TCP/UDP的checksum校检算法实现，亲测与wireshark一致

typedef unsigned short u_short;
typedef unsigned int u_int;
u_short swapI16(u_short v)
{
    return ((v & 0x00FF) << 8) |
            ((v & 0xFF00) >> 8);
}

//1.UDP校检的数据有三块，伪头部(12Bytes)+UDP头(8Bytes)+数据
//2.伪头部包括，IP源地址+IP目的地址+mbz(0x0)+协议类型(UDP:17,TCP:6)+UDP长度
//  长度为4+4+1+1+2=12 Bytes
//3.UDP头包括,源端口+目的端口+UDP长度+checksum
//  长度为2+2+2+2=8 Bytes
//4.注意:
//     a)伪头部中UDP长度为时间UDP头+数据的长度，UDP头部中UDP长度为0x0000
//     b)如果数据长度不是2的整数倍，则必须在最后补一个字节数据，即padding
u_int udp_checksum(void *input, int len)
{
    u_short* tmp = (u_short*)input;
    u_int sum = 0;
    for (int i=0; i<len/2; i++) {
        sum += swapI16(*(tmp+i));
    }
    if (len % 2 == 1) {
        sum += (*((unsigned char*)input+len-1) << 8);
    }
    if (sum > 0xFFFF) {
        sum = (sum & 0xFFFF) + (sum >> 16);
    }

    u_short ret = sum;
    return swapI16(~ret);
}

//1.ip校检只校验IP头20字节
//2.ip校检字段必须为0
u_int ip_checksum(void *input, int len)
{
    if (len != 20) return 0;
    //ip校检置0
    memset((char*)input+10, 0, 2);
    u_short* tmp = (u_short*)input;
    u_int sum = 0;
    for (int i=0; i<len/2; i++) {
        sum += swapI16(*(tmp+i));
    }
    if (sum > 0xFFFF) {
        sum = (sum & 0xFFFF) + (sum >> 16);
    }

    u_short ret = sum;
    return swapI16(~ret);
}