isisd/iso_checksum.c

   1 /*
   2  * IS-IS Rout(e)ing protocol - iso_checksum.c
   3  *                             ISO checksum related routines
   4  *
   5  * Copyright (C) 2001,2002   Sampo Saaristo
   6  *                           Tampere University of Technology
   7  *                           Institute of Communications Engineering
   8  *
   9  * This program is free software; you can redistribute it and/or modify it
  10  * under the terms of the GNU General Public Licenseas published by the Free
  11  * Software Foundation; either version 2 of the License, or (at your option)
  12  * any later version.
  13  *
  14  * This program is distributed in the hope that it will be useful,but WITHOUT
  15  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  16  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
  17  * more details.
  18  *
  19  * You should have received a copy of the GNU General Public License along
  20  * with this program; see the file COPYING; if not, write to the Free Software
  21  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
  22  */
  23
  24 #include <zebra.h>
  25 #include "iso_checksum.h"
  26 #include "checksum.h"
  27
  28 /*
  29  * Calculations of the OSI checksum.
  30  * ISO/IEC 8473 defines the sum as
  31  *
  32  *     L
  33  *  sum  a (mod 255) = 0
  34  *     1  i
  35  *
  36  *     L
  37  *  sum (L-i+1)a (mod 255) = 0
  38  *     1        i
  39  *
  40  */
  41
  42 /*
  43  * Verifies that the checksum is correct.
  44  * Return 0 on correct and 1 on invalid checksum.
  45  * Based on Annex C.4 of ISO/IEC 8473
  46  */
  47
  48 int iso_csum_verify(uint8_t *buffer, int len, uint16_t csum, int offset)
  49 {
  50         uint16_t checksum;
  51         uint32_t c0;
  52         uint32_t c1;
  53
  54         c0 = csum & 0xff00;
  55         c1 = csum & 0x00ff;
  56
  57         /*
  58          * If both are zero return correct
  59          */
  60         if (c0 == 0 && c1 == 0)
  61                 return 0;
  62
  63         /*
  64          * If either, but not both are zero return incorrect
  65          */
  66         if (c0 == 0 || c1 == 0)
  67                 return 1;
  68
  69         checksum = fletcher_checksum(buffer, len, offset);
  70         if (checksum == htons(csum))
  71                 return 0;
  72         return 1;
  73 }