/*	$OpenBSD: in_cksum.c,v 1.1 2014/06/20 04:01:42 lteo Exp $	*/

/*

 * 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. 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/types.h>

#include "interface.h"

/*

 * Given the host-byte-order value of the checksum field in a packet

 * header, and the network-byte-order computed checksum of the data

 * that the checksum covers (including the checksum itself), compute

 * what the checksum field *should* have been.

 */

u_int16_t

in_cksum_shouldbe(u_int16_t sum, u_int16_t computed_sum)

{

	u_int32_t shouldbe;

	/*

	 * The value that should have gone into the checksum field

	 * is the negative of the value gotten by summing up everything

	 * *but* the checksum field.

	 *

	 * We can compute that by subtracting the value of the checksum

	 * field from the sum of all the data in the packet, and then

	 * computing the negative of that value.

	 *

	 * "sum" is the value of the checksum field, and "computed_sum"

	 * is the negative of the sum of all the data in the packets,

	 * so that's -(-computed_sum - sum), or (sum + computed_sum).

	 *

	 * All the arithmetic in question is one's complement, so the

	 * addition must include an end-around carry; we do this by

	 * doing the arithmetic in 32 bits (with no sign-extension),

	 * and then adding the upper 16 bits of the sum, which contain

	 * the carry, to the lower 16 bits of the sum, and then do it

	 * again in case *that* sum produced a carry.

	 *

	 * As RFC 1071 notes, the checksum can be computed without

	 * byte-swapping the 16-bit words; summing 16-bit words

	 * on a big-endian machine gives a big-endian checksum, which

	 * can be directly stuffed into the big-endian checksum fields

	 * in protocol headers, and summing words on a little-endian

	 * machine gives a little-endian checksum, which must be

	 * byte-swapped before being stuffed into a big-endian checksum

	 * field.

	 *

	 * "computed_sum" is a network-byte-order value, so we must put

	 * it in host byte order before subtracting it from the

	 * host-byte-order value from the header; the adjusted checksum

	 * will be in host byte order, which is what we'll return.

	 */

}