From: Johnson Lau <jl2012@xbt.hk>
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Message-Id: <FB8593E6-3CD7-46D5-8FC8-A73A0EF1AE9A@xbt.hk>
Date: Mon, 5 Dec 2016 03:34:00 +0800
To: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
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Subject: [bitcoin-dev] Forcenet: an experimental network with a new header
	format
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Based on Luke Dashjr=E2=80=99s code and BIP: =
https://github.com/luke-jr/bips/blob/bip-mmhf/bip-mmhf.mediawiki , I =
created an experimental network to show how a new header format may be =
implemented.

Basically, the header hash is calculated in a way that non-upgrading =
nodes would see it as a block with only the coinbase tx and zero output =
value. They are effectively broken as they won=E2=80=99t see any =
transactions confirmed. This allows rewriting most of the rules related =
to block and transaction validity. Such technique has different names =
like soft-hardfork, firmfork, evil softfork, and could be itself a =
controversial topic. However, I=E2=80=99d rather not to focus on its =
soft-hardfork property, as that would be trivial to turn this into a =
true hardfork (e.g. setting the sign bit in block nVersion, or setting =
the most significant bit in the dummy coinbase nLockTime)

Instead of its soft-HF property, I think the more interesting thing is =
the new header format. The current bitcoin header has only 80 bytes. It =
provides only 32bits of nonce space and is far not enough for ASICs. It =
also provides no room for committing to additional data. Therefore, =
people are forced to put many different data in the coinbase =
transaction, such as merge-mining commitments, and the segwit =
commitment. It is not a ideal solution, especially for light wallets.

Following the practice of segwit development of making a experimental =
network (segnet), I made something similar and call it the Forcenet (as =
it forces legacy nodes to follow the post-fork chain)

The header of forcenet is mostly described in Luke=E2=80=99s BIP, but I =
have made some amendments as I implemented it. The format is (size in =
parentheses; little endian):

Height (4), BIP9 signalling field (4), hardfork signalling field (3), =
merge-mining hard fork signalling field (1), prev hash (32), timestamp =
(4), nonce1 (4), nonce2 (4), nonce3 (compactSize + variable), Hash TMR =
(32), Hash WMR (32), total tx size (8) , total tx weight (8), total =
sigops (8), number of tx (4), merkle branches leading to header C =
(compactSize + 32 bit hashes)

In addition to increasing the max block size, I also showed how the =
calculation and validation of witness commitment may be changed with a =
new header. For example, since the commitment is no longer in the =
coinbase tx, we don=E2=80=99t need to use a 0000=E2=80=A6.0000 hash for =
the coinbase tx like in BIP141.

Something not yet done:
1. The new merkle root algorithm described in the MMHF BIP
2. The nTxsSigops has no meaning currently
3. Communication with legacy nodes. This version can=E2=80=99t talk to =
legacy nodes through the P2P network, but theoretically they could be =
linked up with a bridge node
4. A new block weight definition to provide incentives for slowing down =
UTXO growth
5. Many other interesting hardfork ideas, and softfork ideas that works =
better with a header redesign

For easier testing, forcenet has the following parameters:

Hardfork at block 200
Segwit is always activated
1 minutes block with 40000 (prefork) and 80000 (postfork) weight limit
50 blocks coinbase maturity
21000 blocks halving
144 blocks retarget

How to join: codes at https://github.com/jl2012/bitcoin/tree/forcenet1 , =
start with "bitcoind =E2=80=94forcenet" .
Connection: I=E2=80=99m running a node at 8333.info with default port =
(38901)
Mining: there is only basic internal mining support. Limited GBT support =
is theoretically possible but needs more hacking. To use the internal =
miner, writeup a shell script to repeatedly call =E2=80=9Cbitcoin-cli =
=E2=80=94forcenet generate 1=E2=80=9D
New RPC commands: getlegacyblock and getlegacyblockheader, which =
generates blocks and headers that are compatible with legacy nodes.

This is largely work-in-progress so expect a reset every couple weeks

jl2012


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