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Author Topic: Warning : Check your system clock (help me)  (Read 9796 times)
joe
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October 14, 2010, 07:55:12 AM
 #21

If someone rejects a block that most of the network accepts, won't the rejecting nodes eventually give up on the rejecting branch and move to the longer branch that accepted it?

If you reject a block, then all blocks after it are also invalid from your point of view, no matter how many other people accept them. If this was not the case, then an attacker could create bitcoins out of thin air by getting more than 50% of the network's CPU power. This effect was demonstrated when the overflow bug was fixed: even though 0.3.10 nodes were in the minority, they rejected all blocks produced by old clients (which contained a fraudulent and impossible transaction for 184 billion bitcoins).

This is not quite the case for timestamp issues, but there is a similar effect. If your clock is off, you won't accept any new blocks, as they'll all be too far in the future. You'll eventually get the blocks when they are no longer too far in the future, but you'll still be unable to generate because your view of the "latest valid block" is wrong.

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The same problem would happen if someone double spends a coin, and nodes disagree on which one came in first. The majority will make a longer chain and the minority nodes will have no choice but to jump over.

You don't reject blocks for transaction timing/ordering issues.

There must be a way to do away with real world time. It is an anonymity risk to give up your computer clock's time, and it shouldn't be necessary to get a transaction or block confirmed on the bitcoin network. If nodes want to compute some time deltas against their own clocks, that's fine, but again, no one should have to fess up to their computer's absolute clock time in order for the system to work right.
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agaumoney
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October 14, 2010, 11:17:13 PM
 #22

There must be a way to do away with real world time. It is an anonymity risk to give up your computer clock's time,

In what way is that a risk?

Is your perception of the risk related to how many bits of precision the time is "given up" ?  For example, any computer synced with a network time server should be accurate to the second.  What about if the time stamp is only given up to the minute?  Or 5 minutes?

It sounds like you believe there is some uniquely identifiable data contained in the time.  If so, then less precision (seconds or minutes instead of nanoseconds) removes that risk.

In addition, since nobody else on the network knows how long it takes you to "give up your computer clock's time" then they have no way to correlate the value they receive to the value in your computer clock with any degree of precision.

And finally, your computer clock's time is not treated as privileged data by anything else.  Why should bitcoin treat it specially?
ByteCoin
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October 14, 2010, 11:55:21 PM
 #23

Just remind me why Bitcoin cares about the time? Is it just to change the difficulty to target a certain average block rate or something more fundamental?

It seems to be quite a bit of hassle for not very much benefit.

Can't the client manage just by measuring time intervals with the actual absolute time value being unused? I imagine that the effects of any systematic drift would be unimportant compared to currently accepted hash-rate related variability.

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March 12, 2011, 12:12:47 AM
 #24

Hi, I am getting this message every time Bitcoin opens: "Please check that your computer's date and time are correct. If your clock is wrong Bitcoin will not work properly." My clock is correct, but I have just installed Win 7 and I am using the Bitcoin data files from the previous Vista install. I tried deleting wallet.dat but it still complains. Any suggestions?

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March 12, 2011, 12:21:43 AM
 #25

Hi, I am getting this message every time Bitcoin opens: "Please check that your computer's date and time are correct. If your clock is wrong Bitcoin will not work properly." My clock is correct, but I have just installed Win 7 and I am using the Bitcoin data files from the previous Vista install. I tried deleting wallet.dat but it still complains. Any suggestions?

Fix your date, time AND time zone.

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MessyCoin
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March 12, 2011, 12:34:26 AM
 #26

Fix your date, time AND time zone.

You're absolutely correct about timezone, thanks. (I guess the brain goes numb installing windows - again)

Jim Hyslop
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March 12, 2011, 02:12:55 AM
 #27

Just remind me why Bitcoin cares about the time? Is it just to change the difficulty to target a certain average block rate or something more fundamental?

It seems to be quite a bit of hassle for not very much benefit.

Can't the client manage just by measuring time intervals with the actual absolute time value being unused? I imagine that the effects of any systematic drift would be unimportant compared to currently accepted hash-rate related variability.

ByteCoin

I'm late to the party, I know, but I'll answer this anyway.

It's quite important for controlling the rate of coin generation. Every 2016 blocks, the computer calculates the difference in time between the most recent block, and the block 2015 before it. The target is exactly two weeks; if the actual time was longer, then the difficulty is reduced. If the actual time was less than two weeks (as just happened a couple of days ago) then the difficulty is increased.

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March 12, 2011, 02:23:19 AM
 #28

I suspect most of the nodes on the network right now are NTP synchronized, though I expect chaos will ensue once we have large numbers of unsynchronized nodes joining the network.

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Jim Hyslop
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March 12, 2011, 03:29:52 AM
 #29

I suspect most of the nodes on the network right now are NTP synchronized, though I expect chaos will ensue once we have large numbers of unsynchronized nodes joining the network.
Well, the mainstream OSes (OSX, Windoze and increasingly Linux) all incorporate some kind of time synchronization, so that shouldn't be an issue. I wouldn't put money on it, though (especially not Bitcoins, since I ain't got none yet :-)

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March 12, 2011, 03:59:00 AM
 #30

I suspect most of the nodes on the network right now are NTP synchronized, though I expect chaos will ensue once we have large numbers of unsynchronized nodes joining the network.
Well, the mainstream OSes (OSX, Windoze and increasingly Linux) all incorporate some kind of time synchronization, so that shouldn't be an issue. I wouldn't put money on it, though (especially not Bitcoins, since I ain't got none yet :-)

Windows by default only synchronizes once a week, and that's only IF the feature is enabled. There's no documented way to make it synchronize more than once a week, and a computer clock can drift quite a bit in that time, especially if the onboard battery is low or dead.

And that's not taking into account the number of people who intentionally desynchronize their clocks in order to play mind games with themselves.

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theymos
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March 12, 2011, 04:55:14 AM
 #31

There's no documented way to make it synchronize more than once a week

It is documented:
http://technet.microsoft.com/en-us/library/cc773263%28WS.10%29.aspx

You need to change this registry value:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Parameters\NtpServer
Change the server flag from 0x9 to 0x0. Then you'll update using "smart adjustments" like normal NTP clients. You could also instead change W32Time\TimeProviders\NtpClient\SpecialPollInterval to update more frequently.

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