1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
|
/*
* IS-IS Rout(e)ing protocol - iso_checksum.c
* ISO checksum related routines
*
* Copyright (C) 2001,2002 Sampo Saaristo
* Tampere University of Technology
* Institute of Communications Engineering
*
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public Licenseas published by the Free
* Software Foundation; either version 2 of the License, or (at your option)
* any later version.
*
* This program is distributed in the hope that it will be useful,but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
* more details.
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
#include <zebra.h>
#include "iso_checksum.h"
/*
* Calculations of the OSI checksum.
* ISO/IEC 8473 defines the sum as
*
* L
* sum a (mod 255) = 0
* 1 i
*
* L
* sum (L-i+1)a (mod 255) = 0
* 1 i
*
*/
/*
* Verifies that the checksum is correct.
* Return 0 on correct and 1 on invalid checksum.
* Based on Annex C.4 of ISO/IEC 8473
*/
int
iso_csum_verify (u_char * buffer, int len, uint16_t * csum)
{
u_int8_t *p;
u_int32_t c0;
u_int32_t c1;
u_int16_t checksum;
int i, partial_len;
p = buffer;
checksum = 0;
c0 = *csum & 0xff00;
c1 = *csum & 0x00ff;
/*
* If both are zero return correct
*/
if (c0 == 0 && c1 == 0)
return 0;
/*
* If either, but not both are zero return incorrect
*/
if (c0 == 0 || c1 == 0)
return 1;
/*
* Otherwise initialize to zero and calculate...
*/
c0 = 0;
c1 = 0;
while (len)
{
partial_len = MIN(len, 5803);
for (i = 0; i < partial_len; i++)
{
c0 = c0 + *(p++);
c1 += c0;
}
c0 = c0 % 255;
c1 = c1 % 255;
len -= partial_len;
}
if (c0 == 0 && c1 == 0)
return 0;
return 1;
}
/*
* Creates the checksum. *csum points to the position of the checksum in the
* PDU.
* Based on Annex C.4 of ISO/IEC 8473
*/
#define FIXED_CODE
u_int16_t
iso_csum_create (u_char * buffer, int len, u_int16_t n)
{
u_int8_t *p;
int x;
int y;
u_int32_t mul;
u_int32_t c0;
u_int32_t c1;
u_int16_t checksum;
u_int16_t *csum;
int i, init_len, partial_len;
checksum = 0;
/*
* Zero the csum in the packet.
*/
csum = (u_int16_t *) (buffer + n);
*(csum) = checksum;
p = buffer;
c0 = 0;
c1 = 0;
init_len = len;
while (len != 0)
{
partial_len = MIN(len, 5803);
for (i = 0; i < partial_len; i++)
{
c0 = c0 + *(p++);
c1 += c0;
}
c0 = c0 % 255;
c1 = c1 % 255;
len -= partial_len;
}
mul = (init_len - n)*(c0);
#ifdef FIXED_CODE
x = mul - c0 - c1;
y = c1 - mul - 1;
if (y > 0)
y++;
if (x < 0)
x--;
x %= 255;
y %= 255;
if (x == 0)
x = 255;
if (y == 0)
y = 1;
checksum = (y << 8) | (x & 0xFF);
#else
x = mul - c0 - c1;
x %= 255;
y = c1 - mul - 1;
y %= 255;
if (x == 0)
x = 255;
if (y == 0)
y = 255;
checksum = ((y << 8) | x);
#endif
/*
* Now we write this to the packet
*/
*(csum) = checksum;
/* return the checksum for user usage */
return checksum;
}
int
iso_csum_modify (u_char * buffer, int len, uint16_t * csum)
{
return 0;
}
|
