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1 | /* | |
2 | * Checksum routine for Internet Protocol family headers (C Version). | |
3 | * | |
4 | * Refer to "Computing the Internet Checksum" by R. Braden, D. Borman and | |
5 | * C. Partridge, Computer Communication Review, Vol. 19, No. 2, April 1989, | |
6 | * pp. 86-101, for additional details on computing this checksum. | |
7 | */ | |
8 | ||
9 | #include <zebra.h> | |
10 | ||
11 | int /* return checksum in low-order 16 bits */ | |
12 | in_cksum(ptr, nbytes) | |
13 | register u_short *ptr; | |
14 | register int nbytes; | |
15 | { | |
16 | register long sum; /* assumes long == 32 bits */ | |
17 | u_short oddbyte; | |
18 | register u_short answer; /* assumes u_short == 16 bits */ | |
19 | ||
20 | /* | |
21 | * Our algorithm is simple, using a 32-bit accumulator (sum), | |
22 | * we add sequential 16-bit words to it, and at the end, fold back | |
23 | * all the carry bits from the top 16 bits into the lower 16 bits. | |
24 | */ | |
25 | ||
26 | sum = 0; | |
27 | while (nbytes > 1) { | |
28 | sum += *ptr++; | |
29 | nbytes -= 2; | |
30 | } | |
31 | ||
32 | /* mop up an odd byte, if necessary */ | |
33 | if (nbytes == 1) { | |
34 | oddbyte = 0; /* make sure top half is zero */ | |
35 | *((u_char *) &oddbyte) = *(u_char *)ptr; /* one byte only */ | |
36 | sum += oddbyte; | |
37 | } | |
38 | ||
39 | /* | |
40 | * Add back carry outs from top 16 bits to low 16 bits. | |
41 | */ | |
42 | ||
43 | sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */ | |
44 | sum += (sum >> 16); /* add carry */ | |
45 | answer = ~sum; /* ones-complement, then truncate to 16 bits */ | |
46 | return(answer); | |
47 | } |