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; if not, write to the Free Software Foundation, Inc.,
  21  * 59 Temple Place - Suite 330, Boston, MA  02111-1307, 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
  49 iso_csum_verify (u_char * buffer, int len, uint16_t * csum)
  50 {
  51   u_int16_t checksum;
  52   u_int32_t c0;
  53   u_int32_t c1;
  54
  55   c0 = *csum & 0xff00;
  56   c1 = *csum & 0x00ff;
  57
  58   /*
  59    * If both are zero return correct
  60    */
  61   if (c0 == 0 && c1 == 0)
  62     return 0;
  63
  64   /*
  65    * If either, but not both are zero return incorrect
  66    */
  67   if (c0 == 0 || c1 == 0)
  68     return 1;
  69
  70   /* Offset of checksum from the start of the buffer */
  71   int offset = (u_char *) csum - buffer;
  72
  73   checksum = fletcher_checksum(buffer, len, offset);
  74   if (checksum == *csum)
  75     return 0;
  76   return 1;
  77 }