Implement ECMP for unsafe_routes (#1332)

routing/balance.go

@@ -0,0 +1,39 @@
+package routing
+
+import (
+	"net/netip"
+
+	"github.com/slackhq/nebula/firewall"
+)
+
+// Hashes the packet source and destination port and always returns a positive integer
+// Based on 'Prospecting for Hash Functions'
+//   - https://nullprogram.com/blog/2018/07/31/
+//   - https://github.com/skeeto/hash-prospector
+//     [16 21f0aaad 15 d35a2d97 15] = 0.10760229515479501
+func hashPacket(p *firewall.Packet) int {
+	x := (uint32(p.LocalPort) << 16) | uint32(p.RemotePort)
+	x ^= x >> 16
+	x *= 0x21f0aaad
+	x ^= x >> 15
+	x *= 0xd35a2d97
+	x ^= x >> 15
+
+	return int(x) & 0x7FFFFFFF
+}
+
+// For this function to work correctly it requires that the buckets for the gateways have been calculated
+// If the contract is violated balancing will not work properly and the second return value will return false
+func BalancePacket(fwPacket *firewall.Packet, gateways []Gateway) (netip.Addr, bool) {
+	hash := hashPacket(fwPacket)
+
+	for i := range gateways {
+		if hash <= gateways[i].BucketUpperBound() {
+			return gateways[i].Addr(), true
+		}
+	}
+
+	// If you land here then the buckets for the gateways are not properly calculated
+	// Fallback to random routing and let the caller know
+	return gateways[hash%len(gateways)].Addr(), false
+}