RE: TCP Checksum Interoperability

[Michael Smith <[e-mail]>]

I'm cross-posting this thread to [e-mail] for archival
purposes, we'll see if any further replies to the ietf list get posted at
[e-mail] too (for instructions to join tcp-impl, see below). My
rationale here is just to try and keep tcp-impl as a good source of material
on tcp/ip implementation issues. It seems you can't post to
'[e-mail]' (see also below for discussion on this group).

Sorry, Chris, forgot to copy you on this.

Aloha

Mike Smith
CTO, iReady

To: [e-mail] 
Subject: Re: TCP Checksum Interoperability 
From: Rob Austein <[e-mail]> 
Date: Fri, 05 Apr 2002 14:29:44 -0500 
In-Reply-To: <[e-mail]> 
References: <[e-mail]> 
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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, but one is
(apparently) more surprising to most implementors than the other, due
to the implementation techniques that suggest themselves on most
modern processors.



-----Original Message-----
From: Michael Smith 
Sent: Friday, April 05, 2002 10:39 AM
To: '[e-mail]'
Cc: Michael Smith
Subject: RE: TCP Checksum Interoperability


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
[e-mail] with one of the following in the body of the message.


subscribe tcp-impl 

unsubscribe tcp-impl 

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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