check sum함수 확인 부탁드립니다.

ip 헤더 20byte 에대한 체크섬 입니다. 제일 간단한건데...
해놓구도 잘 됬는지.. 확신이 안서서.. 이렇게 확인 부탁드립니다.
잘못된 부분 있으면 지적 부탁드립니다.
읽어주신 분들 모두 감사드립니다.

//ip.h
typedef unsigned char    u_int8_t;
typedef unsigned int     u_int16_t;


struct in_addr
{
    u_int8_t s_addr_hi_1;
    u_int8_t s_addr_hi_2;
    u_int8_t s_addr_low_1;
    u_int8_t s_addr_low_2;
};

struct ip {
    u_int8_t        ip_vhl;         /* header length, version */
    u_int8_t        ip_tos;         /* type of service */
    u_int16_t       ip_len;         /* total length */
    u_int16_t       ip_id;          /* identification */
    u_int16_t       ip_off;         /* fragment offset field */
    u_int8_t        ip_ttl;         /* time to live */
    u_int8_t        ip_p;           /* protocol */
    u_int16_t       ip_sum;         /* checksum */
    struct  in_addr ip_src,ip_dst;  /* source and dest address */
};



//dumpcode.h
void printchar(unsigned char c)
{
    if(isprint(c))
    printf("%c",c);
    else
    printf(".");
}

void dumpcode(unsigned char *buff, int len)
{
    int i;
    for(i=0;i<len;i++)
    {
        if(i%16==0)
        printf("0x%08x  ",&buff[i]);
                printf("%02x ",buff[i]);
        if(i%16-15==0)
        {
            int j;
            printf("  ");
            for(j=i-15;j<=i;j++)
                printchar(buff[j]);
                printf("\n");
        }
    }

    if(i%16!=0)
    {
        int j;
        int spaces=(len-i+16-i%16)*3+2;
        for(j=0;j<spaces;j++)
            printf(" ");
        for(j=i-i%16;j<len;j++)
            printchar(buff[j]);
    }

    printf("\n");
}





//check sum 
unsigned short cksum(struct ip *ip, int len)
{
    long sum = 0;


    while(len > 1)
    {

        sum += *((unsigned short *) ip)++;
        if(sum & 0x80000000)
            sum = (sum & 0xFFFF) + (sum >> 16);
        len -= 2;
    }

    if(len)
        sum += (unsigned short) * (unsigned char *) ip;

    while(sum >> 16)
        sum = (sum & 0xFFFF) + (sum >> 16);
  
    return sum;
}


void main(void)
{
    struct ip nip;
    unsigned int check;


    nip.ip_vhl  = 69;
    nip.ip_tos  = 0;
    nip.ip_len = 28;
    nip.ip_id  = 1;
    nip.ip_off = 0;
    nip.ip_ttl  = 4;
    nip.ip_p  = 17;
    nip.ip_sum  = 0;
    nip.ip_src.s_addr_hi_1 = 10;
    nip.ip_src.s_addr_hi_2 = 12;
    nip.ip_src.s_addr_low_1 = 14;
    nip.ip_src.s_addr_low_2 = 5;
    nip.ip_dst.s_addr_hi_1 = 12;
    nip.ip_dst.s_addr_hi_2 = 6;
    nip.ip_dst.s_addr_low_1 = 7;
    nip.ip_dst.s_addr_low_2 = 9;


    printf("address = %x\n", &nip);

    dumpcode(&nip,20);

    check = cksum(&nip, 20);

    printf("%x\n",check);
}


0xbfffea00  45 00 ff bf 1c 00 00 00 01 00 00 00 00 00 00 00   E...............
0xbfffea10  04 11 15 40                                       ...@