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From: Melvin Carvalho <melvincarvalho@gmail.com>
Date: Wed, 19 Mar 2025 10:11:28 +0100
Message-ID: <CAKaEYhLNTbA=qMuQ5W63jxwUoa2+mpn020NC31JqGzbk0wPF-A@mail.gmail.com>
Subject: Re: [bitcoindev] Unbreaking testnet4
To: Sjors Provoost <sjors@sprovoost.nl>
Cc: "'Antoine Poinsot' via Bitcoin Development Mailing List" <bitcoindev@googlegroups.com>, Antoine Poinsot <darosior@protonmail.com>
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st 19. 3. 2025 v 10:04 odes=C3=ADlatel Sjors Provoost <sjors@sprovoost.nl>
napsal:

> Antoine Poinsot wrote:
>
> The given rationale for a difficulty reset was to let developers
> occasionally mine blocks on their laptop. But you cannot have your cake a=
nd
> eat it too: either the network is permissionless (PoW) or you assign
> identities and privileges to some (Signet). By trying to do both at the
> same time testnet4 created a loophole for abuse. As a result it failed on
> both count: it neither mimics mainnet nor allows developers to mine activ=
e
> blocks on their laptop.
>
>
> Let me clarify why developers can't mine on their own laptop with testnet=
4.
>
> The way you would do that is to move your computer clock forward by 20
> minutes (or use faketime), at which point the difficulty drops to 1. You
> would then mine your (presumably non-standard transaction) and broadcast =
it
> to the network.
>
> This is also the strategy used to acquire more testnet coins if you don't
> have an ASIC and don't want to use a faucet.
>
> On testnet3 the latter wasn't very productive because at height 4 million
> the block subsidy is under 10k sats.
>
> But on testnet4 every block yields 50 tBTC. So several people try to mine
> such a block, leading to the many parallel forks.
>
> If you're a developer trying to mine a non-standard transaction, you have
> to be fast, well connected and lucky to be the first block picked up by t=
he
> rest of the network.
>
> But why mine just one if you can mine many?
>
> Some CPU miners are now mining as many testnet4 blocks as they can by
> bumping their clock 20 minutes not just once, but several times in a row.
> Until they run against the limit other nodes put on how far a block can b=
e
> in the future, namely two hours. So when a real difficulty block appears,
> you may see 5 blocks on top of it instantly.
>
> This behavior is even worse from the point of view of a developer trying
> to mine a non-standard transaction. Because the tip of your node is alway=
s
> going to be about two hours in the future, when you mine on top of that b=
y
> moving your clock even further, it will be rejected by your peers.
>
> So this use case of CPU mining non-standard transactions is simply dead a=
s
> long as this behaviour exists. We might as well reduce code complexity.
>
> That said, testnet will never mimic mainnet. It has no value, or worse,
> very little value. So the incentives are different, which leads to
> different behavior. That's a whack-a-mole game, which we should probably
> not dedicate time to.
>

The question I=E2=80=99ve had for some time is why spoofed blocks on testne=
t4 come
in batches of:

- 6 blocks
-20-minute intervals
- A maximum lead time of 2 hours

If these values were set for early Bitcoin network reasons, would it make
sense to tweak them for testnet4 to improve functionality? Reducing the
maximum lead from 2 hours to 1 hour and limiting batches to 3 blocks
instead of 6 seems like it could mitigate some of the observed issues while
maintaining testnet=E2=80=99s purpose.

Would this be feasible?


>
> - Sjors
>
>
>
> --
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<div dir=3D"ltr"><div dir=3D"ltr"><br></div><br><div class=3D"gmail_quote g=
mail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr">st 19. 3. 2025 =
v=C2=A010:04 odes=C3=ADlatel Sjors Provoost &lt;<a href=3D"mailto:sjors@spr=
ovoost.nl">sjors@sprovoost.nl</a>&gt; napsal:<br></div><blockquote class=3D=
"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(2=
04,204,204);padding-left:1ex"><div>Antoine Poinsot wrote:<div><br><div><blo=
ckquote type=3D"cite"><div><div style=3D"font-family:Arial,sans-serif;font-=
size:14px"><div style=3D"font-family:Arial,sans-serif;font-size:14px"><div>=
The
 given rationale for a difficulty reset was to let developers=20
occasionally mine blocks on their laptop. But you cannot have your cake=20
and eat it too: either the network is permissionless (PoW) or you assign
 identities and privileges to some (Signet). By trying to do both at the
 same time testnet4 created a loophole for abuse. As a result it failed=20
on both count: it neither mimics mainnet nor allows developers to mine=20
active blocks on their laptop.<br></div></div></div></div></blockquote><br>=
</div><div>Let me clarify why developers can&#39;t mine on their own laptop=
 with testnet4.</div><div><br></div><div>The way you would do that is to mo=
ve your computer clock forward by 20 minutes (or use faketime), at which po=
int the difficulty drops to 1. You would then mine your (presumably non-sta=
ndard transaction) and broadcast it to the network.</div><div><br></div><di=
v>This is also the strategy used to acquire more testnet coins if you don&#=
39;t have an ASIC and don&#39;t want to use a faucet.</div><div><br></div><=
/div><div>On testnet3 the latter wasn&#39;t very productive because at heig=
ht 4 million the block subsidy is under 10k sats.</div><div><br></div><div>=
But on testnet4 every block yields 50 tBTC. So several people try to mine s=
uch a block, leading to the many parallel forks.=C2=A0</div><div><br></div>=
<div>If you&#39;re a developer trying to mine a non-standard transaction, y=
ou have to be fast, well connected and lucky to be the first block picked u=
p by the rest of the network.</div><div><br></div><div>But why mine just on=
e if you can mine many?</div><div><br></div><div>Some CPU miners are now mi=
ning as many testnet4 blocks as they can by bumping their clock 20 minutes =
not just once, but several times in a row. Until they run against the limit=
 other nodes put on how far a block can be in the future, namely two hours.=
 So when a real difficulty block appears, you may see 5 blocks on top of it=
 instantly.</div><div><br></div><div>This behavior is even worse from the p=
oint of view of a developer trying to mine a non-standard transaction. Beca=
use the tip of your node is always going to be about two hours in the futur=
e, when you mine on top of that by moving your clock even further, it will =
be rejected by your peers.</div><div><br></div><div>So this use case of CPU=
 mining non-standard transactions is simply dead as long as this behaviour =
exists. We might as well reduce code complexity.</div><div><br></div><div>T=
hat said, testnet will never mimic mainnet. It has no value, or worse, very=
 little value. So the incentives are different, which leads to different be=
havior. That&#39;s a whack-a-mole game, which we should probably not dedica=
te time to.</div></div></blockquote><div><br></div>The question I=E2=80=99v=
e had for some time is why spoofed blocks on testnet4 come in batches of:<b=
r><br>- 6 blocks</div><div class=3D"gmail_quote gmail_quote_container">-20-=
minute intervals<br>- A maximum lead time of 2 hours<br><br>If these values=
 were set for early Bitcoin network reasons, would it make sense to tweak t=
hem for testnet4 to improve functionality? Reducing the maximum lead from 2=
 hours to 1 hour and limiting batches to 3 blocks instead of 6 seems like i=
t could mitigate some of the observed issues while maintaining testnet=E2=
=80=99s purpose.<br><br>Would this be feasible? <br><div>=C2=A0</div><block=
quote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1=
px solid rgb(204,204,204);padding-left:1ex"><div><div><br></div><div>- Sjor=
s</div><div><br></div><div><br></div><div>=C2=A0</div></div>



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om/d/msgid/bitcoindev/2064B7F4-B23A-44B0-A361-0EC4187D8E71%40sprovoost.nl</=
a>.<br>
</blockquote></div></div>

<p></p>

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