Consensus fork activation thresholds: Block.nTime vs median time vs block.nHeight

Consensus fork activation thresholds: Block.nTime vs median time vs block.nHeight
2015-07-30 18:16 ` Gavin Andresen
  
From: Jorge Timón @ 2015-07-29 20:27 UTC (permalink / raw)
  To: Bitcoin Dev

When it comes to define thresholds for consensus fork activation there
are 3 options that I know of and each of them has at least a
disadvantage that the other 2 lack:

-Block.nTime: It's not monotonic
-median time: You cannot validate it without context (in contrast,
nTime is contained in the block header and nHeight in the coinbase)
-block.nHeight: You cannot know the exact time when a given height
will be reached.

I personally think that nHeight's disadvantage is the less worse, but
others will likely disagree. The point is we need to find a solid
criteria to decide.

When combining the threshold with a later miner's "voting" (upgrade
confirmation) on top of it, not being monotonic may be a real problem.
Doing that on top of height seems straight forward: check if
(prevBlockIndex.nHeight > threshold &&
IsSuperMajority(...,prevBlockIndex,...))
With median time, seems safe too: if
(prevBlockIndex.CalculateMedianTime > threshold &&
IsSuperMajority(...,prevBlockIndex,...))
Just a little bit less efficient.
It would look more like if (IsSuperMajority(...,prevBlockIndex,...) &&
GetFirstBlockUsedInVoting(prevBlockIndex).nTime > threshold)


But in some cases (say, an emergency consensus fork) you won't combine
the mining confirmation, so you may not have the prevBlockIndex
available and you may need to pass the height or medianTime down.
If the current block is not accessible from wherever the check is
being made, you would need an additional blockTime parameter as well.

Are there any example cases where a rule activation check doesn't have
the block available?

Of course, let's consider the following hardfork example:

before the hardfork: consensus_size(tx) = real_size(tx)
after the hardfork: after consensus_size(tx) = real_size(tx) +
delta_utxo_size(tx)

that would allow miners to create bigger blocks if the transactions
help reducing the size of the utxo (and penalize transactions that
make the utxo grow by considering bigger when it comes to block
inclusion).

Well, at the block validation level (the most important one), you
obviously have block.nTime available.
But what if you're checking an unconfirmed transaction?
It's size (and thus it's validity and the policy relay decisions)
depends on whether the hardfork is activated or not.
So to check an unconfirmed transaction, you would need the block.nTime
of the next block, which is unpredictable (unless you're a miner)
because miners set those.

AcceptToMemoryPool already uses the nHeight (in fact, there's nHeight
and nSpendHeight there, not sure why we need to of them yet), so this
case would be trivial to implement.
Calculating the median time there wouldn't be difficult either: even
if globals weren't so heavily-used in AcceptToMemoryPool, the
prevIndex can always be passed down as parameter.

Some people may think that I'm discussing tiny details, but I would
really like that we can chose whatever is more generic for any type of
consensus fork and always use that from now on (instead of risking of
having to use 2 of them if we find out later that the chosen option is
not general enough).

It would be also nice to have only one uniform type of threshold in
Consensus::Params, and height seems to be the choice for softforks
that have been accepted long ago via miners'
voting/upgrade_confirmation, like in :
https://github.com/bitcoin/bitcoin/pull/5966/files

That doesn't mean it needs to necessarily be height: in a rebased
version of #5966 we could replace consensus.nBIP34Height = 227931 with
consensus.nBIP34Time = <something>.
But I would really like to have a uniform threshold mechanism instead
of using the 3 options depending on the fork.

I had assumed that height was the preferred option for everyone and
that's why I used it in
http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-June/008936.html
But judging from the existing blocksize hardfork proposals (using
block.nTime, the option I like less ) I was too fast there and clearly
I need to reopen the discussion.


 permalink raw reply	[|nested]
2015-07-29 20:27 [bitcoin-dev] Consensus fork activation thresholds: Block.nTime vs median time vs block.nHeight Jorge Timón

  2015-08-04 20:02   ` Jorge Timón
  
From: Gavin Andresen @ 2015-07-30 18:16 UTC (permalink / raw)
  To: Jorge Timón;  Bitcoin Dev

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I still think using the version and timestamp fields in the block header
are simplest and best.

Advantages:
  Available to SPV nodes with no change to the network protocol
  Available after headers downloaded, before full block data is available
  Once well past a fork, allows all block validation except validation
against the UTXO to happen in parallel, out-of-order, independent of any
other block.

Disadvantages:
  Not monotonically increasing


I think discussion about transactions in the memory pool are just a
distraction: no matter what criteria is used (timestamp, height, median
time), a blockchain re-organization could mean the validity of transactions
you've accepted into the memory pool (if you're accepting transactions that
switch from valid to invalid at the consensus change -- Core tries hard not
to do that via IsStandard policy) must be re-evaluated.

I don't strongly care if median time or block timestamp is used, I think
either will work. I don't like height, there are too many cases where the
time is known but the block height isn't (see, for example, the
max-outputs-in-a-transaction sanity check computation at line 190 of
bitcoin-tx.cpp -- bitcoin-tx.cpp has no idea what the current block height
is).


-- 
--
Gavin Andresen

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 permalink raw reply	[|nested]
2015-07-30 18:16 ` Gavin Andresen

  2015-08-04 21:29     ` Peter Todd
  
From: Jorge Timón @ 2015-08-04 20:02 UTC (permalink / raw)
  To: Gavin Andresen;  Bitcoin Dev

On Thu, Jul 30, 2015 at 8:16 PM, Gavin Andresen <gavinandresen@gmail•com> wrote:
> I still think using the version and timestamp fields in the block header are
> simplest and best.

To be clear, all options can use the version.

> Advantages:
>   Available to SPV nodes with no change to the network protocol
>   Available after headers downloaded, before full block data is available
>   Once well past a fork, allows all block validation except validation
> against the UTXO to happen in parallel, out-of-order, independent of any
> other block.

All advantages shared with the height option.

> Disadvantages:
>   Not monotonically increasing
>
>
> I think discussion about transactions in the memory pool are just a
> distraction: no matter what criteria is used (timestamp, height, median
> time), a blockchain re-organization could mean the validity of transactions
> you've accepted into the memory pool (if you're accepting transactions that
> switch from valid to invalid at the consensus change -- Core tries hard not
> to do that via IsStandard policy) must be re-evaluated.

I'm talking about the non-reorg case. Without reorg, you know what the
next height or current median time is, but you don't know what the
next block time is.

> I don't strongly care if median time or block timestamp is used, I think
> either will work. I don't like height, there are too many cases where the
> time is known but the block height isn't (see, for example, the
> max-outputs-in-a-transaction sanity check computation at line 190 of
> bitcoin-tx.cpp -- bitcoin-tx.cpp has no idea what the current block height
> is).

How does bitcoin-tx know about the next block time?
It doesn't. You would need to use the current time as a proxy for the
median time or the block.nTime which you don't know either.
Or just keep the sanity check as it is. Note that this case is
blocksize-specific: other hardforks (like my previous example, or the
code proposal in BIP99) don't share that concern.

One thing I've noticed there seems to be disagreement on is whether
miners' upgrade confirmation (aka voting) is necessary for
uncontroversial hardforks or not.
In BIP99 the advice for uncontroversial hardforks is setting a
threshold (based on height, but we can change it) and then wait for
95% of the hashrate to upgrade to enforce the chain.
But maybe the "voting" can happen first and then the threshold is
added to the "miners' confirmation height/time".
I think that may influence which of the three discussed options
(height, block time and median time) is better, so maybe we should
discuss that first.


 permalink raw reply	[|nested]
2015-08-04 20:02   ` Jorge Timón

  2015-08-05 19:29       ` Jorge Timón
  
From: Peter Todd @ 2015-08-04 21:29 UTC (permalink / raw)
  To: Jorge Timón, Jorge Timón via bitcoin-dev, Gavin Andresen
  Cc: Bitcoin Dev

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On 4 August 2015 16:02:53 GMT-04:00, "Jorge Timón via bitcoin-dev" <bitcoin-dev@lists•linuxfoundation.org> wrote:
>One thing I've noticed there seems to be disagreement on is whether
>miners' upgrade confirmation (aka voting) is necessary for
>uncontroversial hardforks or not.

To be clear, without a strong supermajority of miner support the fork risks attack. Requiring 95% approval - which is actually just a 50% majority vote as the majority can squelch the minority - is an obvious minimum safety requirement.

Another option is Hearn's proposal of using centralised checkpoints to override PoW consensus; obviously that raises serious questions, including legal issues.

For forks without miner approval miners have a number of options to defeat them. For instance, they can make their own fork with a new consensus algorithm that requires miners to prove they're attacking the unwanted chain - Garzik's recent 2MB blocks proposal is a hilarious, and probably accidental, example of such a design, with the original Bitcoin protocol rules having the effect of attacking the Garzik 2MB chain.

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 permalink raw reply	[|nested]
2015-08-04 21:29     ` Peter Todd

  
From: Jorge Timón @ 2015-08-05 19:29 UTC ( / )
  To: Peter Todd;  Jorge Timón via bitcoin-dev

I'm not sure how bip102 is less secure than other blocksize proposal
but please let's keep defects specific to each proposal in their own
threads.
In any case, I understand that you agree that 95% confirmation is a
good idea for uncontroversial hardforks (like in uncontroversial
softforks).
I'm not sure if you prefer that to happen before or after the time
threshold, but I guess you're fine with doing it after the threshold
since you didn't complained about that specifically (you can always
clarify your preferences of course).

On Tue, Aug 4, 2015 at 11:29 PM, Peter Todd <pete@petertodd•org> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA256
>
>
>
> On 4 August 2015 16:02:53 GMT-04:00, "Jorge Timón via bitcoin-dev" <bitcoin-dev@lists•linuxfoundation.org> wrote:
>>One thing I've noticed there seems to be disagreement on is whether
>>miners' upgrade confirmation (aka voting) is necessary for
>>uncontroversial hardforks or not.
>
> To be clear, without a strong supermajority of miner support the fork risks attack. Requiring 95% approval - which is actually just a 50% majority vote as the majority can squelch the minority - is an obvious minimum safety requirement.
>
> Another option is Hearn's proposal of using centralised checkpoints to override PoW consensus; obviously that raises serious questions, including legal issues.
>
> For forks without miner approval miners have a number of options to defeat them. For instance, they can make their own fork with a new consensus algorithm that requires miners to prove they're attacking the unwanted chain - Garzik's recent 2MB blocks proposal is a hilarious, and probably accidental, example of such a design, with the original Bitcoin protocol rules having the effect of attacking the Garzik 2MB chain.
>
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