Question

我正在尝试计算TCP校验和值（显然与IP4校验和相同），但是得到了错误的结果。我认为问题是由于末尾的填充或TCP标头字段引起的，但我找不到有关如何计算它们的任何详细信息（来自RFC）。

为了构建伪头（wiki），我正在使用以下代码将数据整合在一起（field('xyz').bin()返回字段xyz的二进制字符串。）：

public String genChecksum()
{
    String pseudoHeader = "";

    // From IPv4 - Pseudo Header
    pseudoHeader += peekField("source").bin();
    pseudoHeader += peekField("destination").bin();
    pseudoHeader += "00000000"; // reserved block
    pseudoHeader += peekField("protocol").bin();

    /* TCP Length field: 
     * headerSize() = (TCP.dataOffset * 32) to get TCP header length
     * dataSize() = (IP4.totalLength * 8) - ((IP4.internetHeaderLength * 32) + (TCP.dataOffset * 32)) 
     */

    pseudoHeader += Types.hexToBin(Types.numToHex((int) Math.ceil((headerSize() + dataSize()) / 8)));

    // From TCP Header
    pseudoHeader += field("srcPort").bin();
    pseudoHeader += field("dstPort").bin();
    pseudoHeader += field("seqNum").bin();
    pseudoHeader += field("ackNum").bin();
    pseudoHeader += field("dataOffset").bin();
    pseudoHeader += field("reserved").bin();
    pseudoHeader += field("flagNS").bin();
    pseudoHeader += field("flagCWR").bin();
    pseudoHeader += field("flagECE").bin();
    pseudoHeader += field("flagURG").bin();
    pseudoHeader += field("flagACK").bin();
    pseudoHeader += field("flagPSH").bin();
    pseudoHeader += field("flagRST").bin();
    pseudoHeader += field("flagSYN").bin();
    pseudoHeader += field("flagFIN").bin();
    pseudoHeader += field("windowSize").bin();
    pseudoHeader += "0000000000000000"; // checksum zeroed out
    pseudoHeader += field("urgPointer").bin();
    pseudoHeader += field("optional").bin();
    pseudoHeader += field("data").bin();

    // Pad to 16 bit boundary as stated in RFC
    for (int i = 0; i < (pseudoHeader.length() % 16); i++)
    {
        pseudoHeader += "0";
    }

    System.out.println("pseudoHeader: " + Types.binToHex(pseudoHeader));

    return Types.binToHex(pseudoHeader);
}

我非常确定合并在一起的值（field()和peekField()是正确的，我已经用wireshark验证了它们，并且IP4的校验和是正确的。下面包括了实际的校验和方法但是我也有一个测试来检查它是否正确：

public static String CRC16(String hex) 
{
    byte[] buf = Types.hexToBytes(hex);
    int length = buf.length;
    int i = 0;

    long sum = 0;
    long data;

    while (length > 1) 
    {
        data = (((buf[i] << 8) & 0xFF00) | ((buf[i + 1]) & 0xFF));
        sum += data;

        if ((sum & 0xFFFF0000) > 0) 
        {
            sum = sum & 0xFFFF;
            sum += 1;
        }

        i += 2;
        length -= 2;
    }

    if (length > 0) 
    {
        sum += (buf[i] << 8 & 0xFF00);

        if ((sum & 0xFFFF0000) > 0) 
        {
            sum = sum & 0xFFFF;
            sum += 1;
        }
    }

    sum = ~sum;
    sum = sum & 0xFFFF;

    System.out.println("CRCV: " + String.format("%X", sum));
    return String.format("%X", sum);
}

public static void assertTestChecksum()
{
    byte[] buf = {(byte) 0xe3, 0x4f, 0x23, (byte) 0x96, 0x44, 0x27, (byte) 0x99, (byte) 0xf3};

    if (!CRC16(Types.bytesToHex(buf)).equals("1AFF"))
    {
        Utils.exit("Checksum.java", "assertTestChecksum() failed.");
    }       
}

我在做任何明显错误的事情吗？

计算TCP校验和值-TCP长度和填充

0 个答案: