Subject: [bitcoindev] Libre Relay v27.1 released with lower 1.25x replacement threshold

From: Peter Todd <[email-masked]>
Date: Thu, 20 Jun 2024 16:33:47 +0000

https://github.com/petertodd/bitcoin/tree/libre-relay-v27.1

A notable change in this release is I've lowered the replace-by-fee-rate¹
threshold to 1.25x, from the prior 2x. That means if the fee-rate of a
candidate replacement is more than 1.25x higher than the direct conflict, the
replacement can happen. This does two things:

1. Reduces the worst-case cost to defeat transaction pinning.
2. Increases the expected cost of transaction pinning, as 25% is a small enough
   margin that a pin transaction is likely to itself get mined.

Of course, this is a good opportunity for the people who think there are
"massive deficiencies"² in my RBFR proposal to actually prove it with an
exploit. Remember that you have to find a DoS attack that is significantly
better than the existing "free" relay attacks¹³⁴ that we inevitably have
without RBFR.

Libre Relay/RBFR is already mitigating transaction pinning in the real world.
I've personally run into a few cases with LND nodes where anchor outputs were
spent after the 16 block CSV timeout by third parties in a large transaction
that the LND node was not aware of, leading to LND creating a conflicting,
higher-fee, transaction spending the anchor output and other outputs. Normally
the conflict would fail to get mined due to the higher absolute fee pin. But in
each case after propagation via Libre Relay nodes, F2Pool eventually mined the
higher fee-rate transaction after a few hours; I suspect F2Pool has an unusual
short mempool expiration time. Lightning node operators should consider running
Libre Relay for this purpose, as the existing Lightning protocol does have some
pinning vulnerabilities.

1) https://petertodd.org/2024/one-shot-replace-by-fee-rate
2) https://x.com/achow101/status/1767641788192280917
3) https://groups.google.com/g/bitcoindev/c/EJYoeNTPVhg/m/iLVre7MYCAAJ
4) https://groups.google.com/g/bitcoindev/c/3XqfIOYzXqo/m/2bXhHzylBwAJ

-- 
https://petertodd.org 'peter'[:-1]@petertodd.org

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