On 8/15/21 22:02, Anthony Towns wrote:
In
one particular class of applicable routing algorithms you could use for
lightning routing having a base fee makes the algorithm intractably slow,
I don't think of that as the problem, but rather as the base fee having
a multiplicative effect as you split payments.
Yes, matching the real-world costs of forwarding an HTLC.
If every channel has the same (base,proportional) fee pair, and send a
payment along a single path, you're paying n*(base+k*proportional). If
you split the payment, and send half of it one way, and half the other
way, you're paying n*(2*base+k*proportional). If you split the payment
four ways, you're paying n*(4*base+k*proportional). Where's the value
to the network in penalising payment splitting?
Yes. You have to pay the cost of a node. If we're really worried about this, we should be talking about upfront fees
and/or refunds on HTLC fulfillment, not removing the fees entirely.
Being denominated in sats, the base fee also changes in value as the
bitcoin price changes -- c-lightning dropped the base fee to 1sat (from
546 sat!) in Jan 2018, but the value of 1sat has increased about 4x
since then, and it seems unlikely the fixed costs of a successful HTLC
payment have likewise increased 4x. Proportional fees deal with this
factor automatically, of course.
This isn't a protocol issue, implementations can automate this without issue.
There's real cost to distorting the fee structures on the network away from
the costs of node operators,
That's precisely what the base fee is already doing. Yes, we need some
other way of charging fees to prevent using up too many slots or having
transactions not fail in a timely manner, but the base fee does not
do that.
Huh? For values much smaller than a node's liquidity, the cost for nodes is (mostly) a function of HTLCs, not the value.
The cost to nodes is largely (a) the forever-storage that exists, roughly, per HTLC ever on a channel, (b) the HTLC
slots which are highly limited for technical reasons per channel, (c) the disk/cpu/network/etc operations per HTLC on an
channel, (d) the liquidity required per node. I'd argue (c) is basically zero in any realistic context, (a) is pretty
low, but could be non-zero in some cases, so you really just have (b) and (d). For many HTLCs forwarded today, the
liquidity on a channel isn't much, so I'd argue for many HTLCs forwarded today per-payment costs mirror the cost to a
node much, much, much, much better than some proportional fees?
I'm really not sure where you're coming from here.
Imagine we find some great way to address HTLC slot flooding/DoS attacks (or
just chose to do it in a not-great way) by charging for HTLC slot usage, now
we can't fix a critical DoS issue because the routing algorithms we deployed
can't handle the new costing.
I don't think that's true. The two things we don't charge for that can
be abused by probing spam are HTLC slot usage and channel balance usage;
both are problems only in proportion to the amount of time they're held
open, and the latter is also only a problem proportional to the value
being reserved. [0]
Additionally, I don't think HTLC slot usage needs to be kept as a
limitation after we switch to eltoo;
The HTLC slot limit is to keep transactions broadcastable. I don't see why this would change, you still get an output
for each HTLC on the latest commitment in eltoo, AFAIU.
and in the meantime, I think it can
be better managed via adjusting the min_htlc_amount -- at least for the
scenario where problems are being caused by legitimate payment attempts,
which is also the only place base fee can help.
Sure, we could also shift towards upfront fees or similar solutions, though, and that was my point - if we start
dropping absolute fee amounts now in order to make some given routing algorithm work, we box ourselves in here, and
quite needlessly given no one has (yet) done the legwork to show that we even *need* to box ourselves in.
[0] (Well, ln-penalty's requirement to permanently store HTLC information
in order to apply the penalty is in some sense a constant
cost, however the impact is also proportional to value, and for
sufficiently low value HTLCs can be ignored entirely if the HTLC
isn't included in the channel commitment)
Instead, we should investigate how we can
apply the ideas here with the more complicated fee structures we have.
Fee structures should be *simple* not complicated.
I mean, it's kind of great that we started off complicated -- if it
turns out base fee isn't necessary, it's easy to just set it to zero;
if we didn't have it, but needed it, it would be much more annoying to
add it in later.
Fee structures should also match reality, and allow node operators sufficient flexibility to capture their costs. I
think we have a design that does so quite well - its pretty simple, there's only two knobs, but the two knobs capture
exactly the two broad categories of costs a node operator has.
Color me an optimist, but I'm quite confident with sufficient elbow grease
and heuristics we can get 95% of the way there. We can and should revisit
these conversations if such exploration is done and we find that its not
possible, but until then this all feels incredibly premature.
Depends; I don't think it makes sense to try to ban nodes that don't have
a base fee of zero or anything, but random people on twitter advocating
that node operators should set it to zero and just worry about optimising
via the proportional fee and the min htlc amount seems fine.
Sure, the great thing about today is because the protocol exposes decent knobs operators can tune their fee structures
any way they want, providing competition and multiple paths to nodes with potentially very divergent fees depending on
the type of payment. Absent liquidity limits, this should provide better service for all types of payments - routing
them to nodes that will support those types of payments!
For an experimental plugin that aggressively splits payments up, I think
either ignoring channels with >0 base fee entirely, or deciding that
you're happy to spend a total of X sats on base fees, and then ignoring
channels whose base fee is greater than X/paths/path-length sats is fine.
Sure, experimental plugins can do whatever they want!
But long term, I also think that the base fee is an entirely unhelpful
complication that will eventually just be hardcoded to zero by everyone,
and eventually channels that propose non-zero base fees won't even be
gossiped. I don't expect that to happen any time soon though.
I very strongly disagree, as discussed, and am left highly dubious that it is a
practical complication in any case.
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