RFC1146

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:

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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.

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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

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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,

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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:  [email protected]

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

  Phone: (617) 873-2459

  EMail: [email protected]

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