RFC 1146 TCP alternate checksum options

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:

 [] 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.]

 [] 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.

 [] 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.

 [] 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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RFC 1146: TCP alternate checksum options