src/frame.c

   1 /*
   2  * Copyright (c) 2009 Vincent Bernat <bernat@luffy.cx>
   3  *
   4  * Permission to use, copy, modify, and distribute this software for any
   5  * purpose with or without fee is hereby granted, provided that the above
   6  * copyright notice and this permission notice appear in all copies.
   7  *
   8  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
   9  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
  10  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
  11  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
  12  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
  13  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  14  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  15  */
  16
  17 #include "lldpd.h"
  18
  19 u_int16_t
  20 frame_checksum(const u_char *cp, int len, int cisco)
  21 {
  22         unsigned int sum = 0, v = 0;
  23         int oddbyte = 0;
  24
  25         /* We compute in network byte order */
  26         while ((len -= 2) >= 0) {
  27                 sum += *cp++ << 8;
  28                 sum += *cp++;
  29         }
  30         if ((oddbyte = len & 1) != 0)
  31                 v = *cp;
  32
  33         /* The remaining byte seems to be handled oddly by Cisco. Any hint about
  34          * this is welcome. */
  35         if (oddbyte) {
  36                 if (cisco)
  37                         sum += v;
  38                 else
  39                         sum += v << 8;
  40         }
  41         sum = (sum >> 16) + (sum & 0xffff);
  42         sum += sum >> 16;
  43         sum = ntohs(sum);
  44         return (0xffff & ~sum);
  45 }