On Dec 9, 2015 5:40 PM, "Gavin Andresen" <gavinandresen@gmail.com> wrote:
>
> On Wed, Dec 9, 2015 at 3:03 AM, Gregory Maxwell via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
>>
>> I think it would be logical to do as part of a hardfork that moved
>> commitments generally; e.g. a better position for merged mining (such
>> a hardfork was suggested in 2010 as something that could be done if
>> merged mining was used), room for commitments to additional block
>> back-references for compact SPV proofs, and/or UTXO set commitments.
>> Part of the reason to not do it now is that the requirements for the
>> other things that would be there are not yet well defined. For these
>> other applications, the additional overhead is actually fairly
>> meaningful; unlike the fraud proofs.
>
>
> So just design ahead for those future uses. Make the merkle tree:
>
>
>              root_in_block_header
>                      /      \
>   tx_data_root      other_root
>                                /       \
>         segwitness_root     reserved_for_future_use_root

This is basically what I meant by

struct hashRootStruct
{
uint256 hashMerkleRoot;
uint256 hashWitnessesRoot;
uint256 hashextendedHeader;
}

but my design doesn't calculate other_root as it appears in your tree (is not necessary).

Since stop requiring bip34 (height in coinbase) is also a hardfork (and a trivial one) I suggested to move it at the same time. But thinking more about it, since BIP34 also elegantly solves BIP30, I would keep the height in the coinbase (even if we move it to the extented header tree as well for convenience).
That should be able to include future consensus-enforced commitments (extra back-refs for compact proofs, txo/utxo commitments, etc) or non-consensus data (merged mining data, miner-published data).
Greg Maxwell suggested to move those later and I answered fair enough. But thinking more about it, if the extra commitments field is extensible, we don't need to move anything now, and therefore we don't need for those  designs (extra back-refs for compact proofs, txo/utxo commitments, etc) to be ready to deploy a hardfork segregated witness: you just need to make sure that your format is extensible via softfork in the future.

I'm therefore back to the "let's better deploy segregated witness as a hardfork" position.
The change required to the softfork segregated witnesses implementation would be relatively small.

Another option would be to deploy both parts (sw and the movement from the coinbase to the extra header) at the same time but with different activation conditions, for example:

- For sw: deploy as soon as possible with bip9.
- For the hardfork codebase to extra header movement: 1 year grace + bip9 for later miner upgrade confirmation.