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Initial revision

    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: }