RFC 1141 (rfc1141) - Page 2 of 2

Incremental updating of the Internet checksum

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RFC 1141                  Incremental Updating              January 1990


   can bundle updating and checking the ttl.  Compiler mileage may vary.
   Here is a more general and possibly more helpful example which
   updates the ttl by n seconds:

      UpdateTTL(iph,n)
      struct ip_hdr *ipptr;
      unsigned char n;
      {
          unsigned long sum;
          unsigned short old;

          old = ntohs(*(unsigned short *)&ipptr->ttl);
          ipptr->ttl -= n;
          sum = old + (~ntohs(*(unsigned short *)&ipptr->ttl) & 0xffff);
          sum += ntohs(ipptr->Checksum);
          sum = (sum & 0xffff) + (sum>>16);
          ipptr->Checksum = htons(sum + (sum>>16));
      }

Security Considerations

   Security issues are not addressed in this memo.

Authors' Addresses

   Tracy Mallory
   BBN Communications Corporation
   50 Moulton Street
   Cambridge, MA 02238

   Phone: (617) 873-3193

   EMail: 


   A. Kullberg
   BBN Communications Corporation
   50 Moulton Street
   Cambridge, MA 02238

   Phone: (617) 873-4000

   EMail:  








Mallory & Kullberg

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