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/*
 * Recalculate any IP checksum.
 */
unsigned short
svz_raw_ip_checksum (svz_uint8_t *data, int len)
{
  register unsigned checksum = 0;

  /* 
   * Calculate the 16 bit one's complement of the one's complement sum 
   * of all 16 bit words in the header. For computing the checksum, 
   * the checksum field should be zero. This checksum may be replaced in 
   * the future.
   */
  while (len > 1)
    {
      /* This is the inner loop */
      checksum += *data | (*(data + 1) << 8);
      len -= 2;
      data += 2;
    }

  /* Add left-over byte, if any */
  if (len > 0)
    checksum += *data;

  /* Fold 32-bit checksum to 16 bits */
  while (checksum >> 16)
    checksum = (checksum & 0xffff) + (checksum >> 16);
  checksum = ~checksum;

  return htons ((unsigned short) checksum);
}