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/*
* IS-IS Rout(e)ing protocol - iso_checksum.c
* ISO checksum related routines
*
* Copyright (C) 2001,2002 Sampo Saaristo
* Tampere University of Technology
* Institute of Communications Engineering
*
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public Licenseas published by the Free
* Software Foundation; either version 2 of the License, or (at your option)
* any later version.
*
* This program is distributed in the hope that it will be useful,but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
* more details.
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
#include <zebra.h>
#include "iso_checksum.h"
/*
* Calculations of the OSI checksum.
* ISO/IEC 8473 defines the sum as
*
* L
* sum a (mod 255) = 0
* 1 i
*
* L
* sum (L-i+1)a (mod 255) = 0
* 1 i
*
*/
/*
* Verifies that the checksum is correct.
* Return 0 on correct and 1 on invalid checksum.
* Based on Annex C.4 of ISO/IEC 8473
*/
int
iso_csum_verify (u_char * buffer, int len, uint16_t * csum)
{
u_int8_t *p;
u_int32_t c0;
u_int32_t c1;
u_int16_t checksum;
int i, partial_len;
p = buffer;
checksum = 0;
c0 = *csum & 0xff00;
c1 = *csum & 0x00ff;
/*
* If both are zero return correct
*/
if (c0 == 0 && c1 == 0)
return 0;
/*
* If either, but not both are zero return incorrect
*/
if (c0 == 0 || c1 == 0)
return 1;
/*
* Otherwise initialize to zero and calculate...
*/
c0 = 0;
c1 = 0;
while (len)
{
partial_len = MIN(len, 5803);
for (i = 0; i < partial_len; i++)
{
c0 = c0 + *(p++);
c1 += c0;
}
c0 = c0 % 255;
c1 = c1 % 255;
len -= partial_len;
}
if (c0 == 0 && c1 == 0)
return 0;
return 1;
}
/*
* Creates the checksum. *csum points to the position of the checksum in the
* PDU.
* Based on Annex C.4 of ISO/IEC 8473
*/
#define FIXED_CODE
u_int16_t
iso_csum_create (u_char * buffer, int len, u_int16_t n)
{
u_int8_t *p;
int x;
int y;
u_int32_t mul;
u_int32_t c0;
u_int32_t c1;
u_int16_t checksum;
u_int16_t *csum;
int i, init_len, partial_len;
checksum = 0;
/*
* Zero the csum in the packet.
*/
csum = (u_int16_t *) (buffer + n);
*(csum) = checksum;
p = buffer;
c0 = 0;
c1 = 0;
init_len = len;
while (len != 0)
{
partial_len = MIN(len, 5803);
for (i = 0; i < partial_len; i++)
{
c0 = c0 + *(p++);
c1 += c0;
}
c0 = c0 % 255;
c1 = c1 % 255;
len -= partial_len;
}
mul = (init_len - n)*(c0);
#ifdef FIXED_CODE
x = mul - c0 - c1;
y = c1 - mul - 1;
if (y > 0)
y++;
if (x < 0)
x--;
x %= 255;
y %= 255;
if (x == 0)
x = 255;
if (y == 0)
y = 1;
checksum = (y << 8) | (x & 0xFF);
#else
x = mul - c0 - c1;
x %= 255;
y = c1 - mul - 1;
y %= 255;
if (x == 0)
x = 255;
if (y == 0)
y = 255;
checksum = ((y << 8) | x);
#endif
/*
* Now we write this to the packet
*/
*(csum) = checksum;
/* return the checksum for user usage */
return checksum;
}
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