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    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: }