





Network Working Group                                          J. Zweig
Request for Comments: 1145                                         UIUC
                                                           C. Partridge
                                                                    BBN
                                                          February 1990


                     TCP Alternate Checksum Options

Status of This Memo

   This memo suggests a pair of TCP options to allow use of alternate
   data checksum algorithms in the TCP header.  The use of these options
   is experimental, and not recommended for production use.

   Distribution of this memo is unlimited.

Introduction

   Some members of the networking community have expressed interest in
   using checksum-algorithms with different error detection and
   correction properties than the standard TCP checksum.  The option
   described in this memo provides a mechanism to negotiate the use of
   an alternate checksum at connection-establishment time, as well as a
   mechanism to carry additional checksum information for algorithms
   that utilize checksums that are longer than 16 bits.

Definition of the Options

   The TCP Alternate Checksum Request Option may be sent in a SYN
   segment by a TCP to indicate that the TCP is prepared to both
   generate and receive checksums based on an alternate algorithm.
   During communication, the alternate checksum replaces the regular TCP
   checksum in the checksum field of the TCP header.  Should the
   alternate checksum require more than 2 octets to transmit, the
   checksum may either be moved into a TCP Alternate Checksum Data
   Option and the checksum field of the TCP header be sent as 0, or the
   data may be split between the header field and the option.  Alternate
   checksums are computed over the same data as the regular TCP checksum
   (see TCP Alternate Checksum Data Option discussion below).

TCP Alternate Checksum Request Option

   The format of the TCP Alternate Checksum Request Option is:

                 +----------+----------+----------+
                 |  Kind=14 | Length=3 |  chksum  |
                 +----------+----------+----------+



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   Here chksum is a number identifying the type of checksum to be used.

   This option is not copied into each fragment, and appears only in the
   first fragment.

   The currently defined values of chksum are:

                   0  -- TCP checksum
                   1  -- 8-bit  Fletcher's algorithm (see Appendix I)
                   2  -- 16-bit Fletcher's algorithm (see Appendix II)

   Note that the 8-bit Fletcher algorithm gives a 16-bit checksum and
   the 16-bit algorithm gives a 32-bit checksum.

   Alternate checksum negotiation proceeds as follows:

      A SYN segment used to originate a connection may contain the
      Alternate Checksum Request Option, which specifies an alternate
      checksum-calculation algorithm to be used for the connection.  The
      acknowledging SYN-ACK segment may also carry the option.

      If both SYN segments carry the Alternate Checksum Request option,
      and both specify the same algorithm, that algorithm must be used
      for the remainder of the connection.  Otherwise, the standard TCP
      checksum algorithm must be used for the entire connection.  Thus,
      for example, if one TCP specifies type 1 checksums, and the other
      specifies type 2 checksums, then they will use type 0 (the regular
      TCP checksum).  Note that in practice, one TCP will typically be
      responding to the other's SYN, and thus either accepting or
      rejecting the proposed alternate checksum algorithm.

      Any segment with the SYN bit set must always use the standard TCP
      checksum algorithm.  Thus the SYN segment will always be
      understood by the receiving TCP.  The alternate checksum must not
      be used until the first non-SYN segment.  In addition, because RST
      segments may also be received or sent without complete state
      information, any segment with the RST bit set must use the
      standard TCP checksum.

      The option may not be sent in any segment that does not have the
      SYN bit set.

      An implementation of TCP which does not support the option should
      silently ignore it (as RFC 1122 requires).  Ignoring the option
      will force any TCP attempting to use an alternate checksum to use
      the standard TCP checksum algorithm, thus ensuring
      interoperability.




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RFC 1145             TCP Alternate Checksum Options        February 1990


TCP Alternate Checksum Data Option

   The format of the TCP Alternate Checksum Data Option is:

                +---------+---------+---------+     +---------+
                | Kind=15 |Length=N |  data   | ... |  data   |
                +---------+---------+---------+     +---------+

   This field is used only when the alternate checksum that is
   negotiated is longer than 16 bits.  These checksums will not fit in
   the checksum field of the TCP header and thus at least part of them
   must be put in an option.  Whether the checksum is split between the
   checksum field in the TCP header and the option or the entire
   checksum is placed in the option is determined on a checksum by
   checksum basis.

   The length of this option will depend on the choice of alternate
   checksum algorithm for this connection.

   This option is not copied into each fragment, and appears only in the
   first fragment.

   While computing the alternate checksum, the TCP checksum field and
   the data portion TCP Alternate Checksum Data Option are replaced with
   zeros.

   An otherwise acceptable segment carrying this option on a connection
   using a 16-bit checksum algorithm, or carrying this option with an
   inappropriate number of data octets for the chosen alternate checksum
   algorithm is in error and must be discarded; a RST-segment must be
   generated, and the connection aborted.

   Note the requirement above that RST and SYN segments must always use
   the standard TCP checksum.

APPENDIX I:  The 8-bit Fletcher Checksum Algorithm

   The 8-bit Fletcher Checksum Algorithm is calculated over a sequence
   of data octets (call them D[1] through D[N]) by maintaining 2
   unsigned 1's-complement 8-bit accumulators A and B whose contents are
   initially zero, and performing the following loop where i ranges from
   1 to N:

           A := A + D[i]
           B := B + A

   It can be shown that at the end of the loop A will contain the 8-bit
   1's complement sum of all octets in the datagram, and that B will



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   contain (N)D[1] + (N-1)D[2] + ... + D[N].

   The octets covered by this algorithm should be the same as those over
   which the standard TCP checksum calculation is performed, with the
   pseudoheader being D[1] through D[12] and the TCP header beginning at
   D[13].  Note that, for purposes of the checksum computation, the
   checksum field itself must be equal to zero.

   At the end of the loop, the A goes in the first byte of the TCP
   checksum and B goes in the second byte.

   Note that, unlike the OSI version of the Fletcher checksum, this
   checksum does not adjust the check bytes so that the receiver
   checksum is 0.

   There are a number of much faster algorithms for calculating the two
   octets of the 8-bit Fletcher checksum.  For more information see
   [Sklower89], [Nakassis88] and [Fletcher82].  Naturally, any
   computation which computes the same number as would be calculated by
   the loop above may be used to calculate the checksum.  One advantage
   of the Fletcher algorithms over the standard TCP checksum algorithm
   is the ability to detect the transposition of octets/words of any
   size within a datagram.

APPENDIX II:  The 16-bit Fletcher Checksum Algorithm

   The 16-bit Fletcher Checksum algorithm proceeds in precisely the same
   manner as the 8-bit checksum algorithm,, except that A, B and the
   D[i] are 16-bit quantities.  It is necessary (as it is with the
   standard TCP checksum algorithm) to pad a datagram containing an odd
   number of octets with a zero octet.

   Result A should be placed in the TCP header checksum field and Result
   B should appear in an TCP Alternate Checksum Data option.  This
   option must be present in every TCP header. The two bytes reserved
   for B should be set to zero during the calculation of the checksum.

   The checksum field of the TCP header shall contain the contents of A
   at the end of the loop.  The TCP Alternate Checksum Data option must
   be present and contain the contents of B at the end of the loop.

BIBLIOGRAPHY:

   [BrBoPa89]     Braden, R., Borman, D., and C. Partridge, "Computing
                  the Internet Checksum", ACM Computer Communication
                  Review, Vol. 19, No. 2, pp. 86-101, April 1989.
                  [Note that this includes Plummer, W. "IEN-45: TCP
                  Checksum Function Design" (1978) as an appendix.]



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   [Fletcher82]   Fletcher, J., "An Arithmetic Checksum for Serial
                  Transmissions", IEEE Transactions on Communication,
                  Vol. COM-30, No. 1, pp. 247-252, January 1982.

   [Nakassis88]   Nakassis, T., "Fletcher's Error Detection Algorithm:
                  How to implement it efficiently and how to avoid the
                  most common pitfalls", ACM Computer Communication
                  Review, Vol. 18, No. 5, pp. 86-94, October 1988.

   [Sklower89]    Sklower, K., "Improving the Efficiency of the OSI
                  Checksum Calculation", ACM Computer Communication
                  Review, Vol. 19, No. 5, pp. 32-43, October 1989.

Security Considerations

   Security issues are not addressed in this memo.

Authors' Addresses

   Johnny Zweig
   Digital Computer Lab
   University of Illinois (UIUC)
   1304 West Springfield Avenue
   CAMPUS MC 258
   Urbana, IL 61801

   Phone:  (217) 333-7937

   EMail:  zweig@CS.UIUC.EDU


   Craig Partridge
   Bolt Beranek and Newman Inc.
   50 Moulton Street
   Cambridge, MA 02138

   Phone: (617) 873-2459

   EMail: craig@BBN.COM












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