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.

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