在C中计算IP校验和

Question

I looked at RFC 1071 for some information on calculating the IP checksum. For the application I'm making, I need to calculate the IP checksum of a 20 byte IP header (no options). My code is as follows:

uint16_t calculate_ip_chksum(IP *ip)
{
    uint16_t *buf = (uint16_t *) ip;
    uint32_t sum = 0;
    uint16_t checksum = 0;

    int counter = (ip->version & 0xF) * 4;
    while (counter > 1)
     {
            sum += *buf++;
            counter -= 2;
     }

    if (counter == 1)
            sum += * (uint8_t *) buf;

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

I defined an IP header as follows:

typedef struct ip_h
{
    uint8_t version;
    uint8_t tos;
    uint16_t tot_len;
    uint16_t id;
    uint16_t frag_off;
    uint8_t ttl;
    uint8_t proto;
    uint16_t chksum;
    uint32_t src;
    uint32_t dst;

} IP;

I understand the mathematics of how to calculate the checksum, but I do not understand why my code (which is almost exactly the same as the RFC) does not work.

Answer 1

You derive counter from ip->version instead of tot_len.

 int counter = (ip->version & 0xF) * 4;

I assume that you want total number of bytes in the buffer for checksum.