RE: TCP Checksum Interoperability

[Michael Smith <[e-mail]>]

Lloyd: your comment is interesting.

I know there hasn't been much activity on tcp-impl, but it's still there.
Allison sent an email a while back encouraging it's use, but I can't find
that just now.

To subscribe (or unsubscribe) to the tcp-impl mailing list, send mail to
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There's also http://groups.yahoo.com/group/tcp-impl/ but activity there
stopped about July 2001. I never understood how the two mailing lists
related, but I posted your mail (below) to both, I hope you don't mind.

Aloha

Mike Smith
CTO, iReady

-----Original Message-----
From: Lloyd Wood [mailto:[e-mail]]
Sent: Friday, April 05, 2002 4:43 AM
To: Chris Trobridge
Cc: [e-mail]
Subject: Re: TCP Checksum Interoperability


See sections 3-5 of RFC1624 for discussion of the one's complement
problem for the IP header checksum. I presume similar applies for TCP,
although I don't know offhand if it's written down anywhere.

L.


On Fri, 5 Apr 2002, Chris Trobridge wrote:

> 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, which has two 1-s
> complement representations (0000 and ffff).
>
> We don't have source to either stack but I managed to deduce the problem
> from looking at a packet trace.  The receiver drops all datagrams
containing
> a TCP with a TCP checksum of ffff.
>
> The receiver appears to be following the checksum procedure in the RFC
> literally - ie zero checksum, recalculate and check that the result is the
> complement of what the sender sent.  The problem is that the sender and
> receiver don't agree about zero and hence the datagram is dropped
silently.
>
> My view is that the receiver is in error; it should be checking the
special
> case of 0.
>
> All the receiver code I have seen doesn't work this way.  The normal
> approach is to calculate the1-s complement sum with checksum in place and
> check that this is zero.  The methods I konw always return just one
> representation for zero, hence there is no special case.
>
> Any thoughts?
>
> Thanks,
> Chris
>


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