FYI, The 'offending' stacks are PC/TCP (reception) and in Open VMS
(transmission).
This is an age-old issue; the following dates from 1985(!):
http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/8502.mm.www/0389.html
I think RFC 1624 is missing the UDP case, where +0 means not calculated.
I writing to this
list because the TCP workgroup was shutdown a while ago.
We have a
compatibility problem between two third party implementations of the TCP
checksum.
The problem concerns
the representation of zero, whichhas two 1-s complement representations
( and ).
We
