202 lines
6.5 KiB
C
202 lines
6.5 KiB
C
/* in_cksum.c
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* 4.4-Lite-2 Internet checksum routine, modified to take a vector of
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* pointers/lengths giving the pieces to be checksummed. Also using
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* Tahoe/CGI version of ADDCARRY(x) macro instead of from portable version.
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*/
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/*
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* Copyright (c) 1988, 1992, 1993
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* The Regents of the University of California. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. Neither the name of the University nor the names of its contributors
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* may be used to endorse or promote products derived from this software
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* without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*
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* @(#)in_cksum.c 8.1 (Berkeley) 6/10/93
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*/
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#define NETDISSECT_REWORKED
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#ifdef HAVE_CONFIG_H
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# include "config.h"
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#endif
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#include <tcpdump-stdinc.h>
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#include "interface.h"
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/*
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* Checksum routine for Internet Protocol family headers (Portable Version).
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*
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* This routine is very heavily used in the network
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* code and should be modified for each CPU to be as fast as possible.
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*/
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#define ADDCARRY(x) {if ((x) > 65535) (x) -= 65535;}
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#define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);}
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uint16_t
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in_cksum(const struct cksum_vec *vec, int veclen)
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{
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register const uint16_t *w;
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register int sum = 0;
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register int mlen = 0;
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int byte_swapped = 0;
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union {
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uint8_t c[2];
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uint16_t s;
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} s_util;
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union {
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uint16_t s[2];
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uint32_t l;
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} l_util;
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for (; veclen != 0; vec++, veclen--) {
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if (vec->len == 0)
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continue;
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w = (const uint16_t *)(void *)vec->ptr;
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if (mlen == -1) {
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/*
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* The first byte of this chunk is the continuation
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* of a word spanning between this chunk and the
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* last chunk.
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*
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* s_util.c[0] is already saved when scanning previous
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* chunk.
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*/
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s_util.c[1] = *(const uint8_t *)w;
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sum += s_util.s;
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w = (const uint16_t *)(void *)((const uint8_t *)w + 1);
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mlen = vec->len - 1;
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} else
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mlen = vec->len;
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/*
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* Force to even boundary.
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*/
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if ((1 & (unsigned long) w) && (mlen > 0)) {
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REDUCE;
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sum <<= 8;
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s_util.c[0] = *(const uint8_t *)w;
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w = (const uint16_t *)(void *)((const uint8_t *)w + 1);
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mlen--;
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byte_swapped = 1;
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}
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/*
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* Unroll the loop to make overhead from
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* branches &c small.
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*/
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while ((mlen -= 32) >= 0) {
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sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
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sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7];
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sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11];
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sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15];
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w += 16;
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}
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mlen += 32;
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while ((mlen -= 8) >= 0) {
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sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
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w += 4;
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}
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mlen += 8;
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if (mlen == 0 && byte_swapped == 0)
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continue;
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REDUCE;
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while ((mlen -= 2) >= 0) {
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sum += *w++;
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}
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if (byte_swapped) {
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REDUCE;
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sum <<= 8;
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byte_swapped = 0;
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if (mlen == -1) {
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s_util.c[1] = *(const uint8_t *)w;
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sum += s_util.s;
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mlen = 0;
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} else
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mlen = -1;
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} else if (mlen == -1)
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s_util.c[0] = *(const uint8_t *)w;
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}
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if (mlen == -1) {
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/* The last mbuf has odd # of bytes. Follow the
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standard (the odd byte may be shifted left by 8 bits
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or not as determined by endian-ness of the machine) */
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s_util.c[1] = 0;
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sum += s_util.s;
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}
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REDUCE;
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return (~sum & 0xffff);
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}
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/*
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* Given the host-byte-order value of the checksum field in a packet
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* header, and the network-byte-order computed checksum of the data
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* that the checksum covers (including the checksum itself), compute
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* what the checksum field *should* have been.
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*/
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uint16_t
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in_cksum_shouldbe(uint16_t sum, uint16_t computed_sum)
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{
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uint32_t shouldbe;
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/*
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* The value that should have gone into the checksum field
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* is the negative of the value gotten by summing up everything
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* *but* the checksum field.
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*
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* We can compute that by subtracting the value of the checksum
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* field from the sum of all the data in the packet, and then
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* computing the negative of that value.
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*
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* "sum" is the value of the checksum field, and "computed_sum"
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* is the negative of the sum of all the data in the packets,
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* so that's -(-computed_sum - sum), or (sum + computed_sum).
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*
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* All the arithmetic in question is one's complement, so the
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* addition must include an end-around carry; we do this by
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* doing the arithmetic in 32 bits (with no sign-extension),
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* and then adding the upper 16 bits of the sum, which contain
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* the carry, to the lower 16 bits of the sum, and then do it
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* again in case *that* sum produced a carry.
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*
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* As RFC 1071 notes, the checksum can be computed without
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* byte-swapping the 16-bit words; summing 16-bit words
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* on a big-endian machine gives a big-endian checksum, which
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* can be directly stuffed into the big-endian checksum fields
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* in protocol headers, and summing words on a little-endian
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* machine gives a little-endian checksum, which must be
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* byte-swapped before being stuffed into a big-endian checksum
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* field.
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*
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* "computed_sum" is a network-byte-order value, so we must put
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* it in host byte order before subtracting it from the
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* host-byte-order value from the header; the adjusted checksum
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* will be in host byte order, which is what we'll return.
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*/
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shouldbe = sum;
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shouldbe += ntohs(computed_sum);
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shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
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shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
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return shouldbe;
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}
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