>1. Introducing state checkpoints into the chain itself could make it possible 
>for full nodes to skip verification of large sections of historical data when 
>booting up.

What you are suggesting, unless I am mistaken, is that new full nodes should 
have no way of knowing if an output is spent or even if it exists. Since large 
sections of the blockchain will potentially be skipped, the full node will not 
have complete knowledge of utxo's just for starters.


Damian Williamson

From: bitcoin-dev-boun...@lists.linuxfoundation.org 
<bitcoin-dev-boun...@lists.linuxfoundation.org> on behalf of Tao Effect via 
bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Sent: Monday, 19 February 2018 12:29:50 PM
To: Bitcoin Protocol Discussion
Subject: [bitcoin-dev] Some thoughts on removing timestamps in PoW

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# Blockchain Timestamps Unnecessary In Proof-of-Work?

*Author: Greg Slepak 


The Bitcoin blockchain has a 10-minute target blocktime that is achieved by a 
difficulty adjustment algorithm.

I assert, or rather, pose the hypothesis, that the use of timestamps in 
Bitcoin's blockchain may be unnecessary, and that Bitcoin can operate with the 
same security guarantees without it (except as noted in [Risks and 
Mitigations](#risks-and-mitigations)), and therefore does not need miners to 
maintain global clock synchronization.

The alternative difficulty adjustment algorithm would work according to the 
following principles:

- The incentive for miners is and always has been to maximize profit.
- The block reward algorithm is now modified to issue coins into perpetuity (no 
maximum). Any given block can issue _up to_ `X` number of coins per block.
- The number of coins issued per block is now tied directly to the difficulty 
of the block, and the concept of "epocs" or "block reward halving" is removed.
- The chain selection rule remains "chain with most proof of work"
- The difficulty can be modified by miners in an arbitrary direction (up or 
down), but is limited in magnitude by some maximum percentage (e.g. no more 
than 20% deviation from the previous block), we call this `Y%`.

### Observations

- Miners are free to mine blocks of whatever difficulty they choose, up to a 
maximum deviation
- The blockchain may at times produce blocks very quickly, and at other times 
produce blocks more slowly
- Powerful miners are incentivized to raise the difficulty to remove 
competitors (as is true today)
- Whether miners choose to produce blocks quickly or slowly is entirely up to 
them. If they produce blocks quickly, each block has a lower reward, but there 
are more of them. If they produce blocks slowly, each block has a higher 
reward, but there are fewer of them. So an equilibrium will be naturally 
reached to produce blocks at a rate that should minimize orphans.

A timestamp may still be included in blocks, but it no longer needs to be used 
for anything, or represent anything significant other than metadata about when 
the miner claims to have produced the block.

### Risks and Mitigations

Such a system may introduce risks that require further modification of the 
protocol to mitigate.

The most straightforward risk comes from the potential increase in total 
transaction throughput that such a change would introduce (these are the same 
concerns that exist with respect to raising the blocksize). The removal of 
timestamps would allow a cartel of miners to produce high-difficulty blocks at 
a fast rate, potentially resulting in additional centralization pressures not 
only on miners but also on full nodes who then would have greater difficulty 
keeping up with the additional bandwidth and storage demands.

Two equally straightforward mitigations exist to address this if we are given 
the liberty of modifying the protocol as we wish:

1. Introducing state checkpoints into the chain itself could make it possible 
for full nodes to skip verification of large sections of historical data when 
booting up.
2. A sharded protocol, where each shard uses a "sufficiently different" PoW 
algorithm, would create an exit for users should the primary blockchain become 
captured by a cartel providing poor quality-of-service.

Please do not email me anything that you are not comfortable also sharing with 
the NSA.

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