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From: Andy Parkins <andyparkins@gmail.com>
To: Jorge =?iso-8859-1?q?Tim=F3n?= <timon.elviejo@gmail.com>
Date: Wed, 23 Nov 2011 15:29:45 +0000
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Cc: bitcoin-development@lists.sourceforge.net
Subject: Re: [Bitcoin-development] Addressing rapid changes in mining power
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On 2011 November 23 Wednesday, Jorge Tim=F3n wrote:

> Well, I meant "the probability of  your block being the hardest".
> What a miner can do is hash the block (cheating the timestamp) for 2
> more minutes than the rest of the people and then send it to the other
> nodes. Nodes cannot possibly know when did you hashed the block only
> by looking at their clock when they receive it, because there's also
> network latency.

True enough; but then the same is true for everyone else.  If the window is=
 2=20
minutes after the stated time, then everyone _can_ wait until the end of th=
at=20
window.  However, they risk their block being rejected by their peers, and=
=20
their efforts are wasted.  In fact, it can be guaranteed by making the acce=
pt=20
window zero.  There is then no reason to carry on computing after the rewar=
d=20
window closes, since you know your peers will reject it.

> > (2) For the network clock; see util.cpp:GetAdjustedTime().
>=20
> 1) This is part of the satoshi client but not the protocol. A miner
> can rewrite this part of the code and there won't be anything in the
> chain that contradicts the protocol.

Well yes.  What does that matter?  It's only a way of calculating an averag=
e=20
time.  The node can use any clock it wants, as long as the block time is=20
verified by the peers.

> 2) I haven't read the code but I'm pretty sure that's not a perfect
> decentralized clock.

It definitely isn't.  NTP is mentioned in the source as an alternative.

> I will be more specific. Where's the network clock in the chain (in
> the protocol)?

It's nothing to do with the protocol; it's an individual miner choosing=20
whether to accept or reject a block based on the timestamp it claims, and t=
he=20
current time as the miner sees it.  For the sake of compatibility, the clie=
nts=20
currently choose to use a community clock as "current", as established from=
=20
the time they receive from peers in the "version" message (it actually hold=
s=20
offsets between them, which is pretty bad, as a long-connected client will=
=20
drift).  They don't have to, but if miners aren't using time that approxima=
tes=20
what their peers are using, under my system, their blocks would be rejected=
:=20
so an incentive to use that "community clock" exists.



Andy

=2D-=20
Dr Andy Parkins
andyparkins@gmail.com

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