isisd/iso_checksum.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * IS-IS Rout(e)ing protocol - iso_checksum.c
   4  *                             ISO checksum related routines
   5  *
   6  * Copyright (C) 2001,2002   Sampo Saaristo
   7  *                           Tampere University of Technology
   8  *                           Institute of Communications Engineering
   9  */
  10
  11 #include <zebra.h>
  12 #include "iso_checksum.h"
  13 #include "checksum.h"
  14
  15 /*
  16  * Calculations of the OSI checksum.
  17  * ISO/IEC 8473 defines the sum as
  18  *
  19  *     L
  20  *  sum  a (mod 255) = 0
  21  *     1  i
  22  *
  23  *     L
  24  *  sum (L-i+1)a (mod 255) = 0
  25  *     1        i
  26  *
  27  */
  28
  29 /*
  30  * Verifies that the checksum is correct.
  31  * Return 0 on correct and 1 on invalid checksum.
  32  * Based on Annex C.4 of ISO/IEC 8473
  33  */
  34
  35 int iso_csum_verify(uint8_t *buffer, int len, uint16_t csum, int offset)
  36 {
  37         uint16_t checksum;
  38         uint32_t c0;
  39         uint32_t c1;
  40
  41         c0 = csum & 0xff00;
  42         c1 = csum & 0x00ff;
  43
  44         /*
  45          * If both are zero return correct
  46          */
  47         if (c0 == 0 && c1 == 0)
  48                 return 0;
  49
  50         /*
  51          * If either, but not both are zero return incorrect
  52          */
  53         if (c0 == 0 || c1 == 0)
  54                 return 1;
  55
  56         checksum = fletcher_checksum(buffer, len, offset);
  57         if (checksum == htons(csum))
  58                 return 0;
  59         return 1;
  60 }