Network Working Group J. Zweig
Request for Comments: 1146 UIUC
Obsoletes: RFC 1145 C. Partridge
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
Distribution of this memo is unlimited.
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 |
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
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
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
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 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 + (N-1)D + ... + 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 through D and the TCP header beginning at
D. 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.
[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,
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 issues are not addressed in this memo.
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