1 /* -*- mode: c; c-file-style: "openbsd" -*- */
2 /*
3 * Copyright (c) 2009 Vincent Bernat <bernat@luffy.cx>
4 *
5 * Permission to use, copy, modify, and/or distribute this software for any
6 * purpose with or without fee is hereby granted, provided that the above
7 * copyright notice and this permission notice appear in all copies.
8 *
9 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
10 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
11 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
12 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
13 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
14 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
15 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
16 */
17
18 #include "lldpd.h"
19
20 /**
21 * Compute the checksum as 16-bit word.
22 */
23 u_int16_t
frame_checksum(const u_char * cp,int len,int cisco)24 frame_checksum(const u_char *cp, int len, int cisco)
25 {
26 unsigned int sum = 0, v = 0;
27 int oddbyte = 0;
28
29 while ((len -= 2) >= 0) {
30 sum += *cp++ << 8;
31 sum += *cp++;
32 }
33 if ((oddbyte = len & 1) != 0)
34 v = *cp;
35
36 /* The remaining byte seems to be handled oddly by Cisco. From function
37 * dissect_cdp() in wireshark. 2014/6/14,zhengy@yealink.com:
38 *
39 * CDP doesn't adhere to RFC 1071 section 2. (B). It incorrectly assumes
40 * checksums are calculated on a big endian platform, therefore i.s.o.
41 * padding odd sized data with a zero byte _at the end_ it sets the last
42 * big endian _word_ to contain the last network _octet_. This byteswap
43 * has to be done on the last octet of network data before feeding it to
44 * the Internet checksum routine.
45 * CDP checksumming code has a bug in the addition of this last _word_
46 * as a signed number into the long word intermediate checksum. When
47 * reducing this long to word size checksum an off-by-one error can be
48 * made. This off-by-one error is compensated for in the last _word_ of
49 * the network data.
50 */
51 if (oddbyte) {
52 if (cisco) {
53 if (v & 0x80) {
54 sum += 0xff << 8;
55 sum += v - 1;
56 } else {
57 sum += v;
58 }
59 } else {
60 sum += v << 8;
61 }
62 }
63
64 sum = (sum >> 16) + (sum & 0xffff);
65 sum += sum >> 16;
66 return (0xffff & ~sum);
67 }
68