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pathfind.go
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package routing
import (
"bytes"
"encoding/binary"
"fmt"
"math"
"container/heap"
"github.com/btcsuite/btcd/btcec"
"github.com/coreos/bbolt"
"github.com/lightningnetwork/lightning-onion"
"github.com/lightningnetwork/lnd/channeldb"
"github.com/lightningnetwork/lnd/lnwire"
)
const (
// HopLimit is the maximum number hops that is permissible as a route.
// Any potential paths found that lie above this limit will be rejected
// with an error. This value is computed using the current fixed-size
// packet length of the Sphinx construction.
HopLimit = 20
// infinity is used as a starting distance in our shortest path search.
infinity = math.MaxInt64
// RiskFactorBillionths controls the influence of time lock delta
// of a channel on route selection. It is expressed as billionths
// of msat per msat sent through the channel per time lock delta
// block. See edgeWeight function below for more details.
// The chosen value is based on the previous incorrect weight function
// 1 + timelock + fee * fee. In this function, the fee penalty
// diminishes the time lock penalty for all but the smallest amounts.
// To not change the behaviour of path finding too drastically, a
// relatively small value is chosen which is still big enough to give
// some effect with smaller time lock values. The value may need
// tweaking and/or be made configurable in the future.
RiskFactorBillionths = 15
)
// HopHint is a routing hint that contains the minimum information of a channel
// required for an intermediate hop in a route to forward the payment to the
// next. This should be ideally used for private channels, since they are not
// publicly advertised to the network for routing.
type HopHint struct {
// NodeID is the public key of the node at the start of the channel.
NodeID *btcec.PublicKey
// ChannelID is the unique identifier of the channel.
ChannelID uint64
// FeeBaseMSat is the base fee of the channel in millisatoshis.
FeeBaseMSat uint32
// FeeProportionalMillionths is the fee rate, in millionths of a
// satoshi, for every satoshi sent through the channel.
FeeProportionalMillionths uint32
// CLTVExpiryDelta is the time-lock delta of the channel.
CLTVExpiryDelta uint16
}
// Hop represents an intermediate or final node of the route. This naming
// is in line with the definition given in BOLT #4: Onion Routing Protocol.
// The struct houses the channel along which this hop can be reached and
// the values necessary to create the HTLC that needs to be sent to the
// next hop. It is also used to encode the per-hop payload included within
// the Sphinx packet.
type Hop struct {
// PubKeyBytes is the raw bytes of the public key of the target node.
PubKeyBytes Vertex
// ChannelID is the unique channel ID for the channel. The first 3
// bytes are the block height, the next 3 the index within the block,
// and the last 2 bytes are the output index for the channel.
ChannelID uint64
// OutgoingTimeLock is the timelock value that should be used when
// crafting the _outgoing_ HTLC from this hop.
OutgoingTimeLock uint32
// AmtToForward is the amount that this hop will forward to the next
// hop. This value is less than the value that the incoming HTLC
// carries as a fee will be subtracted by the hop.
AmtToForward lnwire.MilliSatoshi
}
// edgePolicyWithSource is a helper struct to keep track of the source node
// of a channel edge. ChannelEdgePolicy only contains to destination node
// of the edge.
type edgePolicyWithSource struct {
sourceNode *channeldb.LightningNode
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
}
// isSamePath returns true if path1 and path2 travel through the exact same
// edges, and false otherwise.
func isSamePath(path1, path2 []*channeldb.ChannelEdgePolicy) bool {
if len(path1) != len(path2) {
return false
}
for i := 0; i < len(path1); i++ {
if path1[i].ChannelID != path2[i].ChannelID {
return false
}
}
return true
}
// Route represents a path through the channel graph which runs over one or
// more channels in succession. This struct carries all the information
// required to craft the Sphinx onion packet, and send the payment along the
// first hop in the path. A route is only selected as valid if all the channels
// have sufficient capacity to carry the initial payment amount after fees are
// accounted for.
type Route struct {
// TotalTimeLock is the cumulative (final) time lock across the entire
// route. This is the CLTV value that should be extended to the first
// hop in the route. All other hops will decrement the time-lock as
// advertised, leaving enough time for all hops to wait for or present
// the payment preimage to complete the payment.
TotalTimeLock uint32
// TotalFees is the sum of the fees paid at each hop within the final
// route. In the case of a one-hop payment, this value will be zero as
// we don't need to pay a fee to ourself.
TotalFees lnwire.MilliSatoshi
// TotalAmount is the total amount of funds required to complete a
// payment over this route. This value includes the cumulative fees at
// each hop. As a result, the HTLC extended to the first-hop in the
// route will need to have at least this many satoshis, otherwise the
// route will fail at an intermediate node due to an insufficient
// amount of fees.
TotalAmount lnwire.MilliSatoshi
// Hops contains details concerning the specific forwarding details at
// each hop.
Hops []*Hop
// nodeIndex is a map that allows callers to quickly look up if a node
// is present in this computed route or not.
nodeIndex map[Vertex]struct{}
// chanIndex is an index that allows callers to determine if a channel
// is present in this route or not. Channels are identified by the
// uint64 version of the short channel ID.
chanIndex map[uint64]struct{}
// nextHop maps a node, to the next channel that it will pass the HTLC
// off to. With this map, we can easily look up the next outgoing
// channel or node for pruning purposes.
nextHopMap map[Vertex]*Hop
// prevHop maps a node, to the channel that was directly before it
// within the route. With this map, we can easily look up the previous
// channel or node for pruning purposes.
prevHopMap map[Vertex]*Hop
}
// HopFee returns the fee charged by the route hop indicated by hopIndex.
func (r *Route) HopFee(hopIndex int) lnwire.MilliSatoshi {
var incomingAmt lnwire.MilliSatoshi
if hopIndex == 0 {
incomingAmt = r.TotalAmount
} else {
incomingAmt = r.Hops[hopIndex-1].AmtToForward
}
// Fee is calculated as difference between incoming and outgoing amount.
return incomingAmt - r.Hops[hopIndex].AmtToForward
}
// nextHopVertex returns the next hop (by Vertex) after the target node. If the
// target node is not found in the route, then false is returned.
func (r *Route) nextHopVertex(n *btcec.PublicKey) (Vertex, bool) {
hop, ok := r.nextHopMap[NewVertex(n)]
return Vertex(hop.PubKeyBytes), ok
}
// nextHopChannel returns the uint64 channel ID of the next hop after the
// target node. If the target node is not found in the route, then false is
// returned.
func (r *Route) nextHopChannel(n *btcec.PublicKey) (*Hop, bool) {
hop, ok := r.nextHopMap[NewVertex(n)]
return hop, ok
}
// prevHopChannel returns the uint64 channel ID of the before hop after the
// target node. If the target node is not found in the route, then false is
// returned.
func (r *Route) prevHopChannel(n *btcec.PublicKey) (*Hop, bool) {
hop, ok := r.prevHopMap[NewVertex(n)]
return hop, ok
}
// containsNode returns true if a node is present in the target route, and
// false otherwise.
func (r *Route) containsNode(v Vertex) bool {
_, ok := r.nodeIndex[v]
return ok
}
// containsChannel returns true if a channel is present in the target route,
// and false otherwise. The passed chanID should be the converted uint64 form
// of lnwire.ShortChannelID.
func (r *Route) containsChannel(chanID uint64) bool {
_, ok := r.chanIndex[chanID]
return ok
}
// ToHopPayloads converts a complete route into the series of per-hop payloads
// that is to be encoded within each HTLC using an opaque Sphinx packet.
func (r *Route) ToHopPayloads() []sphinx.HopData {
hopPayloads := make([]sphinx.HopData, len(r.Hops))
// For each hop encoded within the route, we'll convert the hop struct
// to the matching per-hop payload struct as used by the sphinx
// package.
for i, hop := range r.Hops {
hopPayloads[i] = sphinx.HopData{
// TODO(roasbeef): properly set realm, make sphinx type
// an enum actually?
Realm: 0,
ForwardAmount: uint64(hop.AmtToForward),
OutgoingCltv: hop.OutgoingTimeLock,
}
// As a base case, the next hop is set to all zeroes in order
// to indicate that the "last hop" as no further hops after it.
nextHop := uint64(0)
// If we aren't on the last hop, then we set the "next address"
// field to be the channel that directly follows it.
if i != len(r.Hops)-1 {
nextHop = r.Hops[i+1].ChannelID
}
binary.BigEndian.PutUint64(hopPayloads[i].NextAddress[:],
nextHop)
}
return hopPayloads
}
// 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
// fully payment including fees, then a non-nil error is returned.
//
// NOTE: The passed slice of ChannelHops MUST be sorted in forward order: from
// the source to the target node of the path finding attempt.
func newRoute(amtToSend, feeLimit lnwire.MilliSatoshi, sourceVertex Vertex,
pathEdges []*channeldb.ChannelEdgePolicy, currentHeight uint32,
finalCLTVDelta uint16) (*Route, error) {
var (
hops []*Hop
// totalTimeLock will accumulate the cumulative time lock
// across the entire route. This value represents how long the
// sender will need to wait in the *worst* case.
totalTimeLock = currentHeight
// nextIncomingAmount is the amount that will need to flow into
// the *next* hop. Since we're going to be walking the route
// backwards below, this next hop gets closer and closer to the
// sender of the payment.
nextIncomingAmount lnwire.MilliSatoshi
)
pathLength := len(pathEdges)
for i := pathLength - 1; i >= 0; i-- {
// Now we'll start to calculate the items within the per-hop
// payload for the hop this edge is leading to.
edge := pathEdges[i]
// If this is the last hop, then the hop payload will contain
// the exact amount. In BOLT #4: Onion Routing
// Protocol / "Payload for the Last Node", this is detailed.
amtToForward := amtToSend
// 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)
// If the current hop isn't the last hop, then add enough funds
// to pay for transit over the next link.
if i != len(pathEdges)-1 {
// The amount that the current hop needs to forward is
// equal to the incoming amount of the next hop.
amtToForward = nextIncomingAmount
// The fee that needs to be paid to the current hop is
// based on the amount that this hop needs to forward
// 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])
}
// If this is the last hop, then for verification purposes, the
// value of the outgoing time-lock should be _exactly_ the
// absolute time out they'd expect in the HTLC.
var outgoingTimeLock uint32
if i == len(pathEdges)-1 {
// As this is the last hop, we'll use the specified
// final CLTV delta value instead of the value from the
// last link in the route.
totalTimeLock += uint32(finalCLTVDelta)
outgoingTimeLock = currentHeight + uint32(finalCLTVDelta)
} else {
// Next, increment the total timelock of the entire
// route such that each hops time lock increases as we
// walk backwards in the route, using the delta of the
// previous hop.
delta := uint32(pathEdges[i+1].TimeLockDelta)
totalTimeLock += delta
// Otherwise, the value of the outgoing time-lock will
// be the value of the time-lock for the _outgoing_
// HTLC, so we factor in their specified grace period
// (time lock delta).
outgoingTimeLock = totalTimeLock - delta
}
// Since we're traversing the path backwards atm, we prepend
// each new hop such that, the final slice of hops will be in
// the forwards order.
currentHop := &Hop{
PubKeyBytes: Vertex(edge.Node.PubKeyBytes),
ChannelID: edge.ChannelID,
AmtToForward: amtToForward,
OutgoingTimeLock: outgoingTimeLock,
}
hops = append([]*Hop{currentHop}, hops...)
// 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
}
// With the base routing data expressed as hops, build the full route
newRoute := NewRouteFromHops(
nextIncomingAmount, totalTimeLock, sourceVertex, hops,
)
// Invalidate this route if its total fees exceed our fee limit.
if newRoute.TotalFees > feeLimit {
err := fmt.Sprintf("total route fees exceeded fee "+
"limit of %v", feeLimit)
return nil, newErrf(ErrFeeLimitExceeded, err)
}
return newRoute, nil
}
// NewRouteFromHops creates a new Route structure from the minimally required
// information to perform the payment. It infers fee amounts and populates the
// node, chan and prev/next hop maps.
func NewRouteFromHops(amtToSend lnwire.MilliSatoshi, timeLock uint32,
sourceVertex Vertex, hops []*Hop) *Route {
// First, we'll create a route struct and populate it with the fields
// for which the values are provided as arguments of this function.
// TotalFees is determined based on the difference between the amount
// that is send from the source and the final amount that is received
// by the destination.
route := &Route{
Hops: hops,
TotalTimeLock: timeLock,
TotalAmount: amtToSend,
TotalFees: amtToSend - hops[len(hops)-1].AmtToForward,
nodeIndex: make(map[Vertex]struct{}),
chanIndex: make(map[uint64]struct{}),
nextHopMap: make(map[Vertex]*Hop),
prevHopMap: make(map[Vertex]*Hop),
}
// Then we'll update the node and channel index, to indicate that this
// Vertex and incoming channel link are present within this route.
// Also, the prev and next hop maps will be populated.
prevNode := sourceVertex
for i := 0; i < len(hops); i++ {
hop := hops[i]
v := Vertex(hop.PubKeyBytes)
route.nodeIndex[v] = struct{}{}
route.chanIndex[hop.ChannelID] = struct{}{}
route.prevHopMap[v] = hop
route.nextHopMap[prevNode] = hop
prevNode = v
}
return route
}
// Vertex is a simple alias for the serialization of a compressed Bitcoin
// public key.
type Vertex [33]byte
// NewVertex returns a new Vertex given a public key.
func NewVertex(pub *btcec.PublicKey) Vertex {
var v Vertex
copy(v[:], pub.SerializeCompressed())
return v
}
// 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 {
return fmt.Sprintf("%x", v[:])
}
// edgeWeight computes the weight of an edge. This value is used when searching
// for the shortest path within the channel graph between two nodes. Weight is
// is the fee itself plus a time lock penalty added to it. This benefits
// channels with shorter time lock deltas and shorter (hops) routes in general.
// RiskFactor controls the influence of time lock on route selection. This is
// currently a fixed value, but might be configurable in the future.
func edgeWeight(lockedAmt lnwire.MilliSatoshi, fee lnwire.MilliSatoshi,
timeLockDelta uint16) int64 {
// timeLockPenalty is the penalty for the time lock delta of this channel.
// It is controlled by RiskFactorBillionths and scales proportional
// to the amount that will pass through channel. Rationale is that it if
// a twice as large amount gets locked up, it is twice as bad.
timeLockPenalty := int64(lockedAmt) * int64(timeLockDelta) *
RiskFactorBillionths / 1000000000
return int64(fee) + timeLockPenalty
}
// findPath attempts to find a path from the source node within the
// ChannelGraph to the target node that's capable of supporting a payment of
// `amt` value. The current approach implemented is modified version of
// Dijkstra's algorithm to find a single shortest path between the source node
// and the destination. The distance metric used for edges is related to the
// time-lock+fee costs along a particular edge. If a path is found, this
// function returns a slice of ChannelHop structs which encoded the chosen path
// from the target to the source. The search is performed backwards from
// destination node back to source. This is to properly accumulate fees
// that need to be paid along the path and accurately check the amount
// to forward at every node against the available bandwidth.
func findPath(tx *bolt.Tx, graph *channeldb.ChannelGraph,
additionalEdges map[Vertex][]*channeldb.ChannelEdgePolicy,
sourceNode *channeldb.LightningNode, target *btcec.PublicKey,
ignoredNodes map[Vertex]struct{}, ignoredEdges map[uint64]struct{},
amt lnwire.MilliSatoshi, feeLimit lnwire.MilliSatoshi,
bandwidthHints map[uint64]lnwire.MilliSatoshi) ([]*channeldb.ChannelEdgePolicy, error) {
var err error
if tx == nil {
tx, err = graph.Database().Begin(false)
if err != nil {
return nil, err
}
defer tx.Rollback()
}
// First we'll initialize an empty heap which'll help us to quickly
// locate the next edge we should visit next during our graph
// traversal.
var nodeHeap distanceHeap
// For each node in the graph, we create an entry in the distance map
// for the node set with a distance of "infinity". graph.ForEachNode
// also returns the source node, so there is no need to add the source
// node explicitly.
distance := make(map[Vertex]nodeWithDist)
if err := graph.ForEachNode(tx, func(_ *bolt.Tx, node *channeldb.LightningNode) error {
// TODO(roasbeef): with larger graph can just use disk seeks
// with a visited map
distance[Vertex(node.PubKeyBytes)] = nodeWithDist{
dist: infinity,
node: node,
}
return nil
}); err != nil {
return nil, err
}
additionalEdgesWithSrc := make(map[Vertex][]*edgePolicyWithSource)
for vertex, outgoingEdgePolicies := range additionalEdges {
// We'll also include all the nodes found within the additional
// edges that are not known to us yet in the distance map.
node := &channeldb.LightningNode{PubKeyBytes: vertex}
distance[vertex] = nodeWithDist{
dist: infinity,
node: node,
}
// Build reverse lookup to find incoming edges. Needed because
// search is taken place from target to source.
for _, outgoingEdgePolicy := range outgoingEdgePolicies {
toVertex := outgoingEdgePolicy.Node.PubKeyBytes
incomingEdgePolicy := &edgePolicyWithSource{
sourceNode: node,
edge: outgoingEdgePolicy,
}
additionalEdgesWithSrc[toVertex] =
append(additionalEdgesWithSrc[toVertex],
incomingEdgePolicy)
}
}
sourceVertex := Vertex(sourceNode.PubKeyBytes)
// We can't always assume that the end destination is publicly
// advertised to the network and included in the graph.ForEachNode call
// above, so we'll manually include the target node. The target node
// charges no fee. Distance is set to 0, because this is the starting
// point of the graph traversal. We are searching backwards to get the
// fees first time right and correctly match channel bandwidth.
targetVertex := NewVertex(target)
targetNode := &channeldb.LightningNode{PubKeyBytes: targetVertex}
distance[targetVertex] = nodeWithDist{
dist: 0,
node: targetNode,
amountToReceive: amt,
fee: 0,
}
// We'll use this map as a series of "next" hop pointers. So to get
// from `Vertex` to the target node, we'll take the edge that it's
// mapped to within `next`.
next := make(map[Vertex]*channeldb.ChannelEdgePolicy)
// processEdge is a helper closure that will be used to make sure edges
// satisfy our specific requirements.
processEdge := func(fromNode *channeldb.LightningNode,
edge *channeldb.ChannelEdgePolicy,
bandwidth lnwire.MilliSatoshi, toNode Vertex) {
fromVertex := Vertex(fromNode.PubKeyBytes)
// If the edge is currently disabled, then we'll stop here, as
// we shouldn't attempt to route through it.
edgeFlags := lnwire.ChanUpdateFlag(edge.Flags)
if edgeFlags&lnwire.ChanUpdateDisabled != 0 {
return
}
// If this vertex or edge has been black listed, then we'll
// skip exploring this edge.
if _, ok := ignoredNodes[fromVertex]; ok {
return
}
if _, ok := ignoredEdges[edge.ChannelID]; ok {
return
}
toNodeDist := distance[toNode]
amountToSend := toNodeDist.amountToReceive
// If the estimated band width of the channel edge is not able
// to carry the amount that needs to be send, return.
if bandwidth < amountToSend {
return
}
// If the amountToSend is less than the minimum required
// amount, return.
if amountToSend < edge.MinHTLC {
return
}
// Compute fee that fromNode is charging. It is based on the
// amount that needs to be sent to the next node in the route.
//
// Source node has no predecessor to pay a fee. Therefore set
// fee to zero, because it should not be included in the fee
// limit check and edge weight.
//
// Also determine the time lock delta that will be added to the
// route if fromNode is selected. If fromNode is the source
// node, no additional timelock is required.
var fee lnwire.MilliSatoshi
var timeLockDelta uint16
if fromVertex != sourceVertex {
fee = computeFee(amountToSend, edge)
timeLockDelta = edge.TimeLockDelta
}
// amountToReceive is the amount that the node that is added to
// the distance map needs to receive from a (to be found)
// previous node in the route. That previous node will need to
// pay the amount that this node forwards plus the fee it
// charges.
amountToReceive := amountToSend + fee
// Check if accumulated fees would exceed fee limit when this
// node would be added to the path.
totalFee := amountToReceive - amt
if totalFee > feeLimit {
return
}
// By adding fromNode 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 fromNode.
weight := edgeWeight(amountToReceive, fee, timeLockDelta)
// Compute the tentative distance to this new channel/edge
// which is the distance from our toNode to the target node
// plus the weight of this edge.
tempDist := toNodeDist.dist + weight
// If this new tentative distance is not better than the current
// best known distance to this node, return.
if tempDist >= distance[fromVertex].dist {
return
}
// If the edge has no time lock delta, the payment will always
// fail, so return.
//
// TODO(joostjager): Is this really true? Can't it be that
// nodes take this risk in exchange for a extraordinary high
// fee?
if edge.TimeLockDelta == 0 {
return
}
// All conditions are met and this new tentative distance is
// better than the current best known distance to this node.
// The new better distance is recorded, and also our "next hop"
// map is populated with this edge.
distance[fromVertex] = nodeWithDist{
dist: tempDist,
node: fromNode,
amountToReceive: amountToReceive,
fee: fee,
}
next[fromVertex] = edge
// Add this new node to our heap as we'd like to further
// explore backwards through this edge.
heap.Push(&nodeHeap, distance[fromVertex])
}
// TODO(roasbeef): also add path caching
// * similar to route caching, but doesn't factor in the amount
// To start, our target node will the sole item within our distance
// heap.
heap.Push(&nodeHeap, distance[targetVertex])
for nodeHeap.Len() != 0 {
// Fetch the node within the smallest distance from our source
// from the heap.
partialPath := heap.Pop(&nodeHeap).(nodeWithDist)
bestNode := partialPath.node
// If we've reached our source (or we don't have any incoming
// edges), then we're done here and can exit the graph
// traversal early.
if bytes.Equal(bestNode.PubKeyBytes[:], sourceVertex[:]) {
break
}
// Now that we've found the next potential step to take we'll
// examine all the incoming edges (channels) from this node to
// further our graph traversal.
pivot := Vertex(bestNode.PubKeyBytes)
err := bestNode.ForEachChannel(tx, func(tx *bolt.Tx,
edgeInfo *channeldb.ChannelEdgeInfo,
_, inEdge *channeldb.ChannelEdgePolicy) error {
// If there is no edge policy for this candidate
// node, skip. Note that we are searching backwards
// so this node would have come prior to the pivot
// node in the route.
if inEdge == nil {
return nil
}
// We'll query the lower layer to see if we can obtain
// any more up to date information concerning the
// bandwidth of this edge.
edgeBandwidth, ok := bandwidthHints[edgeInfo.ChannelID]
if !ok {
// If we don't have a hint for this edge, then
// we'll just use the known Capacity as the
// available bandwidth.
edgeBandwidth = lnwire.NewMSatFromSatoshis(
edgeInfo.Capacity,
)
}
// 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.
channelSource, err := edgeInfo.FetchOtherNode(
tx, pivot[:],
)
if err != nil {
return err
}
// Check if this candidate node is better than what we
// already have.
processEdge(channelSource, inEdge, edgeBandwidth, pivot)
return nil
})
if err != nil {
return nil, err
}
// Then, we'll examine all the additional edges from the node
// we're currently visiting. Since we don't know the capacity
// of the private channel, we'll assume it was selected as a
// routing hint due to having enough capacity for the payment
// and use the payment amount as its capacity.
bandWidth := partialPath.amountToReceive
for _, reverseEdge := range additionalEdgesWithSrc[bestNode.PubKeyBytes] {
processEdge(reverseEdge.sourceNode, reverseEdge.edge, bandWidth, pivot)
}
}
// If the source node isn't found in the next hop map, then a path
// doesn't exist, so we terminate in an error.
if _, ok := next[sourceVertex]; !ok {
return nil, newErrf(ErrNoPathFound, "unable to find a path to "+
"destination")
}
// Use the nextHop map to unravel the forward path from source to target.
pathEdges := make([]*channeldb.ChannelEdgePolicy, 0, len(next))
currentNode := sourceVertex
for currentNode != targetVertex { // TODO(roasbeef): assumes no cycles
// Determine the next hop forward using the next map.
nextNode := next[currentNode]
// Add the next hop to the list of path edges.
pathEdges = append(pathEdges, nextNode)
// Advance current node.
currentNode = Vertex(nextNode.Node.PubKeyBytes)
}
// The route is invalid if it spans more than 20 hops. The current
// Sphinx (onion routing) implementation can only encode up to 20 hops
// as the entire packet is fixed size. If this route is more than 20
// hops, then it's invalid.
numEdges := len(pathEdges)
if numEdges > HopLimit {
return nil, newErr(ErrMaxHopsExceeded, "potential path has "+
"too many hops")
}
return pathEdges, nil
}
// findPaths implements a k-shortest paths algorithm to find all the reachable
// paths between the passed source and target. The algorithm will continue to
// traverse the graph until all possible candidate paths have been depleted.
// This function implements a modified version of Yen's. To find each path
// itself, we utilize our modified version of Dijkstra's found above. When
// examining possible spur and root paths, rather than removing edges or
// Vertexes from the graph, we instead utilize a Vertex+edge black-list that
// will be ignored by our modified Dijkstra's algorithm. With this approach, we
// make our inner path finding algorithm aware of our k-shortest paths
// algorithm, rather than attempting to use an unmodified path finding
// algorithm in a block box manner.
func findPaths(tx *bolt.Tx, graph *channeldb.ChannelGraph,
source *channeldb.LightningNode, target *btcec.PublicKey,
amt lnwire.MilliSatoshi, feeLimit lnwire.MilliSatoshi, numPaths uint32,
bandwidthHints map[uint64]lnwire.MilliSatoshi) ([][]*channeldb.ChannelEdgePolicy, error) {
ignoredEdges := make(map[uint64]struct{})
ignoredVertexes := make(map[Vertex]struct{})
// TODO(roasbeef): modifying ordering within heap to eliminate final
// sorting step?
var (
shortestPaths [][]*channeldb.ChannelEdgePolicy
candidatePaths pathHeap
)
// First we'll find a single shortest path from the source (our
// selfNode) to the target destination that's capable of carrying amt
// satoshis along the path before fees are calculated.
startingPath, err := findPath(
tx, graph, nil, source, target, ignoredVertexes, ignoredEdges,
amt, feeLimit, bandwidthHints,
)
if err != nil {
log.Errorf("Unable to find path: %v", err)
return nil, err
}
// Manually insert a "self" edge emanating from ourselves. This
// self-edge is required in order for the path finding algorithm to
// function properly.
firstPath := make([]*channeldb.ChannelEdgePolicy, 0, len(startingPath)+1)
firstPath = append(firstPath, &channeldb.ChannelEdgePolicy{
Node: source,
})
firstPath = append(firstPath, startingPath...)
shortestPaths = append(shortestPaths, firstPath)
// While we still have candidate paths to explore we'll keep exploring
// the sub-graphs created to find the next k-th shortest path.
for k := uint32(1); k < numPaths; k++ {
prevShortest := shortestPaths[k-1]
// We'll examine each edge in the previous iteration's shortest
// path in order to find path deviations from each node in the
// path.
for i := 0; i < len(prevShortest)-1; i++ {
// These two maps will mark the edges and Vertexes
// we'll exclude from the next path finding attempt.
// These are required to ensure the paths are unique
// and loopless.
ignoredEdges = make(map[uint64]struct{})
ignoredVertexes = make(map[Vertex]struct{})
// Our spur node is the i-th node in the prior shortest
// path, and our root path will be all nodes in the
// path leading up to our spurNode.
spurNode := prevShortest[i].Node
rootPath := prevShortest[:i+1]
// Before we kickoff our next path finding iteration,
// we'll find all the edges we need to ignore in this
// next round. This ensures that we create a new unique
// path.
for _, path := range shortestPaths {
// If our current rootPath is a prefix of this
// shortest path, then we'll remove the edge
// directly _after_ our spur node from the
// graph so we don't repeat paths.
if len(path) > i+1 && isSamePath(rootPath, path[:i+1]) {
ignoredEdges[path[i+1].ChannelID] = struct{}{}
}
}
// Next we'll remove all entries in the root path that
// aren't the current spur node from the graph. This
// ensures we don't create a path with loops.
for _, hop := range rootPath {
node := hop.Node.PubKeyBytes
if node == spurNode.PubKeyBytes {
continue
}
ignoredVertexes[Vertex(node)] = struct{}{}
}
// With the edges that are part of our root path, and
// the Vertexes (other than the spur path) within the
// root path removed, we'll attempt to find another
// shortest path from the spur node to the destination.
spurPath, err := findPath(
tx, graph, nil, spurNode, target,
ignoredVertexes, ignoredEdges, amt, feeLimit,
bandwidthHints,
)
// If we weren't able to find a path, we'll continue to
// the next round.
if IsError(err, ErrNoPathFound) {
continue
} else if err != nil {
return nil, err
}
// Create the new combined path by concatenating the
// rootPath to the spurPath.
newPathLen := len(rootPath) + len(spurPath)
newPath := path{
hops: make([]*channeldb.ChannelEdgePolicy, 0, newPathLen),
dist: newPathLen,
}
newPath.hops = append(newPath.hops, rootPath...)
newPath.hops = append(newPath.hops, spurPath...)
// TODO(roasbeef): add and consult path finger print
// We'll now add this newPath to the heap of candidate
// shortest paths.
heap.Push(&candidatePaths, newPath)
}
// If our min-heap of candidate paths is empty, then we can
// exit early.
if candidatePaths.Len() == 0 {
break
}
// To conclude this latest iteration, we'll take the shortest
// path in our set of candidate paths and add it to our
// shortestPaths list as the *next* shortest path.
nextShortestPath := heap.Pop(&candidatePaths).(path).hops
shortestPaths = append(shortestPaths, nextShortestPath)
}
return shortestPaths, nil
}