htlcswitch: randomize non-strict forwarding

htlcswitch/switch.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"math/rand"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -1010,9 +1011,9 @@ func (s *Switch) handlePacketForward(packet *htlcPacket) error {
 		// trying all links to utilize our available bandwidth.
 		linkErrs := make(map[lnwire.ShortChannelID]*LinkError)
 
-		// Try to find destination channel link with appropriate
+		// Find all destination channel links with appropriate
 		// bandwidth.
-		var destination ChannelLink
+		var destinations []ChannelLink
 		for _, link := range interfaceLinks {
 			var failure *LinkError
 
@@ -1035,10 +1036,11 @@ func (s *Switch) handlePacketForward(packet *htlcPacket) error {
 				)
 			}
 
-			// Stop searching if this link can forward the htlc.
+			// If this link can forward the htlc, add it to the set
+			// of destinations.
 			if failure == nil {
-				destination = link
-				break
+				destinations = append(destinations, link)
+				continue
 			}
 
 			linkErrs[link.ShortChanID()] = failure
@@ -1048,7 +1050,7 @@ func (s *Switch) handlePacketForward(packet *htlcPacket) error {
 		// satisfying the current policy, then we'll send back an
 		// error, but ensure we send back the error sourced at the
 		// *target* link.
-		if destination == nil {
+		if len(destinations) == 0 {
 			// At this point, some or all of the links rejected the
 			// HTLC so we couldn't forward it. So we'll try to look
 			// up the error that came from the source.
@@ -1075,6 +1077,12 @@ func (s *Switch) handlePacketForward(packet *htlcPacket) error {
 			return s.failAddPacket(packet, linkErr)
 		}
 
+		// Choose a random link out of the set of links that can forward
+		// this htlc. The reason for randomization is to evenly
+		// distribute the htlc load without making assumptions about
+		// what the best channel is.
+		destination := destinations[rand.Intn(len(destinations))]
+
 		// Send the packet to the destination channel link which
 		// manages the channel.
 		packet.outgoingChanID = destination.ShortChanID()