Re: [secdir] review of draft-ietf-6man-predictable-fragment-id-09

Fernando Gont <fgont@si6networks.com> Thu, 01 October 2015 22:36 UTC

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To: "Klaas Wierenga (kwiereng)" <kwiereng@cisco.com>
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From: Fernando Gont <fgont@si6networks.com>
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Subject: Re: [secdir] review of draft-ietf-6man-predictable-fragment-id-09
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Hi, Klaas,

On 09/09/2015 07:02 AM, Klaas Wierenga (kwiereng) wrote:
>> On 09/09/2015 07:12 AM, Klaas Wierenga (kwiereng) wrote:
>>> I believe the document has some issues, see below.
>>> 
>>> The document does an analysis of the security implications of 
>>> predictable identification fields and I believe (not being an
>>> IPv6 expert) that it does a good job at that. The analysis of
>>> the potential exploits is convincing. Where I am struggling a bit
>>> is the algorithms for selecting fragment identification values
>>> (5).
>>> 
>>> The intro text states that there are ‘a number of algorithms',
>>> but really there are only 3: 1- per destination counter random 
>>> initialised 2- random value 3- hash over source, destination,
>>> secret with a counter
>> 
>> FWIW, these are three concrete algorithms, but that doesn't mean
>> they are the only possible ones…
> 
> Sure, I understand that. It is just when I read it I was preparing
> for a long list to come, so I think it would be good to state
> something like:
> 
> OLD
> 
> This section specifies a number of algorithms that may be used for 
> selecting Fragment Identification values.
> 
> NEW
> 
> There are a number of algorithms that may be used for selecting
> Fragment Identification values. This section presents three of
> those.

Will do. (thanks for proposing a way forward, btw).



>>> 1 and 3 are essentially the same, the hash function in 3 performs
>>> the same function as the pseudo random generated initial value in
>>> 1 if I am not mistaken.
>> 
>> Yes and no. 1 requires state, but 3 doesn't. That means that, e.g.,
>> if you have lot's of flows to many different destinations, you may
>> need to remove some entries from the Dest Cache (and then you run
>> the risk of Frag ID collisions). However, this is not the case with
>> algorithm #3.
> 
> good point, so is there any compelling reason to select 1 over 3?

It is generally the other way around: for #1, if you need to remove the
state from the Destinations Cache, you run the risk of colliding Frag
IDs. So you could say that #1 is more trivial to implement, whereas #3
has better properties (when there are ongoing communications with
multiple destinations that make you hit the limit of entries in the
Destination Cache).



>>> So really the choice is between a random value for every datagram
>>> or a random value at initialisation of a connection and
>>> increasing by 1 for every subsequent datagram.
>>> 
>>> I’d really like to see some quantitative analysis as to the
>>> impact of a random value per packet as well as between 1 and 3.
>> 
>> Impact in terms of what?
> 
> Well, as an implementer I want to choose between one of the
> algorithms you propose. But since I have no clue what the penalty is
> for doing per packet randomisation as opposed to per flow that is
> hard.

Wel, the thing is that, to a large extent, it depends on the details of
implementation. e.g., what's the algorithm you use for the randon()
function, etc.



> If the cost of a pseudorandom operation is outweighed by other
> factors involved in sending a packet I would probably choose option
> 2. My gut feeling says however that it is a pretty expensive
> operation to do on a per packet basis, so I would expect the advise
> to be “use 1 or 3” unless…..

We tried to provide options rather than pushing one specific algorithm.


> And similarly, what is the cost of the
> hash versus the prg? If they are comparable would option 3 not be
> better?

It depends on which hash function vs PRG. For instance, you could employ
a hash function for the PRG.

So any assertion on performace would really be questionable...

Thoughts?

Thanks!

Cheers,
-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
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