Network Working Group J. Zweig
Request for Comments: 1146 UIUC
Obsoletes: RFC 1145 C. Partridge
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:
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
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: