typedef struct iphdr
{
u_char ip_hl:4, /* header length */
ip_v:4; /* version */
u_char ip_tos; /* type of service */
short ip_len; /* total length */
u_short ip_id; /* identification */
short ip_off; /* fragment offset field */
#define IP_DF 0x4000 /* dont fragment flag */
#define IP_MF 0x2000 /* more fragments flag */
u_char ip_ttl; /* time to live */
u_char ip_p; /* protocol */
u_short ip_sum; /* checksum */
struct in_addr ip_src,ip_dst; /* source and dest address */
} iphdr, *iphdr_ptr;


VOID RecalculateIPChecksum (
iphdr_ptr pIpHeader
)
{
unsigned short word16;
unsigned int sum = 0;
unsigned int i = 0;
PUCHAR buff;

// Initialize checksum to zero
pIpHeader->ip_sum = 0;
buff = (PUCHAR)pIpHeader;

// Calculate IP header checksum
for (i = 0; i < pIpHeader->ip_hl*sizeof(DWORD); i=i+2)
{
word16 = ((buff[i]<<8)&0xFF00)+(buff[i+1]&0xFF);
sum = sum+word16;
}

// keep only the last 16 bits of the 32 bit calculated sum and add the carries
while (sum>>16)
sum = (sum & 0xFFFF)+(sum >> 16);

// Take the one's complement of sum
sum = ~sum;

pIpHeader->ip_sum = htons((unsigned short) sum);
}