RFC1141

Network Working Group T. Mallory Request for Comments: 1141 A. Kullberg Obsoletes: RFC 1071 BBN Communications

                                                           January 1990

            Incremental Updating of the Internet Checksum

Status of this Memo

  This memo correctly describes the incremental update procedure for
  use with the standard Internet checksum.  It is intended to replace
  the description of Incremental Update in RFC 1071.  This is not a
  standard but rather, an implementation technique.  Distribution of
  this memo is unlimited.

Description

  In RFC 1071 on pages 4 and 5, there is a description of a method to
  update the IP checksum in the IP header without having to completely
  recompute the checksum.  In particular, the RFC recommends the
  following equation for computing the update checksum C' from the
  original checksum C, and the old and new values of byte m:

        C' = C + (-m) + m' = C + (m' - m)

  While the equation above is correct, it is not very useful for
  incremental updates since the equation above updates the checksum C,
  rather than the 1's complement of the checksum, ~C, which is the
  value stored in the checksum field.  In addition, it suffers because
  the notation does not clearly specify that all arithmetic, including
  the unary negation, must be performed one's complement, and so is
  difficult to use to build working code.  The useful calculation for
  2's complement machines is:

        ~C' = ~(C + (-m) + m') = ~C + (m - m') = ~C + m + ~m'

  In the oft-mentioned case of updating the IP TTL field, subtracting
  one from the TTL means ADDING 1 or 256 as appropriate to the checksum
  field in the packet, using one's complement addition.  One big-endian
  non-portable implementation in C looks like:

     unsigned long sum;
     ipptr->ttl--;                  /* decrement ttl */
     sum = ipptr->Checksum + 0x100;  /* increment checksum high byte*/
     ipptr->Checksum = (sum + (sum>>16)) /* add carry */

  This special case can be optimized in many ways: for instance, you

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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: [email protected]

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

  Phone: (617) 873-4000

  EMail:  [email protected]

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