From: Luke Dashjr <luke@dashjr.org>
To: bitcoin-dev@lists.linuxfoundation.org,
 Tier Nolan <tier.nolan@gmail.com>
Date: Sat, 10 Dec 2016 21:41:57 +0000
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Subject: Re: [bitcoin-dev] Forcenet: an experimental network with a new
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On Saturday, December 10, 2016 9:29:09 PM Tier Nolan via bitcoin-dev wrote:
> On Sun, Dec 4, 2016 at 7:34 PM, Johnson Lau via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
> > Something not yet done:
> > 1. The new merkle root algorithm described in the MMHF BIP
> 
> Any new merkle algorithm should use a sum tree for partial validation and
> fraud proofs.

PR welcome.

> Is there something special about 216 bits?  I guess at most 448 bits total
> means only one round of SHA256.  16 bits for flags would give 216 for each
> child.

See https://github.com/luke-jr/bips/blob/bip-mmhf/bip-mmhf.mediawiki#Merkle_tree_algorithm

But yes, the 448 bits total target is to optimise the tree-building.

> Even better would be to make the protocol extendable.  Allow blocks to
> indicate new trees and legacy nodes would just ignore the extra ones.  If
> Bitcoin supported that then the segregated witness tree could have been
> added as a easier soft fork.

It already is. This is a primary goal of the new protocol.

> The sum-tree could be added later as an extra tree.

Adding new trees means more hashing to validate blocks, so it'd be better to 
keep it at a minimum.

Luke