WinDivertHelperCalcChecksums Not modify of packet check bits and length bits， thx for help

[boyyao]

code：

    public static string Filter = "outbound && tcp && tcp.DstPort == 8888 && tcp.PayloadLength > 120";
    public unsafe static void RunDiversion()
    {
        IntPtr handle = WinDivertMethods.WinDivertOpen(Filter, WINDIVERT_LAYER.WINDIVERT_LAYER_NETWORK, -1, 0);
        IntPtr INVALID_HANDLE_VALUE = new IntPtr(-1);
        if (handle == INVALID_HANDLE_VALUE)
            Console.WriteLine("open error:" + Marshal.GetLastWin32Error());

        const uint MAXBUF = 0xFFFF;
        byte[] buffer = new byte[MAXBUF];

        while (true)
        {
            Array.Clear(buffer, 0, buffer.Length);
            WINDIVERT_ADDRESS windivert_ADDRESS = default(WINDIVERT_ADDRESS);
            uint packetLen = 0u;

            if (WinDivertMethods.WinDivertRecv(handle, buffer, MAXBUF, ref windivert_ADDRESS, ref packetLen))
            {
                fixed (byte* buffer_fixed = buffer)
                {
                    WINDIVERT_IPHDR* IPHDR = null;
                    WINDIVERT_TCPHDR* TCPHDR = null;
                    byte* ppData = null;
                    WinDivertMethods.WinDivertHelperParsePacket(buffer_fixed, packetLen, &IPHDR, null, null, null, &TCPHDR, null, &ppData, null);
                    if (IPHDR != null && TCPHDR != null && ppData != null)
                    {
                        string data = Marshal.PtrToStringAnsi((IntPtr)((void*)ppData));

                        if (data.Contains("need modify data"))
                        {
                            Console.WriteLine("old data:");
                            for (int i = 0; i < packetLen; i++)
                            {
                                Console.Write("{0:X2}", buffer[i]);
                            }
                            Console.WriteLine("");

                            byte[] newbody = System.Text.Encoding.ASCII.GetBytes("new data");

                            Buffer.BlockCopy(newbody, 0, buffer, 40, newbody.Length);
                            packetLen = (uint)newbody.Length + 40;

                            Console.WriteLine("new data:");
                            for (int i = 0; i < packetLen; i++)
                            {
                                Console.Write("{0:X2}", buffer[i]);
                            }
                            Console.WriteLine("");
                        }
                    }
                }
                ....invalid....... buffer not modify   need help.
                WinDivertMethods.WinDivertHelperCalcChecksums(buffer, packetLen, 0UL);
                Console.WriteLine("send data:");
                for (int i = 0; i < packetLen; i++)
                {
                    Console.Write("{0:X2}", buffer[i]);
                }
                Console.WriteLine("");
                WinDivertMethods.WinDivertSendEx(handle, buffer, packetLen, 0UL, ref windivert_ADDRESS, IntPtr.Zero, IntPtr.Zero);
            }
        }

    }