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
  27 /*
  28  * Calculations of the OSI checksum.
  29  * ISO/IEC 8473 defines the sum as
  30  *
  31  *     L
  32  *  sum  a (mod 255) = 0
  33  *     1  i
  34  *
  35  *     L
  36  *  sum (L-i+1)a (mod 255) = 0
  37  *     1        i
  38  *
  39  */
  40
  41 /*
  42  * Verifies that the checksum is correct.
  43  * Return 0 on correct and 1 on invalid checksum.
  44  * Based on Annex C.4 of ISO/IEC 8473
  45  */
  46
  47 int
  48 iso_csum_verify (u_char * buffer, int len, uint16_t * csum)
  49 {
  50   u_int8_t *p;
  51   u_int32_t c0;
  52   u_int32_t c1;
  53   u_int16_t checksum;
  54   int i, partial_len;
  55
  56   p = buffer;
  57   checksum = 0;
  58   c0 = *csum & 0xff00;
  59   c1 = *csum & 0x00ff;
  60
  61   /*
  62    * If both are zero return correct
  63    */
  64   if (c0 == 0 && c1 == 0)
  65     return 0;
  66
  67   /*
  68    * If either, but not both are zero return incorrect
  69    */
  70   if (c0 == 0 || c1 == 0)
  71     return 1;
  72
  73   /*
  74    * Otherwise initialize to zero and calculate...
  75    */
  76   c0 = 0;
  77   c1 = 0;
  78
  79   while (len)
  80     {
  81       partial_len = MIN(len, 5803);
  82
  83       for (i = 0; i < partial_len; i++)
  84         {
  85           c0 = c0 + *(p++);
  86           c1 += c0;
  87         }
  88
  89       c0 = c0 % 255;
  90       c1 = c1 % 255;
  91
  92       len -= partial_len;
  93     }
  94
  95   if (c0 == 0 && c1 == 0)
  96     return 0;
  97
  98   return 1;
  99 }
 100
 101 /*
 102  * Creates the checksum. *csum points to the position of the checksum in the
 103  * PDU.
 104  * Based on Annex C.4 of ISO/IEC 8473
 105  */
 106 #define FIXED_CODE
 107 u_int16_t
 108 iso_csum_create (u_char * buffer, int len, u_int16_t n)
 109 {
 110
 111   u_int8_t *p;
 112   int x;
 113   int y;
 114   u_int32_t mul;
 115   u_int32_t c0;
 116   u_int32_t c1;
 117   u_int16_t checksum;
 118   u_int16_t *csum;
 119   int i, init_len, partial_len;
 120
 121   checksum = 0;
 122
 123   /*
 124    * Zero the csum in the packet.
 125    */
 126   csum = (u_int16_t *) (buffer + n);
 127   *(csum) = checksum;
 128
 129   p = buffer;
 130   c0 = 0;
 131   c1 = 0;
 132   init_len = len;
 133
 134   while (len != 0)
 135     {
 136       partial_len = MIN(len, 5803);
 137
 138       for (i = 0; i < partial_len; i++)
 139         {
 140           c0 = c0 + *(p++);
 141           c1 += c0;
 142         }
 143
 144       c0 = c0 % 255;
 145       c1 = c1 % 255;
 146
 147       len -= partial_len;
 148     }
 149
 150   mul = (init_len - n)*(c0);
 151
 152 #ifdef FIXED_CODE
 153   x = mul - c0 - c1;
 154   y = c1 - mul - 1;
 155
 156   if (y > 0)
 157     y++;
 158   if (x < 0)
 159     x--;
 160
 161   x %= 255;
 162   y %= 255;
 163
 164   if (x == 0)
 165     x = 255;
 166   if (y == 0)
 167     y = 1;
 168
 169   checksum = (y << 8) | (x & 0xFF);
 170
 171 #else
 172   x = mul - c0 - c1;
 173   x %= 255;
 174
 175   y = c1 - mul - 1;
 176   y %= 255;
 177
 178   if (x == 0)
 179     x = 255;
 180   if (y == 0)
 181     y = 255;
 182
 183   checksum = ((y << 8) | x);
 184 #endif
 185
 186   /*
 187    * Now we write this to the packet
 188    */
 189   *(csum) = checksum;
 190
 191   /* return the checksum for user usage */
 192   return checksum;
 193 }