Cisco's Statement about IPR Claimed in draft-ietf-tcpm-tcpsecure

Thu May 13 17:47:15 UTC 2004

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.