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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; see the file COPYING; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*/
#include <zebra.h>
#include "iso_checksum.h"
#include "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(uint8_t *buffer, int len, uint16_t csum, int offset)
{
uint16_t checksum;
uint32_t c0;
uint32_t c1;
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;
checksum = fletcher_checksum(buffer, len, offset);
if (checksum == htons(csum))
return 0;
return 1;
}
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