Re: [Lightning-dev] Solving the Price Of Anarchy Problem, Or: Cheap AND Reliable Payments Via Forwarding Fee Economic Rationality
Good morning aj, > On Wed, Jun 29, 2022 at 12:38:17PM +, ZmnSCPxj wrote: > > > > > ### Inverting The Filter: Feerate Cards > > > > Basically, a feerate card is a mapping between a probability-of-success > > > > range and a feerate. > > > > * 00%->25%: -10ppm > > > > * 26%->50%: 1ppm > > > > * 51%->75%: 5ppm > > > > * 76%->100%: 50ppm > > > > The economic insight here is this: > > > > * The payer wants to pay because it values a service / product more > > > > highly than the sats they are spending. > > > * If payment fails, then the payer incurs an opportunity cost, as it is > > unable to utilize the difference in subjective value between the service / > > product and the sats being spent. > > > (If payment fails, the only opportunity cost they incur is that they > can't use the funds that they locked up for the payment. The opportunity > cost is usually considered to occur when the payment succeeds: at that > point you've lost the ability to use those funds for any other purpose) I think you misunderstand me completely. The "payment fails" term here means that *all possible routes below the fee budget have failed*, i.e. a complete payment failure that will cause your `pay` command to error out with a frownie face. In that case, the payer is unable to purchase the service or product. The opportunity cost they lose is the lack of the service or product; they keep the value of the sats that did not get paid, but lose the value of the service or product they wanted to buy in the first place. In that case, the payer loses the subjective difference in value between the service / product, and the sats they would have paid. Regards, ZmnSCPxj ___ Lightning-dev mailing list Lightning-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
Re: [Lightning-dev] Solving the Price Of Anarchy Problem, Or: Cheap AND Reliable Payments Via Forwarding Fee Economic Rationality
On Wed, Jun 29, 2022 at 12:38:17PM +, ZmnSCPxj wrote: > > > ### Inverting The Filter: Feerate Cards > > > Basically, a feerate card is a mapping between a probability-of-success > > > range and a feerate. > > > * 00%->25%: -10ppm > > > * 26%->50%: 1ppm > > > * 51%->75%: 5ppm > > > * 76%->100%: 50ppm > The economic insight here is this: > * The payer wants to pay because it values a service / product more highly > than the sats they are spending. > * If payment fails, then the payer incurs an opportunity cost, as it is > unable to utilize the difference in subjective value between the service / > product and the sats being spent. (If payment fails, the only opportunity cost they incur is that they can't use the funds that they locked up for the payment. The opportunity cost is usually considered to occur when the payment succeeds: at that point you've lost the ability to use those funds for any other purpose) > * Thus, the subjective difference in value between the service / product > being bought, and the sats to be paid, is the cost of payment failure. If you couldn't successfully route the payment at any price, you never had the opportunity to buy whatever the thing was. > We can now use the left-hand side of the feerate card table, by multiplying > `100% - middle_probability_of_success` (i.e. probability of failure) by the > fee budget (i.e. cost of failure), and getting the > cost-of-failure-for-this-entry. I don't think that makes much sense; your expected gain if you just try one option is: (1-p)*0 + p*cost*(benefit/cost - fee) where p is the probability of success that corresponds with the fee. I don't think you can do that calculation with a range; if I fix the probabilities as: 12.5% -10ppm 27.5%1ppm 62.5%5ppm 87.5% 50ppm then that approach chooses: -10 ppm if the benefit/cost is in (-10ppm, 8.77ppm) 5 ppm if the benefit/cost is in [8.77ppm, 162.52ppm) 50 ppm if the benefit/cost is >= 162.52ppm so for that policy, one of those entries is already irrelevant. But that just feels super unrealistic to me. If your benefit is 8ppm, and you try at -10ppm, and that fails, why wouldn't you try again at 5ppm? That means the real calculation is: p1*(benefit/cost - fee1) + (p2-p1)*(benefit/cost - fee2 - retry_delay) - (1-p2)*(2*retry_delay) Which is: p2*(benefit/cost) - p1*fee1 - (p2-p1)*fee2 - (2-p1-p2)*retry_delay My feeling is that the retry_delay factor's going to dominate... That's also only considering one hop; to get the entire path, you need them all to succeed, giving an expected benefit (for a particular combination of rate card entries) of: (p1*p2*p3*p4*p5)*cost*(benefit/cost - (fee1 + fee2 + fee3 + fee4 + fee5) And (p1*..*p5) is going to be pretty small in most cases -- 5 hops at 87.5% each already gets you down to only a 51% total chance of success. And there's an exponential explosion of combinations, if each of the 5 hops has 4 options on their rate card, that's up to 1024 different options to be evaluated... > We then evaluate the fee card by plugging this in to each entry of the > feerate card, and picking which entry gives the lowest total fee. I don't think that combines hops correctly. For example, if the rate cards for hop1 and hop2 are both: 10% 10ppm 100% 92ppm and your expected benefit/cost is 200ppm (so 100ppm per hop), then treated individually you get: 10%*(100ppm - 10ppm) = 9ppm <-- this one! 100%*(100ppm - 92ppm) = 8ppm but treated together, you get: 1%*(200ppm - 20ppm) = 1.8ppm 10%*(200ppm - 102ppm) = 9.8ppm (twice) 100%*(200ppm - 184ppm) = 16ppm <-- this one! > This is then added as a fee in payment algorithms, thus translated down to > "just optimize for low fee". You're not optimising for low fee though, you're optimising for maximal expected value, assuming you can't retry. But you can retry, and probably in reality also want to minimise the chance of failure up to some threshold. For example: if I buy a coffee with lightning every week day for a year, that's 250 days, so maybe I'd like to choose a fee so that my payment failure rate is <0.4%, to avoid embarassment and holding up the queue. > * Nodes utilizing wall strategies and doing lots of rebalancing put low > limits on the fee budget of the rebalancing cost. > * These nodes are willing to try lots of possible routes, hoping to nab the > liquidity of a low-fee node on the cheap in order to resell it later. > * Such nodes are fine with low probability of success. Sure. But in that case, they don't care about delays, so why wouldn't they just try the lowest fee rates all the time, no matter what their expected value is? They can retry once an hour indefinitely, and eventually they should get lucky, if the rate card's even remotely accurate. (Though chances are they won't get -10ppm lucky for the entire path) Finding out that you're paying 50ppm at the exact same time someone else is
Re: [Lightning-dev] Solving the Price Of Anarchy Problem, Or: Cheap AND Reliable Payments Via Forwarding Fee Economic Rationality
Good morning aj, > On Sun, Jun 05, 2022 at 02:29:28PM +, ZmnSCPxj via Lightning-dev wrote: > > Just sharing my thoughts on this. > > > Introduction > > > > Optimize for reliability+ > > uncertainty+fee+drain+uptime... > > .--~~--. > > / \ > > / \ > > / \ > > / \ > > / \ > > --' `-- > > Just Just > > optimize optimize > > for for > > low fee low fee > > > I think ideally you want to optimise for some combination of fee, speed > and reliability (both liklihood of a clean failure that you can retry > and of generating stuck payments). As Matt/Peter suggest in another > thread, maybe for some uses you can accept low speed for low fees, > while in others you'd rather pay more and get near-instant results. I > think drain should just go to fee, and uncertainty/uptime are just ways > of estimating reliability. > > It might be reasonable to generate local estimates for speed/reliability > by regularly sending onion messages or designed-to-fail htlcs. > > Sorry if that makes me a midwit :) Actually feerate cards help with this; it just requires an economic insight to translate probability-of-success to an actual cost that the payer incurs. > > ### Inverting The Filter: Feerate Cards > > Basically, a feerate card is a mapping between a probability-of-success > > range and a feerate. > > * 00%->25%: -10ppm > > * 26%->50%: 1ppm > > * 51%->75%: 5ppm > > * 76%->100%: 50ppm > > > Feerate cards don't really make sense to me; "probability of success" > isn't a real measure the payer can use -- naively, if it were, they could > just retry at 1ppm 10 times and get to 95% chances of success. But if > they can afford to retry (background rebalancing?), they might as well > just try at -10ppm, 1ppm, 5ppm, 10ppm (or perhaps with a binary search?), > and see if they're lucky; but if they want a 1s response time, and can't > afford retries, what good is even a 75% chance of success if that's the > individual success rate on each hop of their five hop path? The economic insight here is this: * The payer wants to pay because it values a service / product more highly than the sats they are spending. * There is a subjective difference in value between the service / product being bought and the amount to be spent. * In short, if the payment succeeds and the service / product is acquired, then the payer perceives itself as richer (increased utilons) by that subjective difference. * If payment fails, then the payer incurs an opportunity cost, as it is unable to utilize the difference in subjective value between the service / product and the sats being spent. * Thus, the subjective difference in value between the service / product being bought, and the sats to be paid, is the cost of payment failure. * That difference in value is the "fee budget" that Lightning Network payment algorithms all require as an argument. * If the LN fee total is greater than the fee budget, the payment algorithm will reject that path outright. * If the LN fee total is greater than the subjective difference in value between the service / product being bought and the amount to be delivered at the destination, then the payer gets negative utility and would prefer not to continue paying --- which is exactly what the payment algorithm does, it rejects such paths. Therefore the fee budget is the cost of failure. We can now use the left-hand side of the feerate card table, by multiplying `100% - middle_probability_of_success` (i.e. probability of failure) by the fee budget (i.e. cost of failure), and getting the cost-of-failure-for-this-entry. We then evaluate the fee card by plugging this in to each entry of the feerate card, and picking which entry gives the lowest total fee. This is then added as a fee in payment algorithms, thus translated down to "just optimize for low fee". If the above logic seems dubious, consider this: * Nodes utilizing wall strategies and doing lots of rebalancing put low limits on the fee budget of the rebalancing cost. * These nodes are willing to try lots of possible routes, hoping to nab the liquidity of a low-fee node on the cheap in order to resell it later. * i.e. those nodes are fine with taking a long time to successfully route a payment from themselves to themselves; they absolutely insist on low fees or else they will not earn anything. * Such nodes are fine with low probability of success. * Being fine with low probability of success means that the effect of the left-hand side of the feerate card is smaller and such nodes will tend to get the low probability of success entries. * Buyers getting FOMOed into buying some neat new widget want to get their grubby hands on the widget ASAP. * These nodes are willing to pay a premium to get the neat new widget RIGHT NOW. * i.e. these nodes will be willing to provide a higher fee budget. * Being fine with a higher fee budget means that the effect of the left-hand side of the feerate card is larger and such nodes
Re: [Lightning-dev] Solving the Price Of Anarchy Problem, Or: Cheap AND Reliable Payments Via Forwarding Fee Economic Rationality
On Sun, Jun 05, 2022 at 02:29:28PM +, ZmnSCPxj via Lightning-dev wrote: Just sharing my thoughts on this. > Introduction > > Optimize for reliability+ >uncertainty+fee+drain+uptime... > .--~~--. > /\ >/ \ > /\ > / \ > /\ > _--' `--_ > Just Just > optimize optimize > for for > low fee low fee I think ideally you want to optimise for some combination of fee, speed and reliability (both liklihood of a clean failure that you can retry and of generating stuck payments). As Matt/Peter suggest in another thread, maybe for some uses you can accept low speed for low fees, while in others you'd rather pay more and get near-instant results. I think drain should just go to fee, and uncertainty/uptime are just ways of estimating reliability. It might be reasonable to generate local estimates for speed/reliability by regularly sending onion messages or designed-to-fail htlcs. Sorry if that makes me a midwit :) > Rene Pickhardt also presented the idea of leaking friend-of-a-friend > balances, to help payers increase their payment reliability. I think foaf (as opposed to global) gossip of *fee rates* is a very interesting approach to trying to give nodes more *current* information, without flooding the entire network with more traffic than it can cope with. > Now we can consider that *every channel is a marketplace*. > What is being sold is the sats inside the channel. (Really, the marketplace is a channel pair (the incoming channel and the outgoing channel), and what's being sold is their relative balance) > So my concrete proposal is that we can do the same friend-of-a-friend balance > leakage proposed by Rene, except we leak it using *existing* mechanisms --- > i.e. gossiping a `channel_update` with new feerates adjusted according to the > supply on the channel --- rather than having a new message to leak > friend-of-a-friend balance directly. +42 > Because we effectively leak the balance of channels by the feerates on the > channel, this totally leaks the balance of channels. I don't think this is true -- you ideally want to adjust fees not to maintain a balanced channel (50% on each side), but a balanced *flow* (1:1 incoming/outgoing payment volume) -- it doesn't really matter if you get the balanced flow that results in an average of a 50:50, 80:20 or 20:80 ratio of channel balances (at least, it doesn't as long as your channel capacity is 10 or 100 times the payment size, and your variance is correspondingly low). Further, you have two degrees of freedom when setting fee rates: one is how balanced the flows are, which controls how long your channel can remain useful, but the other is how *much* flow there is -- if halving your fee rate doubles the flow rate in sats/hour, then that will still increase your profit. That also doesn't leak balance information. > ### Inverting The Filter: Feerate Cards > Basically, a feerate card is a mapping between a probability-of-success range > and a feerate. > * 00%->25%: -10ppm > * 26%->50%: 1ppm > * 51%->75%: 5ppm > * 76%->100%: 50ppm Feerate cards don't really make sense to me; "probability of success" isn't a real measure the payer can use -- naively, if it were, they could just retry at 1ppm 10 times and get to 95% chances of success. But if they can afford to retry (background rebalancing?), they might as well just try at -10ppm, 1ppm, 5ppm, 10ppm (or perhaps with a binary search?), and see if they're lucky; but if they want a 1s response time, and can't afford retries, what good is even a 75% chance of success if that's the individual success rate on each hop of their five hop path? And if you're not just going by odds of having to retry, then you need to get some current information about the channel to plug into the formula; but if you're getting *current* information, why not let that information be the feerate directly? > More concretely, we set some high feerate, impose some kind of constant > "gravity" that pulls down the feerate over time, then we measure the relative > loss of outgoing liquidity to serve as "lift" to the feerate. If your current fee rate is F (ppm), and your current volume (flow) is V (sats forwarded per hour), then your profit is FV. If dropping your fee rate by dF (<0) results in an increase of V by dV (>0), then you want: (F+dF)(V+dV) > FV FV + VdF + FdV + dFdV > FV FdV > -VdF dV/dF < -V/F (flip the inequality because dF is negative) (dV/V)/(dF/F) < -1 (fee-elasticity of volume is in the elastic region) (<-1 == elastic == flow changes more than the fee does == drop the fee rate; >-1 == ineleastic == flow changes less than the fee does == raise the fee rate; =-1 == unit elastic ==
