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+Network Working Group                                          J. Zweig
+Request for Comments: 1146                                         UIUC
+Obsoletes: RFC 1145                                        C. Partridge
+                                                                    BBN
+                                                             March 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.
+
+   Note:  This RFC corrects errors introduced in the editing process in
+   RFC 1145.
+
+   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:
+
+
+
+
+Zweig & Partridge                                               [Page 1]
+
+RFC 1146             TCP Alternate Checksum Options           March 1990
+
+
+                 +----------+----------+----------+
+                 |  Kind=14 | Length=3 |  chksum  |
+                 +----------+----------+----------+
+
+   Here chksum is a number identifying the type of checksum to be used.
+
+   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.
+
+
+
+Zweig & Partridge                                               [Page 2]
+
+RFC 1146             TCP Alternate Checksum Options           March 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.
+
+   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
+   contain (N)D[1] + (N-1)D[2] + ... + D[N].
+
+   The octets covered by this algorithm should be the same as those over
+
+
+
+Zweig & Partridge                                               [Page 3]
+
+RFC 1146             TCP Alternate Checksum Options           March 1990
+
+
+   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.]
+
+   [Fletcher82]   Fletcher, J., "An Arithmetic Checksum for Serial
+                  Transmissions", IEEE Transactions on Communication,
+
+
+
+Zweig & Partridge                                               [Page 4]
+
+RFC 1146             TCP Alternate Checksum Options           March 1990
+
+
+                  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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+Zweig & Partridge                                               [Page 5]
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