Subject: Re: [bitcoin-dev] [Lightning-dev] On the scalability issues of onboarding millions of LN mobile clients

From: Antoine Riard <[email-masked]>
Date: Wed, 6 May 2020 04:27:45 -0400

I didn't trust myself and verify. In fact the [3] is the real [2].

Le mar. 5 mai 2020 à 06:28, Andrés G. Aragoneses <[email-masked]> a
écrit :

> Hey Antoine, just a small note, [3] is missing in your footnotes, can you
> add it? Thanks
>
> On Tue, 5 May 2020 at 18:17, Antoine Riard <[email-masked]>
> wrote:
>
>> Hi,
>>
>> (cross-posting as it's really both layers concerned)
>>
>> Ongoing advancement of BIP 157 implementation in Core maybe the
>> opportunity to reflect on the future of light client protocols and use this
>> knowledge to make better-informed decisions about what kind of
>> infrastructure is needed to support mobile clients at large scale.
>>
>> 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. Assuming a
>> user adoption path where a full-node is required to benefit for LN may
>> deprive a lot of users, especially those who are already denied a real
>> financial infrastructure access. It doesn't mean we shouldn't foster node
>> adoption when people are able to do so, and having a LN wallet maybe even a
>> first-step to it.
>>
>> Designing a mobile-first LN experience opens its own gap of challenges
>> especially in terms of security and privacy. The problem can be scoped as
>> how to build a scalable, secure, private chain access backend for millions
>> of LN clients ?
>>
>> Light client protocols for LN exist (either BIP157 or Electrum are used),
>> although their privacy and security guarantees with regards to
>> implementation on the client-side may still be an object of concern
>> (aggressive tx-rebroadcast, sybillable outbound peer selection, trusted fee
>> estimation). That said, one of the bottlenecks is likely the number of
>> full-nodes being willingly to dedicate resources to serve those clients.
>> It's not about _which_ protocol is deployed but more about _incentives_ for
>> node operators to dedicate long-term resources to client they have lower
>> reasons to care about otherwise.
>>
>> Even with cheaper, more efficient protocols like BIP 157, you may have a
>> huge discrepancy between what is asked and what is offered. Assuming 10M
>> light clients [0] each of them consuming ~100MB/month for filters/headers,
>> that means you're asking 1PB/month of traffic to the backbone network. If
>> you assume 10K public nodes, like today, assuming _all_ of them opt-in to
>> signal BIP 157, that's an increase of 100GB/month for each. Which is
>> consequent with regards to the estimated cost of 350GB/month for running an
>> actual public node. Widening full-node adoption, specially in term of
>> geographic distribution means as much as we can to bound its operational
>> cost.
>>
>> Obviously,  deployment of more efficient tx-relay protocol like Erlay
>> will free up some resources but it maybe wiser to dedicate them to increase
>> health and security of the backbone network like deploying more outbound
>> connections.
>>
>> Unless your light client protocol is so ridiculous cheap to rely on
>> niceness of a subset of node operators offering free resources, it won't
>> scale. And it's likely you will always have a ratio disequilibrium between
>> numbers of clients and numbers of full-node, even worst their growth rate
>> won't be the same, first ones are so much easier to setup.
>>
>> It doesn't mean servicing filters for free won't work for now, numbers of
>> BIP157 clients is still pretty low, but what is worrying is  wallet vendors
>> building such chain access backend, hitting a bandwidth scalability wall
>> few years from now instead of pursuing better solutions. And if this
>> happen, maybe suddenly, isn't the quick fix going to be to rely on
>> centralized services, so much easier to deploy ?
>>
>> Of course, it may be brought that actually current full-node operators
>> don't get anything back from servicing blocks, transactions, addresses...
>> It may be replied that you have an indirect incentive to participate in
>> network relay and therefore guarantee censorship-resistance, instead of
>> directly connecting to miners. You do have today ways to select your
>> resources exposure like pruning, block-only or being private but the wider
>> point is the current (non?)-incentives model seems to work for the base
>> layer. For light clients data, are node operators going to be satisfied to
>> serve this new *class* of traffic en masse ?
>>
>> This doesn't mean you won't find BIP157 servers, ready to serve you with
>> unlimited credit, but it's more likely their intentions maybe not aligned,
>> like spying on your transaction broadcast or block fetched. And you do want
>> peer diversity to avoid every BIP157 servers being on few ASNs for
>> fault-tolerance. Do people expect a scenario a la Cloudflare, where
>> everyone connections is to far or less the same set of entities ?
>>
>> Moreover, the LN security model diverges hugely from basic on-chain
>> transactions. Worst-case attack on-chain a malicious light client server
>> showing a longest, invalid, PoW-signed chain to double-spend the user. On
>> LN, the *liveliness* requirement means the entity owning your view of the
>> chain can lie to you on whether your channel has been spent by a revoked
>> commitment, the real tip of the blockchain or even dry-up block
>> announcement to trigger unexpected behavior in the client logic. A
>> malicious light client server may just drop any filters/utxos spends, what
>> your LN client should do in this case ? [1]
>>
>> Therefore, you may want to introduce monetary compensation in exchange of
>> servicing filters. Light client not dedicating resources to maintain the
>> network but free-riding on it, you may use their micro-payment capabilities
>> to price chain access resources [3]. This proposition may suit within the
>> watchtower paradigm, where another entity is delegated some part of
>> protocol execution, alleviating client onliness requirement. It needs
>> further analysis but how your funds may be compromised by a watchtower are
>> likely to be the same scenario that how a chain validation provider can
>> compromise you. That said, how do you avoid such "chain access" market
>> turning as an oligopoly is an open question. You may "bind" them to
>> internet topology or ask for fidelity bonds and create some kind of
>> scarcity but still...
>>
>> Maybe I'm completely wrong, missing some numbers, and it's maybe fine to
>> just rely on few thousands of full-node operators being nice and servicing
>> friendly millions of LN mobiles clients. But just in case it may be good to
>> consider a reasonable alternative.
>>
>> Thanks Gleb for many points exposed here but all mistakes are my own.
>>
>> Cheers,
>>
>> Antoine
>>
>> [0] UTXO set size may be a bottleneck, but still if you have 2 channels
>> by clients that's 20M utxos, just roughly ~x3 than today.
>>
>> [1] And committing filters as part of headers may not solve everything as
>> an attacker can just delay or slow announcements to you, so you still need
>> network access to at least one honest node.
>>
>> [2]  It maybe argue that distinction client-vs-peer doesn't hold because
>> you may start as a client and start synchronizing the chain, relaying
>> blocks, etc. AFAIK, there is no such hybrid implementation and that's not
>> what you want to run in a mobile.
>>
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