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Network Working Group                                          J. ZweigRequest for Comments: 1146                                         UIUCObsoletes: RFC1145                                        C. Partridge                                                                    BBN                                                             March 1990TCP Alternate Checksum OptionsStatus 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 inRFC 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 (seeAppendix I)                   2  -- 16-bit Fletcher's algorithm (seeAppendix 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 (asRFC 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 1990TCP 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 overZweig & 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.COMZweig & Partridge                                               [Page 5]