Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!GATEWAY.MITRE.ORG!hal
From: hal@GATEWAY.MITRE.ORG (Hal Feinstein)
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  Sequence numbers provide security?? (Bellovin's article)
Message-ID: <8905091426.AA17619@gateway.mitre.org>
Date: 9 May 89 14:26:19 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 12

Although your point on "proper" authentication is well taken, you must be
aware that many networks use a variety of ad hoc mechanisms to
provide security.  The Morris TCP number attack described by Bellovin 
is real  and takes advantage a near deterministic nature of TCP serial
numbers.  Sequence numbers in TCP is part of its reliable mechanism 
but in addition provide a poor man's data origin authentication. 

The point is that you can't trust (what ever that means) the TCP 
sequence numbers to resist an attack; however, in low threat environments 
it can be OK.  In a high threat environment with an attacker willing to do 
the work it takes to subvert TCP sequence numbers, then it's not appropriate 
and you'd want something cryptographically protected.