Handle retries for confirmed uplinks

Resolves #272

core/broker/uplink.go

@@ -167,13 +167,25 @@ func (b *broker) HandleUplink(uplink *pb.UplinkMessage) (err error) {
 		ctx = ctx.WithField("RealFCnt", macPayload.FHDR.FCnt)
 	}
 
-	if device.DisableFCntCheck {
-		// TODO: Add warning to message?
-	} else if device.FCntUp == 0 {
-
-	} else if macPayload.FHDR.FCnt <= device.FCntUp || macPayload.FHDR.FCnt-device.FCntUp > maxFCntGap {
-		// Replay attack or FCnt gap too big
-		return errors.NewErrNotFound("device with matching FCnt")
+	switch {
+	case macPayload.FHDR.FCnt > device.FCntUp && macPayload.FHDR.FCnt-device.FCntUp <= maxFCntGap:
+		// FCnt higher than latest and within max FCnt gap (normal case)
+	case device.DisableFCntCheck:
+		// FCnt Check disabled. Rely on MIC check only
+	case device.FCntUp == 0:
+		// FCntUp is reset. We don't know where the device will start sending.
+	case macPayload.FHDR.FCnt == device.FCntUp:
+		if phyPayload.MHDR.MType == lorawan.ConfirmedDataUp {
+			// Retry of confirmed uplink
+			break
+		}
+		fallthrough
+	case macPayload.FHDR.FCnt <= device.FCntUp:
+		return errors.NewErrInvalidArgument("FCnt", "not high enough")
+	case macPayload.FHDR.FCnt-device.FCntUp > maxFCntGap:
+		return errors.NewErrInvalidArgument("FCnt", "too high")
+	default:
+		return errors.NewErrInternal("FCnt check failed")
 	}
 
 	// Add FCnt to Metadata (because it's not marshaled in lorawan payload)

core/broker/uplink_test.go

@@ -112,7 +112,7 @@ func TestHandleUplink(t *testing.T) {
 		GatewayMetadata:  &gateway.RxMetadata{Snr: 1.2, GatewayId: gtwID},
 		ProtocolMetadata: &protocol.RxMetadata{Protocol: &protocol.RxMetadata_Lorawan{Lorawan: &pb_lorawan.Metadata{}}},
 	})
-	a.So(err, ShouldHaveSameTypeAs, &errors.ErrNotFound{})
+	a.So(err, ShouldHaveSameTypeAs, &errors.ErrInvalidArgument{})
 
 	// Disable FCnt Check
 	b.uplinkDeduplicator = NewDeduplicator(10 * time.Millisecond)