[e2e] Forbes.com story highlights how NATs destroy end-to-end reliability

David P. Reed dpreed at reed.com
Wed Jul 11 08:21:10 PDT 2001

Thought that many on this list don't get Forbes, so I'd forward a link to a 
Stephen Manes column in the latest issue about 3Com's NAT software and how 
it changes data at whim and corrects the checksum to spoof the recipient.

Fortunately, the mission critical backup application that discovered the 
problem was programmed according to the end-to-end principle:  it didn't 
depend on the TCP checksum for reliability.

While Manes calls it a "programming error", it appears more likely that it 
was a design feature - the NAT software scanned the entire TCP segment for 
addresses that matched the local address that must be translated.  This 
kind of solution has been used in some NAT software I've seen on machines 
that try to be "automagical" thinking that the byte sequence is 
pretty unlikely to appear in most data packets, so it should be corrected 
everywhere.  3Com probably bought the rights to a package like that one 
(anyone on this list know the truth?)

This is a good argument for adversary-proof checksums (like one-way signed 
message digests) I suggested in a recent exchange - clearly there are 
devices that behave like adversaries out there in the real world today, 
designed by real programmers to change bits in a way that is not 
statistically independent of the data.

It is also becoming clear that patching the symptoms of a bad design choice 
(NAT in this case) is going to be never-ending, and it's time to obviate 
the need to perpetuate such kludges.  I realize that this (beginning with 
IPv6, end-to-end encryption, etc.) is a big job and the 
Cisco/3Com/Microsoft axis don't seem to have the guts to do it.  But it is 


The Four-Byte Shuffle
Stephen Manes, Forbes Magazine, 07.23.01, 12:00 AM ET

Digital bits are supposed to be sacrosanct. An error in just one among 
billions can create unknowable consequences, from innocuous to disastrous. 
I know this firsthand. Four little bytes of data corruption--just enough to 
spell a choice expletive--recently wasted many hours of my life.


- David
WWW Page: http://www.reed.com/dpr.html

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