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 }