[Bitcoin-development] Reconsidering block version number use

[Gavin Andresen]

> The current block height in coinbase addition currently proposes to use block
> version 2. However, the protocol change is in fact to the coinbase
> transaction, not the block itself (which really doesn't have any extensibility
> without a hardfork anyway). Perhaps we should consider bumping the coinbase
> transaction version number to 2 for this instead?

I'd thought about bumping the coinbase transaction version, but the
problem is if we want a smooth rollout then, during the rollout, every
time a new block comes in the percentage of the last 1,000 blocks that
support the new version has to be computed.

If that means looking in the coinbase transaction, then either the
last 1,000 coinbases have to be stored in memory or they have to be
fetched from disk. Which isn't a huge deal, unless we start
aggressively pruning spent transactions, and that coinbase 900 blocks
back got spent and pruned.

On Sun, Jul 22, 2012 at 4:52 PM, Luke-Jr <luke at dashjr.org> wrote:
> It just occurred to me that the block version number could easily be used as a
> cheap "extra nonce" right now. Considering that we will probably see lots of
> ASIC miners running at 1 TH/s per rig before the end of 2012, it might be
> desirable to save the block version for this purpose.

Hmm...  I think it'd be ok to give 3 of the 4 block version bytes as a
simple extranonce, so version=0x00000001 is what we have now, version
2 blocks are any with 0x02 in the low byte, 0x03 is version 3, etc.  I
don't think we'll go through 253 block versions before we're all dead.

That'd be 7 bytes of nonce in the block header, which is
  72,057,594,037,927,936  ~ 72 petahashes = 72,000 terahashes

So: the changes for version 2 blocks would be "has height in the
coinbase, and has a 1-byte version number with a 3-byte extranonce."

> Also, Jeff noticed that block 190192 has version==2 without a valid block
> height in the coinbase. I suspect this may be the result of combining the
> current blockheight-in-coinbase pullreq with P2Pool. This means that if we go
> forward with the version==2 marker, we will forever need to make an exception
> for that block.

No, the rules are "enforce the rules when the chain has a
super-majority."  Since block 190192 is in a part of the chain with
zero other version==2 blocks, the height-in-the-coinbase rule will not
be enforced.

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Gavin Andresen