diff --git a/core/stack/udp.go b/core/stack/udp.go
index ab5b762..74763e9 100755
--- a/core/stack/udp.go
+++ b/core/stack/udp.go
@@ -86,33 +86,30 @@ func (p *udpPacket) WriteBack(b []byte, addr net.Addr) (int, error) {
 		return 0, fmt.Errorf("%s", &tcpip.ErrMessageTooLong{})
 	}
 
-	route, err := p.s.FindRoute(p.nicID, p.netHdr.DestinationAddress(), p.netHdr.SourceAddress(), p.netProto, false /* multicastLoop */)
+	var (
+		localAddress tcpip.Address
+		localPort    uint16
+	)
+
+	if udpAddr, ok := addr.(*net.UDPAddr); !ok {
+		localAddress = p.netHdr.DestinationAddress()
+		localPort = p.id.LocalPort
+	} else if ipv4 := udpAddr.IP.To4(); ipv4 != nil {
+		localAddress = tcpip.Address(ipv4)
+		localPort = uint16(udpAddr.Port)
+	} else {
+		localAddress = tcpip.Address(udpAddr.IP)
+		localPort = uint16(udpAddr.Port)
+	}
+
+	route, err := p.s.FindRoute(p.nicID, localAddress, p.netHdr.SourceAddress(), p.netProto, false /* multicastLoop */)
 	if err != nil {
 		return 0, fmt.Errorf("%#v find route: %s", p.id, err)
 	}
 	defer route.Release()
 
 	data := v.ToVectorisedView()
-	// if addr is not provided, write back use original dst Addr as src Addr.
-	if addr == nil {
-		if err = sendUDP(route, data, p.id.LocalPort, p.id.RemotePort, udpNoChecksum); err != nil {
-			return 0, fmt.Errorf("%v", err)
-		}
-		return data.Size(), nil
-	}
-
-	udpAddr, ok := addr.(*net.UDPAddr)
-	if !ok {
-		return 0, fmt.Errorf("type %T is not a valid udp address", addr)
-	}
-
-	if ipv4 := udpAddr.IP.To4(); ipv4 != nil {
-		route.LocalAddress = tcpip.Address(ipv4)
-	} else {
-		route.LocalAddress = tcpip.Address(udpAddr.IP)
-	}
-
-	if err = sendUDP(route, data, uint16(udpAddr.Port), p.id.RemotePort, udpNoChecksum); err != nil {
+	if err = sendUDP(route, data, localPort, p.id.RemotePort, udpNoChecksum); err != nil {
 		return 0, fmt.Errorf("%v", err)
 	}
 	return data.Size(), nil
@@ -142,7 +139,7 @@ func sendUDP(r *stack.Route, data buffer.VectorisedView, localPort, remotePort u
 	// transmitter skipped the checksum generation (RFC768).
 	// On IPv6, UDP checksum is not optional (RFC2460 Section 8.1).
 	if r.RequiresTXTransportChecksum() &&
-		(!noChecksum || r.NetProto == header.IPv6ProtocolNumber) {
+		(!noChecksum || r.NetProto() == header.IPv6ProtocolNumber) {
 		xsum := r.PseudoHeaderChecksum(udp.ProtocolNumber, length)
 		for _, v := range data.Views() {
 			xsum = header.Checksum(v, xsum)
@@ -152,7 +149,7 @@ func sendUDP(r *stack.Route, data buffer.VectorisedView, localPort, remotePort u
 
 	ttl := r.DefaultTTL()
 
-	if err := r.WritePacket(nil /* gso */, stack.NetworkHeaderParams{
+	if err := r.WritePacket(stack.NetworkHeaderParams{
 		Protocol: udp.ProtocolNumber,
 		TTL:      ttl,
 		TOS:      0, /* default */