lib/checksum.c

   1 /*
   2  * Checksum routine for Internet Protocol family headers (C Version).
   3  *
   4  * Refer to "Computing the Internet Checksum" by R. Braden, D. Borman and
   5  * C. Partridge, Computer Communication Review, Vol. 19, No. 2, April 1989,
   6  * pp. 86-101, for additional details on computing this checksum.
   7  */
   8
   9 #include <zebra.h>
  10 #include "checksum.h"
  11
  12 int                     /* return checksum in low-order 16 bits */
  13 in_cksum(void *parg, int nbytes)
  14 {
  15         u_short *ptr = parg;
  16         register long           sum;            /* assumes long == 32 bits */
  17         u_short                 oddbyte;
  18         register u_short        answer;         /* assumes u_short == 16 bits */
  19
  20         /*
  21          * Our algorithm is simple, using a 32-bit accumulator (sum),
  22          * we add sequential 16-bit words to it, and at the end, fold back
  23          * all the carry bits from the top 16 bits into the lower 16 bits.
  24          */
  25
  26         sum = 0;
  27         while (nbytes > 1)  {
  28                 sum += *ptr++;
  29                 nbytes -= 2;
  30         }
  31
  32                                 /* mop up an odd byte, if necessary */
  33         if (nbytes == 1) {
  34                 oddbyte = 0;            /* make sure top half is zero */
  35                 *((u_char *) &oddbyte) = *(u_char *)ptr;   /* one byte only */
  36                 sum += oddbyte;
  37         }
  38
  39         /*
  40          * Add back carry outs from top 16 bits to low 16 bits.
  41          */
  42
  43         sum  = (sum >> 16) + (sum & 0xffff);    /* add high-16 to low-16 */
  44         sum += (sum >> 16);                     /* add carry */
  45         answer = ~sum;          /* ones-complement, then truncate to 16 bits */
  46         return(answer);
  47 }
  48
  49 /* Fletcher Checksum -- Refer to RFC1008. */
  50 #define MODX                 4102   /* 5802 should be fine */
  51
  52 /* To be consistent, offset is 0-based index, rather than the 1-based
  53    index required in the specification ISO 8473, Annex C.1 */
  54 u_int16_t
  55 fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
  56 {
  57   u_int8_t *p;
  58   int x, y, c0, c1;
  59   u_int16_t checksum;
  60   u_int16_t *csum;
  61   size_t partial_len, i, left = len;
  62
  63   checksum = 0;
  64
  65   assert (offset < len);
  66
  67   /*
  68    * Zero the csum in the packet.
  69    */
  70   csum = (u_int16_t *) (buffer + offset);
  71   *(csum) = 0;
  72
  73   p = buffer;
  74   c0 = 0;
  75   c1 = 0;
  76
  77   while (left != 0)
  78     {
  79       partial_len = MIN(left, MODX);
  80
  81       for (i = 0; i < partial_len; i++)
  82         {
  83           c0 = c0 + *(p++);
  84           c1 += c0;
  85         }
  86
  87       c0 = c0 % 255;
  88       c1 = c1 % 255;
  89
  90       left -= partial_len;
  91     }
  92
  93   /* The cast is important, to ensure the mod is taken as a signed value. */
  94   x = (int)((len - offset - 1) * c0 - c1) % 255;
  95
  96   if (x <= 0)
  97     x += 255;
  98   y = 510 - c0 - x;
  99   if (y > 255)
 100     y -= 255;
 101
 102   /*
 103    * Now we write this to the packet.
 104    * We could skip this step too, since the checksum returned would
 105    * be stored into the checksum field by the caller.
 106    */
 107   buffer[offset] = x;
 108   buffer[offset + 1] = y;
 109
 110   /* Take care of the endian issue */
 111   checksum = htons((x << 8) | (y & 0xFF));
 112
 113   return checksum;
 114 }