From: Tamas Blummer <tamas.blummer@gmail.com>
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Message-Id: <9509A1AC-287A-48E9-84E9-D507D6A2A4CD@gmail.com>
Date: Sun, 3 Feb 2019 21:15:25 +0100
To: bitcoin-dev@lists.linuxfoundation.org
Cc: jimpo@coinbase.com
Subject: [bitcoin-dev] BIP157 server Murmel introduced,
	enchancement suggestion to BIP158
Precedence: list

TLDR: I suggest to add outpoint filter to BIP158 as it proved to be =
useful while developing a filter server and allows further checks in =
filter client.

Murmel is my project within the rust-bitcoin community. =
https://github.com/rust-bitcoin/murmel

Its goal is to provide a lightweight, at least SPV security, settlement =
layer for the Lightning Network implementation in Rust. =
https://github.com/rust-bitcoin/rust-lightning

Murmel relies on BIP157 (Client Side Block Filtering). Since Bitcoin =
Core does not yet support this protocol extension, I also added filter =
and block server functionality to Murmel and this might be useful for=20
development purposes any other BIP157 client project.

You may compile and run a Murmel filter server to support your client =
development. It bootstraps within a few hours. Follow the instructions =
at: https://github.com/rust-bitcoin/murmel

While implementing both client and server side I made an observation =
that should be considered for BIP158:

BIP158 specifies base filter containing all scripts spent or created by =
a block (except those with OP_RETURN). I found it useful to also compute =
a filter on spent and created outpoints.
The Murmel filter server consults these outpoint filters to find the =
transactions with the spent scripts while computing the base script =
filter. Since outpoints usually getting spent shortly after created, =
this approach works well enough to
keep up with the blockchain, although far too slow to rely on it while =
bootstrapping. An advantage of this approach of looking up UTXO is that =
there is nothing to be recomputed at re-org, filters are consulted =
following
the path from current tip back to genesis. This fits well with =
Murmel=E2=80=99s storage, that is my other project Hammersbald =
https://github.com/rust-bitcoin/hammersbald, a highly efficient truly =
append only blockchain store in Rust.
Filter matching is also nicely parallelizable looking up subsets of =
spent outputs in parallel.=20

A lightweight client can use outpoint filters to efficiently validate =
spent coins or miner reward, which goes beyond SPV guarantees. This is =
probabilistically possible now, and definitely once filters are =
committed.

For above reasons I suggest to also add outpoint filter to BIP158, so =
filter servers may support it, as does Murmel. Murmel is moving quickly; =
I tagged the version as of this mail with DEVLIST for later reference.

Tamas Blummer=