问数据包的getOffset()在接收端返回0

EN

        byte[] buf = text.getBytes();
        InetAddress address = InetAddress.getByName("localhost");

        DatagramPacket packet = new DatagramPacket(buf, 500, BYTES_PER_PACKET, address, udp_port_num);

        System.out.println("Offset at server:  " + packet.getOffset());
        udpSocket.send(packet);

                byte[] buf = new byte[BYTES_PER_PACKET];                    
                DatagramPacket packet = new DatagramPacket(buf, BYTES_PER_PACKET);

                udpSocket.receive(packet);

                String received = new String(packet.getData());

                System.out.println(received);
                System.out.println("Offset:  " + packet.getOffset());