routing: add inbound fee support to pathfinding

Add sender-side support for inbound fees in pathfinding
and route building.

channeldb/graph.go

@@ -490,6 +490,16 @@ func (c *ChannelGraph) ForEachNodeDirectedChannel(tx kvdb.RTx,
 			cachedInPolicy.ToNodeFeatures = toNodeFeatures
 		}
 
+		var inboundFee lnwire.Fee
+		if p1 != nil {
+			// Extract inbound fee. If there is a decoding error,
+			// skip this edge.
+			_, err := p1.ExtraOpaqueData.ExtractRecords(&inboundFee)
+			if err != nil {
+				return nil
+			}
+		}
+
 		directedChannel := &DirectedChannel{
 			ChannelID:    e.ChannelID,
 			IsNode1:      node == e.NodeKey1Bytes,
@@ -497,6 +507,7 @@ func (c *ChannelGraph) ForEachNodeDirectedChannel(tx kvdb.RTx,
 			Capacity:     e.Capacity,
 			OutPolicySet: p1 != nil,
 			InPolicy:     cachedInPolicy,
+			InboundFee:   inboundFee,
 		}
 
 		if node == e.NodeKey2Bytes {

channeldb/graph_cache.go

@@ -59,6 +59,9 @@ type DirectedChannel struct {
 	// source, so we're always interested in the edge that arrives to us
 	// from the other node.
 	InPolicy *models.CachedEdgePolicy
+
+	// Inbound fees of this node.
+	InboundFee lnwire.Fee
 }
 
 // DeepCopy creates a deep copy of the channel, including the incoming policy.
@@ -220,6 +223,14 @@ func (c *GraphCache) updateOrAddEdge(node route.Vertex, edge *DirectedChannel) {
 func (c *GraphCache) UpdatePolicy(policy *models.ChannelEdgePolicy, fromNode,
 	toNode route.Vertex, edge1 bool) {
 
+	// Extract inbound fee if possible and available. If there is a decoding
+	// error, ignore this policy.
+	var inboundFee lnwire.Fee
+	_, err := policy.ExtraOpaqueData.ExtractRecords(&inboundFee)
+	if err != nil {
+		return
+	}
+
 	c.mtx.Lock()
 	defer c.mtx.Unlock()
 
@@ -240,11 +251,13 @@ func (c *GraphCache) UpdatePolicy(policy *models.ChannelEdgePolicy, fromNode,
 		// policy for node 1.
 		case channel.IsNode1 && edge1:
 			channel.OutPolicySet = true
+			channel.InboundFee = inboundFee
 
 		// This is node 2, and it is edge 2, so this is the outgoing
 		// policy for node 2.
 		case !channel.IsNode1 && !edge1:
 			channel.OutPolicySet = true
+			channel.InboundFee = inboundFee
 
 		// The other two cases left mean it's the inbound policy for the
 		// node.

channeldb/graph_cache_test.go

@@ -75,6 +75,10 @@ func TestGraphCacheAddNode(t *testing.T) {
 			ChannelID:    1000,
 			ChannelFlags: lnwire.ChanUpdateChanFlags(channelFlagA),
 			ToNode:       nodeB,
+			// Define an inbound fee.
+			ExtraOpaqueData: []byte{
+				253, 217, 3, 8, 0, 0, 0, 10, 0, 0, 0, 20,
+			},
 		}
 		inPolicy1 := &models.ChannelEdgePolicy{
 			ChannelID:    1000,
@@ -124,8 +128,18 @@ func TestGraphCacheAddNode(t *testing.T) {
 			edges map[uint64]*DirectedChannel) error {
 
 			nodes[node] = struct{}{}
-			for chanID := range edges {
+			for chanID, directedChannel := range edges {
 				chans[chanID] = struct{}{}
+
+				if node == nodeA {
+					require.NotZero(
+						t, directedChannel.InboundFee,
+					)
+				} else {
+					require.Zero(
+						t, directedChannel.InboundFee,
+					)
+				}
 			}
 
 			return nil

channeldb/graph_test.go

@@ -674,7 +674,7 @@ func createChannelEdge(db kvdb.Backend, node1, node2 *LightningNode) (
 		FeeBaseMSat:               4352345,
 		FeeProportionalMillionths: 3452352,
 		ToNode:                    secondNode,
-		ExtraOpaqueData:           []byte("new unknown feature2"),
+		ExtraOpaqueData:           []byte{1, 0},
 	}
 	edge2 := &models.ChannelEdgePolicy{
 		SigBytes:                  testSig.Serialize(),
@@ -688,7 +688,7 @@ func createChannelEdge(db kvdb.Backend, node1, node2 *LightningNode) (
 		FeeBaseMSat:               4352345,
 		FeeProportionalMillionths: 90392423,
 		ToNode:                    firstNode,
-		ExtraOpaqueData:           []byte("new unknown feature1"),
+		ExtraOpaqueData:           []byte{1, 0},
 	}
 
 	return edgeInfo, edge1, edge2
@@ -3929,7 +3929,17 @@ func TestGraphCacheForEachNodeChannel(t *testing.T) {
 	require.Nil(t, err)
 
 	// Create an edge and add it to the db.
-	edgeInfo, _, _ := createChannelEdge(graph.db, node1, node2)
+	edgeInfo, e1, e2 := createChannelEdge(graph.db, node1, node2)
+
+	// Because of lexigraphical sorting and the usage of random node keys in
+	// this test, we need to determine which edge belongs to node 1 at
+	// runtime.
+	var edge1 *models.ChannelEdgePolicy
+	if e1.ToNode == node2.PubKeyBytes {
+		edge1 = e1
+	} else {
+		edge1 = e2
+	}
 
 	// Add the channel, but only insert a single edge into the graph.
 	require.NoError(t, graph.AddChannelEdge(edgeInfo))
@@ -3952,6 +3962,28 @@ func TestGraphCacheForEachNodeChannel(t *testing.T) {
 	// We should be able to accumulate the single channel added, even
 	// though we have a nil edge policy here.
 	require.NotNil(t, getSingleChannel())
+
+	// Set an inbound fee and check that it is properly returned.
+	edge1.ExtraOpaqueData = []byte{
+		253, 217, 3, 8, 0, 0, 0, 10, 0, 0, 0, 20,
+	}
+	require.NoError(t, graph.UpdateEdgePolicy(edge1))
+
+	directedChan := getSingleChannel()
+	require.NotNil(t, directedChan)
+	require.Equal(t, directedChan.InboundFee, lnwire.Fee{
+		BaseFee: 10,
+		FeeRate: 20,
+	})
+
+	// Set an invalid inbound fee and check that the edge is no longer
+	// returned.
+	edge1.ExtraOpaqueData = []byte{
+		253, 217, 3, 8, 0,
+	}
+	require.NoError(t, graph.UpdateEdgePolicy(edge1))
+
+	require.Nil(t, getSingleChannel())
 }
 
 // TestGraphLoading asserts that the cache is properly reconstructed after a

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

@@ -117,9 +117,8 @@
   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
+  fees. [Send support](https://github.com/lightningnetwork/lnd/pull/6934) is
-  added yet. We first want to learn more about the impact that inbound fees have
+  implemented as well.
-  on the routing economy.
 
 * A new config value,
   [sweeper.maxfeerate](https://github.com/lightningnetwork/lnd/pull/7823), is

itest/lnd_multi-hop-payments_test.go

@@ -70,7 +70,7 @@ func testMultiHopPayments(ht *lntest.HarnessTest) {
 	// channel edges to relatively large non default values. This makes it
 	// possible to pick up more subtle fee calculation errors.
 	maxHtlc := lntest.CalculateMaxHtlc(chanAmt)
-	const aliceBaseFeeSat = 1
+	const aliceBaseFeeSat = 20
 	const aliceFeeRatePPM = 100000
 	updateChannelPolicy(
 		ht, alice, chanPointAlice, aliceBaseFeeSat*1000,
@@ -81,8 +81,8 @@ func testMultiHopPayments(ht *lntest.HarnessTest) {
 	// Define a negative inbound fee for Alice, to verify that this is
 	// backwards compatible with an older sender ignoring the discount.
 	const (
-		aliceInboundBaseFeeMsat = -1
+		aliceInboundBaseFeeMsat = -2000
-		aliceInboundFeeRate     = -10000
+		aliceInboundFeeRate     = -50000 // 5%
 	)
 
 	updateChannelPolicy(
@@ -119,12 +119,11 @@ func testMultiHopPayments(ht *lntest.HarnessTest) {
 	ht.AssertAmountPaid("Alice(local) => Bob(remote)", alice,
 		chanPointAlice, expectedAmountPaidAtoB, int64(0))
 
-	// To forward a payment of 1000 sat, Alice is charging a fee of 1 sat +
+	// To forward a payment of 1000 sat, Alice is charging a fee of 20 sat +
-	// 10% = 101 sat. Note that this does not include the inbound fee
+	// 10% = 120 sat, plus the inbound fee over 1120 (= 1000 + 120) sat of
-	// (discount) because there is no sender support yet.
+	// -2 sat - 5% = -58 sat. This makes a total of 62 sat per payment. For
-	const aliceFeePerPayment = aliceBaseFeeSat +
+	// 5 payments, it works out to 310 sat.
-		(paymentAmt * aliceFeeRatePPM / 1_000_000)
+	const expectedFeeAlice = 310
-	const expectedFeeAlice = numPayments * aliceFeePerPayment
 
 	// Dave needs to pay what Alice pays plus Alice's fee.
 	expectedAmountPaidDtoA := expectedAmountPaidAtoB + expectedFeeAlice
@@ -134,12 +133,10 @@ func testMultiHopPayments(ht *lntest.HarnessTest) {
 	ht.AssertAmountPaid("Dave(local) => Alice(remote)", dave,
 		chanPointDave, expectedAmountPaidDtoA, int64(0))
 
-	// To forward a payment of 1101 sat, Dave is charging a fee of
+	// To forward a payment of 1062 sat, Dave is charging a fee of 5 sat +
-	// 5 sat + 15% = 170.15 sat. This is rounded down in rpcserver to 170.
+	// 15% = 164.3 sat. For 5 payments this is 821.5 sat. This test works
-	const davePaymentAmt = paymentAmt + aliceFeePerPayment
+	// with sats, so we need to round down to 821.
-	const daveFeePerPayment = daveBaseFeeSat +
+	const expectedFeeDave = 821
-		(davePaymentAmt * daveFeeRatePPM / 1_000_000)
-	const expectedFeeDave = numPayments * daveFeePerPayment
 
 	// Carol needs to pay what Dave pays plus Dave's fee.
 	expectedAmountPaidCtoD := expectedAmountPaidDtoA + expectedFeeDave

routing/graph.go

@@ -103,7 +103,10 @@ func (g *CachedGraph) FetchAmountPairCapacity(nodeFrom, nodeTo route.Vertex,
 	amount lnwire.MilliSatoshi) (btcutil.Amount, error) {
 
 	// Create unified edges for all incoming connections.
-	u := newNodeEdgeUnifier(g.sourceNode(), nodeTo, nil)
+	//
+	// Note: Inbound fees are not used here because this method is only used
+	// by a deprecated router rpc.
+	u := newNodeEdgeUnifier(g.sourceNode(), nodeTo, false, nil)
 
 	err := u.addGraphPolicies(g)
 	if err != nil {
@@ -116,7 +119,7 @@ func (g *CachedGraph) FetchAmountPairCapacity(nodeFrom, nodeTo route.Vertex,
 			nodeFrom, nodeTo)
 	}
 
-	edge := edgeUnifier.getEdgeNetwork(amount)
+	edge := edgeUnifier.getEdgeNetwork(amount, 0)
 	if edge == nil {
 		return 0, fmt.Errorf("no edge for node pair %v -> %v "+
 			"(amount %v)", nodeFrom, nodeTo, amount)

routing/heap.go

@@ -18,10 +18,16 @@ type nodeWithDist struct {
 	// outgoing edges (channels) emanating from a node.
 	node route.Vertex
 
-	// amountToReceive is the amount that should be received by this node.
+	// netAmountReceived is the amount that should be received by this node.
 	// Either as final payment to the final node or as an intermediate
-	// amount that includes also the fees for subsequent hops.
+	// amount that includes also the fees for subsequent hops. This node's
-	amountToReceive lnwire.MilliSatoshi
+	// inbound fee is already subtracted from the htlc amount - if
+	// applicable.
+	netAmountReceived lnwire.MilliSatoshi
+
+	// outboundFee is the fee that this node charges on the outgoing
+	// channel.
+	outboundFee lnwire.MilliSatoshi
 
 	// incomingCltv is the expected absolute expiry height for the incoming
 	// htlc of this node. This value should already include the final cltv

routing/pathfind.go

@@ -11,6 +11,7 @@ import (
 	"github.com/btcsuite/btcd/btcutil"
 	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/feature"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/record"
@@ -118,7 +119,7 @@ type finalHopParams struct {
 // makes calculating the totals during route construction difficult if we
 // include blinded paths on the first pass).
 //
-// NOTE: The passed slice of ChannelHops MUST be sorted in forward order: from
+// NOTE: The passed slice of unified edges MUST be sorted in forward order: from
 // the source to the target node of the path finding attempt. It is assumed that
 // any feature vectors on all hops have been validated for transitive
 // dependencies.
@@ -157,7 +158,7 @@ func newRoute(sourceVertex route.Vertex,
 		// we compute the route in reverse.
 		var (
 			amtToForward        lnwire.MilliSatoshi
-			fee                 lnwire.MilliSatoshi
+			fee                 int64
 			totalAmtMsatBlinded lnwire.MilliSatoshi
 			outgoingTimeLock    uint32
 			tlvPayload          bool
@@ -195,7 +196,7 @@ func newRoute(sourceVertex route.Vertex,
 
 			// Fee is not part of the hop payload, but only used for
 			// reporting through RPC. Set to zero for the final hop.
-			fee = lnwire.MilliSatoshi(0)
+			fee = 0
 
 			// As this is the last hop, we'll use the specified
 			// final CLTV delta value instead of the value from the
@@ -244,7 +245,18 @@ func newRoute(sourceVertex route.Vertex,
 			// and its policy for the outgoing channel. This policy
 			// is stored as part of the incoming channel of
 			// the next hop.
-			fee = pathEdges[i+1].policy.ComputeFee(amtToForward)
+			outboundFee := pathEdges[i+1].policy.ComputeFee(
+				amtToForward,
+			)
+
+			inboundFee := pathEdges[i].inboundFees.CalcFee(
+				amtToForward + outboundFee,
+			)
+
+			fee = int64(outboundFee) + inboundFee
+			if fee < 0 {
+				fee = 0
+			}
 
 			// We'll take the total timelock of the preceding hop as
 			// the outgoing timelock or this hop. Then we'll
@@ -275,7 +287,7 @@ func newRoute(sourceVertex route.Vertex,
 		// Finally, we update the amount that needs to flow into the
 		// *next* hop, which is the amount this hop needs to forward,
 		// accounting for the fee that it takes.
-		nextIncomingAmount = amtToForward + fee
+		nextIncomingAmount = amtToForward + lnwire.MilliSatoshi(fee)
 	}
 
 	// If we are creating a route to a blinded path, we need to add some
@@ -660,13 +672,13 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 	// Don't record the initial partial path in the distance map and reserve
 	// that key for the source key in the case we route to ourselves.
 	partialPath := &nodeWithDist{
-		dist:            0,
+		dist:              0,
-		weight:          0,
+		weight:            0,
-		node:            target,
+		node:              target,
-		amountToReceive: amt,
+		netAmountReceived: amt,
-		incomingCltv:    finalHtlcExpiry,
+		incomingCltv:      finalHtlcExpiry,
-		probability:     1,
+		probability:       1,
-		routingInfoSize: lastHopPayloadSize,
+		routingInfoSize:   lastHopPayloadSize,
 	}
 
 	// Calculate the absolute cltv limit. Use uint64 to prevent an overflow
@@ -703,9 +715,27 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 
 		edgesExpanded++
 
+		// Calculate inbound fee charged by "to" node. The exit hop
+		// doesn't charge inbound fees. If the "to" node is the exit
+		// hop, its inbound fees have already been set to zero by
+		// nodeEdgeUnifier.
+		inboundFee := edge.inboundFees.CalcFee(
+			toNodeDist.netAmountReceived,
+		)
+
+		// Make sure that the node total fee is never negative.
+		// Routing nodes treat a total fee that turns out
+		// negative as a zero fee and pathfinding should do the
+		// same.
+		minInboundFee := -int64(toNodeDist.outboundFee)
+		if inboundFee < minInboundFee {
+			inboundFee = minInboundFee
+		}
+
 		// Calculate amount that the candidate node would have to send
 		// out.
-		amountToSend := toNodeDist.amountToReceive
+		amountToSend := toNodeDist.netAmountReceived +
+			lnwire.MilliSatoshi(inboundFee)
 
 		// Request the success probability for this edge.
 		edgeProbability := r.ProbabilitySource(
@@ -735,10 +765,15 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		// Also determine the time lock delta that will be added to the
 		// route if fromVertex is selected. If fromVertex is the source
 		// node, no additional timelock is required.
-		var fee lnwire.MilliSatoshi
+		var (
-		var timeLockDelta uint16
+			timeLockDelta uint16
+			outboundFee   int64
+		)
+
 		if fromVertex != source {
-			fee = edge.policy.ComputeFee(amountToSend)
+			outboundFee = int64(
+				edge.policy.ComputeFee(amountToSend),
+			)
 			timeLockDelta = edge.policy.TimeLockDelta
 		}
 
@@ -749,17 +784,19 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 			return
 		}
 
-		// amountToReceive is the amount that the node that is added to
+		// netAmountToReceive is the amount that the node that is added
-		// the distance map needs to receive from a (to be found)
+		// to the distance map needs to receive from a (to be found)
-		// previous node in the route. That previous node will need to
+		// previous node in the route. The inbound fee of the receiving
-		// pay the amount that this node forwards plus the fee it
+		// node is already subtracted from this value. The previous node
-		// charges.
+		// will need to pay the amount that this node forwards plus the
-		amountToReceive := amountToSend + fee
+		// fee it charges plus this node's inbound fee.
+		netAmountToReceive := amountToSend +
+			lnwire.MilliSatoshi(outboundFee)
 
 		// Check if accumulated fees would exceed fee limit when this
 		// node would be added to the path.
-		totalFee := amountToReceive - amt
+		totalFee := int64(netAmountToReceive) - int64(amt)
-		if totalFee > r.FeeLimit {
+		if totalFee > 0 && lnwire.MilliSatoshi(totalFee) > r.FeeLimit {
 			return
 		}
 
@@ -775,11 +812,21 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 			return
 		}
 
+		// Calculate the combined fee for this edge. Dijkstra does not
+		// support negative edge weights. Because this fee feeds into
+		// the edge weight calculation, we don't allow it to be
+		// negative.
+		signedFee := inboundFee + outboundFee
+		fee := lnwire.MilliSatoshi(0)
+		if signedFee > 0 {
+			fee = lnwire.MilliSatoshi(signedFee)
+		}
+
 		// By adding fromVertex in the route, there will be an extra
 		// weight composed of the fee that this node will charge and
 		// the amount that will be locked for timeLockDelta blocks in
 		// the HTLC that is handed out to fromVertex.
-		weight := edgeWeight(amountToReceive, fee, timeLockDelta)
+		weight := edgeWeight(netAmountToReceive, fee, timeLockDelta)
 
 		// Compute the tentative weight to this new channel/edge
 		// which is the weight from our toNode to the target node
@@ -787,7 +834,10 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		tempWeight := toNodeDist.weight + weight
 
 		// Add an extra factor to the weight to take into account the
-		// probability.
+		// probability. Another reason why we rounded the fee up to zero
+		// is to prevent a highly negative fee from cancelling out the
+		// extra factor. We don't want an always-failing node to attract
+		// traffic using a highly negative fee and escape penalization.
 		tempDist := getProbabilityBasedDist(
 			tempWeight, probability,
 			absoluteAttemptCost,
@@ -854,14 +904,15 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		// The new better distance is recorded, and also our "next hop"
 		// map is populated with this edge.
 		withDist := &nodeWithDist{
-			dist:            tempDist,
+			dist:              tempDist,
-			weight:          tempWeight,
+			weight:            tempWeight,
-			node:            fromVertex,
+			node:              fromVertex,
-			amountToReceive: amountToReceive,
+			netAmountReceived: netAmountToReceive,
-			incomingCltv:    incomingCltv,
+			outboundFee:       lnwire.MilliSatoshi(outboundFee),
-			probability:     probability,
+			incomingCltv:      incomingCltv,
-			nextHop:         edge,
+			probability:       probability,
-			routingInfoSize: routingInfoSize,
+			nextHop:           edge,
+			routingInfoSize:   routingInfoSize,
 		}
 		distance[fromVertex] = withDist
 
@@ -920,9 +971,13 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		nodesVisited++
 
 		pivot := partialPath.node
+		isExitHop := partialPath.nextHop == nil
 
-		// Create unified edges for all incoming connections.
+		// Create unified policies for all incoming connections. Don't
-		u := newNodeEdgeUnifier(self, pivot, outgoingChanMap)
+		// use inbound fees for the exit hop.
+		u := newNodeEdgeUnifier(
+			self, pivot, !isExitHop, outgoingChanMap,
+		)
 
 		err := u.addGraphPolicies(g.graph)
 		if err != nil {
@@ -931,6 +986,11 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 
 		// We add hop hints that were supplied externally.
 		for _, reverseEdge := range additionalEdgesWithSrc[pivot] {
+			// Assume zero inbound fees for route hints. If inbound
+			// fees would apply, they couldn't be communicated in
+			// bolt11 invoices currently.
+			inboundFee := models.InboundFee{}
+
 			// Hop hints don't contain a capacity. We set one here,
 			// since a capacity is needed for probability
 			// calculations. We set a high capacity to act as if
@@ -942,12 +1002,13 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 			u.addPolicy(
 				reverseEdge.sourceNode,
 				reverseEdge.edge.EdgePolicy(),
+				inboundFee,
 				fakeHopHintCapacity,
 				reverseEdge.edge.IntermediatePayloadSize,
 			)
 		}
 
-		amtToSend := partialPath.amountToReceive
+		netAmountReceived := partialPath.netAmountReceived
 
 		// Expand all connections using the optimal policy for each
 		// connection.
@@ -969,7 +1030,8 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 			}
 
 			edge := edgeUnifier.getEdge(
-				amtToSend, g.bandwidthHints,
+				netAmountReceived, g.bandwidthHints,
+				partialPath.outboundFee,
 			)
 
 			if edge == nil {
@@ -1050,7 +1112,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 
 	log.Debugf("Found route: probability=%v, hops=%v, fee=%v",
 		distance[source].probability, len(pathEdges),
-		distance[source].amountToReceive-amt)
+		distance[source].netAmountReceived-amt)
 
 	return pathEdges, distance[source].probability, nil
 }

routing/pathfind_test.go

@@ -417,14 +417,16 @@ func parseTestGraph(t *testing.T, useCache bool, path string) (
 }
 
 type testChannelPolicy struct {
-	Expiry      uint16
+	Expiry             uint16
-	MinHTLC     lnwire.MilliSatoshi
+	MinHTLC            lnwire.MilliSatoshi
-	MaxHTLC     lnwire.MilliSatoshi
+	MaxHTLC            lnwire.MilliSatoshi
-	FeeBaseMsat lnwire.MilliSatoshi
+	FeeBaseMsat        lnwire.MilliSatoshi
-	FeeRate     lnwire.MilliSatoshi
+	FeeRate            lnwire.MilliSatoshi
-	LastUpdate  time.Time
+	InboundFeeBaseMsat int64
-	Disabled    bool
+	InboundFeeRate     int64
-	Features    *lnwire.FeatureVector
+	LastUpdate         time.Time
+	Disabled           bool
+	Features           *lnwire.FeatureVector
 }
 
 type testChannelEnd struct {
@@ -662,6 +664,19 @@ func createTestGraphFromChannels(t *testing.T, useCache bool,
 			return nil, err
 		}
 
+		getExtraData := func(
+			end *testChannelEnd) lnwire.ExtraOpaqueData {
+
+			var extraData lnwire.ExtraOpaqueData
+			inboundFee := lnwire.Fee{
+				BaseFee: int32(end.InboundFeeBaseMsat),
+				FeeRate: int32(end.InboundFeeRate),
+			}
+			require.NoError(t, extraData.PackRecords(&inboundFee))
+
+			return extraData
+		}
+
 		if node1.testChannelPolicy != nil {
 			var msgFlags lnwire.ChanUpdateMsgFlags
 			if node1.MaxHTLC != 0 {
@@ -684,6 +699,7 @@ func createTestGraphFromChannels(t *testing.T, useCache bool,
 				FeeBaseMSat:               node1.FeeBaseMsat,
 				FeeProportionalMillionths: node1.FeeRate,
 				ToNode:                    node2Vertex,
+				ExtraOpaqueData:           getExtraData(node1),
 			}
 			if err := graph.UpdateEdgePolicy(edgePolicy); err != nil {
 				return nil, err
@@ -713,6 +729,7 @@ func createTestGraphFromChannels(t *testing.T, useCache bool,
 				FeeBaseMSat:               node2.FeeBaseMsat,
 				FeeProportionalMillionths: node2.FeeRate,
 				ToNode:                    node1Vertex,
+				ExtraOpaqueData:           getExtraData(node2),
 			}
 			if err := graph.UpdateEdgePolicy(edgePolicy); err != nil {
 				return nil, err
@@ -809,6 +826,9 @@ func TestPathFinding(t *testing.T) {
 	}, {
 		name: "with metadata",
 		fn:   runFindPathWithMetadata,
+	}, {
+		name: "inbound fees",
+		fn:   runInboundFees,
 	}}
 
 	// Run with graph cache enabled.
@@ -3129,6 +3149,147 @@ func runRouteToSelf(t *testing.T, useCache bool) {
 	ctx.assertPath(path, []uint64{1, 3, 2})
 }
 
+// runInboundFees tests whether correct routes are built when inbound fees
+// apply.
+func runInboundFees(t *testing.T, useCache bool) {
+	// Setup a test network.
+	chanCapSat := btcutil.Amount(100000)
+	features := lnwire.NewFeatureVector(
+		lnwire.NewRawFeatureVector(
+			lnwire.PaymentAddrOptional,
+			lnwire.TLVOnionPayloadRequired,
+		),
+		lnwire.Features,
+	)
+
+	testChannels := []*testChannel{
+		asymmetricTestChannel("a", "b", chanCapSat,
+			&testChannelPolicy{
+				MinHTLC:  lnwire.NewMSatFromSatoshis(2),
+				Features: features,
+			},
+			&testChannelPolicy{
+				Features:           features,
+				InboundFeeRate:     -60000,
+				InboundFeeBaseMsat: -5000,
+			}, 10,
+		),
+		asymmetricTestChannel("b", "c", chanCapSat,
+			&testChannelPolicy{
+				Expiry:      20,
+				FeeRate:     50000,
+				FeeBaseMsat: 1000,
+				MinHTLC:     lnwire.NewMSatFromSatoshis(2),
+				Features:    features,
+			},
+			&testChannelPolicy{
+				Features:           features,
+				InboundFeeRate:     0,
+				InboundFeeBaseMsat: 5000,
+			}, 11,
+		),
+		asymmetricTestChannel("c", "d", chanCapSat,
+			&testChannelPolicy{
+				Expiry:      20,
+				FeeRate:     50000,
+				FeeBaseMsat: 0,
+				MinHTLC:     lnwire.NewMSatFromSatoshis(2),
+				Features:    features,
+			},
+			&testChannelPolicy{
+				Features:           features,
+				InboundFeeRate:     -50000,
+				InboundFeeBaseMsat: -8000,
+			}, 12,
+		),
+		asymmetricTestChannel("d", "e", chanCapSat,
+			&testChannelPolicy{
+				Expiry:      20,
+				FeeRate:     100000,
+				FeeBaseMsat: 9000,
+				MinHTLC:     lnwire.NewMSatFromSatoshis(2),
+				Features:    features,
+			},
+			&testChannelPolicy{
+				Features:           features,
+				InboundFeeRate:     80000,
+				InboundFeeBaseMsat: 2000,
+			}, 13,
+		),
+	}
+
+	ctx := newPathFindingTestContext(t, useCache, testChannels, "a")
+
+	payAddr := [32]byte{1}
+	ctx.restrictParams.PaymentAddr = &payAddr
+	ctx.restrictParams.DestFeatures = tlvPayAddrFeatures
+
+	const (
+		startingHeight = 100
+		finalHopCLTV   = 1
+	)
+
+	paymentAmt := lnwire.MilliSatoshi(100_000)
+	target := ctx.keyFromAlias("e")
+	path, err := ctx.findPath(target, paymentAmt)
+	require.NoError(t, err, "unable to find path")
+
+	rt, err := newRoute(
+		ctx.source, path, startingHeight,
+		finalHopParams{
+			amt:         paymentAmt,
+			cltvDelta:   finalHopCLTV,
+			records:     nil,
+			paymentAddr: &payAddr,
+			totalAmt:    paymentAmt,
+		},
+		nil,
+	)
+	require.NoError(t, err, "unable to create path")
+
+	expectedHops := []*route.Hop{
+		{
+			PubKeyBytes: ctx.keyFromAlias("b"),
+			ChannelID:   10,
+			// The amount that c forwards (105_050) plus the out fee
+			// (5_252) and in fee (5_000) of c.
+			AmtToForward:     115_302,
+			OutgoingTimeLock: 141,
+		},
+		{
+			PubKeyBytes: ctx.keyFromAlias("c"),
+			ChannelID:   11,
+			// The amount that d forwards (100_000) plus the out fee
+			// (19_000) and in fee (-13_950) of d.
+			AmtToForward:     105_050,
+			OutgoingTimeLock: 121,
+		},
+		{
+			PubKeyBytes:      ctx.keyFromAlias("d"),
+			ChannelID:        12,
+			AmtToForward:     100_000,
+			OutgoingTimeLock: 101,
+		},
+		{
+			PubKeyBytes:      ctx.keyFromAlias("e"),
+			ChannelID:        13,
+			AmtToForward:     100_000,
+			OutgoingTimeLock: 101,
+			MPP:              record.NewMPP(100_000, payAddr),
+		},
+	}
+
+	expectedRt := &route.Route{
+		// The amount that b forwards (115_302) plus the out fee (6_765)
+		// and in fee (-12324) of b. The total fee is floored at zero.
+		TotalAmount:   115_302,
+		TotalTimeLock: 161,
+		SourcePubKey:  ctx.keyFromAlias("a"),
+		Hops:          expectedHops,
+	}
+	require.Equal(t, expectedRt, rt)
+}
+
 type pathFindingTestContext struct {
 	t                 *testing.T
 	graph             *channeldb.ChannelGraph

routing/router.go

@@ -3161,9 +3161,13 @@ func getRouteUnifiers(source route.Vertex, hops []route.Vertex,
 
 		localChan := i == 0
 
-		// Build unified edges for this hop based on the channels known
+		// Build unified policies for this hop based on the channels
-		// in the graph.
+		// known in the graph. Don't use inbound fees.
-		u := newNodeEdgeUnifier(source, toNode, outgoingChans)
+		//
+		// TODO: Add inbound fees support for BuildRoute.
+		u := newNodeEdgeUnifier(
+			source, toNode, false, outgoingChans,
+		)
 
 		err := u.addGraphPolicies(graph)
 		if err != nil {
@@ -3189,7 +3193,7 @@ func getRouteUnifiers(source route.Vertex, hops []route.Vertex,
 		}
 
 		// Get an edge for the specific amount that we want to forward.
-		edge := edgeUnifier.getEdge(runningAmt, bandwidthHints)
+		edge := edgeUnifier.getEdge(runningAmt, bandwidthHints, 0)
 		if edge == nil {
 			log.Errorf("Cannot find policy with amt=%v for node %v",
 				runningAmt, fromNode)
@@ -3227,7 +3231,7 @@ func getPathEdges(source route.Vertex, receiverAmt lnwire.MilliSatoshi,
 	// amount ranges re-checked.
 	var pathEdges []*unifiedEdge
 	for i, unifier := range unifiers {
-		edge := unifier.getEdge(receiverAmt, bandwidthHints)
+		edge := unifier.getEdge(receiverAmt, bandwidthHints, 0)
 		if edge == nil {
 			fromNode := source
 			if i > 0 {

routing/unified_edges.go

@@ -1,6 +1,8 @@
 package routing
 
 import (
+	"math"
+
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
@@ -21,6 +23,9 @@ type nodeEdgeUnifier struct {
 	// toNode is the node for which the edge unifiers are instantiated.
 	toNode route.Vertex
 
+	// useInboundFees indicates whether to take inbound fees into account.
+	useInboundFees bool
+
 	// outChanRestr is an optional outgoing channel restriction for the
 	// local channel to use.
 	outChanRestr map[uint64]struct{}
@@ -28,14 +33,15 @@ type nodeEdgeUnifier struct {
 
 // newNodeEdgeUnifier instantiates a new nodeEdgeUnifier object. Channel
 // policies can be added to this object.
-func newNodeEdgeUnifier(sourceNode, toNode route.Vertex,
+func newNodeEdgeUnifier(sourceNode, toNode route.Vertex, useInboundFees bool,
 	outChanRestr map[uint64]struct{}) *nodeEdgeUnifier {
 
 	return &nodeEdgeUnifier{
-		edgeUnifiers: make(map[route.Vertex]*edgeUnifier),
+		edgeUnifiers:   make(map[route.Vertex]*edgeUnifier),
-		toNode:       toNode,
+		toNode:         toNode,
-		sourceNode:   sourceNode,
+		useInboundFees: useInboundFees,
-		outChanRestr: outChanRestr,
+		sourceNode:     sourceNode,
+		outChanRestr:   outChanRestr,
 	}
 }
 
@@ -44,8 +50,8 @@ func newNodeEdgeUnifier(sourceNode, toNode route.Vertex,
 // graceful shutdown if it is not provided as this indicates that edges are
 // incorrectly specified.
 func (u *nodeEdgeUnifier) addPolicy(fromNode route.Vertex,
-	edge *models.CachedEdgePolicy, capacity btcutil.Amount,
+	edge *models.CachedEdgePolicy, inboundFee models.InboundFee,
-	hopPayloadSizeFn PayloadSizeFunc) {
+	capacity btcutil.Amount, hopPayloadSizeFn PayloadSizeFunc) {
 
 	localChan := fromNode == u.sourceNode
 
@@ -75,10 +81,16 @@ func (u *nodeEdgeUnifier) addPolicy(fromNode route.Vertex,
 		return
 	}
 
+	// Zero inbound fee for exit hops.
+	if !u.useInboundFees {
+		inboundFee = models.InboundFee{}
+	}
+
 	unifier.edges = append(unifier.edges, &unifiedEdge{
 		policy:           edge,
 		capacity:         capacity,
 		hopPayloadSizeFn: hopPayloadSizeFn,
+		inboundFees:      inboundFee,
 	})
 }
 
@@ -97,9 +109,13 @@ func (u *nodeEdgeUnifier) addGraphPolicies(g routingGraph) error {
 		// to the clear hop payload size function because
 		// `addGraphPolicies` is only used for cleartext intermediate
 		// hops in a route.
+		inboundFee := models.NewInboundFeeFromWire(
+			channel.InboundFee,
+		)
+
 		u.addPolicy(
-			channel.OtherNode, channel.InPolicy, channel.Capacity,
+			channel.OtherNode, channel.InPolicy, inboundFee,
-			defaultHopPayloadSize,
+			channel.Capacity, defaultHopPayloadSize,
 		)
 
 		return nil
@@ -112,8 +128,9 @@ func (u *nodeEdgeUnifier) addGraphPolicies(g routingGraph) error {
 // unifiedEdge is the individual channel data that is kept inside an edgeUnifier
 // object.
 type unifiedEdge struct {
-	policy   *models.CachedEdgePolicy
+	policy      *models.CachedEdgePolicy
-	capacity btcutil.Amount
+	capacity    btcutil.Amount
+	inboundFees models.InboundFee
 
 	// hopPayloadSize supplies an edge with the ability to calculate the
 	// exact payload size if this edge would be included in a route. This
@@ -163,20 +180,41 @@ type edgeUnifier struct {
 // getEdge returns the optimal unified edge to use for this connection given a
 // specific amount to send. It differentiates between local and network
 // channels.
-func (u *edgeUnifier) getEdge(amt lnwire.MilliSatoshi,
+func (u *edgeUnifier) getEdge(netAmtReceived lnwire.MilliSatoshi,
-	bandwidthHints bandwidthHints) *unifiedEdge {
+	bandwidthHints bandwidthHints,
+	nextOutFee lnwire.MilliSatoshi) *unifiedEdge {
 
 	if u.localChan {
-		return u.getEdgeLocal(amt, bandwidthHints)
+		return u.getEdgeLocal(
+			netAmtReceived, bandwidthHints, nextOutFee,
+		)
 	}
 
-	return u.getEdgeNetwork(amt)
+	return u.getEdgeNetwork(netAmtReceived, nextOutFee)
+}
+
+// calcCappedInboundFee calculates the inbound fee for a channel, taking into
+// account the total node fee for the "to" node.
+func calcCappedInboundFee(edge *unifiedEdge, amt lnwire.MilliSatoshi,
+	nextOutFee lnwire.MilliSatoshi) int64 {
+
+	// Calculate the inbound fee charged for the amount that passes over the
+	// channel.
+	inboundFee := edge.inboundFees.CalcFee(amt)
+
+	// Take into account that the total node fee cannot be negative.
+	if inboundFee < -int64(nextOutFee) {
+		inboundFee = -int64(nextOutFee)
+	}
+
+	return inboundFee
 }
 
 // getEdgeLocal returns the optimal unified edge to use for this local
 // connection given a specific amount to send.
-func (u *edgeUnifier) getEdgeLocal(amt lnwire.MilliSatoshi,
+func (u *edgeUnifier) getEdgeLocal(netAmtReceived lnwire.MilliSatoshi,
-	bandwidthHints bandwidthHints) *unifiedEdge {
+	bandwidthHints bandwidthHints,
+	nextOutFee lnwire.MilliSatoshi) *unifiedEdge {
 
 	var (
 		bestEdge     *unifiedEdge
@@ -184,10 +222,20 @@ func (u *edgeUnifier) getEdgeLocal(amt lnwire.MilliSatoshi,
 	)
 
 	for _, edge := range u.edges {
+		// Calculate the inbound fee charged at the receiving node.
+		inboundFee := calcCappedInboundFee(
+			edge, netAmtReceived, nextOutFee,
+		)
+
+		// Add inbound fee to get to the amount that is sent over the
+		// local channel.
+		amt := netAmtReceived + lnwire.MilliSatoshi(inboundFee)
+
 		// Check valid amount range for the channel.
 		if !edge.amtInRange(amt) {
 			log.Debugf("Amount %v not in range for edge %v",
-				amt, edge.policy.ChannelID)
+				netAmtReceived, edge.policy.ChannelID)
+
 			continue
 		}
 
@@ -249,6 +297,7 @@ func (u *edgeUnifier) getEdgeLocal(amt lnwire.MilliSatoshi,
 			policy:           edge.policy,
 			capacity:         edge.capacity,
 			hopPayloadSizeFn: edge.hopPayloadSizeFn,
+			inboundFees:      edge.inboundFees,
 		}
 	}
 
@@ -259,16 +308,27 @@ func (u *edgeUnifier) getEdgeLocal(amt lnwire.MilliSatoshi,
 // given a specific amount to send. The goal is to return a unified edge with a
 // policy that maximizes the probability of a successful forward in a non-strict
 // forwarding context.
-func (u *edgeUnifier) getEdgeNetwork(amt lnwire.MilliSatoshi) *unifiedEdge {
+func (u *edgeUnifier) getEdgeNetwork(netAmtReceived lnwire.MilliSatoshi,
+	nextOutFee lnwire.MilliSatoshi) *unifiedEdge {
+
 	var (
-		bestPolicy       *models.CachedEdgePolicy
+		bestPolicy       *unifiedEdge
-		maxFee           lnwire.MilliSatoshi
+		maxFee           int64 = math.MinInt64
 		maxTimelock      uint16
 		maxCapMsat       lnwire.MilliSatoshi
 		hopPayloadSizeFn PayloadSizeFunc
 	)
 
 	for _, edge := range u.edges {
+		// Calculate the inbound fee charged at the receiving node.
+		inboundFee := calcCappedInboundFee(
+			edge, netAmtReceived, nextOutFee,
+		)
+
+		// Add inbound fee to get to the amount that is sent over the
+		// channel.
+		amt := netAmtReceived + lnwire.MilliSatoshi(inboundFee)
+
 		// Check valid amount range for the channel.
 		if !edge.amtInRange(amt) {
 			log.Debugf("Amount %v not in range for edge %v",
@@ -302,9 +362,12 @@ func (u *edgeUnifier) getEdgeNetwork(amt lnwire.MilliSatoshi) *unifiedEdge {
 		maxTimelock = lntypes.Max(
 			maxTimelock, edge.policy.TimeLockDelta,
 		)
+
+		outboundFee := int64(edge.policy.ComputeFee(amt))
+		fee := outboundFee + inboundFee
+
 		// Use the policy that results in the highest fee for this
 		// specific amount.
-		fee := edge.policy.ComputeFee(amt)
 		if fee < maxFee {
 			log.Debugf("Skipped edge %v due to it produces less "+
 				"fee: fee=%v, maxFee=%v",
@@ -314,7 +377,11 @@ func (u *edgeUnifier) getEdgeNetwork(amt lnwire.MilliSatoshi) *unifiedEdge {
 		}
 		maxFee = fee
 
-		bestPolicy = edge.policy
+		bestPolicy = &unifiedEdge{
+			policy:      edge.policy,
+			inboundFees: edge.inboundFees,
+		}
+
 		// The payload size function for edges to a connected peer is
 		// always the same hence there is not need to find the maximum.
 		// This also counts for blinded edges where we only have one
@@ -337,8 +404,11 @@ func (u *edgeUnifier) getEdgeNetwork(amt lnwire.MilliSatoshi) *unifiedEdge {
 	// get forwarded. Because we penalize pair-wise, there won't be a second
 	// chance for this node pair. But this is all only needed for nodes that
 	// have distinct policies for channels to the same peer.
-	policyCopy := *bestPolicy
+	policyCopy := *bestPolicy.policy
-	modifiedEdge := unifiedEdge{policy: &policyCopy}
+	modifiedEdge := unifiedEdge{
+		policy:      &policyCopy,
+		inboundFees: bestPolicy.inboundFees,
+	}
 	modifiedEdge.policy.TimeLockDelta = maxTimelock
 	modifiedEdge.capacity = maxCapMsat.ToSatoshis()
 	modifiedEdge.hopPayloadSizeFn = hopPayloadSizeFn

routing/unified_edges_test.go

@@ -46,31 +46,75 @@ func TestNodeEdgeUnifier(t *testing.T) {
 	c1 := btcutil.Amount(7)
 	c2 := btcutil.Amount(8)
 
-	unifierFilled := newNodeEdgeUnifier(source, toNode, nil)
+	inboundFee1 := models.InboundFee{
-	unifierFilled.addPolicy(fromNode, &p1, c1, defaultHopPayloadSize)
+		Base: 5,
-	unifierFilled.addPolicy(fromNode, &p2, c2, defaultHopPayloadSize)
+		Rate: 10000,
+	}
 
-	unifierNoCapacity := newNodeEdgeUnifier(source, toNode, nil)
+	inboundFee2 := models.InboundFee{
-	unifierNoCapacity.addPolicy(fromNode, &p1, 0, defaultHopPayloadSize)
+		Base: 10,
-	unifierNoCapacity.addPolicy(fromNode, &p2, 0, defaultHopPayloadSize)
+		Rate: 10000,
+	}
 
-	unifierNoInfo := newNodeEdgeUnifier(source, toNode, nil)
+	unifierFilled := newNodeEdgeUnifier(source, toNode, false, nil)
-	unifierNoInfo.addPolicy(
+
-		fromNode, &models.CachedEdgePolicy{}, 0, defaultHopPayloadSize,
+	unifierFilled.addPolicy(
+		fromNode, &p1, inboundFee1, c1, defaultHopPayloadSize,
+	)
+	unifierFilled.addPolicy(
+		fromNode, &p2, inboundFee2, c2, defaultHopPayloadSize,
 	)
 
-	unifierLocal := newNodeEdgeUnifier(fromNode, toNode, nil)
+	unifierNoCapacity := newNodeEdgeUnifier(source, toNode, false, nil)
-	unifierLocal.addPolicy(fromNode, &p1, c1, defaultHopPayloadSize)
+	unifierNoCapacity.addPolicy(
+		fromNode, &p1, inboundFee1, 0, defaultHopPayloadSize,
+	)
+	unifierNoCapacity.addPolicy(
+		fromNode, &p2, inboundFee2, 0, defaultHopPayloadSize,
+	)
+
+	unifierNoInfo := newNodeEdgeUnifier(source, toNode, false, nil)
+	unifierNoInfo.addPolicy(
+		fromNode, &models.CachedEdgePolicy{}, models.InboundFee{},
+		0, defaultHopPayloadSize,
+	)
+
+	unifierInboundFee := newNodeEdgeUnifier(source, toNode, true, nil)
+	unifierInboundFee.addPolicy(
+		fromNode, &p1, inboundFee1, c1, defaultHopPayloadSize,
+	)
+	unifierInboundFee.addPolicy(
+		fromNode, &p2, inboundFee2, c2, defaultHopPayloadSize,
+	)
+
+	unifierLocal := newNodeEdgeUnifier(fromNode, toNode, true, nil)
+	unifierLocal.addPolicy(
+		fromNode, &p1, inboundFee1, c1, defaultHopPayloadSize,
+	)
+
+	inboundFeeZero := models.InboundFee{}
+	inboundFeeNegative := models.InboundFee{
+		Base: -150,
+	}
+	unifierNegInboundFee := newNodeEdgeUnifier(source, toNode, true, nil)
+	unifierNegInboundFee.addPolicy(
+		fromNode, &p1, inboundFeeZero, c1, defaultHopPayloadSize,
+	)
+	unifierNegInboundFee.addPolicy(
+		fromNode, &p2, inboundFeeNegative, c2, defaultHopPayloadSize,
+	)
 
 	tests := []struct {
-		name             string
+		name               string
-		unifier          *nodeEdgeUnifier
+		unifier            *nodeEdgeUnifier
-		amount           lnwire.MilliSatoshi
+		amount             lnwire.MilliSatoshi
-		expectedFeeBase  lnwire.MilliSatoshi
+		expectedFeeBase    lnwire.MilliSatoshi
-		expectedFeeRate  lnwire.MilliSatoshi
+		expectedFeeRate    lnwire.MilliSatoshi
-		expectedTimeLock uint16
+		expectedInboundFee models.InboundFee
-		expectNoPolicy   bool
+		expectedTimeLock   uint16
-		expectedCapacity btcutil.Amount
+		expectNoPolicy     bool
+		expectedCapacity   btcutil.Amount
+		nextOutFee         lnwire.MilliSatoshi
 	}{
 		{
 			name:           "amount below min htlc",
@@ -132,13 +176,49 @@ func TestNodeEdgeUnifier(t *testing.T) {
 			expectNoPolicy: true,
 		},
 		{
-			name:             "local",
+			name:               "local",
-			unifier:          unifierLocal,
+			unifier:            unifierLocal,
-			amount:           100,
+			amount:             100,
-			expectedFeeBase:  p1.FeeBaseMSat,
+			expectedFeeBase:    p1.FeeBaseMSat,
-			expectedFeeRate:  p1.FeeProportionalMillionths,
+			expectedFeeRate:    p1.FeeProportionalMillionths,
-			expectedTimeLock: p1.TimeLockDelta,
+			expectedTimeLock:   p1.TimeLockDelta,
-			expectedCapacity: c1,
+			expectedCapacity:   c1,
+			expectedInboundFee: inboundFee1,
+		},
+		{
+			name: "use p2 with highest fee " +
+				"including inbound",
+			unifier:            unifierInboundFee,
+			amount:             200,
+			expectedFeeBase:    p2.FeeBaseMSat,
+			expectedFeeRate:    p2.FeeProportionalMillionths,
+			expectedInboundFee: inboundFee2,
+			expectedTimeLock:   p1.TimeLockDelta,
+			expectedCapacity:   c2,
+		},
+		// Choose inbound fee exactly so that max htlc is just exceeded.
+		// In this test, the amount that must be sent is 5001 msat.
+		{
+			name:           "inbound fee exceeds max htlc",
+			unifier:        unifierInboundFee,
+			amount:         4947,
+			expectNoPolicy: true,
+		},
+		// The outbound fee of p2 is higher than p1, but because of the
+		// inbound fee on p2 it is brought down to 0. Purely based on
+		// total channel fee, p1 would be selected as the highest fee
+		// channel. However, because the total node fee can never be
+		// negative and the next outgoing fee is zero, the effect of the
+		// inbound discount is cancelled out.
+		{
+			name:               "inbound fee that is rounded up",
+			unifier:            unifierNegInboundFee,
+			amount:             500,
+			expectedFeeBase:    p2.FeeBaseMSat,
+			expectedFeeRate:    p2.FeeProportionalMillionths,
+			expectedInboundFee: inboundFeeNegative,
+			expectedTimeLock:   p1.TimeLockDelta,
+			expectedCapacity:   c2,
 		},
 	}
 
@@ -149,7 +229,7 @@ func TestNodeEdgeUnifier(t *testing.T) {
 			t.Parallel()
 
 			edge := test.unifier.edgeUnifiers[fromNode].getEdge(
-				test.amount, bandwidthHints,
+				test.amount, bandwidthHints, test.nextOutFee,
 			)
 
 			if test.expectNoPolicy {
@@ -163,6 +243,8 @@ func TestNodeEdgeUnifier(t *testing.T) {
 				policy.FeeBaseMSat, "base fee")
 			require.Equal(t, test.expectedFeeRate,
 				policy.FeeProportionalMillionths, "fee rate")
+			require.Equal(t, test.expectedInboundFee,
+				edge.inboundFees, "inbound fee")
 			require.Equal(t, test.expectedTimeLock,
 				policy.TimeLockDelta, "timelock")
 			require.Equal(t, test.expectedCapacity, edge.capacity,