.  This is far longer then necessary to smooth the variance in finding times and makes difficulty changes unnecessarily sluggish and vulnerable to rate crashes.

This was 1 part of how TRC fixed this problem for them... they've had asics coming in and out for days now, difficulty ramps up pretty quick and then when they bail difficulty comes down to normal miner level pretty quick.  The other part of their solution involved how they determine the next difficulty.  IMHO bitcoin would be better off rolling this algo into it and start reaping some reward from the numerous alts that were inspired by it.....

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April 23, 2013, 05:49:59 AM
 #8

A simple decrease in the number of blocks between difficulty changes would go a long was to address this as an issue.  In BTC it is 2016 which is 2 weeks at target rate.  This is far longer then necessary to smooth the variance in finding times and makes difficulty changes unnecessarily sluggish and vulnerable to rate crashes.

Don't forget that decreasing the number of blocks between difficulty changes would also make it easier to upset the difficulty repeatedly and on purpose. Simple fixes tend to have unintended consequences.

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April 23, 2013, 11:13:52 AM
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A simple decrease in the number of blocks between difficulty changes would go a long was to address this as an issue.  In BTC it is 2016 which is 2 weeks at target rate.  This is far longer then necessary to smooth the variance in finding times and makes difficulty changes unnecessarily sluggish and vulnerable to rate crashes.

Don't forget that decreasing the number of blocks between difficulty changes would also make it easier to upset the difficulty repeatedly and on purpose. Simple fixes tend to have unintended consequences.

And how is that easier than upsetting the difficulty repeatedly on purpose over a span of Months vs. a span of Days/Hours before having to come back and repeat the task again?....  This is effectively what is happening to Terracoin right now, and I bet if you ask most users, would you prefer the namecoin experience (where it was taking many months to have a difficulty drop, and block times that were numbering many hours) or the Terracoin experience (where, ya hashing could hop in and out repeating in a shorter time than before but at least difficulty fixes itself fairly rapidly when the disproportionate hash rate comes and goes)

What I'm trying to say is that upsetting difficulty is easy both ways, the "Terracoin" way though leaves the end user with a workable network, whereas the current bitcoin way leaves the end user with a horrible system to operate on for a long time....  I would think the latter is a lot more damaging to the adoption rate.

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April 23, 2013, 11:37:05 AM
 #10

A simple rule is build on the valid chain with highest POW.

A chain is valid if
- the head of the chain meets the standard difficulty
- the head of the chain was seen by the miner at least f(block difficulty) minutes ago

Re-targets would be every 2016 blocks, but would take total POW between the 2 endpoints as the target.

The function could be a decaying exponential.  For example, the target could drop by 50% every 10 minutes, after the first 20. 

f(block difficulty) = 20 + 10 * log2( (standard difficulty) / (block difficulty) )

Blocks which meet the standard difficulty are zero.

If the block has 1/64 of the difficulty, then it would be forwarded to peers, and then held in a queue for 20 minutes + 60 minutes.  In the normal situation, another block would arrive that meets the standard difficulty and thus that block would be discarded.

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April 23, 2013, 11:47:31 AM
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And how is that easier than upsetting the difficulty repeatedly on purpose over a span of Months vs. a span of Days/Hours before having to come back and repeat the task again?....  This is effectively what is happening to Terracoin right now, and I bet if you ask most users, would you prefer the namecoin experience (where it was taking many months to have a difficulty drop, and block times that were numbering many hours) or the Terracoin experience (where, ya hashing could hop in and out repeating in a shorter time than before but at least difficulty fixes itself fairly rapidly when the disproportionate hash rate comes and goes)

What I'm trying to say is that upsetting difficulty is easy both ways, the "Terracoin" way though leaves the end user with a workable network, whereas the current bitcoin way leaves the end user with a horrible system to operate on for a long time....  I would think the latter is a lot more damaging to the adoption rate.

Terracoin is weak and desperate and needs to defend against the top dogs somehow, even if it has drawbacks. The best defense of bitcoin is being  the ultimate top dog.
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April 23, 2013, 12:56:31 PM
 #12

A simple rule is build on the valid chain with highest POW.

A chain is valid if
- the head of the chain meets the standard difficulty
- the head of the chain was seen by the miner at least f(block difficulty) minutes ago

Re-targets would be every 2016 blocks, but would take total POW between the 2 endpoints as the target.

The function could be a decaying exponential.  For example, the target could drop by 50% every 10 minutes, after the first 20. 

f(block difficulty) = 20 + 10 * log2( (standard difficulty) / (block difficulty) )

Blocks which meet the standard difficulty are zero.

If the block has 1/64 of the difficulty, then it would be forwarded to peers, and then held in a queue for 20 minutes + 60 minutes.  In the normal situation, another block would arrive that meets the standard difficulty and thus that block would be discarded.

This is an insanely bad idea, just like it was the last 20 or so times it was suggested.

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April 23, 2013, 01:22:10 PM
 #13

This is an insanely bad idea, just like it was the last 20 or so times it was suggested.

Total proof of work still increases and the "real" main chain should still be the highest.  Is there a specific flaw in it?

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April 23, 2013, 01:24:53 PM
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And how is that easier than upsetting the difficulty repeatedly on purpose over a span of Months vs. a span of Days/Hours before having to come back and repeat the task again?....  This is effectively what is happening to Terracoin right now, and I bet if you ask most users, would you prefer the namecoin experience (where it was taking many months to have a difficulty drop, and block times that were numbering many hours) or the Terracoin experience (where, ya hashing could hop in and out repeating in a shorter time than before but at least difficulty fixes itself fairly rapidly when the disproportionate hash rate comes and goes)

What I'm trying to say is that upsetting difficulty is easy both ways, the "Terracoin" way though leaves the end user with a workable network, whereas the current bitcoin way leaves the end user with a horrible system to operate on for a long time....  I would think the latter is a lot more damaging to the adoption rate.

Terracoin is weak and desperate and needs to defend against the top dogs somehow, even if it has drawbacks. The best defense of bitcoin is being  the ultimate top dog.

YET TRC survived and thrives....

worth taking a good look at the retarget code used

legit concern here

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April 23, 2013, 01:49:31 PM
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And how is that easier than upsetting the difficulty repeatedly on purpose over a span of Months vs. a span of Days/Hours before having to come back and repeat the task again?....  This is effectively what is happening to Terracoin right now, and I bet if you ask most users, would you prefer the namecoin experience (where it was taking many months to have a difficulty drop, and block times that were numbering many hours) or the Terracoin experience (where, ya hashing could hop in and out repeating in a shorter time than before but at least difficulty fixes itself fairly rapidly when the disproportionate hash rate comes and goes)

What I'm trying to say is that upsetting difficulty is easy both ways, the "Terracoin" way though leaves the end user with a workable network, whereas the current bitcoin way leaves the end user with a horrible system to operate on for a long time....  I would think the latter is a lot more damaging to the adoption rate.

Terracoin is weak and desperate and needs to defend against the top dogs somehow, even if it has drawbacks. The best defense of bitcoin is being  the ultimate top dog.

YET TRC survived and thrives....

worth taking a good look at the retarget code used

legit concern here

No it's not legit. To make it painfully clear: If the ASIC miners coming from bitcoin pumping the difficulty in Terracoin would have instead 51%'ed it, Terracoin would be dead. They probably didn't because Terracoin has not enough value (yet).

On the other hand, all Terra- and other altcoin miners together are not strong enough to do the same to bitcoin, not even close (or a 51% attack would be close), and that's the best defense there is.

But almost everything is better than taking experimental code requiring a hard fork.
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April 23, 2013, 02:03:26 PM
 #16

This is an insanely bad idea, just like it was the last 20 or so times it was suggested.

Total proof of work still increases and the "real" main chain should still be the highest.  Is there a specific flaw in it?

Any asymmetric function for difficulty adjustment can be gamed.  If anyone with a bit of hashing power cared enough about the scamcoin de jour, they could use it to overwrite the current chain.  If they don't have quite enough hashing power for that, they can manipulate the future chain to exclude whatever they want excluded.  If you compensate for either of those attacks by adding state to the block acceptance mechanism, they can fork your network into oblivion.

Note that most of these things have already been done.  Feel free to search for the various threads where they have been discussed.  Sooner or later, everyone comes to the conclusion that Satoshi got it exactly right*, but there is usually a bunch of whining and crying along the way.

*  Well, almost exactly.  There is an off-by-1 bug in the difficulty calculation that throws things of by about a twentieth of a percent, but the bug is symmetric, so it doesn't change the security of the system.

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April 23, 2013, 03:52:59 PM
 #17

Any asymmetric function for difficulty adjustment can be gamed.

Your concerns are inherent if someone has > 51% of the hash power.

Rather than show 6 blocks required for security require 6 blocks worth of POW is required.  Clients would show that the chain has lost hashing power without completely locking up.

So, the conclusion is that if you can't generate enough hashing power, you can't assume the chain is safe.  However, the standard difficulty rule does help there anyway.

Quote
*  Well, almost exactly.  There is an off-by-1 bug in the difficulty calculation that throws things of by about a twentieth of a percent, but the bug is symmetric, so it doesn't change the security of the system.

Yeah, they should soft fork that one out.  Require that the two blocks have the same timestamp (or the one after must have timestamp + 10 minutes of the old one).

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April 23, 2013, 03:57:01 PM
 #18

yes but firmware crash on all ASICs them what!!!! or hardware flaw, they let the smoke out...dead.


then what.




And how is that easier than upsetting the difficulty repeatedly on purpose over a span of Months vs. a span of Days/Hours before having to come back and repeat the task again?....  This is effectively what is happening to Terracoin right now, and I bet if you ask most users, would you prefer the namecoin experience (where it was taking many months to have a difficulty drop, and block times that were numbering many hours) or the Terracoin experience (where, ya hashing could hop in and out repeating in a shorter time than before but at least difficulty fixes itself fairly rapidly when the disproportionate hash rate comes and goes)

What I'm trying to say is that upsetting difficulty is easy both ways, the "Terracoin" way though leaves the end user with a workable network, whereas the current bitcoin way leaves the end user with a horrible system to operate on for a long time....  I would think the latter is a lot more damaging to the adoption rate.

Terracoin is weak and desperate and needs to defend against the top dogs somehow, even if it has drawbacks. The best defense of bitcoin is being  the ultimate top dog.



YET TRC survived and thrives....

worth taking a good look at the retarget code used

legit concern here

No it's not legit. To make it painfully clear: If the ASIC miners coming from bitcoin pumping the difficulty in Terracoin would have instead 51%'ed it, Terracoin would be dead. They probably didn't because Terracoin has not enough value (yet).

On the other hand, all Terra- and other altcoin miners together are not strong enough to do the same to bitcoin, not even close (or a 51% attack would be close), and that's the best defense there is.

But almost everything is better than taking experimental code requiring a hard fork.


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April 23, 2013, 04:05:53 PM
 #19

Any asymmetric function for difficulty adjustment can be gamed.

Your concerns are inherent if someone has > 51% of the hash power.

Rather than show 6 blocks required for security require 6 blocks worth of POW is required.  Clients would show that the chain has lost hashing power without completely locking up.

So, the conclusion is that if you can't generate enough hashing power, you can't assume the chain is safe.  However, the standard difficulty rule does help there anyway.

Quote
*  Well, almost exactly.  There is an off-by-1 bug in the difficulty calculation that throws things of by about a twentieth of a percent, but the bug is symmetric, so it doesn't change the security of the system.

Yeah, they should soft fork that one out.  Require that the two blocks have the same timestamp (or the one after must have timestamp + 10 minutes of the old one).

You are missing my point.  A poorly chosen difficulty mechanism is a force multiplier.  It lets someone with considerably less than 50% of the network power fuck with you.

P.S.  The bug cannot be fixed with a soft fork.  It is an absolutely hard fork to change the number of blocks used in the calculation.

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April 23, 2013, 04:17:37 PM
 #20

P.S.  The bug cannot be fixed with a soft fork.  It is an absolutely hard fork to change the number of blocks used in the calculation.

A soft fork is banning something that was previously allowed.  However, since that exploit assumes that the attackers have > 51% of the hashing power, the distinction soft and hand fork changes.

You could add a rule that the last block in a difficulty period and the first block in the next difficulty period must have the a similar timestamp and reject the first block in the new re-targeting period if it doesn't meet that rule.  You could scan the blockchain and see what the largest difference was and use that (or say 30 minutes if lower).

All new clients could have that rule.  Forking the chain means that only the old clients are kicked and only then for as long as that chain maintains hashing power.  If the "economic majority" decides that the non-forked chain is the real one, then the only benefit to the attackers is that they can fool users of the old client.

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