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". Indeed, RFC 768 asserts that the two values (+0 and -0) are equivalent. Part of the problem is that there isn't an RFC standard that deals with the issue of a zero checksum, apart from for UDP. The protocol RFCs don't specify the details and the implementation RFCs (1071, 1141, 1624) state explicitly that they are not standards, "It is not a standard, but a set of useful implementation techniques". They are a very good idea but not mandatory. That's why I said I thought the receiver wasn't compliant: it rejects -0 as invalid. Chris > -----Original Message----- > From: Lloyd Wood [mailto:l.wood@eim.surrey.ac.uk] > Sent: 05 April 2002 21:45 > To: Michael Smith > Cc: 'CTrobridge@baltimore.com'; 'sra+ietf@hactrn.net'; > 'ietf@IETF.ORG'; > 'tcp-impl@grc.nasa.gov' > Subject: RE: TCP Checksum Interoperability > > > On Fri, 5 Apr 2002, Michael Smith wrote: > > > The last time this came up for a TCP implementation I used to > > maintain, our interpretation of Robustness Principle applied to this > > problem dictated that we shouldn't send segments with > checksum fields > > set to all ones (that is, we shouldn't send ~(+0)), but > that we had to > > accept either ~(+0) or ~(-0) in received segments. > > > > Strictly speaking, either zero state is completely legal > > Not so. Please read RFC1624 sections 3-5. > > L. > > <L.Wood@surrey.ac.uk>PGP<http://www.ee.surrey.ac.uk/Personal/L.Wood/> > > ----------------------------------------------------------------------------------------------------------------- The information contained in this message is confidential and is intended for the addressee(s) only. If you have received this message in error or there are any problems please notify the originator immediately. The unauthorized use, disclosure, copying or alteration of this message is strictly forbidden. Baltimore Technologies plc will not be liable for direct, special, indirect or consequential damages arising from alteration of the contents of this message by a third party or as a result of any virus being passed on. This footnote confirms that this email message has been swept for Content Security threats, including computer viruses. http://www.baltimore.com