routing: fix route fee calculation and channel capacity check
This commit fixes the logic inside the newRoute function to address the following problems: - Fee calculation for a hop does not include the fee that needs to be paid to the next hop. - The incoming channel capacity "sanity" check does not include the fee to be paid to the current hop.
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Joost Jager committed
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@@ -50,10 +50,11 @@ type HopHint struct {
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CLTVExpiryDelta uint16
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}
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// ChannelHop is an intermediate hop within the network with a greater
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// multi-hop payment route. This struct contains the relevant routing policy of
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// the particular edge, as well as the total capacity, and origin chain of the
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// channel itself.
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// ChannelHop describes the channel through which an intermediate or final
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// hop can be reached. This struct contains the relevant routing policy of
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// the particular edge (which is a property of the source node of the channel
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// edge), as well as the total capacity. It also includes the origin chain of
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// the channel itself.
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type ChannelHop struct {
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// Capacity is the total capacity of the channel being traversed. This
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// value is expressed for stability in satoshis.
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@@ -69,13 +70,15 @@ type ChannelHop struct {
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*channeldb.ChannelEdgePolicy
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}
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// Hop represents the forwarding details at a particular position within the
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// final route. This struct houses the values necessary to create the HTLC
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// which will travel along this hop, and also encode the per-hop payload
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// included within the Sphinx packet.
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// Hop represents an intermediate or final node of the route. This naming
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// is in line with the definition given in BOLT #4: Onion Routing Protocol.
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// The struct houses the channel along which this hop can be reached and
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// the values necessary to create the HTLC that needs to be sent to the
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// next hop. It is also used to encode the per-hop payload included within
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// the Sphinx packet.
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type Hop struct {
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// Channel is the active payment channel edge that this hop will travel
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// along.
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// Channel is the active payment channel edge along which the packet
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// travels to reach this hop. This is the _incoming_ channel to this hop.
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Channel *ChannelHop
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// OutgoingTimeLock is the timelock value that should be used when
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@@ -269,11 +272,6 @@ func newRoute(amtToSend, feeLimit lnwire.MilliSatoshi, sourceVertex Vertex,
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// information for the first hop so the mapping is sound.
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route.nextHopMap[sourceVertex] = pathEdges[0]
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// The running amount is the total amount of satoshis required at this
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// point in the route. We start this value at the amount we want to
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// send to the destination. This value will then get successively
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// larger as we compute the fees going backwards.
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runningAmt := amtToSend
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pathLength := len(pathEdges)
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for i := pathLength - 1; i >= 0; i-- {
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edge := pathEdges[i]
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@@ -285,59 +283,56 @@ func newRoute(amtToSend, feeLimit lnwire.MilliSatoshi, sourceVertex Vertex,
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route.nodeIndex[v] = struct{}{}
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route.chanIndex[edge.ChannelID] = struct{}{}
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// If this isn't a direct payment, and this isn't the last hop
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// in the route, then we'll also populate the nextHop map to
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// allow easy route traversal by callers.
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// If this isn't a direct payment, and this isn't the edge to
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// the last hop in the route, then we'll also populate the
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// nextHop map to allow easy route traversal by callers.
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if len(pathEdges) > 1 && i != len(pathEdges)-1 {
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route.nextHopMap[v] = route.Hops[i+1].Channel
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}
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// Now we'll start to calculate the items within the per-hop
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// payload for this current hop.
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//
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// If this is the last hop, then we send the exact amount and
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// pay no fee, as we're paying directly to the receiver, and
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// there're no additional hops.
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amtToForward := runningAmt
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// payload for the hop this edge is leading to. This hop will
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// be called the 'current hop'.
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// If it is the last hop, then the hop payload will contain
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// the exact amount. In BOLT #4: Onion Routing
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// Protocol / "Payload for the Last Node", this is detailed.
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amtToForward := amtToSend
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// Fee is not part of the hop payload, but only used for
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// reporting through RPC. Set to zero for the final hop.
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fee := lnwire.MilliSatoshi(0)
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// If this isn't the last hop, to add enough funds to pay for
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// transit over the next link.
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// If the current hop isn't the last hop, then add enough funds
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// to pay for transit over the next link.
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if i != len(pathEdges)-1 {
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// We'll grab the edge policy and per-hop payload of
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// the prior hop so we can calculate fees properly.
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prevEdge := pathEdges[i+1]
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prevHop := route.Hops[i+1]
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// We'll grab the per-hop payload of the next hop (the
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// hop _after_ the hop this edge leads to) in the
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// route so we can calculate fees properly.
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nextHop := route.Hops[i+1]
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// The fee for this hop, will be based on how much the
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// prior hop carried, as we'll need to increase the
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// amount of satoshis incoming into this hop to
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// properly pay the required fees.
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prevAmount := prevHop.AmtToForward
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fee = computeFee(prevAmount, prevEdge.ChannelEdgePolicy)
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// The amount that the current hop needs to forward is
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// based on how much the next hop forwards plus the fee
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// that needs to be paid to the next hop.
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amtToForward = nextHop.AmtToForward + nextHop.Fee
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// With the fee computed, we increment the total amount
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// as we need to pay this fee. This value represents
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// the amount of funds that will come _into_ this edge.
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runningAmt += fee
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// Otherwise, for a node to forward an HTLC, then
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// following inequality most hold true:
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//
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// * amt_in - fee >= amt_to_forward
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amtToForward = runningAmt - fee
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// The fee that needs to be paid to the current hop is
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// based on the amount that this hop needs to forward
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// and its policy for the outgoing channel. This policy
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// is stored as part of the incoming channel of
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// the next hop.
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fee = computeFee(amtToForward, nextHop.Channel.ChannelEdgePolicy)
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}
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// Now we create the hop struct for this point in the route.
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// The amount to forward is the running amount, and we compute
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// the required fee based on this amount.
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nextHop := &Hop{
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// Now we create the hop struct for the current hop.
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currentHop := &Hop{
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Channel: edge,
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AmtToForward: amtToForward,
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Fee: fee,
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}
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route.TotalFees += nextHop.Fee
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// Accumulate all fees.
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route.TotalFees += currentHop.Fee
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// Invalidate this route if its total fees exceed our fee limit.
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if route.TotalFees > feeLimit {
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@@ -346,14 +341,17 @@ func newRoute(amtToSend, feeLimit lnwire.MilliSatoshi, sourceVertex Vertex,
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return nil, newErrf(ErrFeeLimitExceeded, err)
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}
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// As a sanity check, we ensure that the selected channel has
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// enough capacity to forward the required amount which
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// includes the fee dictated at each hop.
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if nextHop.AmtToForward.ToSatoshis() > nextHop.Channel.Capacity {
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// As a sanity check, we ensure that the incoming channel has
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// enough capacity to carry the required amount which
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// includes the fee dictated at each hop. Make the comparison
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// in msat to prevent rounding errors.
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if currentHop.AmtToForward + fee > lnwire.NewMSatFromSatoshis(
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currentHop.Channel.Capacity) {
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err := fmt.Sprintf("channel graph has insufficient "+
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"capacity for the payment: need %v, have %v",
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nextHop.AmtToForward.ToSatoshis(),
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nextHop.Channel.Capacity)
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currentHop.AmtToForward.ToSatoshis(),
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currentHop.Channel.Capacity)
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return nil, newErrf(ErrInsufficientCapacity, err)
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}
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@@ -367,7 +365,7 @@ func newRoute(amtToSend, feeLimit lnwire.MilliSatoshi, sourceVertex Vertex,
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// last link in the route.
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route.TotalTimeLock += uint32(finalCLTVDelta)
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nextHop.OutgoingTimeLock = currentHeight + uint32(finalCLTVDelta)
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currentHop.OutgoingTimeLock = currentHeight + uint32(finalCLTVDelta)
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} else {
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// Next, increment the total timelock of the entire
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// route such that each hops time lock increases as we
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@@ -380,10 +378,10 @@ func newRoute(amtToSend, feeLimit lnwire.MilliSatoshi, sourceVertex Vertex,
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// be the value of the time-lock for the _outgoing_
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// HTLC, so we factor in their specified grace period
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// (time lock delta).
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nextHop.OutgoingTimeLock = route.TotalTimeLock - delta
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currentHop.OutgoingTimeLock = route.TotalTimeLock - delta
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}
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route.Hops[i] = nextHop
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route.Hops[i] = currentHop
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}
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// We'll then make a second run through our route in order to set up
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@@ -393,9 +391,11 @@ func newRoute(amtToSend, feeLimit lnwire.MilliSatoshi, sourceVertex Vertex,
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route.prevHopMap[vertex] = hop.Channel
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}
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// The total amount required for this route will be the value the
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// source extends to the first hop in the route.
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route.TotalAmount = runningAmt
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// The total amount required for this route will be the value
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// that the first hop needs to forward plus the fee that
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// the first hop charges for this. Note that the sender of the
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// payment is not a hop in the route.
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route.TotalAmount = route.Hops[0].AmtToForward + route.Hops[0].Fee
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return route, nil
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}
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