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From: Karl Johan Alm <karljohan-alm@garage.co.jp>
Date: Tue, 9 May 2017 10:15:25 +0900
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To: Erik Aronesty <erik@q32.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] BIP Proposal: Rate Limiting with server specified
 Proof of Work challenges
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Erik,

On Tue, May 9, 2017 at 3:58 AM, Erik Aronesty <erik@q32.com> wrote:
> - It would be cool if any rate-limiting POW was specified as bytecode ... so
> nodes can plug in as many "machine-captcha" things as they please, and
> solvers can choose to solve... or just say "nope too hard".

I'm not entirely sure what you mean, but right now you can make an
arbitrary chain of challenges, and the BIP includes methods for
determining an approximate time to solve (nodes will, at the very
least, discard any challenge which will on average take longer time to
solve than the expiration of the challenge itself, for example, i.e.
the "nope too hard" part).

> - Alternately, it would be a lot nicer if you just required people to pay a
> nanobit .... that could prevent DDOS even better, and generate a revenue
> stream for nodes.

Others mentioned this approach. I haven't given it much thought.
Admittedly it would be an effective way to prevent DoS but it also has
some unwanted side effects that need to be cleared up (e.g. in a
no-gains scenario like the BIP proposes, the node requesting PoW done
doesn't *gain* anything from lying to the node performing the work).