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/*
* Checksum routine for Internet Protocol family headers (C Version).
*
* Refer to "Computing the Internet Checksum" by R. Braden, D. Borman and
* C. Partridge, Computer Communication Review, Vol. 19, No. 2, April 1989,
* pp. 86-101, for additional details on computing this checksum.
*/
#include <zebra.h>
#include "checksum.h"
int /* return checksum in low-order 16 bits */
in_cksum(void *parg, int nbytes)
{
u_short *ptr = parg;
register long sum; /* assumes long == 32 bits */
u_short oddbyte;
register u_short answer; /* assumes u_short == 16 bits */
/*
* Our algorithm is simple, using a 32-bit accumulator (sum),
* we add sequential 16-bit words to it, and at the end, fold back
* all the carry bits from the top 16 bits into the lower 16 bits.
*/
sum = 0;
while (nbytes > 1) {
sum += *ptr++;
nbytes -= 2;
}
/* mop up an odd byte, if necessary */
if (nbytes == 1) {
oddbyte = 0; /* make sure top half is zero */
*((u_char *) &oddbyte) = *(u_char *)ptr; /* one byte only */
sum += oddbyte;
}
/*
* Add back carry outs from top 16 bits to low 16 bits.
*/
sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */
sum += (sum >> 16); /* add carry */
answer = ~sum; /* ones-complement, then truncate to 16 bits */
return(answer);
}
/* Fletcher Checksum -- Refer to RFC1008. */
#define MODX 4102 /* 5802 should be fine */
/* To be consistent, offset is 0-based index, rather than the 1-based
index required in the specification ISO 8473, Annex C.1 */
/* calling with offset == FLETCHER_CHECKSUM_VALIDATE will validate the checksum
without modifying the buffer; a valid checksum returns 0 */
u_int16_t
fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
{
u_int8_t *p;
int x, y, c0, c1;
u_int16_t checksum = 0;
u_int16_t *csum;
size_t partial_len, i, left = len;
if (offset != FLETCHER_CHECKSUM_VALIDATE)
/* Zero the csum in the packet. */
{
assert (offset < (len - 1)); /* account for two bytes of checksum */
csum = (u_int16_t *) (buffer + offset);
*(csum) = 0;
}
p = buffer;
c0 = 0;
c1 = 0;
while (left != 0)
{
partial_len = MIN(left, MODX);
for (i = 0; i < partial_len; i++)
{
c0 = c0 + *(p++);
c1 += c0;
}
c0 = c0 % 255;
c1 = c1 % 255;
left -= partial_len;
}
/* The cast is important, to ensure the mod is taken as a signed value. */
x = (int)((len - offset - 1) * c0 - c1) % 255;
if (x <= 0)
x += 255;
y = 510 - c0 - x;
if (y > 255)
y -= 255;
if (offset == FLETCHER_CHECKSUM_VALIDATE)
{
checksum = (c1 << 8) + c0;
}
else
{
/*
* Now we write this to the packet.
* We could skip this step too, since the checksum returned would
* be stored into the checksum field by the caller.
*/
buffer[offset] = x;
buffer[offset + 1] = y;
/* Take care of the endian issue */
checksum = htons((x << 8) | (y & 0xFF));
}
return checksum;
}
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