Personally I use this one
//
// Function recalculates IP checksum
//
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<<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);
}