htlcswitch: add receiver-side inbound fee 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/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))
+}

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
+	// Using the outgoing HTLC amount, we'll calculate the outgoing
-	// fee this incoming HTLC must carry in order to satisfy the
+	// fee this incoming HTLC must carry in order to satisfy the constraints
-	// constraints of the outgoing link.
+	// of the outgoing link.
-	expectedFee := ExpectedFee(policy, amtToForward)
+	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
+	// any case, we'll cancel this HTLC.
-	// to leak as little information as possible.
+	actualFee := int64(incomingHtlcAmt) - int64(amtToForward)
-	actualFee := incomingHtlcAmt - amtToForward
 	if incomingHtlcAmt < amtToForward || actualFee < expectedFee {
 		l.log.Warnf("outgoing htlc(%x) has insufficient fee: "+
-			"expected %v, got %v",
+			"expected %v, got %v: incoming=%v, outgoing=%v, "+
-			payHash[:], int64(expectedFee), int64(actualFee))
+			"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)

htlcswitch/link_test.go

@@ -643,6 +643,206 @@ func testChannelLinkMultiHopPayment(t *testing.T,
 	}
 }
 
+func TestChannelLinkInboundFee(t *testing.T) {
+	t.Parallel()
+
+	t.Run("negative", func(t *testing.T) {
+		t.Parallel()
+
+		bobInboundFee := models.InboundFee{
+			Base: -500,
+			Rate: -100,
+		}
+
+		// Bob is supposed to sent Carol 1000000 msats. For this, he
+		// will charge an out fee of 1000 msat (the default hop network
+		// policy). Bob's inbound fee is based on the sum of outgoing
+		// htlc amount and the out fee that Bob charges. The value of
+		// this sum is 1001000. The proportional component of the
+		// inbound fee is -0.01% of the sum, which is -100 (rounded
+		// up). Added to this is the base inbound fee of -500, making
+		// for a total inbound fee of -600.
+		const expectedBobInFee = -600
+
+		testChannelLinkInboundFee(
+			t, bobInboundFee, expectedBobInFee, false,
+		)
+	})
+
+	t.Run("negative overpaid", func(t *testing.T) {
+		t.Parallel()
+
+		bobInboundFee := models.InboundFee{
+			Base: -500,
+			Rate: -100,
+		}
+
+		// Alice is not aware of the inbound discount and pays the full
+		// outbound fee.
+		const expectedBobInFee = 0
+
+		testChannelLinkInboundFee(
+			t, bobInboundFee, expectedBobInFee, false,
+		)
+	})
+
+	t.Run("negative total", func(t *testing.T) {
+		t.Parallel()
+
+		bobInboundFee := models.InboundFee{
+			Base: -5000,
+		}
+
+		const expectedBobInFee = -5000
+
+		// Bob's inbound discount exceeds his outbound fee. Forwards
+		// carrying a negative total fee should be rejected.
+		testChannelLinkInboundFee(
+			t, bobInboundFee, expectedBobInFee, true,
+		)
+	})
+
+	t.Run("positive", func(t *testing.T) {
+		t.Parallel()
+
+		bobInboundFee := models.InboundFee{
+			Base: 1_000,
+			Rate: 100_000,
+		}
+
+		const expectedBobInFee = 101_100
+
+		testChannelLinkInboundFee(
+			t, bobInboundFee, expectedBobInFee, false,
+		)
+	})
+}
+
+func testChannelLinkInboundFee(t *testing.T, //nolint:thelper
+	bobInboundFee models.InboundFee, expectedBobInFee int64,
+	expectedFail bool) {
+
+	channels, _, err := createClusterChannels(
+		t, btcutil.SatoshiPerBitcoin*3, btcutil.SatoshiPerBitcoin*5,
+	)
+	require.NoError(t, err, "unable to create channel")
+
+	n := newThreeHopNetwork(t, channels.aliceToBob, channels.bobToAlice,
+		channels.bobToCarol, channels.carolToBob, testStartingHeight)
+
+	require.NoError(t, n.start())
+	defer n.stop()
+
+	bobPolicy := n.globalPolicy
+	bobPolicy.InboundFee = bobInboundFee
+	n.firstBobChannelLink.UpdateForwardingPolicy(bobPolicy)
+
+	// Set an inbound fee for Carol. Because Carol is the payee, the fee
+	// should not be applied.
+	carolPolicy := n.globalPolicy
+	carolPolicy.InboundFee = models.InboundFee{
+		Base: -2_000,
+		Rate: -200_000,
+	}
+	n.carolChannelLink.UpdateForwardingPolicy(carolPolicy)
+
+	carolBandwidthBefore := n.carolChannelLink.Bandwidth()
+	firstBobBandwidthBefore := n.firstBobChannelLink.Bandwidth()
+	secondBobBandwidthBefore := n.secondBobChannelLink.Bandwidth()
+	aliceBandwidthBefore := n.aliceChannelLink.Bandwidth()
+
+	const (
+		expectedCarolInboundFee = 0
+
+		// Expect Bob's outbound fee to match the default hop network
+		// policy.
+		expectedBobOutboundFee = 1_000
+	)
+
+	amount := lnwire.MilliSatoshi(1_000_000)
+	htlcAmt := lnwire.MilliSatoshi(1_000_000 +
+		expectedCarolInboundFee + expectedBobOutboundFee +
+		expectedBobInFee,
+	)
+	totalTimelock := uint32(112)
+
+	hops := []*hop.Payload{
+		{
+			FwdInfo: hop.ForwardingInfo{
+				NextHop: n.carolChannelLink.
+					ShortChanID(),
+				AmountToForward: 1_000_000,
+				OutgoingCTLV:    106,
+			},
+		},
+		{
+			FwdInfo: hop.ForwardingInfo{
+				AmountToForward: 1_000_000,
+				OutgoingCTLV:    106,
+			},
+		},
+	}
+
+	receiver := n.carolServer
+	firstHop := n.firstBobChannelLink.ShortChanID()
+	rhash, err := makePayment(
+		n.aliceServer, n.carolServer, firstHop, hops, amount, htlcAmt,
+		totalTimelock,
+	).Wait(30 * time.Second)
+
+	if expectedFail {
+		require.Error(t, err)
+
+		return
+	}
+
+	require.NoError(t, err, "unable to send payment")
+
+	// Wait for Alice and Bob's second link to receive the revocation.
+	time.Sleep(2 * time.Second)
+
+	// Check that Carol invoice was settled and bandwidth of HTLC
+	// links were changed.
+	invoice, err := receiver.registry.LookupInvoice(
+		context.Background(), rhash,
+	)
+	require.NoError(t, err, "unable to get invoice")
+	require.Equal(t, invpkg.ContractSettled, invoice.State,
+		"carol invoice haven't been settled")
+
+	expectedAliceBandwidth := aliceBandwidthBefore - htlcAmt
+	require.Equalf(t,
+		expectedAliceBandwidth, n.aliceChannelLink.Bandwidth(),
+		"channel bandwidth incorrect: expected %v, got %v",
+		expectedAliceBandwidth, n.aliceChannelLink.Bandwidth(),
+	)
+
+	expectedBobBandwidth1 := firstBobBandwidthBefore + htlcAmt
+	require.Equalf(t,
+		expectedBobBandwidth1, n.firstBobChannelLink.Bandwidth(),
+		"channel bandwidth incorrect: expected %v, got %v",
+		expectedBobBandwidth1, n.firstBobChannelLink.Bandwidth(),
+	)
+
+	bobCarolDelta := lnwire.MilliSatoshi(
+		int64(amount) + expectedCarolInboundFee,
+	)
+
+	expectedBobBandwidth2 := secondBobBandwidthBefore - bobCarolDelta
+	require.Equalf(t,
+		expectedBobBandwidth2, n.secondBobChannelLink.Bandwidth(),
+		"channel bandwidth incorrect: expected %v, got %v",
+		expectedBobBandwidth2, n.secondBobChannelLink.Bandwidth(),
+	)
+
+	expectedCarolBandwidth := carolBandwidthBefore + bobCarolDelta
+	require.Equalf(t,
+		expectedCarolBandwidth, n.carolChannelLink.Bandwidth(),
+		"channel bandwidth incorrect: expected %v, got %v",
+		expectedCarolBandwidth, n.carolChannelLink.Bandwidth(),
+	)
+}
+
 // TestChannelLinkCancelFullCommitment tests the ability for links to cancel
 // forwarded HTLCs once all of their commitment slots are full.
 func TestChannelLinkCancelFullCommitment(t *testing.T) {
@@ -5994,7 +6194,9 @@ func TestCheckHtlcForward(t *testing.T) {
 
 	t.Run("satisfied", func(t *testing.T) {
 		result := link.CheckHtlcForward(hash, 1500, 1000,
-			200, 150, 0, lnwire.ShortChannelID{})
+			200, 150, models.InboundFee{}, 0,
+			lnwire.ShortChannelID{},
+		)
 		if result != nil {
 			t.Fatalf("expected policy to be satisfied")
 		}
@@ -6002,7 +6204,9 @@ func TestCheckHtlcForward(t *testing.T) {
 
 	t.Run("below minhtlc", func(t *testing.T) {
 		result := link.CheckHtlcForward(hash, 100, 50,
-			200, 150, 0, lnwire.ShortChannelID{})
+			200, 150, models.InboundFee{}, 0,
+			lnwire.ShortChannelID{},
+		)
 		if _, ok := result.WireMessage().(*lnwire.FailAmountBelowMinimum); !ok {
 			t.Fatalf("expected FailAmountBelowMinimum failure code")
 		}
@@ -6010,7 +6214,9 @@ func TestCheckHtlcForward(t *testing.T) {
 
 	t.Run("above maxhtlc", func(t *testing.T) {
 		result := link.CheckHtlcForward(hash, 1500, 1200,
-			200, 150, 0, lnwire.ShortChannelID{})
+			200, 150, models.InboundFee{}, 0,
+			lnwire.ShortChannelID{},
+		)
 		if _, ok := result.WireMessage().(*lnwire.FailTemporaryChannelFailure); !ok {
 			t.Fatalf("expected FailTemporaryChannelFailure failure code")
 		}
@@ -6018,7 +6224,9 @@ func TestCheckHtlcForward(t *testing.T) {
 
 	t.Run("insufficient fee", func(t *testing.T) {
 		result := link.CheckHtlcForward(hash, 1005, 1000,
-			200, 150, 0, lnwire.ShortChannelID{})
+			200, 150, models.InboundFee{}, 0,
+			lnwire.ShortChannelID{},
+		)
 		if _, ok := result.WireMessage().(*lnwire.FailFeeInsufficient); !ok {
 			t.Fatalf("expected FailFeeInsufficient failure code")
 		}
@@ -6031,7 +6239,7 @@ func TestCheckHtlcForward(t *testing.T) {
 
 		result := link.CheckHtlcForward(
 			hash, 100005, 100000, 200,
-			150, 0, lnwire.ShortChannelID{},
+			150, models.InboundFee{}, 0, lnwire.ShortChannelID{},
 		)
 		_, ok := result.WireMessage().(*lnwire.FailFeeInsufficient)
 		require.True(t, ok, "expected FailFeeInsufficient failure code")
@@ -6039,7 +6247,9 @@ func TestCheckHtlcForward(t *testing.T) {
 
 	t.Run("expiry too soon", func(t *testing.T) {
 		result := link.CheckHtlcForward(hash, 1500, 1000,
-			200, 150, 190, lnwire.ShortChannelID{})
+			200, 150, models.InboundFee{}, 190,
+			lnwire.ShortChannelID{},
+		)
 		if _, ok := result.WireMessage().(*lnwire.FailExpiryTooSoon); !ok {
 			t.Fatalf("expected FailExpiryTooSoon failure code")
 		}
@@ -6047,7 +6257,9 @@ func TestCheckHtlcForward(t *testing.T) {
 
 	t.Run("incorrect cltv expiry", func(t *testing.T) {
 		result := link.CheckHtlcForward(hash, 1500, 1000,
-			200, 190, 0, lnwire.ShortChannelID{})
+			200, 190, models.InboundFee{}, 0,
+			lnwire.ShortChannelID{},
+		)
 		if _, ok := result.WireMessage().(*lnwire.FailIncorrectCltvExpiry); !ok {
 			t.Fatalf("expected FailIncorrectCltvExpiry failure code")
 		}
@@ -6057,11 +6269,37 @@ func TestCheckHtlcForward(t *testing.T) {
 	t.Run("cltv expiry too far in the future", func(t *testing.T) {
 		// Check that expiry isn't too far in the future.
 		result := link.CheckHtlcForward(hash, 1500, 1000,
-			10200, 10100, 0, lnwire.ShortChannelID{})
+			10200, 10100, models.InboundFee{}, 0,
+			lnwire.ShortChannelID{},
+		)
 		if _, ok := result.WireMessage().(*lnwire.FailExpiryTooFar); !ok {
 			t.Fatalf("expected FailExpiryTooFar failure code")
 		}
 	})
+
+	t.Run("inbound fee satisfied", func(t *testing.T) {
+		t.Parallel()
+
+		result := link.CheckHtlcForward(hash, 1000+10-2-1, 1000,
+			200, 150, models.InboundFee{Base: -2, Rate: -1_000},
+			0, lnwire.ShortChannelID{})
+		if result != nil {
+			t.Fatalf("expected policy to be satisfied")
+		}
+	})
+
+	t.Run("inbound fee insufficient", func(t *testing.T) {
+		t.Parallel()
+
+		result := link.CheckHtlcForward(hash, 1000+10-10-101-1, 1000,
+			200, 150, models.InboundFee{Base: -10, Rate: -100_000},
+			0, lnwire.ShortChannelID{})
+
+		msg := result.WireMessage()
+		if _, ok := msg.(*lnwire.FailFeeInsufficient); !ok {
+			t.Fatalf("expected FailFeeInsufficient failure code")
+		}
+	})
 }
 
 // TestChannelLinkCanceledInvoice in this test checks the interaction

htlcswitch/mock.go

@@ -834,7 +834,7 @@ func (f *mockChannelLink) HandleChannelUpdate(lnwire.Message) {
 func (f *mockChannelLink) UpdateForwardingPolicy(_ models.ForwardingPolicy) {
 }
 func (f *mockChannelLink) CheckHtlcForward([32]byte, lnwire.MilliSatoshi,
-	lnwire.MilliSatoshi, uint32, uint32, uint32,
+	lnwire.MilliSatoshi, uint32, uint32, models.InboundFee, uint32,
 	lnwire.ShortChannelID) *LinkError {
 
 	return f.checkHtlcForwardResult

htlcswitch/packet.go

@@ -2,6 +2,7 @@ package htlcswitch
 
 import (
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/record"
@@ -103,6 +104,9 @@ type htlcPacket struct {
 	// but receives a channel_update with the alias SCID. Instead, the
 	// payer should receive a channel_update with the public SCID.
 	originalOutgoingChanID lnwire.ShortChannelID
+
+	// inboundFee is the fee schedule of the incoming channel.
+	inboundFee models.InboundFee
 }
 
 // inKey returns the circuit key used to identify the incoming htlc.

htlcswitch/switch.go

@@ -1178,7 +1178,9 @@ func (s *Switch) handlePacketForward(packet *htlcPacket) error {
 				failure = link.CheckHtlcForward(
 					htlc.PaymentHash, packet.incomingAmount,
 					packet.amount, packet.incomingTimeout,
-					packet.outgoingTimeout, currentHeight,
+					packet.outgoingTimeout,
+					packet.inboundFee,
+					currentHeight,
 					packet.originalOutgoingChanID,
 				)
 			}

peer/brontide.go

@@ -961,12 +961,26 @@ func (p *Brontide) loadActiveChannels(chans []*channeldb.OpenChannel) (
 		// routing policy into a forwarding policy.
 		var forwardingPolicy *models.ForwardingPolicy
 		if selfPolicy != nil {
+			var inboundWireFee lnwire.Fee
+			_, err := selfPolicy.ExtraOpaqueData.ExtractRecords(
+				&inboundWireFee,
+			)
+			if err != nil {
+				return nil, err
+			}
+
+			inboundFee := models.NewInboundFeeFromWire(
+				inboundWireFee,
+			)
+
 			forwardingPolicy = &models.ForwardingPolicy{
 				MinHTLCOut:    selfPolicy.MinHTLC,
 				MaxHTLC:       selfPolicy.MaxHTLC,
 				BaseFee:       selfPolicy.FeeBaseMSat,
 				FeeRate:       selfPolicy.FeeProportionalMillionths,
 				TimeLockDelta: uint32(selfPolicy.TimeLockDelta),
+
+				InboundFee: inboundFee,
 			}
 		} else {
 			p.log.Warnf("Unable to find our forwarding policy "+