IPV6上的TCP校验和

Question

我试图找出TCP over IPV6的校验和。请参阅以下代码。考虑的领域是：

1. 源地址128位
2. 目标地址128位
3. TCP段长度
4. 协议类型（TCP为6）

代码：

char src_addr[] = {
  0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0xa1, 0xe4, 0x22, 0x2c, 0x0c, 0x9b, 0x57, 0x22
};

char dest_addr[] = {
  0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x18, 0xd7, 0xc9, 0x57, 0x52, 0xd7, 0x0f, 0xcd
};


_inline unsigned short TOWORD(unsigned int a)
{
    return (unsigned short)( ((a>>8)&0x00FFL) + ((a<<8)&0xFF00L) );
}


int _tmain(int argc, _TCHAR* argv[])
{
    unsigned int sum=0;

    for(int i = 0; i < 16; i += 2)
    {
        sum += src_addr[i] + (src_addr[i+1] << 8U);
    }

    for(int i = 0; i < 16; i += 2)
    {
        sum += dest_addr[i] + (dest_addr[i+1] << 8U);
    }


    unsigned short type = 0x06; // TCP protocol
    sum += TOWORD(type);

    unsigned int tcplen = 0x1c; // 28 bytes length
    sum += ((tcplen <<8) & 0xFFFF) + (tcplen >> 8);

    sum = (sum>>16)+(sum&0xffff);
    sum += (sum>>16);

    sum = (~sum & 0xFFFF);
    unsigned short sum1 =  sum;

    printf("Checksum = %x\n", sum1);
    return 0;
}

结果为3e9c的校验和。

然而，WireShark显示0x3c3b的校验和。不知道这里缺少什么。 你能帮忙吗？