UDP checksum

udp_checksum.c

uint16_t udp_checksum(struct udphdr *p_udp_header, size_t len, uint32_t src_addr, uint32_t dest_addr)
{
  const uint16_t *buf = (const uint16_t*)p_udp_header;
  uint16_t *ip_src = (void*)&src_addr, *ip_dst = (void*)&dest_addr;
  uint32_t sum;
  size_t length = len;

  // Calculate the sum
  sum = 0;
  while (len > 1)
  {
    sum += *buf++;
    if (sum & 0x80000000)
      sum = (sum & 0xFFFF) + (sum >> 16);
    len -= 2;
  }

  if (len & 1)
    // Add the padding if the packet lenght is odd
    sum += *((uint8_t*)buf);

  // Add the pseudo-header
  sum += *(ip_src++);
  sum += *ip_src;

  sum += *(ip_dst++);
  sum += *ip_dst;

  sum += htons(IPPROTO_UDP);
  sum += htons(length);

  // Add the carries
  while (sum >> 16)
    sum = (sum & 0xFFFF) + (sum >> 16);

  // Return the one's complement of sum
  return (uint16_t)~sum;
}