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Subject: Re: [bitcoindev] Does anyone still need testnet3?
From: Sjors Provoost <sjors@sprovoost.nl>
In-Reply-To: <7c28f8e9-d221-4633-8b71-53b4db07fa78@schildbach.de>
Date: Fri, 14 Mar 2025 10:31:50 +0100
Cc: Andreas Schildbach <google.com@schildbach.de>
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Hi Andreas,

> By the way, testnet3 works great for us (for testing). I consider block s=
torms a feature, because it tests our code in rough conditions. We had plan=
ned to support both testnets in the forseeable future.
>=20
> https://github.com/bitcoinj/bitcoinj/

The "good" news is that testnet4 has very frequent short reorgs. [0]

It appears that people are trying to CPU mine by exploiting the rule that d=
ifficulty drops to 1 after 20 minutes. For every real difficulty block that=
's mined, they instantly publish 5 or 6 blocks, right up against the rule t=
hat blocks can't be more than 2 hours in the future.=20

Another side-effect of that behavior is that on testnet4 MTP is typically i=
n the future, rather than an hour in the past. Combined with the timewarp a=
ttack mitigation rule helped reveal some bugs. [1, 2, 3]

There is a potential counter measure [4] that involves re-orging out those =
low difficulty blocks, though it's just a hackathon proof-of-concept and no=
t likely to be included in Bitcoin Core itself. If anyone were to run it, y=
ou'd get even more reorgs to test :-)

- Sjors

[0] https://fork.observer/?network=3D4
[1] https://github.com/benjamin-wilson/public-pool/commit/4282233d2f11ceecb=
d0d142e8292ccc9c37ea999
[2] https://github.com/stratum-mining/stratum/pull/1325
[3] https://github.com/bitcoin/bitcoin/pull/31600
[4] https://github.com/bitcoin/bitcoin/pull/31117

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