Merge pull request #3440 from joostjager/buildroute

routing: add build route functionality

channeldb/graph.go

@@ -155,6 +155,9 @@ const (
 	// would be possible for a node to create a ton of updates and slowly
 	// fill our disk, and also waste bandwidth due to relaying.
 	MaxAllowedExtraOpaqueBytes = 10000
+
+	// feeRateParts is the total number of parts used to express fee rates.
+	feeRateParts = 1e6
 )
 
 // ChannelGraph is a persistent, on-disk graph representation of the Lightning
@@ -2828,6 +2831,31 @@ func (c *ChannelEdgePolicy) IsDisabled() bool {
 		lnwire.ChanUpdateDisabled
 }
 
+// ComputeFee computes the fee to forward an HTLC of `amt` milli-satoshis over
+// the passed active payment channel. This value is currently computed as
+// specified in BOLT07, but will likely change in the near future.
+func (c *ChannelEdgePolicy) ComputeFee(
+	amt lnwire.MilliSatoshi) lnwire.MilliSatoshi {
+
+	return c.FeeBaseMSat + (amt*c.FeeProportionalMillionths)/feeRateParts
+}
+
+// divideCeil divides dividend by factor and rounds the result up.
+func divideCeil(dividend, factor lnwire.MilliSatoshi) lnwire.MilliSatoshi {
+	return (dividend + factor - 1) / factor
+}
+
+// ComputeFeeFromIncoming computes the fee to forward an HTLC given the incoming
+// amount.
+func (c *ChannelEdgePolicy) ComputeFeeFromIncoming(
+	incomingAmt lnwire.MilliSatoshi) lnwire.MilliSatoshi {
+
+	return incomingAmt - divideCeil(
+		feeRateParts*(incomingAmt-c.FeeBaseMSat),
+		feeRateParts+c.FeeProportionalMillionths,
+	)
+}
+
 // FetchChannelEdgesByOutpoint attempts to lookup the two directed edges for
 // the channel identified by the funding outpoint. If the channel can't be
 // found, then ErrEdgeNotFound is returned. A struct which houses the general

channeldb/graph_test.go

@@ -3173,3 +3173,25 @@ func TestLightningNodeSigVerification(t *testing.T) {
 		t.Fatalf("unable to verify sig")
 	}
 }
+
+// TestComputeFee tests fee calculation based on both in- and outgoing amt.
+func TestComputeFee(t *testing.T) {
+	var (
+		policy = ChannelEdgePolicy{
+			FeeBaseMSat:               10000,
+			FeeProportionalMillionths: 30000,
+		}
+		outgoingAmt = lnwire.MilliSatoshi(1000000)
+		expectedFee = lnwire.MilliSatoshi(40000)
+	)
+
+	fee := policy.ComputeFee(outgoingAmt)
+	if fee != expectedFee {
+		t.Fatalf("expected fee %v, got %v", expectedFee, fee)
+	}
+
+	fwdFee := policy.ComputeFeeFromIncoming(outgoingAmt + fee)
+	if fwdFee != expectedFee {
+		t.Fatalf("expected fee %v, but got %v", fee, fwdFee)
+	}
+}

cmd/lncli/cmd_build_route.go

@@ -0,0 +1,94 @@
+// +build routerrpc
+
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/lightningnetwork/lnd"
+	"github.com/lightningnetwork/lnd/lnrpc/routerrpc"
+	"github.com/lightningnetwork/lnd/routing/route"
+	"github.com/urfave/cli"
+)
+
+var buildRouteCommand = cli.Command{
+	Name:     "buildroute",
+	Category: "Payments",
+	Usage:    "Build a route from a list of hop pubkeys.",
+	Action:   actionDecorator(buildRoute),
+	Flags: []cli.Flag{
+		cli.Int64Flag{
+			Name: "amt",
+			Usage: "the amount to send expressed in satoshis. If" +
+				"not set, the minimum routable amount is used",
+		},
+		cli.Int64Flag{
+			Name: "final_cltv_delta",
+			Usage: "number of blocks the last hop has to reveal " +
+				"the preimage",
+			Value: lnd.DefaultBitcoinTimeLockDelta,
+		},
+		cli.StringFlag{
+			Name:  "hops",
+			Usage: "comma separated hex pubkeys",
+		},
+		cli.Uint64Flag{
+			Name: "outgoing_chan_id",
+			Usage: "short channel id of the outgoing channel to " +
+				"use for the first hop of the payment",
+			Value: 0,
+		},
+	},
+}
+
+func buildRoute(ctx *cli.Context) error {
+	conn := getClientConn(ctx, false)
+	defer conn.Close()
+
+	client := routerrpc.NewRouterClient(conn)
+
+	if !ctx.IsSet("hops") {
+		return errors.New("hops required")
+	}
+
+	// Build list of hop addresses for the rpc.
+	hops := strings.Split(ctx.String("hops"), ",")
+	rpcHops := make([][]byte, 0, len(hops))
+	for _, k := range hops {
+		pubkey, err := route.NewVertexFromStr(k)
+		if err != nil {
+			return fmt.Errorf("error parsing %v: %v", k, err)
+		}
+		rpcHops = append(rpcHops, pubkey[:])
+	}
+
+	var amtMsat int64
+	hasAmt := ctx.IsSet("amt")
+	if hasAmt {
+		amtMsat = ctx.Int64("amt") * 1000
+		if amtMsat == 0 {
+			return fmt.Errorf("non-zero amount required")
+		}
+	}
+
+	// Call BuildRoute rpc.
+	req := &routerrpc.BuildRouteRequest{
+		AmtMsat:        amtMsat,
+		FinalCltvDelta: int32(ctx.Int64("final_cltv_delta")),
+		HopPubkeys:     rpcHops,
+		OutgoingChanId: ctx.Uint64("outgoing_chan_id"),
+	}
+
+	rpcCtx := context.Background()
+	route, err := client.BuildRoute(rpcCtx, req)
+	if err != nil {
+		return err
+	}
+
+	printJSON(route)
+
+	return nil
+}

cmd/lncli/commands.go

@@ -21,6 +21,7 @@ import (
 	"github.com/golang/protobuf/jsonpb"
 	"github.com/golang/protobuf/proto"
 	"github.com/lightningnetwork/lnd/lnrpc"
+	"github.com/lightningnetwork/lnd/lnrpc/routerrpc"
 	"github.com/lightningnetwork/lnd/walletunlocker"
 	"github.com/urfave/cli"
 	"golang.org/x/crypto/ssh/terminal"
@@ -2381,22 +2382,23 @@ var sendToRouteCommand = cli.Command{
 	Usage:    "Send a payment over a predefined route.",
 	Description: `
 	Send a payment over Lightning using a specific route. One must specify
-	a list of routes to attempt and the payment hash. This command can even
+	the route to attempt and the payment hash. This command can even
-	be chained with the response to queryroutes. This command can be used
+	be chained with the response to queryroutes or buildroute. This command
-	to implement channel rebalancing by crafting a self-route, or even
+	can be used to implement channel rebalancing by crafting a self-route, 
-	atomic swaps using a self-route that crosses multiple chains.
+	or even atomic swaps using a self-route that crosses multiple chains.
 
-	There are three ways to specify routes:
+	There are three ways to specify a route:
 	   * using the --routes parameter to manually specify a JSON encoded
-	     set of routes in the format of the return value of queryroutes:
+	     route in the format of the return value of queryroutes or
+	     buildroute:
 	         (lncli sendtoroute --payment_hash=<pay_hash> --routes=<route>)
 
-	   * passing the routes as a positional argument:
+	   * passing the route as a positional argument:
 	         (lncli sendtoroute --payment_hash=pay_hash <route>)
 
-	   * or reading in the routes from stdin, which can allow chaining the
+	   * or reading in the route from stdin, which can allow chaining the
-	     response from queryroutes, or even read in a file with a set of
+	     response from queryroutes or buildroute, or even read in a file
-	     pre-computed routes:
+	     with a pre-computed route:
 	         (lncli queryroutes --args.. | lncli sendtoroute --payment_hash= -
 
 	     notice the '-' at the end, which signals that lncli should read
@@ -2474,13 +2476,13 @@ func sendToRoute(ctx *cli.Context) error {
 		jsonRoutes = string(b)
 	}
 
+	// Try to parse the provided json both in the legacy QueryRoutes format
+	// that contains a list of routes and the single route BuildRoute
+	// format.
+	var route *lnrpc.Route
 	routes := &lnrpc.QueryRoutesResponse{}
 	err = jsonpb.UnmarshalString(jsonRoutes, routes)
-	if err != nil {
+	if err == nil {
-		return fmt.Errorf("unable to unmarshal json string "+
-			"from incoming array of routes: %v", err)
-	}
-
 		if len(routes.Routes) == 0 {
 			return fmt.Errorf("no routes provided")
 		}
@@ -2490,9 +2492,21 @@ func sendToRoute(ctx *cli.Context) error {
 				len(routes.Routes))
 		}
 
+		route = routes.Routes[0]
+	} else {
+		routes := &routerrpc.BuildRouteResponse{}
+		err = jsonpb.UnmarshalString(jsonRoutes, routes)
+		if err != nil {
+			return fmt.Errorf("unable to unmarshal json string "+
+				"from incoming array of routes: %v", err)
+		}
+
+		route = routes.Route
+	}
+
 	req := &lnrpc.SendToRouteRequest{
 		PaymentHash: rHash,
-		Route:       routes.Routes[0],
+		Route:       route,
 	}
 
 	return sendToRouteRequest(ctx, req)

cmd/lncli/routerrpc_active.go

@@ -6,5 +6,9 @@ import "github.com/urfave/cli"
 
 // routerCommands will return nil for non-routerrpc builds.
 func routerCommands() []cli.Command {
-	return []cli.Command{queryMissionControlCommand, resetMissionControlCommand}
+	return []cli.Command{
+		queryMissionControlCommand,
+		resetMissionControlCommand,
+		buildRouteCommand,
+	}
 }

lnrpc/routerrpc/router.pb.go

@@ -1197,6 +1197,122 @@ func (m *PairHistory) GetLastAttemptSuccessful() bool {
 	return false
 }
 
+type BuildRouteRequest struct {
+	//*
+	//The amount to send expressed in msat. If set to zero, the minimum routable
+	//amount is used.
+	AmtMsat int64 `protobuf:"varint,1,opt,name=amt_msat,json=amtMsat,proto3" json:"amt_msat,omitempty"`
+	//*
+	//CLTV delta from the current height that should be used for the timelock
+	//of the final hop
+	FinalCltvDelta int32 `protobuf:"varint,2,opt,name=final_cltv_delta,json=finalCltvDelta,proto3" json:"final_cltv_delta,omitempty"`
+	//*
+	//The channel id of the channel that must be taken to the first hop. If zero,
+	//any channel may be used.
+	OutgoingChanId uint64 `protobuf:"varint,3,opt,name=outgoing_chan_id,json=outgoingChanId,proto3" json:"outgoing_chan_id,omitempty"`
+	//*
+	//A list of hops that defines the route. This does not include the source hop
+	//pubkey.
+	HopPubkeys           [][]byte `protobuf:"bytes,4,rep,name=hop_pubkeys,json=hopPubkeys,proto3" json:"hop_pubkeys,omitempty"`
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
+}
+
+func (m *BuildRouteRequest) Reset()         { *m = BuildRouteRequest{} }
+func (m *BuildRouteRequest) String() string { return proto.CompactTextString(m) }
+func (*BuildRouteRequest) ProtoMessage()    {}
+func (*BuildRouteRequest) Descriptor() ([]byte, []int) {
+	return fileDescriptor_7a0613f69d37b0a5, []int{15}
+}
+
+func (m *BuildRouteRequest) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_BuildRouteRequest.Unmarshal(m, b)
+}
+func (m *BuildRouteRequest) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_BuildRouteRequest.Marshal(b, m, deterministic)
+}
+func (m *BuildRouteRequest) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_BuildRouteRequest.Merge(m, src)
+}
+func (m *BuildRouteRequest) XXX_Size() int {
+	return xxx_messageInfo_BuildRouteRequest.Size(m)
+}
+func (m *BuildRouteRequest) XXX_DiscardUnknown() {
+	xxx_messageInfo_BuildRouteRequest.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_BuildRouteRequest proto.InternalMessageInfo
+
+func (m *BuildRouteRequest) GetAmtMsat() int64 {
+	if m != nil {
+		return m.AmtMsat
+	}
+	return 0
+}
+
+func (m *BuildRouteRequest) GetFinalCltvDelta() int32 {
+	if m != nil {
+		return m.FinalCltvDelta
+	}
+	return 0
+}
+
+func (m *BuildRouteRequest) GetOutgoingChanId() uint64 {
+	if m != nil {
+		return m.OutgoingChanId
+	}
+	return 0
+}
+
+func (m *BuildRouteRequest) GetHopPubkeys() [][]byte {
+	if m != nil {
+		return m.HopPubkeys
+	}
+	return nil
+}
+
+type BuildRouteResponse struct {
+	//*
+	//Fully specified route that can be used to execute the payment.
+	Route                *lnrpc.Route `protobuf:"bytes,1,opt,name=route,proto3" json:"route,omitempty"`
+	XXX_NoUnkeyedLiteral struct{}     `json:"-"`
+	XXX_unrecognized     []byte       `json:"-"`
+	XXX_sizecache        int32        `json:"-"`
+}
+
+func (m *BuildRouteResponse) Reset()         { *m = BuildRouteResponse{} }
+func (m *BuildRouteResponse) String() string { return proto.CompactTextString(m) }
+func (*BuildRouteResponse) ProtoMessage()    {}
+func (*BuildRouteResponse) Descriptor() ([]byte, []int) {
+	return fileDescriptor_7a0613f69d37b0a5, []int{16}
+}
+
+func (m *BuildRouteResponse) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_BuildRouteResponse.Unmarshal(m, b)
+}
+func (m *BuildRouteResponse) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_BuildRouteResponse.Marshal(b, m, deterministic)
+}
+func (m *BuildRouteResponse) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_BuildRouteResponse.Merge(m, src)
+}
+func (m *BuildRouteResponse) XXX_Size() int {
+	return xxx_messageInfo_BuildRouteResponse.Size(m)
+}
+func (m *BuildRouteResponse) XXX_DiscardUnknown() {
+	xxx_messageInfo_BuildRouteResponse.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_BuildRouteResponse proto.InternalMessageInfo
+
+func (m *BuildRouteResponse) GetRoute() *lnrpc.Route {
+	if m != nil {
+		return m.Route
+	}
+	return nil
+}
+
 func init() {
 	proto.RegisterEnum("routerrpc.PaymentState", PaymentState_name, PaymentState_value)
 	proto.RegisterEnum("routerrpc.Failure_FailureCode", Failure_FailureCode_name, Failure_FailureCode_value)
@@ -1216,123 +1332,131 @@ func init() {
 	proto.RegisterType((*QueryMissionControlResponse)(nil), "routerrpc.QueryMissionControlResponse")
 	proto.RegisterType((*NodeHistory)(nil), "routerrpc.NodeHistory")
 	proto.RegisterType((*PairHistory)(nil), "routerrpc.PairHistory")
+	proto.RegisterType((*BuildRouteRequest)(nil), "routerrpc.BuildRouteRequest")
+	proto.RegisterType((*BuildRouteResponse)(nil), "routerrpc.BuildRouteResponse")
 }
 
 func init() { proto.RegisterFile("routerrpc/router.proto", fileDescriptor_7a0613f69d37b0a5) }
 
 var fileDescriptor_7a0613f69d37b0a5 = []byte{
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+	0x57, 0x7b, 0x56, 0x39, 0x98, 0xaa, 0xf6, 0x33, 0xd8, 0x99, 0xb8, 0x3e, 0x9d, 0x72, 0x9f, 0x79,
-	0x4d, 0x3a, 0x94, 0x9c, 0x22, 0xae, 0xb5, 0xa1, 0x3a, 0x54, 0x96, 0x4e, 0x92, 0x57, 0x27, 0x59,
+	0xee, 0x1f, 0x39, 0x4d, 0x26, 0x94, 0x9c, 0x52, 0x5c, 0x29, 0x43, 0x75, 0xa8, 0x2c, 0xdd, 0x24,
-	0xc2, 0xd0, 0x4b, 0x38, 0x50, 0x51, 0x60, 0x42, 0xf0, 0xc9, 0x54, 0x24, 0x07, 0xbc, 0x9e, 0x79,
+	0xaf, 0x6e, 0xb2, 0x84, 0xa1, 0x97, 0xb0, 0xa7, 0xa2, 0xc0, 0x84, 0xe0, 0x93, 0xa9, 0x48, 0x2e,
-	0xaa, 0x06, 0x8a, 0xe4, 0x31, 0xf3, 0x8b, 0xbf, 0x66, 0xa0, 0x92, 0xee, 0xd2, 0x50, 0x15, 0x4a,
+	0x78, 0x35, 0xf3, 0x54, 0x0f, 0x14, 0xc9, 0x63, 0xe2, 0xfa, 0xdf, 0x32, 0xb0, 0xf5, 0x6a, 0xe6,
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+	0x7a, 0xa3, 0xa5, 0x39, 0xe5, 0x29, 0x14, 0xe5, 0xf1, 0xa9, 0x39, 0x48, 0x0e, 0x53, 0xaa, 0x60,
-	0x2d, 0xdc, 0x32, 0x32, 0x08, 0xc1, 0x96, 0x14, 0x12, 0xdc, 0xa2, 0x7d, 0xab, 0x8b, 0x9d, 0x81,
+	0x57, 0x0d, 0xf6, 0x6b, 0x2b, 0x07, 0xfb, 0x55, 0x23, 0x76, 0x76, 0xe5, 0x88, 0xfd, 0x0c, 0xca,
-	0x7c, 0x83, 0x76, 0x61, 0x3b, 0xc6, 0x6c, 0x87, 0x12, 0x67, 0xd0, 0xc7, 0x46, 0x16, 0x19, 0x50,
+	0xe3, 0x60, 0x4a, 0x75, 0xa2, 0x23, 0x33, 0x77, 0x94, 0x3d, 0xae, 0x10, 0x18, 0x07, 0xd3, 0x0b,
-	0x89, 0x41, 0x4c, 0x88, 0x43, 0x8c, 0x9c, 0x14, 0xce, 0x18, 0x79, 0xf8, 0x9e, 0x25, 0xcf, 0x5d,
+	0x8d, 0xd4, 0x5f, 0x02, 0x4a, 0x3b, 0x19, 0x57, 0xe5, 0x7c, 0x54, 0xca, 0x3c, 0x3a, 0x2a, 0xbd,
-	0xfe, 0xfc, 0x2f, 0x39, 0xd8, 0x54, 0x5d, 0x4d, 0x88, 0x2e, 0xa0, 0x9c, 0x6a, 0x85, 0xd1, 0xd1,
+	0xf8, 0x4b, 0x06, 0x2a, 0xe9, 0x29, 0x14, 0x55, 0xa1, 0x64, 0xd9, 0xb4, 0xdd, 0xb1, 0xbe, 0x3f,
-	0x47, 0x5b, 0xe4, 0x9a, 0xb9, 0xbe, 0xed, 0x9c, 0x45, 0xdf, 0x64, 0xd0, 0x6b, 0xa8, 0xa4, 0x9b,
+	0xef, 0x1b, 0x9f, 0xc8, 0x65, 0x6f, 0xd0, 0x6c, 0x62, 0xdc, 0xc2, 0x2d, 0x23, 0x83, 0x10, 0x6c,
-	0x5d, 0x94, 0x6e, 0x62, 0xd6, 0x74, 0xc1, 0x1f, 0xf5, 0xf5, 0x06, 0x0c, 0x1c, 0x09, 0x77, 0x22,
+	0x48, 0xa2, 0xc4, 0x2d, 0xda, 0xb7, 0xba, 0xd8, 0x19, 0xc8, 0x37, 0x76, 0x1b, 0x36, 0x63, 0xcc,
-	0x9b, 0x96, 0xb8, 0x8d, 0x44, 0xb5, 0x14, 0x7f, 0xa5, 0x37, 0xad, 0x1d, 0xae, 0xb5, 0xc5, 0x75,
+	0x76, 0x28, 0x71, 0x06, 0x7d, 0x6c, 0x64, 0x91, 0x01, 0x95, 0x18, 0xc4, 0x84, 0x38, 0xc4, 0xc8,
-	0xd5, 0xd1, 0x47, 0x8c, 0x1b, 0xb9, 0x07, 0x47, 0x5c, 0xee, 0x1e, 0x6b, 0x9f, 0x3e, 0x66, 0x8e,
+	0xc9, 0x87, 0x21, 0x46, 0x1e, 0xbe, 0xd7, 0xc9, 0x73, 0x9e, 0x3f, 0xfb, 0x47, 0x0e, 0xd6, 0x95,
-	0xbd, 0x8d, 0x60, 0x77, 0x8d, 0x02, 0xa0, 0x5f, 0xa4, 0x77, 0xf0, 0xa8, 0x7e, 0xd4, 0x9e, 0xff,
+	0x83, 0x21, 0x3a, 0x87, 0x72, 0x6a, 0xd4, 0x47, 0x87, 0x1f, 0xfd, 0x17, 0xa0, 0x66, 0xae, 0x1e,
-	0x2f, 0xda, 0x62, 0x95, 0x35, 0x52, 0xb1, 0xb4, 0xca, 0xe3, 0x42, 0xb3, 0xb4, 0xca, 0x47, 0x14,
+	0xab, 0x67, 0xd1, 0xd7, 0x19, 0xf4, 0x1a, 0x2a, 0xe9, 0x61, 0x1e, 0xa5, 0x87, 0xb4, 0x15, 0x53,
-	0xe7, 0xd5, 0xaf, 0x7e, 0x77, 0x76, 0xe3, 0x8a, 0xf1, 0xec, 0xea, 0x74, 0x18, 0x4c, 0xce, 0x3c,
+	0xfe, 0x47, 0x6d, 0xbd, 0x01, 0x03, 0x47, 0xc2, 0x9d, 0xc8, 0xa1, 0x2c, 0x1e, 0x93, 0x51, 0x2d,
-	0xd9, 0x52, 0xf9, 0xae, 0x7f, 0xe3, 0x73, 0xf1, 0x87, 0x20, 0xbc, 0x3d, 0xf3, 0xfc, 0xd1, 0x99,
+	0xa5, 0x7f, 0x6f, 0xf6, 0xae, 0xed, 0xaf, 0x94, 0xc5, 0x19, 0xea, 0xe8, 0x2b, 0xc6, 0x83, 0xea,
-	0x6a, 0x8c, 0xcf, 0xe6, 0xee, 0xae, 0x36, 0xd5, 0x3f, 0xdb, 0x5f, 0xff, 0x37, 0x00, 0x00, 0xff,
+	0x83, 0x2b, 0x2e, 0x4f, 0xc7, 0xb5, 0x4f, 0x1f, 0x13, 0xc7, 0xd6, 0x46, 0xb0, 0xbd, 0x82, 0xe1,
-	0xff, 0x3c, 0xe4, 0x5c, 0x67, 0x09, 0x0f, 0x00, 0x00,
+	0xd0, 0x2f, 0xd2, 0x1e, 0x3c, 0xca, 0x8f, 0xb5, 0xe7, 0xff, 0x4f, 0x6d, 0x71, 0xca, 0x0a, 0x2a,
+	0x5c, 0x3a, 0xe5, 0x71, 0x22, 0x5d, 0x3a, 0xe5, 0x63, 0x8c, 0x6a, 0x01, 0x2c, 0x2a, 0x1a, 0x1d,
+	0xa4, 0x76, 0x3d, 0xe8, 0xc6, 0xda, 0xe1, 0x23, 0x52, 0x6d, 0xea, 0xd5, 0xaf, 0x7e, 0x77, 0x7a,
+	0xed, 0x8a, 0xf1, 0xec, 0xf2, 0x64, 0x18, 0x4c, 0x4e, 0x3d, 0x39, 0x7d, 0xfa, 0xae, 0x7f, 0xed,
+	0x73, 0xf1, 0x87, 0x20, 0xbc, 0x39, 0xf5, 0xfc, 0xd1, 0xa9, 0x6a, 0x8c, 0xd3, 0xb9, 0x95, 0xcb,
+	0x75, 0xf5, 0x23, 0xc0, 0xaf, 0xff, 0x17, 0x00, 0x00, 0xff, 0xff, 0x85, 0xe2, 0x3f, 0x31, 0x34,
+	0x10, 0x00, 0x00,
 }
 
 // Reference imports to suppress errors if they are not otherwise used.
@@ -1373,6 +1497,11 @@ type RouterClient interface {
 	//QueryMissionControl exposes the internal mission control state to callers.
 	//It is a development feature.
 	QueryMissionControl(ctx context.Context, in *QueryMissionControlRequest, opts ...grpc.CallOption) (*QueryMissionControlResponse, error)
+	//*
+	//BuildRoute builds a fully specified route based on a list of hop public
+	//keys. It retrieves the relevant channel policies from the graph in order to
+	//calculate the correct fees and time locks.
+	BuildRoute(ctx context.Context, in *BuildRouteRequest, opts ...grpc.CallOption) (*BuildRouteResponse, error)
 }
 
 type routerClient struct {
@@ -1483,6 +1612,15 @@ func (c *routerClient) QueryMissionControl(ctx context.Context, in *QueryMission
 	return out, nil
 }
 
+func (c *routerClient) BuildRoute(ctx context.Context, in *BuildRouteRequest, opts ...grpc.CallOption) (*BuildRouteResponse, error) {
+	out := new(BuildRouteResponse)
+	err := c.cc.Invoke(ctx, "/routerrpc.Router/BuildRoute", in, out, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
 // RouterServer is the server API for Router service.
 type RouterServer interface {
 	//*
@@ -1511,6 +1649,11 @@ type RouterServer interface {
 	//QueryMissionControl exposes the internal mission control state to callers.
 	//It is a development feature.
 	QueryMissionControl(context.Context, *QueryMissionControlRequest) (*QueryMissionControlResponse, error)
+	//*
+	//BuildRoute builds a fully specified route based on a list of hop public
+	//keys. It retrieves the relevant channel policies from the graph in order to
+	//calculate the correct fees and time locks.
+	BuildRoute(context.Context, *BuildRouteRequest) (*BuildRouteResponse, error)
 }
 
 func RegisterRouterServer(s *grpc.Server, srv RouterServer) {
@@ -1631,6 +1774,24 @@ func _Router_QueryMissionControl_Handler(srv interface{}, ctx context.Context, d
 	return interceptor(ctx, in, info, handler)
 }
 
+func _Router_BuildRoute_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(BuildRouteRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(RouterServer).BuildRoute(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: "/routerrpc.Router/BuildRoute",
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(RouterServer).BuildRoute(ctx, req.(*BuildRouteRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
 var _Router_serviceDesc = grpc.ServiceDesc{
 	ServiceName: "routerrpc.Router",
 	HandlerType: (*RouterServer)(nil),
@@ -1651,6 +1812,10 @@ var _Router_serviceDesc = grpc.ServiceDesc{
 			MethodName: "QueryMissionControl",
 			Handler:    _Router_QueryMissionControl_Handler,
 		},
+		{
+			MethodName: "BuildRoute",
+			Handler:    _Router_BuildRoute_Handler,
+		},
 	},
 	Streams: []grpc.StreamDesc{
 		{

lnrpc/routerrpc/router.proto

@@ -380,6 +380,39 @@ message PairHistory {
     bool last_attempt_successful = 6 [json_name = "last_attempt_successful"];
 }
 
+message BuildRouteRequest {
+    /**
+    The amount to send expressed in msat. If set to zero, the minimum routable
+    amount is used.
+    */
+    int64 amt_msat = 1;
+
+    /**
+    CLTV delta from the current height that should be used for the timelock
+    of the final hop
+    */
+    int32 final_cltv_delta = 2;
+
+    /**
+    The channel id of the channel that must be taken to the first hop. If zero,
+    any channel may be used.
+    */
+    uint64 outgoing_chan_id = 3;
+
+    /**
+    A list of hops that defines the route. This does not include the source hop
+    pubkey.
+    */
+    repeated bytes hop_pubkeys = 4;
+}
+
+message BuildRouteResponse {
+    /**
+    Fully specified route that can be used to execute the payment.
+    */
+    lnrpc.Route route = 1;
+}
+
 service Router {
     /**
     SendPayment attempts to route a payment described by the passed
@@ -419,4 +452,11 @@ service Router {
     It is a development feature.
     */
     rpc QueryMissionControl(QueryMissionControlRequest) returns (QueryMissionControlResponse);
+
+    /**
+    BuildRoute builds a fully specified route based on a list of hop public
+    keys. It retrieves the relevant channel policies from the graph in order to
+    calculate the correct fees and time locks.
+    */
+    rpc BuildRoute(BuildRouteRequest) returns (BuildRouteResponse);
 }

lnrpc/routerrpc/router_server.go

@@ -72,6 +72,10 @@ var (
 			Entity: "offchain",
 			Action: "write",
 		}},
+		"/routerrpc.Router/BuildRoute": {{
+			Entity: "offchain",
+			Action: "read",
+		}},
 	}
 
 	// DefaultRouterMacFilename is the default name of the router macaroon
@@ -612,3 +616,49 @@ func marshallFailureReason(reason channeldb.FailureReason) (
 
 	return 0, errors.New("unknown failure reason")
 }
+
+// BuildRoute builds a route from a list of hop addresses.
+func (s *Server) BuildRoute(ctx context.Context,
+	req *BuildRouteRequest) (*BuildRouteResponse, error) {
+
+	// Unmarshall hop list.
+	hops := make([]route.Vertex, len(req.HopPubkeys))
+	for i, pubkeyBytes := range req.HopPubkeys {
+		pubkey, err := route.NewVertexFromBytes(pubkeyBytes)
+		if err != nil {
+			return nil, err
+		}
+		hops[i] = pubkey
+	}
+
+	// Prepare BuildRoute call parameters from rpc request.
+	var amt *lnwire.MilliSatoshi
+	if req.AmtMsat != 0 {
+		rpcAmt := lnwire.MilliSatoshi(req.AmtMsat)
+		amt = &rpcAmt
+	}
+
+	var outgoingChan *uint64
+	if req.OutgoingChanId != 0 {
+		outgoingChan = &req.OutgoingChanId
+	}
+
+	// Build the route and return it to the caller.
+	route, err := s.cfg.Router.BuildRoute(
+		amt, hops, outgoingChan, req.FinalCltvDelta,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	rpcRoute, err := s.cfg.RouterBackend.MarshallRoute(route)
+	if err != nil {
+		return nil, err
+	}
+
+	routeResp := &BuildRouteResponse{
+		Route: rpcRoute,
+	}
+
+	return routeResp, nil
+}

routing/pathfind.go

@@ -66,15 +66,6 @@ type edgePolicyWithSource struct {
 	edge       *channeldb.ChannelEdgePolicy
 }
 
-// computeFee computes the fee to forward an HTLC of `amt` milli-satoshis over
-// the passed active payment channel. This value is currently computed as
-// specified in BOLT07, but will likely change in the near future.
-func computeFee(amt lnwire.MilliSatoshi,
-	edge *channeldb.ChannelEdgePolicy) lnwire.MilliSatoshi {
-
-	return edge.FeeBaseMSat + (amt*edge.FeeProportionalMillionths)/1000000
-}
-
 // newRoute returns a fully valid route between the source and target that's
 // capable of supporting a payment of `amtToSend` after fees are fully
 // computed. If the route is too long, or the selected path cannot support the
@@ -129,7 +120,7 @@ func newRoute(amtToSend lnwire.MilliSatoshi, 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 = computeFee(amtToForward, pathEdges[i+1])
+			fee = pathEdges[i+1].ComputeFee(amtToForward)
 		}
 
 		// If this is the last hop, then for verification purposes, the
@@ -482,7 +473,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		var fee lnwire.MilliSatoshi
 		var timeLockDelta uint16
 		if fromVertex != source {
-			fee = computeFee(amountToSend, edge)
+			fee = edge.ComputeFee(amountToSend)
 			timeLockDelta = edge.TimeLockDelta
 		}
 

routing/route/route.go

@@ -3,6 +3,7 @@ package route
 import (
 	"bytes"
 	"encoding/binary"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"strconv"
@@ -47,6 +48,22 @@ func NewVertexFromBytes(b []byte) (Vertex, error) {
 	return v, nil
 }
 
+// NewVertexFromStr returns a new Vertex given its hex-encoded string format.
+func NewVertexFromStr(v string) (Vertex, error) {
+	// Return error if hex string is of incorrect length.
+	if len(v) != VertexSize*2 {
+		return Vertex{}, fmt.Errorf("invalid vertex string length of "+
+			"%v, want %v", len(v), VertexSize*2)
+	}
+
+	vertex, err := hex.DecodeString(v)
+	if err != nil {
+		return Vertex{}, err
+	}
+
+	return NewVertexFromBytes(vertex)
+}
+
 // String returns a human readable version of the Vertex which is the
 // hex-encoding of the serialized compressed public key.
 func (v Vertex) String() string {

routing/router.go

@@ -2253,3 +2253,297 @@ func generateBandwidthHints(sourceNode *channeldb.LightningNode,
 
 	return bandwidthHints, nil
 }
+
+// runningAmounts keeps running amounts while the route is traversed.
+type runningAmounts struct {
+	// amt is the intended amount to send via the route.
+	amt lnwire.MilliSatoshi
+
+	// max is the running maximum that the route can carry.
+	max lnwire.MilliSatoshi
+}
+
+// prependChannel returns a new set of running amounts that would result from
+// prepending the given channel to the route. If canIncreaseAmt is set, the
+// amount may be increased if it is too small to satisfy the channel's minimum
+// htlc amount.
+func (r *runningAmounts) prependChannel(policy *channeldb.ChannelEdgePolicy,
+	capacity btcutil.Amount, localChan bool, canIncreaseAmt bool) (
+	runningAmounts, error) {
+
+	// Determine max htlc value.
+	maxHtlc := lnwire.NewMSatFromSatoshis(capacity)
+	if policy.MessageFlags.HasMaxHtlc() {
+		maxHtlc = policy.MaxHTLC
+	}
+
+	amt := r.amt
+
+	// If we have a specific amount for which we are building the route,
+	// validate it against the channel constraints and return the new
+	// running amount.
+	if !canIncreaseAmt {
+		if amt < policy.MinHTLC || amt > maxHtlc {
+			return runningAmounts{}, fmt.Errorf("channel htlc "+
+				"constraints [%v - %v] violated with amt %v",
+				policy.MinHTLC, maxHtlc, amt)
+		}
+
+		// Update running amount by adding the fee for non-local
+		// channels.
+		if !localChan {
+			amt += policy.ComputeFee(amt)
+		}
+
+		return runningAmounts{
+			amt: amt,
+		}, nil
+	}
+
+	// Adapt the minimum amount to what this channel allows.
+	if policy.MinHTLC > r.amt {
+		amt = policy.MinHTLC
+	}
+
+	// Update the maximum amount too to be able to detect incompatible
+	// channels.
+	max := r.max
+	if maxHtlc < r.max {
+		max = maxHtlc
+	}
+
+	// If we get in the situation that the minimum amount exceeds the
+	// maximum amount (enforced further down stream), we have incompatible
+	// channel policies.
+	//
+	// There is possibility with pubkey addressing that we should have
+	// selected a different channel downstream, but we don't backtrack to
+	// try to fix that. It would complicate path finding while we expect
+	// this situation to be rare. The spec recommends to keep all policies
+	// towards a peer identical. If that is the case, there isn't a better
+	// channel that we should have selected.
+	if amt > max {
+		return runningAmounts{},
+			fmt.Errorf("incompatible channel policies: %v "+
+				"exceeds %v", amt, max)
+	}
+
+	// Add fees to the running amounts. Skip the source node fees as
+	// those do not need to be paid.
+	if !localChan {
+		amt += policy.ComputeFee(amt)
+		max += policy.ComputeFee(max)
+	}
+
+	return runningAmounts{amt: amt, max: max}, nil
+}
+
+// ErrNoChannel is returned when a route cannot be built because there are no
+// channels that satisfy all requirements.
+type ErrNoChannel struct {
+	position int
+	fromNode route.Vertex
+}
+
+// Error returns a human readable string describing the error.
+func (e ErrNoChannel) Error() string {
+	return fmt.Sprintf("no matching outgoing channel available for "+
+		"node %v (%v)", e.position, e.fromNode)
+}
+
+// BuildRoute returns a fully specified route based on a list of pubkeys. If
+// amount is nil, the minimum routable amount is used. To force a specific
+// outgoing channel, use the outgoingChan parameter.
+func (r *ChannelRouter) BuildRoute(amt *lnwire.MilliSatoshi,
+	hops []route.Vertex, outgoingChan *uint64,
+	finalCltvDelta int32) (*route.Route, error) {
+
+	log.Tracef("BuildRoute called: hopsCount=%v, amt=%v",
+		len(hops), amt)
+
+	// If no amount is specified, we need to build a route for the minimum
+	// amount that this route can carry.
+	useMinAmt := amt == nil
+
+	// We'll attempt to obtain a set of bandwidth hints that helps us select
+	// the best outgoing channel to use in case no outgoing channel is set.
+	bandwidthHints, err := generateBandwidthHints(
+		r.selfNode, r.cfg.QueryBandwidth,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	// Allocate a list that will contain the selected channels for this
+	// route.
+	edges := make([]*channeldb.ChannelEdgePolicy, len(hops))
+
+	// Keep a running amount and the maximum for this route.
+	amts := runningAmounts{
+		max: lnwire.MilliSatoshi(^uint64(0)),
+	}
+	if useMinAmt {
+		// For minimum amount routes, aim to deliver at least 1 msat to
+		// the destination. There are nodes in the wild that have a
+		// min_htlc channel policy of zero, which could lead to a zero
+		// amount payment being made.
+		amts.amt = 1
+	} else {
+		// If an amount is specified, we need to build a route that
+		// delivers exactly this amount to the final destination.
+		amts.amt = *amt
+	}
+
+	// Traverse hops backwards to accumulate fees in the running amounts.
+	source := r.selfNode.PubKeyBytes
+	for i := len(hops) - 1; i >= 0; i-- {
+		toNode := hops[i]
+
+		var fromNode route.Vertex
+		if i == 0 {
+			fromNode = source
+		} else {
+			fromNode = hops[i-1]
+		}
+
+		localChan := i == 0
+
+		// Iterate over candidate channels to select the channel
+		// to use for the final route.
+		var (
+			bestEdge      *channeldb.ChannelEdgePolicy
+			bestAmts      *runningAmounts
+			bestBandwidth lnwire.MilliSatoshi
+		)
+
+		cb := func(tx *bbolt.Tx,
+			edgeInfo *channeldb.ChannelEdgeInfo,
+			_, inEdge *channeldb.ChannelEdgePolicy) error {
+
+			chanID := edgeInfo.ChannelID
+
+			// Apply outgoing channel restriction is active.
+			if localChan && outgoingChan != nil &&
+				chanID != *outgoingChan {
+
+				return nil
+			}
+
+			// No unknown policy channels.
+			if inEdge == nil {
+				return nil
+			}
+
+			// Before we can process the edge, we'll need to
+			// fetch the node on the _other_ end of this
+			// channel as we may later need to iterate over
+			// the incoming edges of this node if we explore
+			// it further.
+			chanFromNode, err := edgeInfo.FetchOtherNode(
+				tx, toNode[:],
+			)
+			if err != nil {
+				return err
+			}
+
+			// Continue searching if this channel doesn't
+			// connect with the previous hop.
+			if chanFromNode.PubKeyBytes != fromNode {
+				return nil
+			}
+
+			// Validate whether this channel's policy is satisfied
+			// and obtain the new running amounts if this channel
+			// was to be selected.
+			newAmts, err := amts.prependChannel(
+				inEdge, edgeInfo.Capacity, localChan,
+				useMinAmt,
+			)
+			if err != nil {
+				log.Tracef("Skipping chan %v: %v",
+					inEdge.ChannelID, err)
+
+				return nil
+			}
+
+			// If we already have a best edge, check whether this
+			// edge is better.
+			bandwidth := bandwidthHints[chanID]
+			if bestEdge != nil {
+				if localChan {
+					// For local channels, better is defined
+					// as having more bandwidth. We try to
+					// maximize the chance that the returned
+					// route succeeds.
+					if bandwidth < bestBandwidth {
+						return nil
+					}
+				} else {
+					// For other channels, better is defined
+					// as lower fees for the amount to send.
+					// Normally all channels between two
+					// nodes should have the same policy,
+					// but in case not we minimize our cost
+					// here. Regular path finding would do
+					// the same.
+					if newAmts.amt > bestAmts.amt {
+						return nil
+					}
+				}
+			}
+
+			// If we get here, the current edge is better. Replace
+			// the best.
+			bestEdge = inEdge
+			bestAmts = &newAmts
+			bestBandwidth = bandwidth
+
+			return nil
+		}
+
+		err := r.cfg.Graph.ForEachNodeChannel(nil, toNode[:], cb)
+		if err != nil {
+			return nil, err
+		}
+
+		// There is no matching channel. Stop building the route here.
+		if bestEdge == nil {
+			return nil, ErrNoChannel{
+				fromNode: fromNode,
+				position: i,
+			}
+		}
+
+		log.Tracef("Select channel %v at position %v", bestEdge.ChannelID, i)
+
+		edges[i] = bestEdge
+		amts = *bestAmts
+	}
+
+	_, height, err := r.cfg.Chain.GetBestBlock()
+	if err != nil {
+		return nil, err
+	}
+
+	var receiverAmt lnwire.MilliSatoshi
+	if useMinAmt {
+		// We've calculated the minimum amount for the htlc that the
+		// source node hands out. The newRoute call below expects the
+		// amount that must reach the receiver after subtraction of fees
+		// along the way. Iterate over all edges to calculate the
+		// receiver amount.
+		receiverAmt = amts.amt
+		for _, edge := range edges[1:] {
+			receiverAmt -= edge.ComputeFeeFromIncoming(receiverAmt)
+		}
+	} else {
+		// Deliver the specified amount to the receiver.
+		receiverAmt = *amt
+	}
+
+	// Build and return the final route.
+	return newRoute(
+		receiverAmt, source, edges, uint32(height),
+		uint16(finalCltvDelta), nil,
+	)
+}

routing/router_test.go

@@ -3330,3 +3330,146 @@ func TestSendToRouteStructuredError(t *testing.T) {
 		t.Fatalf("initPayment not called")
 	}
 }
+
+// TestBuildRoute tests whether correct routes are built.
+func TestBuildRoute(t *testing.T) {
+	// Setup a three node network.
+	chanCapSat := btcutil.Amount(100000)
+	testChannels := []*testChannel{
+		// Create two local channels from a. The bandwidth is estimated
+		// in this test as the channel capacity. For building routes, we
+		// expected the channel with the largest estimated bandwidth to
+		// be selected.
+		symmetricTestChannel("a", "b", chanCapSat, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 20000,
+			MinHTLC: lnwire.NewMSatFromSatoshis(5),
+			MaxHTLC: lnwire.NewMSatFromSatoshis(chanCapSat),
+		}, 1),
+		symmetricTestChannel("a", "b", chanCapSat/2, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 20000,
+			MinHTLC: lnwire.NewMSatFromSatoshis(5),
+			MaxHTLC: lnwire.NewMSatFromSatoshis(chanCapSat / 2),
+		}, 6),
+
+		// Create two channels from b to c. For building routes, we
+		// expect the lowest cost channel to be selected. Note that this
+		// isn't a situation that we are expecting in reality. Routing
+		// nodes are recommended to keep their channel policies towards
+		// the same peer identical.
+		symmetricTestChannel("b", "c", chanCapSat, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 50000,
+			MinHTLC: lnwire.NewMSatFromSatoshis(20),
+			MaxHTLC: lnwire.NewMSatFromSatoshis(120),
+		}, 2),
+		symmetricTestChannel("b", "c", chanCapSat, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 60000,
+			MinHTLC: lnwire.NewMSatFromSatoshis(20),
+			MaxHTLC: lnwire.NewMSatFromSatoshis(120),
+		}, 7),
+
+		symmetricTestChannel("a", "e", chanCapSat, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 80000,
+			MinHTLC: lnwire.NewMSatFromSatoshis(5),
+			MaxHTLC: lnwire.NewMSatFromSatoshis(10),
+		}, 5),
+		symmetricTestChannel("e", "c", chanCapSat, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 100000,
+			MinHTLC: lnwire.NewMSatFromSatoshis(20),
+			MaxHTLC: lnwire.NewMSatFromSatoshis(chanCapSat),
+		}, 4),
+	}
+
+	testGraph, err := createTestGraphFromChannels(testChannels, "a")
+	if err != nil {
+		t.Fatalf("unable to create graph: %v", err)
+	}
+	defer testGraph.cleanUp()
+
+	const startingBlockHeight = 101
+
+	ctx, cleanUp, err := createTestCtxFromGraphInstance(
+		startingBlockHeight, testGraph,
+	)
+	if err != nil {
+		t.Fatalf("unable to create router: %v", err)
+	}
+	defer cleanUp()
+
+	checkHops := func(rt *route.Route, expected []uint64) {
+		if len(rt.Hops) != len(expected) {
+			t.Fatal("hop count mismatch")
+		}
+		for i, hop := range rt.Hops {
+			if hop.ChannelID != expected[i] {
+				t.Fatalf("expected channel %v at pos %v, but "+
+					"got channel %v",
+					expected[i], i, hop.ChannelID)
+			}
+		}
+	}
+
+	// Create hop list from the route node pubkeys.
+	hops := []route.Vertex{
+		ctx.aliases["b"], ctx.aliases["c"],
+	}
+	amt := lnwire.NewMSatFromSatoshis(100)
+
+	// Build the route for the given amount.
+	rt, err := ctx.router.BuildRoute(
+		&amt, hops, nil, 40,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Check that we get the expected route back. The total amount should be
+	// the amount to deliver to hop c (100 sats) plus the fee for hop b (5
+	// sats).
+	checkHops(rt, []uint64{1, 2})
+	if rt.TotalAmount != 105000 {
+		t.Fatalf("unexpected total amount %v", rt.TotalAmount)
+	}
+
+	// Build the route for the minimum amount.
+	rt, err = ctx.router.BuildRoute(
+		nil, hops, nil, 40,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Check that we get the expected route back. The minimum that we can
+	// send from b to c is 20 sats. Hop b charges 1 sat for the forwarding.
+	// The channel between hop a and b can carry amounts in the range [5,
+	// 100], so 21 sats is the minimum amount for this route.
+	checkHops(rt, []uint64{1, 2})
+	if rt.TotalAmount != 21000 {
+		t.Fatalf("unexpected total amount %v", rt.TotalAmount)
+	}
+
+	// Test a route that contains incompatible channel htlc constraints.
+	// There is no amount that can pass through both channel 5 and 4.
+	hops = []route.Vertex{
+		ctx.aliases["e"], ctx.aliases["c"],
+	}
+	_, err = ctx.router.BuildRoute(
+		nil, hops, nil, 40,
+	)
+	errNoChannel, ok := err.(ErrNoChannel)
+	if !ok {
+		t.Fatalf("expected incompatible policies error, but got %v",
+			err)
+	}
+	if errNoChannel.position != 0 {
+		t.Fatalf("unexpected no channel error position")
+	}
+	if errNoChannel.fromNode != ctx.aliases["a"] {
+		t.Fatalf("unexpected no channel error node")
+	}
+}