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

From: ZmnSCPxj <[email-masked]>
Date: Mon, 11 May 2020 05:44:08 +0000

Good morning Richard, and all,

> 2) a light client can query an ISP connected full node on the same unmetered local WiFi network and exchange differences in block headers opportunistically or pay for large missing ranges of headers, filters or full blocks using a payment channel. Cost is reduced and privacy is enhanced for the light client by not using a centralized ISP. Bandwidth for running the full node can be amortized and subsidized by payments from light clients who they resell data to.

A relatively pointless observation, but it seems to me that:

* The light client is requesting for validation information, because...
* ...its direct peers might be defrauding it, leading to...
* ...the money it *thinks* it has in its channels being valueless.

Thus, if the light client opportunistically pays for validation information (whether full blocks, headers, or filters), the direct peers it has could just as easily not forward any payments, thus preventing the light client from paying for the validation information.

Indeed, if the direct peer *is* defrauding the light client, the direct peer has no real incentive to actually forward *any* payments --- to do so would be to reduce the possible earnings it gets from defrauding the light client.
("Simulating" the payments so that the light client will not suspect anything runs the risk that the light client will be able to forward all its money out of the channel, and the cheating peer is still potentially liable for any funds it originally had in the channel if it gets caught.)

What would work would be to use a system similar to watchtowers, wherein the validation-information-provider is prepaid and issues tokens that can be redeemed later.
But this is not suitable for opportunistic on-same-WiFi where, say, a laptop is running a validation-information-provider-for-payment program on the same WiFi as a light-client mobile phone, if we consider that the laptop and mobile may have never met before and may never meet again.
It would work if the laptop altruistically serves the blocks, but not if it were for (on-Lightning) payment.

So it seems to me that this kind of service is best ridden on top of watchtower service providers.

Regards,
ZmnSCPxj