date and time created 86/01/23 12:59:52 by sam

[unix-history] / usr / src / sys / netns / ns_cksum.c

/*      ns_cksum.c      1.1     86/01/23        */

/*
* ns_cksum.c - Xerox Internet Datagram protocol checksum
*/
#include "types.h"
#include "mbuf.h"

/*
* Perform (slowly) the Xerox Internet checksum algorithm on a
*      chain of mbufs.  This means we add all the 16-bits words,
*      shifting the sum after each 16-bit add.  Ones-complement
*      arithmetic is required, so we fold the carry bits after
*      each 16-bit add as well.
* If the result is the *no-checksum* value 0xffff, return zero instead.
*
* Chris Torek <chris@maryland>
* James O'Toole <james@maryland>
*/
u_short
ns_cksum(m, len)
       register struct mbuf *m;
       register int len;
{
       register int cksum = 0;
       register int shift = 8;
       register u_char *p;
       register int mlen;

       if (len & 1)
               printf("ns_cksum: odd length\n");
       while (m && len) {
               p = mtod(m, u_char *);
               mlen = m->m_len;
               if ((len -= mlen) < 0)
                       mlen += len, len = 0;
               while (--mlen >= 0) {
                       cksum += *p++ << shift;
                       if ((shift = 8 - shift) != 0) {
                               cksum <<= 1;
                               cksum = (cksum & 0xffff) + (cksum >> 16);
                       }
               }
               m = m->m_next;
       }
#ifdef notdef
       if (len)
               printf("ns_cksum: out of data\n");
#endif
       return (cksum == 0xffff ? 0 : cksum);
}