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Date: Wed, 30 Apr 2003 16:05:39 -0700
From: "Crist J. Clark" <crist.clark@attbi.com>
To: "."@babolo.ru
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cc: Garrett Wollman <wollman@lcs.mit.edu>
cc: net@FreeBSD.org
Subject: Re: Reducing ip_id information leakage
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On Thu, May 01, 2003 at 02:23:44AM +0400, "."@babolo.ru wrote:
> > <<On Wed, 30 Apr 2003 16:35:24 -0500 (CDT), Mike Silbersack <silby@silby.com> said:
[snip]

> > The trouble is that we need sequences that are guaranteed not to
> > repeat too fast -- and even then we'll still break on modern networks
> > anyway, as I noted in my comment.
> Why not to use 16 bit of 32 bit pseudorandom generator?

Uhh... I might be missing a joke here, but the problem is that after
you put 65536 packets onto the wire, the next one _must_ have a
repeated IP ID (since there are only 65536 possible). Choosing a
random IP ID only can make this problem worse. As many of the
references perviously discussed or a very simple calculation on your
own will show, with a perfect random generator, after about 300
packets, there is a 50-50 chance the next packet's IP ID will
collide (good ol' "birthday paradox").
-- 
Crist J. Clark                     |     cjclark@alum.mit.edu
                                   |     cjclark@jhu.edu
http://people.freebsd.org/~cjc/    |     cjc@freebsd.org