htlcswitch: add inbound routing fees receive support

channeldb/models/channel.go

@@ -115,6 +115,9 @@ type ForwardingPolicy struct {
 	// used to compute the required fee for a given HTLC.
 	FeeRate lnwire.MilliSatoshi
 
+	// InboundFee is the fee that must be paid for incoming HTLCs.
+	InboundFee InboundFee
+
 	// TimeLockDelta is the absolute time-lock value, expressed in blocks,
 	// that will be subtracted from an incoming HTLC's timelock value to
 	// create the time-lock value for the forwarded outgoing HTLC. The

channeldb/models/channel_edge_policy.go

@@ -70,7 +70,7 @@ type ChannelEdgePolicy struct {
 	// properly validate the set of signatures that cover these new fields,
 	// and ensure we're able to make upgrades to the network in a forwards
 	// compatible manner.
-	ExtraOpaqueData []byte
+	ExtraOpaqueData lnwire.ExtraOpaqueData
 }
 
 // Signature is a channel announcement signature, which is needed for proper

channeldb/models/inbound_fee.go

@@ -0,0 +1,53 @@
+package models
+
+import "github.com/lightningnetwork/lnd/lnwire"
+
+const (
+	// maxFeeRate is the maximum fee rate that we allow. It is set to allow
+	// a variable fee component of up to 10x the payment amount.
+	maxFeeRate = 10 * feeRateParts
+)
+
+type InboundFee struct {
+	Base int32
+	Rate int32
+}
+
+// NewInboundFeeFromWire constructs an inbound fee structure from a wire fee.
+func NewInboundFeeFromWire(fee lnwire.Fee) InboundFee {
+	return InboundFee{
+		Base: fee.BaseFee,
+		Rate: fee.FeeRate,
+	}
+}
+
+// ToWire converts the inbound fee to a wire fee structure.
+func (i *InboundFee) ToWire() lnwire.Fee {
+	return lnwire.Fee{
+		BaseFee: i.Base,
+		FeeRate: i.Rate,
+	}
+}
+
+// CalcFee calculates what the inbound fee should minimally be for forwarding
+// the given amount. This amount is the total of the outgoing amount plus the
+// outbound fee, which is what the inbound fee is based on.
+func (i *InboundFee) CalcFee(amt lnwire.MilliSatoshi) int64 {
+	fee := int64(i.Base)
+	rate := int64(i.Rate)
+
+	// Cap the rate to prevent overflows.
+	switch {
+	case rate > maxFeeRate:
+		rate = maxFeeRate
+
+	case rate < -maxFeeRate:
+		rate = -maxFeeRate
+	}
+
+	// Calculate proportional component. To keep the integer math simple,
+	// positive fees are rounded down while negative fees are rounded up.
+	fee += rate * int64(amt) / feeRateParts
+
+	return fee
+}

channeldb/models/inbound_fee_test.go

@@ -0,0 +1,33 @@
+package models
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestInboundFee(t *testing.T) {
+	t.Parallel()
+
+	// Test positive fee.
+	i := InboundFee{
+		Base: 5,
+		Rate: 500000,
+	}
+
+	require.Equal(t, int64(6), i.CalcFee(2))
+
+	// Expect fee to be rounded down.
+	require.Equal(t, int64(6), i.CalcFee(3))
+
+	// Test negative fee.
+	i = InboundFee{
+		Base: -5,
+		Rate: -500000,
+	}
+
+	require.Equal(t, int64(-6), i.CalcFee(2))
+
+	// Expect fee to be rounded up.
+	require.Equal(t, int64(-6), i.CalcFee(3))
+}

cmd/lncli/commands.go

@@ -2166,6 +2166,31 @@ var updateChannelPolicyCommand = cli.Command{
 				"0.000001 (millionths). Can not be set at " +
 				"the same time as fee_rate",
 		},
+		cli.Int64Flag{
+			Name: "inbound_base_fee_msat",
+			Usage: "the base inbound fee in milli-satoshis that " +
+				"will be charged for each forwarded HTLC, " +
+				"regardless of payment size. Its value must " +
+				"be zero or negative - it is a discount " +
+				"for using a particular incoming channel. " +
+				"Note that forwards will be rejected if the " +
+				"discount exceeds the outbound fee " +
+				"(forward at a loss), and lead to " +
+				"penalization by the sender",
+		},
+		cli.Int64Flag{
+			Name: "inbound_fee_rate_ppm",
+			Usage: "the inbound fee rate that will be charged " +
+				"proportionally based on the value of each " +
+				"forwarded HTLC and the outbound fee. Fee " +
+				"rate is expressed in parts per million and " +
+				"must be zero or negative - it is a discount " +
+				"for using a particular incoming channel." +
+				"Note that forwards will be rejected if the " +
+				"discount exceeds the outbound fee " +
+				"(forward at a loss), and lead to " +
+				"penalization by the sender",
+		},
 		cli.Uint64Flag{
 			Name: "time_lock_delta",
 			Usage: "the CLTV delta that will be applied to all " +
@@ -2318,10 +2343,26 @@ func updateChannelPolicy(ctx *cli.Context) error {
 		}
 	}
 
+	inboundBaseFeeMsat := ctx.Int64("inbound_base_fee_msat")
+	if inboundBaseFeeMsat < math.MinInt32 ||
+		inboundBaseFeeMsat > 0 {
+
+		return errors.New("inbound_base_fee_msat out of range")
+	}
+
+	inboundFeeRatePpm := ctx.Int64("inbound_fee_rate_ppm")
+	if inboundFeeRatePpm < math.MinInt32 ||
+		inboundFeeRatePpm > 0 {
+
+		return errors.New("inbound_fee_rate_ppm out of range")
+	}
+
 	req := &lnrpc.PolicyUpdateRequest{
-		BaseFeeMsat:   baseFee,
-		TimeLockDelta: uint32(timeLockDelta),
-		MaxHtlcMsat:   ctx.Uint64("max_htlc_msat"),
+		BaseFeeMsat:        baseFee,
+		TimeLockDelta:      uint32(timeLockDelta),
+		MaxHtlcMsat:        ctx.Uint64("max_htlc_msat"),
+		InboundBaseFeeMsat: int32(inboundBaseFeeMsat),
+		InboundFeeRatePpm:  int32(inboundFeeRatePpm),
 	}
 
 	if ctx.IsSet("min_htlc_msat") {

docs/release-notes/release-notes-0.18.0.md

@@ -1,17 +1,20 @@
 # Release Notes
+- [Release Notes](#release-notes)
 - [Bug Fixes](#bug-fixes)
 - [New Features](#new-features)
   - [Functional Enhancements](#functional-enhancements)
   - [RPC Additions](#rpc-additions)
   - [lncli Additions](#lncli-additions)
 - [Improvements](#improvements)
   - [Functional Updates](#functional-updates)
+    - [Tlv](#tlv)
+  - [Misc](#misc)
+    - [Logging](#logging)
   - [RPC Updates](#rpc-updates)
   - [lncli Updates](#lncli-updates)
   - [Code Health](#code-health)
   - [Breaking Changes](#breaking-changes)
   - [Performance Improvements](#performance-improvements)
-  - [Misc](#misc)
 - [Technical and Architectural Updates](#technical-and-architectural-updates)
   - [BOLT Spec Updates](#bolt-spec-updates)
   - [Testing](#testing)
@@ -109,6 +112,15 @@
 # New Features
 ## Functional Enhancements
 
+* Experimental support for [inbound routing
+  fees](https://github.com/lightningnetwork/lnd/pull/6703) is added. This allows
+  node operators to require senders to pay an inbound fee for forwards and
+  payments. It is recommended to only use negative fees (an inbound "discount")
+  initially to keep the channels open for senders that do not recognize inbound
+  fees. In this release, no send support for pathfinding and route building is
+  added yet. We first want to learn more about the impact that inbound fees have
+  on the routing economy.
+
 * A new config value,
   [sweeper.maxfeerate](https://github.com/lightningnetwork/lnd/pull/7823), is
   added so users can specify the max allowed fee rate when sweeping on-chain
@@ -421,6 +433,7 @@ bitcoin peers' feefilter values into account](https://github.com/lightningnetwor
 * Elle Mouton
 * ErikEk
 * Jesse de Wit
+* Joost Jager
 * Keagan McClelland
 * Marcos Fernandez Perez
 * Matt Morehouse

htlcswitch/interfaces.go

@@ -247,6 +247,7 @@ type ChannelLink interface {
 	CheckHtlcForward(payHash [32]byte, incomingAmt lnwire.MilliSatoshi,
 		amtToForward lnwire.MilliSatoshi,
 		incomingTimeout, outgoingTimeout uint32,
+		inboundFee models.InboundFee,
 		heightNow uint32, scid lnwire.ShortChannelID) *LinkError
 
 	// CheckHtlcTransit should return a nil error if the passed HTLC details

htlcswitch/link.go

@@ -2780,28 +2780,43 @@ func (l *channelLink) UpdateForwardingPolicy(
 func (l *channelLink) CheckHtlcForward(payHash [32]byte,
 	incomingHtlcAmt, amtToForward lnwire.MilliSatoshi,
 	incomingTimeout, outgoingTimeout uint32,
+	inboundFee models.InboundFee,
 	heightNow uint32, originalScid lnwire.ShortChannelID) *LinkError {
 
 	l.RLock()
 	policy := l.cfg.FwrdingPolicy
 	l.RUnlock()
 
-	// Using the amount of the incoming HTLC, we'll calculate the expected
-	// fee this incoming HTLC must carry in order to satisfy the
-	// constraints of the outgoing link.
-	expectedFee := ExpectedFee(policy, amtToForward)
+	// Using the outgoing HTLC amount, we'll calculate the outgoing
+	// fee this incoming HTLC must carry in order to satisfy the constraints
+	// of the outgoing link.
+	outFee := ExpectedFee(policy, amtToForward)
+
+	// Then calculate the inbound fee that we charge based on the sum of
+	// outgoing HTLC amount and outgoing fee.
+	inFee := inboundFee.CalcFee(amtToForward + outFee)
+
+	// Add up both fee components. It is important to calculate both fees
+	// separately. An alternative way of calculating is to first determine
+	// an aggregate fee and apply that to the outgoing HTLC amount. However,
+	// rounding may cause the result to be slightly higher than in the case
+	// of separately rounded fee components. This potentially causes failed
+	// forwards for senders and is something to be avoided.
+	expectedFee := inFee + int64(outFee)
 
 	// If the actual fee is less than our expected fee, then we'll reject
 	// this HTLC as it didn't provide a sufficient amount of fees, or the
 	// values have been tampered with, or the send used incorrect/dated
 	// information to construct the forwarding information for this hop. In
-	// any case, we'll cancel this HTLC. We're checking for this case first
-	// to leak as little information as possible.
-	actualFee := incomingHtlcAmt - amtToForward
+	// any case, we'll cancel this HTLC.
+	actualFee := int64(incomingHtlcAmt) - int64(amtToForward)
 	if incomingHtlcAmt < amtToForward || actualFee < expectedFee {
 		l.log.Warnf("outgoing htlc(%x) has insufficient fee: "+
-			"expected %v, got %v",
-			payHash[:], int64(expectedFee), int64(actualFee))
+			"expected %v, got %v: incoming=%v, outgoing=%v, "+
+			"inboundFee=%v",
+			payHash[:], expectedFee, actualFee,
+			incomingHtlcAmt, amtToForward, inboundFee,
+		)
 
 		// As part of the returned error, we'll send our latest routing
 		// policy so the sending node obtains the most up to date data.
@@ -3330,6 +3345,8 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 				// round of processing.
 				chanIterator.EncodeNextHop(buf)
 
+				inboundFee := l.cfg.FwrdingPolicy.InboundFee
+
 				updatePacket := &htlcPacket{
 					incomingChanID:  l.ShortChanID(),
 					incomingHTLCID:  pd.HtlcIndex,
@@ -3342,6 +3359,7 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 					incomingTimeout: pd.Timeout,
 					outgoingTimeout: fwdInfo.OutgoingCTLV,
 					customRecords:   pld.CustomRecords(),
+					inboundFee:      inboundFee,
 				}
 				switchPackets = append(
 					switchPackets, updatePacket,
@@ -3394,6 +3412,8 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 			// have been added to switchPackets at the top of this
 			// section.
 			if fwdPkg.State == channeldb.FwdStateLockedIn {
+				inboundFee := l.cfg.FwrdingPolicy.InboundFee
+
 				updatePacket := &htlcPacket{
 					incomingChanID:  l.ShortChanID(),
 					incomingHTLCID:  pd.HtlcIndex,
@@ -3406,6 +3426,7 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 					incomingTimeout: pd.Timeout,
 					outgoingTimeout: fwdInfo.OutgoingCTLV,
 					customRecords:   pld.CustomRecords(),
+					inboundFee:      inboundFee,
 				}
 
 				fwdPkg.FwdFilter.Set(idx)