Configure htlc_max via plugin or API

[NCrashed]

Hi, I am an operator of routing Eclair node. There is a common practice to set higher fees when channel is depleted. That helps to reduce failing payment rate as wallets tend to select routes with lower fees. But, the approach conflicts with my marketing strategy to sustain low fees on my node. Dynamic fees skew statistics on explorers services.

There is also another way to signal wallets that the channel is not suited for payment. It is setting max_htlc parameter dynamically. The advantage of the method is that I can set constant fees and get encourage natural rebalancing simultaneously.

I want to implement a plugin for Eclair to set max_htlc dynamically, but I struggle to find part of sources that I should dig in. Could someone help me by pointing where to start?

[pm47]

There is also another way to signal wallets that the channel is not suited for payment

The "proper" way of signalling that a channel shouldn't be used would actually be to set the disable bit of channel_update. This is what we do at channel creation when you are fundee, the channel will only be enabled after your side goes above reserve (after you receive a few payments). That's a similar use case as the one you are describing. But apart from that, this bit is managed by eclair and used mostly in relation to connections/disconnections, so whatever you do with a plugin would be overridden.

I want to implement a plugin for Eclair to set max_htlc dynamically, but I struggle to find part of sources that I should dig in. Could someone help me where to start?

You can't customize this value, as per the protocol spec for privacy reasons:

Note that the htlc_maximum_msat field is static in the current protocol over the life of the channel: it is not designed to be indicative of real-time channel capacity in each direction, which would be both a massive data leak and uselessly spam the network.

Let's say you change your mind and choose to implement it with custom fees, here is how you would do it:

1. listen to the AvailableBalanceChanged event
2. upon receiving a AvailableBalanceChanged, look at event.commitments.availableBalanceForSend and maybe send a CMD_UPDATE_RELAY_FEE to the channel

[NCrashed]

@pm47 thanks for clearing that out! So, could I use disable/enable mechanism to mark depleted channels?

[pm47]

No, as I mentioned, this bit is managed by eclair, so your only option today is to use the fees.

[DerEwige]

@pm47

Note that the htlc_maximum_msat field is static in the current protocol over the life of the channel: it is not designed to be indicative of real-time channel capacity in each direction, which would be both a massive data leak and uselessly spam the network.

If I were to only update the max_htlc when I update the fee anyway, that would not increase the traffic on the network, right? The value is present in each update anyway afaik?

If I am ok with my channel state being leaked out to others, wouldn’t this massively help in route finding?

[DerEwige]

@t-bast
I have been running the dynamic maxHTLC for about a week now.
It works pretty well, but I have one major issue (which is regarding my implementation)

I just could not figure out how to change the state of my local channel (fees) without triggering an announcement to the public.

So right now, I use this to change my fees:
eclair.updateRelayFee(one, new MilliSatoshi(currentBaseFee), currentPercentFee, myTimeout).result(myTimeout.duration(), null);

This line updates the fees and releases an announcement (with the default maxHTLC)

Then I send another announcement a bit later with my wanted maxHTLCs.
I’m really unhappy with this, because I have to send 2 announcements, when one would be enough and better (because of spam measures)

So if anybody could lend me a hand and show me how to change the fees on my channel without triggering an automated announcement, that would be great.

[t-bast]

It doesn't make sense to update fees without telling the network, that's why this triggers the broadcast of a channel_update.
You can simply do it in the other order: modify max htlc first, then call updateRelayFee which will send the channel_update for you.

[DerEwige]

That would actually be pretty good.
But right now I do not really change the maxHTLC I just anounce it. (so the actuall enfroces HTLC stayes unchanged and I just annouce a smaller one)
I have the same problem for maxHTLC as for fee. I just could not figure out how to actually change the state in the channel by hand =/

[t-bast]

This is protected on purpose, because otherwise there are many ways plugins could completely mess up their channel state and trigger force-closes (or worse, loss of funds). We can't provide unrestricted edit access to the channel state, this is too dangerous.

However we can reasonably provide tools to update specific channel parameters such as max htlc. You can experiment with changing eclair to allow this and run your modified version of eclair to see if that works for you.

[DerEwige]

Ok Thanks,
Will have to push this one back a bit then