Re: Pseudorandom Flow Labels

[Shane Amante <[e-mail]>]

Fernando, Thomas,

On Apr 6, 2011, at 12:22 MDT, Fernando Gont wrote:
> Thomas,
> 
> On 05/04/2011 05:36 p.m., Thomas Narten wrote:
>> Case in point about how we are being *extremely* loose in using the
>> term "pseudo random".
> [....]
>> Part of my objection to the term "pseudo random" is that the term has
>> not been defined within the context of the Flow Label.
> 
> You raise a very good point, indeed. For instance, when we talk about
> e.g. "port randomization", we're really talking about "producing port
> numbers that are unpredictable by off-path attackers".
> 
> To make this terminology issue worse, it has been argued a few times (by
> some mathematician IETFers) that the properties that we need for the
> "hash" functions in the hash-based algorithms are really that of PRFs
> (Pseudo Random Functions) (i.e., hash functions being a specific example).
> 
> In summary, I agree with the terminology issue that you've raised. I'd
> probably argue that the best way to go is to specify which properties we
> want for Flow Labels, such as they have been specified for port numbers
> in RFC 6056. Namely:
> 
> * We want Flow Labels that unpredictable by off-path attackers (history
> has taught us that this is a good proactive measure)
> * We want an algorithm for generating FL that produces FLs that do not
> repeat with a high frequency (i.e., they are distributed normally)

I like your (attempt at) a more precise definition that Thomas has been asking for.

I would think another desirable property of (host-generated?) flow-labels might be that, by default, they strive to preserve privacy of the transmitter.  IOW, flow-labels cannot be used to track individuals (over time), because they are traceable back to a particular implementation or, worse, a specific device.


> One possible algorithm for achieving these properties is calling a
> random()-like function. But there are others, such as the hash-based
> algorithms specified in draft-gont-6man-flowlabel-security.

Right.

-shane


> Thanks,
> -- 
> Fernando Gont
> e-mail: [e-mail] || [e-mail]
> PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1
> 
> 
> 
>