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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 | ||
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 | |
49 | iso_csum_verify (u_char *buffer, int len, uint16_t *csum) | |
50 | { | |
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; | |
73 | ||
74 | /* | |
75 | * Otherwise initialize to zero and calculate... | |
76 | */ | |
77 | c0 = 0; | |
78 | c1 = 0; | |
79 | ||
80 | for (i = 0; i < len; i++) { | |
81 | c0 = c0 + *(p++); | |
82 | c1 += c0; | |
83 | } | |
84 | ||
85 | c0 = c0 % 255; | |
86 | c1 = c1 % 255; | |
87 | ||
88 | if ( c0 == 0 && c1 == 0) | |
89 | return 0; | |
90 | ||
91 | return 1; | |
92 | } | |
93 | ||
94 | ||
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 | |
105 | iso_csum_create (u_char *buffer, int len, u_int16_t n) | |
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; | |
115 | u_int16_t *csum; | |
116 | int i; | |
117 | ||
118 | checksum = 0; | |
119 | ||
120 | /* | |
121 | * Zero the csum in the packet. | |
122 | */ | |
123 | csum = (u_int16_t*)(buffer + n); | |
124 | *(csum) = checksum; | |
125 | ||
126 | /* for the limitation of our implementation */ | |
127 | if (len > 5000) { | |
128 | return 0; | |
129 | } | |
130 | ||
131 | p = buffer; | |
132 | c0 = 0; | |
133 | c1 = 0; | |
134 | ||
135 | for (i = 0; i < len; i++) { | |
136 | c0 = c0 + *(p++); | |
137 | c1 += c0; | |
138 | } | |
139 | ||
140 | c0 = c0 % 255; | |
141 | c1 = c1 % 255; | |
142 | ||
143 | mul = (len - n)*(c0); | |
144 | ||
145 | #ifdef FIXED_CODE | |
146 | x = mul - c0 - c1; | |
147 | y = c1 - mul - 1; | |
148 | ||
149 | if ( y >= 0 ) y++; | |
150 | if ( x < 0 ) x--; | |
151 | ||
152 | x %= 255; | |
153 | y %= 255; | |
154 | ||
155 | if (x == 0) x = 255; | |
156 | if (y == 0) y = 255; | |
157 | ||
158 | x &= 0x00FF; | |
159 | ||
160 | checksum = ((y << 8) | x); | |
161 | ||
162 | #else | |
163 | x = mul - c0 - c1; | |
164 | x %= 255; | |
165 | ||
166 | y = c1 - mul - 1; | |
167 | y %= 255; | |
168 | ||
169 | if (x == 0) x = 255; | |
170 | if (y == 0) y = 255; | |
171 | ||
172 | checksum = ((y << 8) | x); | |
173 | #endif | |
174 | ||
175 | /* | |
176 | * Now we write this to the packet | |
177 | */ | |
178 | *(csum) = checksum; | |
179 | ||
180 | /* return the checksum for user usage */ | |
181 | return checksum; | |
182 | } | |
183 | ||
184 | ||
185 | int | |
186 | iso_csum_modify (u_char *buffer, int len, uint16_t *csum) | |
187 | { | |
188 | ||
189 | return 0; | |
190 | } | |
191 | ||
192 |