[Bitcoin-development] Proposal for extra nonce in block header

Melvin Carvalho melvincarvalho at gmail.com
Sun Apr 27 08:17:11 UTC 2014

On 27 April 2014 09:07, Timo Hanke <timo.hanke at web.de> wrote:

> I'd like to put the following draft of a BIP up for discussion.
> Timo
> # Abstract
> There are incentives for miners to find cheap, non-standard ways to
> generate new work, which are not necessarily in the best interest of the
> protocol.
> In order to reduce these incentives this proposal re-assigns 2 bytes from
> the version field of the block header to a new extra nonce field.
> # Copyright
> # Specification
> The block version number field in the block header is reduced in size from
> 4 to 2 bytes.
> The third and fourth byte in the block header are assigned to the new
> extra nonce field inside the block header.
> # Motivation
> The motivation of this proposal is to provide miners with a cheap
> constant-complexity method to create new work that does not require
> altering the transaction tree.
> Furthermore, the motivation is to protect the version and timestamp fields
> in the block header from abuse.
> # Rationale
> Traditionally, the extra nonce is part of the coinbase field of the
> generation transaction, which is always the very first transaction of a
> block.
> After incrementing the extra nonce the minimum amount of work a miner has
> to do to re-calculate the block header is a) to hash the coinbase
> transaction and b) to re-calculate the left-most branch of the merkle tree
> all the way to the merkle root.
> This is necessary overhead a miner has to do besides hashing the block
> header itself.
> We shall call the process that leads to a new block header from the same
> transaction set the _pre-hashing_.
> First it should be noted that the relative cost of pre-hashing in its
> traditional form depends
> on the block size, which may create an unwanted incentive for miners
> to keep the block size small. However, this is not the main motivation for
> the current proposal.
> While the block header is hashed by ASICs, pre-hashing typically happens
> on a CPU because of the greater flexibility required.
> Consequently, as ASIC cost per hash performance drops the relative cost of
> pre-hashing increases.
> This creates an incentive for miners to find cheaper ways to create new
> work than by means of pre-hashing.
> An example of this currently happening is the on-device rolling of the
> timestamp into the future.
> These ways of creating new work are unlikely to be in the best interest of
> the protocol.
> For example, rolling the timestamp faster than the real time is unwanted
> (more so on faster blockchains).
> The version number in the block header is a possible target for alteration
> with the goal of cheaply creating new work.
> Currently, blocks with arbitrarily large version numbers get relayed and
> accepted by the network.
> As this is unwanted behaviour, there should not exist any incentive for a
> miner to abuse the version number in this way.
> The solution is to reduce the range of version numbers from 2^32 to 2^16
> and to declare the third and forth bytes of the block header as legitimate
> space for an extra nonce.
> This will reduce the incentive for a miner to abuse the shortened version
> number by a factor in the order of 2^16.
> As a side effect, this proposal greatly reduces the bandwidth requirements
> of a blind pool protocol by only submitting the block header to the miner.
> # Backwards Compatibility
> Old versions of the client will accept blocks of this kind but will throw
> an alert at the user to upgrade.
> The only code change would be a cast of the version number to a short.
> Besides the upgrade alert, old and new versions of the client can co-exist
> and there is no need to introduce a new block version number or to
> phase-out old block versions.
> # Reference Implementation
> # Final implementation

If changing the structure of the block header, wouldnt you also need to
increment the version number to 3?

> --
> Timo Hanke
> PGP 1EFF 69BC 6FB7 8744 14DB  631D 1BB5 D6E3 AB96 7DA8
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