Telechat Review of draft-ietf-6man-predictable-fragment-id-10
review-ietf-6man-predictable-fragment-id-10-secdir-telechat-wierenga-2015-10-22-00

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review-ietf-6man-predictable-fragment-id-10-secdir-telechat-wierenga-2015-10-22

Hi,

I am happy with the updated draft.

Klaas

--
Klaas Wierenga
Identity Architect
Cisco Cloud Services






> On 01 Oct 2015, at 23:51, Fernando Gont <fgont at si6networks.com> wrote:
> 
> 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 at si6networks.com
> PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492
> 
> 
> 
>