Subject: Re: [tsv-area] Comments on draft-blake-ipv6-flow-label-nonce-01

From: Bob Briscoe <[email-masked]>
Date: Wed, 07 Jan 2009 18:37:50 +0000

Steven,

Another thought on this.

You cite that draft-weaver-dnsext-comprehensive-resolver
   "argues that 40 bits of entropy is desirable"
To be precise, he actually says:
   "We argue that 40b of entropy is more than sufficient."
he also says
   "32b of entropy may in practice currently suffice to mitigate
    most attacks today, as attackers will prefer victims that do not
    implement proper randomization."
Also, there's no real reasoning for any particular number. It's based 
on an arbitrary limit to an attackers arsenal of "a terapacket of 
attacks" and the probability of the attack being "reasonably successful".

I think it would be safer to give the formula he gives and give some 
argumentation why different numbers might be reasonable in different 
circumstances.

It would also be worth saying that this is all about a transport's 
vulnerability to blind single packet attacks. Some attacks need more 
than one packet close together in time, in order to hit. Other 
attacks aren't any use unless the attacker knows when there's been a hit.

Bob

____________________________________________________________________________
Bob Briscoe,                           Networks Research Centre, BT Research