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eb5d44eb | 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 | * FIXME: Check for overflow | |
46 | */ | |
47 | ||
48 | int | |
f390d2c7 | 49 | iso_csum_verify (u_char * buffer, int len, uint16_t * csum) |
50 | { | |
eb5d44eb | 51 | u_int8_t *p; |
52 | u_int32_t c0; | |
53 | u_int32_t c1; | |
54 | u_int16_t checksum; | |
55 | int i; | |
56 | ||
57 | p = buffer; | |
58 | checksum = 0; | |
59 | c0 = *csum & 0xff00; | |
60 | c1 = *csum & 0x00ff; | |
61 | ||
62 | /* | |
63 | * If both are zero return correct | |
64 | */ | |
65 | if (c0 == 0 && c1 == 0) | |
66 | return 0; | |
67 | ||
68 | /* | |
69 | * If either, but not both are zero return incorrect | |
70 | */ | |
71 | if (c0 == 0 || c1 == 0) | |
72 | return 1; | |
f390d2c7 | 73 | |
eb5d44eb | 74 | /* |
75 | * Otherwise initialize to zero and calculate... | |
76 | */ | |
77 | c0 = 0; | |
78 | c1 = 0; | |
79 | ||
f390d2c7 | 80 | for (i = 0; i < len; i++) |
81 | { | |
82 | c0 = c0 + *(p++); | |
83 | c1 += c0; | |
84 | } | |
eb5d44eb | 85 | |
86 | c0 = c0 % 255; | |
87 | c1 = c1 % 255; | |
f390d2c7 | 88 | |
89 | if (c0 == 0 && c1 == 0) | |
eb5d44eb | 90 | return 0; |
91 | ||
92 | return 1; | |
93 | } | |
94 | ||
eb5d44eb | 95 | /* |
96 | * Creates the checksum. *csum points to the position of the checksum in the | |
97 | * PDU. | |
98 | * Based on Annex C.4 of ISO/IEC 8473 | |
99 | * we will not overflow until about length of 6000, | |
100 | * which is the answer to (255+255n)*n/2 > 2^32 | |
101 | * so if we have a length of over 5000 we will return zero (for now) | |
102 | */ | |
103 | #define FIXED_CODE | |
104 | u_int16_t | |
f390d2c7 | 105 | iso_csum_create (u_char * buffer, int len, u_int16_t n) |
eb5d44eb | 106 | { |
107 | ||
108 | u_int8_t *p; | |
109 | int x; | |
110 | int y; | |
111 | u_int32_t mul; | |
112 | u_int32_t c0; | |
113 | u_int32_t c1; | |
114 | u_int16_t checksum; | |
f390d2c7 | 115 | u_int16_t *csum; |
eb5d44eb | 116 | int i; |
117 | ||
118 | checksum = 0; | |
119 | ||
120 | /* | |
121 | * Zero the csum in the packet. | |
122 | */ | |
f390d2c7 | 123 | csum = (u_int16_t *) (buffer + n); |
eb5d44eb | 124 | *(csum) = checksum; |
125 | ||
126 | /* for the limitation of our implementation */ | |
f390d2c7 | 127 | if (len > 5000) |
128 | { | |
129 | return 0; | |
130 | } | |
eb5d44eb | 131 | |
132 | p = buffer; | |
133 | c0 = 0; | |
134 | c1 = 0; | |
135 | ||
f390d2c7 | 136 | for (i = 0; i < len; i++) |
137 | { | |
138 | c0 = c0 + *(p++); | |
139 | c1 += c0; | |
140 | } | |
eb5d44eb | 141 | |
142 | c0 = c0 % 255; | |
143 | c1 = c1 % 255; | |
144 | ||
f390d2c7 | 145 | mul = (len - n) * (c0); |
146 | ||
eb5d44eb | 147 | #ifdef FIXED_CODE |
148 | x = mul - c0 - c1; | |
149 | y = c1 - mul - 1; | |
150 | ||
f390d2c7 | 151 | if (y >= 0) |
152 | y++; | |
153 | if (x < 0) | |
154 | x--; | |
eb5d44eb | 155 | |
156 | x %= 255; | |
157 | y %= 255; | |
158 | ||
f390d2c7 | 159 | if (x == 0) |
160 | x = 255; | |
161 | if (y == 0) | |
162 | y = 255; | |
eb5d44eb | 163 | |
164 | x &= 0x00FF; | |
165 | ||
166 | checksum = ((y << 8) | x); | |
167 | ||
168 | #else | |
169 | x = mul - c0 - c1; | |
170 | x %= 255; | |
171 | ||
172 | y = c1 - mul - 1; | |
173 | y %= 255; | |
174 | ||
f390d2c7 | 175 | if (x == 0) |
176 | x = 255; | |
177 | if (y == 0) | |
178 | y = 255; | |
eb5d44eb | 179 | |
180 | checksum = ((y << 8) | x); | |
181 | #endif | |
f390d2c7 | 182 | |
eb5d44eb | 183 | /* |
184 | * Now we write this to the packet | |
185 | */ | |
186 | *(csum) = checksum; | |
187 | ||
188 | /* return the checksum for user usage */ | |
189 | return checksum; | |
190 | } | |
191 | ||
eb5d44eb | 192 | int |
f390d2c7 | 193 | iso_csum_modify (u_char * buffer, int len, uint16_t * csum) |
eb5d44eb | 194 | { |
eb5d44eb | 195 | return 0; |
196 | } |