Subject: Re: [Bitcoin-development] Distributing low POW headers

From: Tier Nolan <[email-masked]>
Date: Wed, 24 Jul 2013 12:55:33 +0100

On Wed, Jul 24, 2013 at 10:42 AM, Peter Todd <[email-masked]> wrote:

> Please provide equations and data justifying the 'magic constants' in
> this proposal.
>

The are a range of workable values.  Ideally, there would first need to be
agreement on the general principle.

Distributing headers with 1/64 of the standard POW means that a header
would be broadcast approximately once every 9 seconds (assuming a 10 minute
block time).  This was picked because sending 80 byte headers every 9
seconds shouldn't represent much load on the network.

The second magic number is how much credit to give for mini-headers.
Setting it at 1/16 means that the headers will be worth around 4 times as
much as a block (since there would be around 63 low POW headers for each
full POW one).

This creates an incentive for miners to take headers into account.  If all
the headers were worth less than a full block, then a fork which was losing
would suddenly be winning if a block is found.  A fork will only become the
main chain due to a new block, if it is within 16 mini-confirms.

Miners don't have to mine against the absolute best fork, but they do need
to make sure they stay within 16 of the best one (so if they find a block,
that block would be considered part of the main chain).  Some hysteresis
might be helpful.  The rule could be to only switch unless the current fork
is losing by at least 4 mini-confirms.

In most cases, this won't be a problem, since orphans don't happen that
often anyway.

Since it isn't a chain, this doesn't give the full benefits of a 9 second
block, but it should bring things to consensus faster.  6 full confirms
would be much more secure against random and hostile reversals.

It doesn't have the risks of 9 second blocks in causing network collapse,
since it isn't a chain, the headers are short, and there is no
confirmations of the required (other than checking the hash).

Each "mini" confirms adds to the strength of leaf blocks of the tree.  If
there is a tie, and 20% of the network is mining one block and 80% is
mining the other, the mining power of the network will be split until the
next block arrives.

With mini confirms, the entire network is aware of the 2 blocks (since the
headers would be forwarded) and the mini-confirms would show which one has
majority hashing power.

The least risk option would be to make them purely advisory.  The proposal
takes it further than that.

The proposal means that if the network is split 80/20, then miners should
stick with the 80% fork, even if the 20% fork wins the race for the next
block.

Winning a few rounds is easier than wining many rounds worth of
mini-confirms.

The key is that as long as the honest miners stay on the main chain, they
will eventually overwhelm any rewrite attack with less than 50% of the
mining power.  This is a system to agree on what is the main chain in the
face of a re-write attack.

>
> Currently we do not relay blocks to peers if they conflict with blocks
> in the best known chain. What changes exactly are you proposing to that
> behavior?
>

The (sub) proposal is that headers would still be broadcast.  The blocks
would not be forwarded.

If a header extends the header tree, meets full POW and is "near" the end
of the chain, then it is broadcast.  This means that all nodes will have
the entire header tree, including orphans.

The full blocks would only be sent if they extend the main chain.

Second, if a header builds on a header that is in the header tree, then it
is broadcast, even if it doesn't meet full POW (only 1/64 required).  This
gives information on which fork is getting the most power.

It gives information about potential "consensus loss" forks, where a
significant number of miners are following an alternative chain.

In fact, this is probably worth doing as an initial step.

A warning could be displayed on the client if a fork is getting more than
15% of the hashing power.