Re: Comments on draft-bagnulo-multi6dt-hba-00.txt

Hi Mohan,

thanks for the review
replies inline....

El 15/11/2004, a las 0:22, Mohan Parthasarathy escribió:

> I had a few comments/questions on this draft. Sorry if some of them 
> have already been
> asked (i did not look closely at the archives).
>
> 1) In section 1,
>
>    In addition, it should also be noted that it
>    is not required that all interface identifiers of the addresses of 
> an
>    HBA set are equal, preserving some degree of privacy through changes
>    in the addresses used during the communications.
>
> To create all addresses (for various prefixes) with same interface 
> identifiers, what is the
> change that would be needed ? Does including the multi-prefix 
> extension and removing the subnet-prefix
> in the second hash calculation (Hash1 in CGA draft) sufficient ?

Yes, but this imply not to be compliant with CGA spec.
in other words, if we are compliant with CGAs, the iid will be 
different and it wouldn't be possible to have equal iids

we felt that cga compatibility was more important than supporting equal 
iids in the HBAs of a given set (note that we do see some value in 
having equal iids, for instance for simplified management, but this 
cannot be provided in the CGA format)

>  It might be worth mentioning the procedure.
>
> 2) If HBA is used and the node wants RFC 3041 privacy addresses, it 
> means that it needs to
> periodically regnerate the HBA ?

i think so

>  Though this is more expensive than RFC 3041 privacy addresses,

I guess that it depends in the Sec value...
If Sec =0 then i guess than generating an HBA is similar to generate a 
random number. I mean, you only need to add 1 to the modifier and 
perform a hash operation (probably generating random addresses requires 
a similar effort)
If Sec>0 then i agree

> there is not much we can do here i guess. Perhaps, this should be 
> mentioned somewhere.
>

Ok, i will mention this in the privacy consideration, thanks

> 3) In section 2,
>
>       First, the current Secure Neighbor Discovery specification [2] 
> uses
>       the CGAs defined in [1] to prove address ownership.  If HBAs are 
> not
>       compatible with CGAs, then nodes using HBAs for multihoming 
> wouldn't
>       be able to do Secure Neighbor Discovery using the same addresses 
> (at
>       least the parts of SeND that require CGAs).  This would imply 
> that
>       nodes would have to choose between security (from SeND) and
>       reliability (from multi6)
>
> What do you mean by reliability here ?

fault tolerance i.e. the fault tolerance and other features provided by 
the  multi6 protocol based on the usage of HBAs

i will try to reword this in the draft



>
> 4)When the multi6 state is setup, i assume that the CGA parameter is 
> exchanged between the
>   communicating parties.

if you mean the CGA parameter data structure including the multiprefix 
extension, then yes
(BTW i need to state this explicitly in the draft)

> I was wondering whether it would make any difference between including
>   the prefix Vs the whole address itself in the CGA parameter data 
> structrure ?

i am not following you here.
The CGA parameter data structure is used to generate the iid of the 
addresses, so how can you include the complete address in it if you 
still haven't generate it yet.... i am probably missing your point here

>  I am not able
>   to see anything less secure by including the whole address itself. 
> One possibility is that
>   the peer can compute and validate the address ahead of time (which 
> is of no use if rehoming
>   will never occur) and hash verification becomes a simple comparison
>   when a packet with different locator comes in.

but this can be done with the current CGA parameter data structure, 
right?
i mean, the HBA set generation only uses the info contained in the CGA 
parameter data structure, so once the peer has the CGA parameter data 
structure, it can regenerate the whole HBA set before the rehoming, as 
you suggest.

i mean, i think you can do this that you suggest already

>   I am not saying we should do it this way.
>   I was mainly trying to understand what it means  to include the 
> address instead of prefix.
>
>
> 5) Security considerations need to be improved.

agree

>  It would be good to discuss what attacks it
>    addresses with respect to the threats document.

agree, but note that HBAs are not a complete multihoming solution, and 
are useful to prevent certain attacks, not all the threats described in 
the threats drafts

> In the beginning of the document,
>
>       This memo describes a tool that can be used to provide 
> protection against some
>       of the potential attacks, in particular against future/ 
> premeditated
>       attacks (a.k.a.  time shifting attacks in [6]).
>
>    I hope the future version of the document would explain more on 
> this. Is there a document
>    that explains what threat needs to be addressed by multi6 solution 
> ? (the threats document
>    mentions this in a few places, but does not look like requirements 
> to me).

well, i guess that the arch document that we are suppose to write next 
should at least specify the security goals that the designed solution 
is trying to achieve

>
>    One of the inferences from the security consideration section is 
> that the attacker cannot create an HBA
>    set given a set of victim's addresses. It was not very obvious by 
> reading the security consideration section.
>

agree, i will include an example of usage and possible attempts of 
attacks, and see if this improves this point

>    Also, how important is return routability for the locators  before 
> they are used ? It is not discussed here.
>    As it is hard for the attacker to redirect packets to a victim, is 
> it needed ? Can it be optional ?

very good point
HBAs are mainly used to prevent hijacking attacks and to prevent 
flooding attacks. We will need reachability tests to prevent flooding 
attacks. I will include this also in the doc.

Thank you very much for your constructive comments
I hope i can address them in a satisfactory manner in the next version. 
Please let me know if i wasn't successful

regards, marcelo

>
> -mohan
>
>
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