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/* -*- mode: c; c-file-style: "openbsd" -*- */
/*
* Copyright (c) 2009 Vincent Bernat <bernat@luffy.cx>
*
* Permission to use, copy, modify, and/or distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
#include "lldpd.h"
/**
* Compute the checksum as 16-bit word.
*/
u_int16_t
frame_checksum(const u_char *cp, int len, int cisco)
{
unsigned int sum = 0, v = 0;
int oddbyte = 0;
while ((len -= 2) >= 0) {
sum += *cp++ << 8;
sum += *cp++;
}
if ((oddbyte = len & 1) != 0) v = *cp;
/* The remaining byte seems to be handled oddly by Cisco. From function
* dissect_cdp() in wireshark. 2014/6/14,zhengy@yealink.com:
*
* CDP doesn't adhere to RFC 1071 section 2. (B). It incorrectly assumes
* checksums are calculated on a big endian platform, therefore i.s.o.
* padding odd sized data with a zero byte _at the end_ it sets the last
* big endian _word_ to contain the last network _octet_. This byteswap
* has to be done on the last octet of network data before feeding it to
* the Internet checksum routine.
* CDP checksumming code has a bug in the addition of this last _word_
* as a signed number into the long word intermediate checksum. When
* reducing this long to word size checksum an off-by-one error can be
* made. This off-by-one error is compensated for in the last _word_ of
* the network data.
*/
if (oddbyte) {
if (cisco) {
if (v & 0x80) {
sum += 0xff << 8;
sum += v - 1;
} else {
sum += v;
}
} else {
sum += v << 8;
}
}
sum = (sum >> 16) + (sum & 0xffff);
sum += sum >> 16;
return (0xffff & ~sum);
}
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