libopenbios/ipchecksum.c

   1 /* Taken from Etherboot */
   2
   3 #include "libopenbios/ipchecksum.h"
   4
   5 unsigned short ipchksum(const void *data, unsigned long length)
   6 {
   7         unsigned long sum;
   8         unsigned long i;
   9         const unsigned char *ptr;
  10         union {
  11             unsigned char byte[2];
  12             unsigned short word;
  13         } u;
  14
  15         /* In the most straight forward way possible,
  16          * compute an ip style checksum.
  17          */
  18         sum = 0;
  19         ptr = data;
  20         for(i = 0; i < length; i++) {
  21                 unsigned long value;
  22                 value = ptr[i];
  23                 if (i & 1) {
  24                         value <<= 8;
  25                 }
  26                 /* Add the new value */
  27                 sum += value;
  28                 /* Wrap around the carry */
  29                 if (sum > 0xFFFF) {
  30                         sum = (sum + (sum >> 16)) & 0xFFFF;
  31                 }
  32         }
  33         u.byte[0] = (unsigned char) sum;
  34         u.byte[1] = (unsigned char) (sum >> 8);
  35         return (unsigned short) ~u.word;
  36 }
  37
  38 unsigned short add_ipchksums(unsigned long offset, unsigned short sum, unsigned short new)
  39 {
  40         unsigned long checksum;
  41         sum = ~sum & 0xFFFF;
  42         new = ~new & 0xFFFF;
  43         if (offset & 1) {
  44                 /* byte swap the sum if it came from an odd offset
  45                  * since the computation is endian independant this
  46                  * works.
  47                  */
  48                 new = (new << 8) | (new >> 8);
  49         }
  50         checksum = sum + new;
  51         if (checksum > 0xFFFF) {
  52                 checksum -= 0xFFFF;
  53         }
  54         return (~checksum) & 0xFFFF;
  55 }