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 48int 49iso_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} 78