2 * 4.4-Lite-2 Internet checksum routine, modified to take a vector of
3 * pointers/lengths giving the pieces to be checksummed. Also using
4 * Tahoe/CGI version of ADDCARRY(x) macro instead of from portable version.
6 * Copyright (c) 1988, 1992, 1993
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33 * @(#)in_cksum.c 8.1 (Berkeley) 6/10/93
40 #include <epan/in_cksum.h>
43 * Checksum routine for Internet Protocol family headers (Portable Version).
45 * This routine is very heavily used in the network
46 * code and should be modified for each CPU to be as fast as possible.
49 #define ADDCARRY(x) {if ((x) > 65535) (x) -= 65535;}
50 #define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);}
53 in_cksum(const vec_t *vec, int veclen)
55 register const guint16 *w;
57 register int mlen = 0;
69 for (; veclen != 0; vec++, veclen--) {
72 w = (const guint16 *)(const void *)vec->ptr;
75 * The first byte of this chunk is the continuation
76 * of a word spanning between this chunk and the
79 * s_util.c[0] is already saved when scanning previous
82 s_util.c[1] = *(const guint8 *)w;
84 w = (const guint16 *)(const void *)((const guint8 *)w + 1);
89 * Force to even boundary.
91 if ((1 & (unsigned long) w) && (mlen > 0)) {
94 s_util.c[0] = *(const guint8 *)w;
95 w = (const guint16 *)(const void *)((const guint8 *)w + 1);
100 * Unroll the loop to make overhead from
103 while ((mlen -= 32) >= 0) {
104 sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
105 sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7];
106 sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11];
107 sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15];
111 while ((mlen -= 8) >= 0) {
112 sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
116 if (mlen == 0 && byte_swapped == 0)
119 while ((mlen -= 2) >= 0) {
127 s_util.c[1] = *(const guint8 *)w;
132 } else if (mlen == -1)
133 s_util.c[0] = *(const guint8 *)w;
136 /* The last mbuf has odd # of bytes. Follow the
137 standard (the odd byte may be shifted left by 8 bits
138 or not as determined by endian-ness of the machine) */
143 return (~sum & 0xffff);
147 * Given the host-byte-order value of the checksum field in a packet
148 * header, and the network-byte-order computed checksum of the data
149 * that the checksum covers (including the checksum itself), compute
150 * what the checksum field *should* have been.
153 in_cksum_shouldbe(guint16 sum, guint16 computed_sum)
158 * The value that should have gone into the checksum field
159 * is the negative of the value gotten by summing up everything
160 * *but* the checksum field.
162 * We can compute that by subtracting the value of the checksum
163 * field from the sum of all the data in the packet, and then
164 * computing the negative of that value.
166 * "sum" is the value of the checksum field, and "computed_sum"
167 * is the negative of the sum of all the data in the packets,
168 * so that's -(-computed_sum - sum), or (sum + computed_sum).
170 * All the arithmetic in question is one's complement, so the
171 * addition must include an end-around carry; we do this by
172 * doing the arithmetic in 32 bits (with no sign-extension),
173 * and then adding the upper 16 bits of the sum, which contain
174 * the carry, to the lower 16 bits of the sum, and then do it
175 * again in case *that* sum produced a carry.
177 * As RFC 1071 notes, the checksum can be computed without
178 * byte-swapping the 16-bit words; summing 16-bit words
179 * on a big-endian machine gives a big-endian checksum, which
180 * can be directly stuffed into the big-endian checksum fields
181 * in protocol headers, and summing words on a little-endian
182 * machine gives a little-endian checksum, which must be
183 * byte-swapped before being stuffed into a big-endian checksum
186 * "computed_sum" is a network-byte-order value, so we must put
187 * it in host byte order before subtracting it from the
188 * host-byte-order value from the header; the adjusted checksum
189 * will be in host byte order, which is what we'll return.
192 shouldbe += g_ntohs(computed_sum);
193 shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
194 shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);