There doesn't seem to be anything in the original email that's specific to BIP 157. It's a restatement of the arguments against light clients:

- light clients are a burden on the full nodes that serve them
- if light clients become more popular, there won't be enough full nodes to serve them
- people might build products that depend on altruistic nodes serving data, which is unsustainable
- maybe at some point in the future, light clients will need to pay for services

The choice isn't between people using light clients or not. People already use light clients. The choice between whether we offer them a light client technology that is better or worse for privacy and scalability.

The arguments for why BIP 157 is better than the existing light client technologies are available elsewhere, but to summarize:

- they're unique for a block, which means they can easily be cached. Serving a filter requires no computation, just i/o (or memory access for cached filter/header data) and bandwidth. There are plenty of other services that a full node offers that use i/o and bandwidth, such as serving blocks.
- unique-for-block means clients can download from multiple sources

- the linked-headers/filters model allows hybrid approaches, where headers checkpoints can be fetched from trusted/signed nodes, with intermediate headers and filters fetched from untrusted sources
- less possibilities to DoS/waste resources on the serving node
- better for privacy

> The intention, as I understood it, of putting BIP157 directly into bitcoind was to essentially force all `bitcoind` users to possibly service BIP157 clients

Please. No-one is forcing anyone to do anything. To serve filters, a node user needs to download the latest version, set `-blockfilterindex=basic` to build the compact filters index, and set `-peercfilters` to serve them over P2P. This is an optional, off-by-default feature.

Regards,

John

On Tue, May 5, 2020 at 9:50 AM ZmnSCPxj via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

Good morning ariard and luke-jr

> > Trust-minimization of Bitcoin security model has always relied first and
> > above on running a full-node. This current paradigm may be shifted by LN
> > where fast, affordable, confidential, censorship-resistant payment services
> > may attract a lot of adoption without users running a full-node.
>
> No, it cannot be shifted. This would compromise Bitcoin itself, which for
> security depends on the assumption that a supermajority of the economy is
> verifying their incoming transactions using their own full node.
>
> The past few years has seen severe regressions in this area, to the point
> where Bitcoin's future seems quite bleak. Without serious improvements to the
> full node ratio, Bitcoin is likely to fail.
>
> Therefore, all efforts to improve the "full node-less" experience are harmful,
> and should be actively avoided. BIP 157 improves privacy of fn-less usage,
> while providing no real benefits to full node users (compared to more
> efficient protocols like Stratum/Electrum).
>
> For this reason, myself and a few others oppose merging support for BIP 157 in
> Core.

BIP 157 can be implemented as a separate daemon that processes the blocks downloaded by an attached `bitcoind`, i.e. what Wasabi does.

The intention, as I understood it, of putting BIP157 directly into bitcoind was to essentially force all `bitcoind` users to possibly service BIP157 clients, in the hope that a BIP157 client can contact any arbitrary fullnode to get BIP157 service.
This is supposed to improve to the situation relative to e.g. Electrum, where there are far fewer Electrum servers than fullnodes.

Of course, as ariard computes, deploying BIP157 could lead to an effective DDoS on the fullnode network if a large number of BIP157 clients arise.
Though maybe this will not occur very fast? We hope?

It seems to me that the thing that *could* be done would be to have watchtowers provide light-client services, since that seems to be the major business model of watchtowers, as suggested by ariard as well.
This is still less than ideal, but maybe is better than nothing.

Regards,
ZmnSCPxj
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