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UDP Checksum implementation

Posted on 1997-03-11
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Last Modified: 2008-03-10
udp checksum implementation in c++ programming
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Question by:pongchai
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Expert Comment

by:avramov
Comment Utility
Please look at
http://www.cis.ohio-state.edu/hypertext/information/rfc.html
There are different RFCs about all the protocols,
so U can easily find the UDP specification and to implement it.
   Best
     Alex Abramovich (avramov)
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Accepted Solution

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yonat earned 20 total points
Comment Utility
The UDP checksum is "the 16-bit one's complement of the one's
complement sum of a pseudo header of information from the IP
header, the UDP header, and the data, padded with zero octets at
the end (if necessary) to make a multiple of two octets."
as defined in http://www.internic.net/std/std6.txt .
Below is an implementation. If you have problems with line breaks
(since Experts-Exchange allows only 62 chars in a line), I can
mail you the file.

/*
 * Copyright (c) 1988, 1992, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)in_cksum.c      8.1 (Berkeley) 6/10/93
 */

#include <sys/param.h>
#include <sys/mbuf.h>

/*
 * Checksum routine for Internet Protocol family headers (Portable Version).
 *
 * This routine is very heavily used in the network
 * code and should be modified for each CPU to be as fast as possible.
 */

#define ADDCARRY(x)  (x > 65535 ? x -= 65535 : x)
#define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);}

int
in_cksum(m, len)
      register struct mbuf *m;
      register int len;
{
      register u_short *w;
      register int sum = 0;
      register int mlen = 0;
      int byte_swapped = 0;

      union {
            char      c[2];
            u_short      s;
      } s_util;
      union {
            u_short s[2];
            long      l;
      } l_util;

      for (;m && len; m = m->m_next) {
            if (m->m_len == 0)
                  continue;
            w = mtod(m, u_short *);
            if (mlen == -1) {
                  /*
                   * The first byte of this mbuf is the continuation
                   * of a word spanning between this mbuf and the
                   * last mbuf.
                   *
                   * s_util.c[0] is already saved when scanning previous
                   * mbuf.
                   */
                  s_util.c[1] = *(char *)w;
                  sum += s_util.s;
                  w = (u_short *)((char *)w + 1);
                  mlen = m->m_len - 1;
                  len--;
            } else
                  mlen = m->m_len;
            if (len < mlen)
                  mlen = len;
            len -= mlen;
            /*
             * Force to even boundary.
             */
            if ((1 & (int) w) && (mlen > 0)) {
                  REDUCE;
                  sum <<= 8;
                  s_util.c[0] = *(u_char *)w;
                  w = (u_short *)((char *)w + 1);
                  mlen--;
                  byte_swapped = 1;
            }
            /*
             * Unroll the loop to make overhead from
             * branches &c small.
             */
            while ((mlen -= 32) >= 0) {
                  sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
                  sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7];
                  sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11];
                  sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15];
                  w += 16;
            }
            mlen += 32;
            while ((mlen -= 8) >= 0) {
                  sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
                  w += 4;
            }
            mlen += 8;
            if (mlen == 0 && byte_swapped == 0)
                  continue;
            REDUCE;
            while ((mlen -= 2) >= 0) {
                  sum += *w++;
            }
            if (byte_swapped) {
                  REDUCE;
                  sum <<= 8;
                  byte_swapped = 0;
                  if (mlen == -1) {
                        s_util.c[1] = *(char *)w;
                        sum += s_util.s;
                        mlen = 0;
                  } else
                        mlen = -1;
            } else if (mlen == -1)
                  s_util.c[0] = *(char *)w;
      }
      if (len)
            printf("cksum: out of data\n");
      if (mlen == -1) {
            /* The last mbuf has odd # of bytes. Follow the
               standard (the odd byte may be shifted left by 8 bits
               or not as determined by endian-ness of the machine) */
            s_util.c[1] = 0;
            sum += s_util.s;
      }
      REDUCE;
      return (~sum & 0xffff);
}

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