[70398] in North American Network Operators' Group
Re: Cisco's Statement about IPR Claimed in draft-ietf-tcpm-tcpsecure
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Thu May 13 13:48:24 2004
In-Reply-To: <Pine.NEB.4.58.0405131259330.29046@server.duh.org>
Cc: North American Noise and Off-topic Gripes <nanog@merit.edu>
From: Iljitsch van Beijnum <iljitsch@muada.com>
Date: Thu, 13 May 2004 19:47:15 +0200
To: Todd Vierling <tv@duh.org>
Errors-To: owner-nanog-outgoing@merit.edu
On 13-mei-04, at 19:07, Todd Vierling wrote:
> Whereas the Internet-Draft claims, by assuming that both source and
> dest
> ports are knowns, the number of bits required for the attack is 16 (or
> even
> lower) and thus can cause connection resets "even at DSL speed."
Guess what, they call them drafts because they're not finished yet. So
why don't you say something to the author?
> A 2^[28..33] problem is much more difficult to attack than a 2^[14..16]
> problem. It's amazing that such a cheap source of entropy --
> randomizing
> the source port appropriately -- is being so readily discounted.
> (In case you're curious, 2^33 is achievable for things like BGP, where
> it's
> not certain which end initiated the connection. You get one extra bit
> for
> the originator choice, on top of a fully randomized 16-bit port and a
> 16-bit
> window size: 2^33.)
I don't think you can fully randomize the source port as it might clash
with well-known ports. Also, it may be somewhat expensive to make ports
truly random. (But not as expensive as doing MD5 for the whole
session.)
But why are you assuming the window size is 64k? This is completely
unnecessary, and not done in practice by "real" routers: those
typically use a 16k window. It should even be possible to set the
window to a very small size, such as 64 bytes. That's enough to receive
the initial BGP header, after which the window can be set to a larger
size until the session is idle again.
