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

below...

El 16/11/2004, a las 19:12, Mohan Parthasarathy escribió:

Marcelo,

Comments inline..


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)

I thought we have 3 different types of addresses now. The procedure to
compute the hash is different depending on HBA or CGA i.e include the
public key or extended modifier depending on CGA or HBA. So, why does
it matter if we can specify something different for HBA address ? Both ends
know they are operating on an HBA address. So, what is the compatibility
issue ? Could you elaborate on that ?


Well, the HBA is defined always (in the 3 cases) as an extension of the CGA.
i agree with you that we could define the HBA as an extension of the CGA when the CGA is also used (i.e. the HBA/CGA hybrid) and define the HBA in a different way when the CGA are not used (HBA only mode)
But i am not sure this is worthy, since i guess it would be more complex since you need to handle multiple generation algorithms, depending in the case. suppose the case when a a communication has a CGA/HBA and a HBA only in the other side, this would probably would be more complex either. I mean, having a different generation for the generation of the HBA only case would essentially provide that all the iids are the same in an HBA set, but with an additional complexity in the implementation, do you think that it is worthy?



 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


Agreed.

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

Okay.



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)

Yes, that's what i meant. I assumed that whatever is not there in this
draft is present in one of the other 5 drafts :-)

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 mean, you can generate the address and include it..but it is not needed as
you mention below..

 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

Agreed. I was thinking in a slightly different direction which may not
be needed i guess. Somebody in one of the multi6 meeting raised a point about
computing the hash and verifying it as expensive. First, the hash is not computed
always i guess. Only after the rehoming occurs. But once the rehoming occurs, you
have to recompute always as long as the locator does not match the ULID. Is that right ?
Then i was wondering whether it is a real problem because it affects the fast path processing
of the packet. Perhaps not because it can be pre-computed.

But it may not be necessary
i guess. The first packet after rehoming will trigger the hash computation, but that can
be cached i guess. Hence the subsequent packets can be compared directly
with the cached value. Does that make sense ?


Yes,
I mean, i would do the following.
first the communication is set up, and at some moment in time the CGA parameters are exchanged. For now, no hash calculation.
Suddenly, an different locator needs to be used. So at this point, i would run the verification process, implying a hash calculation.
then as long this is the locator that is used, i don't think that additional verification are needed, you just need to remember that this locator is OK
So, i would say that for a HBA set with n prefixes, the maximum number of hash verifications needed is n (independently of the number of packets exchanged with each locator)


  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

That should be  sufficient.


   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.

I am not sure i understood the above sentence.

well as it is stated it is impossible to understand (i am sorry for that)

What i wanted to say was:

HBAs are mainly used to prevent hijacking attacks.
We will need reachability tests to prevent flooding
attacks.


However, an additional consideration about this...
Note that if HBAs are used, flooding a specific target host is very difficult, since i would need to find an HBA set that contains an address with the attacker prefix and that the address of the victim is included in the HBA set, agree?

this is very difficult (O^(59+(16*Sec)) difficult

However, the flooding attack can be easily directed to the access link or access router of the victim, since the attacker only need to include the prefix of the victim's network in the prefix set.

This means that the resulting HBA set won't contain any address of a real host within the victim's network, but in any case the flood will be routed to the access router of the victim's network flooding the access link and the access router.

So even if the flooding attack is slightly different, i guess that it would still make sense to provide some protection from it, agree?

regards, marcelo


 First you say that HBA can
be used to prevent flooding attacks and then you say that we need
reachaility tests to prevent flooding attacks.

-mohan

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


------------------------------------------
Please note that my former email address
mbagnulo@ing.uc3m.es is no longer in use
Please send mail to:
marcelo at it dot uc3m dot es
------------------------------------------



------------------------------------------
Please note that my former email address
mbagnulo@ing.uc3m.es is no longer in use
Please send mail to:
marcelo at it dot uc3m dot es
------------------------------------------