src/lib/compute_ip_checksum.c

   1 #include <stdint.h>
   2 #include <ip_checksum.h>
   3
   4 unsigned long compute_ip_checksum(void *addr, unsigned long length)
   5 {
   6         uint8_t *ptr;
   7         volatile union {
   8                 uint8_t  byte[2];
   9                 uint16_t word;
  10         } value;
  11         unsigned long sum;
  12         unsigned long i;
  13         /* In the most straight forward way possible,
  14          * compute an ip style checksum.
  15          */
  16         sum = 0;
  17         ptr = addr;
  18         for(i = 0; i < length; i++) {
  19                 unsigned long v;
  20                 v = ptr[i];
  21                 if (i & 1) {
  22                         v <<= 8;
  23                 }
  24                 /* Add the new value */
  25                 sum += v;
  26                 /* Wrap around the carry */
  27                 if (sum > 0xFFFF) {
  28                         sum = (sum + (sum >> 16)) & 0xFFFF;
  29                 }
  30         }
  31         value.byte[0] = sum & 0xff;
  32         value.byte[1] = (sum >> 8) & 0xff;
  33         return (~value.word) & 0xFFFF;
  34 }
  35
  36 unsigned long add_ip_checksums(unsigned long offset, unsigned long sum, unsigned long new)
  37 {
  38         unsigned long checksum;
  39         sum = ~sum & 0xFFFF;
  40         new = ~new & 0xFFFF;
  41         if (offset & 1) {
  42                 /* byte swap the sum if it came from an odd offset
  43                  * since the computation is endian independent this
  44                  * works.
  45                  */
  46                 new = ((new >> 8) & 0xff) | ((new << 8) & 0xff00);
  47         }
  48         checksum = sum + new;
  49         if (checksum > 0xFFFF) {
  50                 checksum -= 0xFFFF;
  51         }
  52         return (~checksum) & 0xFFFF;
  53 }