RFC 1145 (rfc1145) - Page 2 of 5

TCP alternate checksum options

RFC 1145             TCP Alternate Checksum Options        February 1990


   Here chksum is a number identifying the type of checksum to be used.

   This option is not copied into each fragment, and appears only in the
   first fragment.

   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.




Zweig & Partridge