Re: [dtn-security] Including fragment offset in the correlator doesn't prevent all fragment collisions.

[Elwyn Davies <[e-mail]>]

Hi, Amy.

I grok the problem properly now (I hope).

Does pushing a metadata block into the bundle with the EID (or if too
long) a short nonce or abbreviated digest of some part of the bundle
(e.g. some part of the PIB) into all the fragments at the node where it
is fragmented help?

Not a very satisfactory solution but it would help tie the two parts of
the PIB together.

/Elwyn


On Wed, 2013-03-20 at 19:36 -0400, Amy Alford wrote:
> A 100 byte bundle is created.  Different nodes independently fragment
> it into 50 byte fragments.  A subsequent node adds PIB and PCB (so we
> have correlated PCB blocks) to each fragment with a security
> destination of the bundle destination.   The correlators can collide.
> (In DTN2, they will collide.)     
> 
> 
> If the fragments are reassembled before the destination, the
> destination has no way to distinguish the PCB blocks that should
> correlate with each other.
> 
> 
> Alternatively, if we assume reassembly only happens at the destination
> (which 5050 doesn't guarantee), we still have a problem if we use a
> two block PIB ciphersuite where one block comes before the payload and
> one comes after.  
> 
> 
> A node creates a 100 byte bundle.  It's independently fragmented into
> 50 byte bundles.  A subsequent node adds a PIB ciphersuite with a
> correlated block trailing the payload.  Again, the correlators
> collide.  Now, each 50 byte fragment is fragmented again.  So, we have
> fragments:
> 
> 
> (I'm numbering the PIB instances so that we can distinguish them in
> the discussion.  The numbers don't correspond to anything identifiable
> in the blocks themselves.)
> 
> 
> PIB 1 with correlator x -> 0-25
> 25-50 -> PIB 1 with correlator x
> PIB 2 with correlator y -> 50-75
> 75-100 -> PIB 2 with correlator y
> 
> 
> PIB 3 with correlator x -> 0-25
> 25-50 -> PIB 3 with correlator x
> PIB 4 with correlator y -> 50-75
> 75-100 -> PIB 4 with correlator y
> 
> 
> The receiving node has no way to realize that that pre-payload PIB 1
> with correlator x doesn't match up to the post-payload PIB 3 with
> correlator x.
> 
> 
> - Amy
> 
> On Tue, Mar 19, 2013 at 9:11 PM, Peter Lovell <[e-mail]> wrote:
>         Amy Alford <[e-mail]> wrote:
>         
>         >A bundle can be fragmented multiple times independently, so a
>         node may
>         >receive multiple fragments with the same offset and length
>         that have
>         >traveled different paths (and accumulated different BSP
>         blocks along the
>         >way).  Collisions in the correlator values once the bundle is
>         reassembled
>         >are inevitable.
>         >- Amy
>         
>         
>         Hi Amy,
>         
>         my thought is that we have covered the problem of
>         multiple-fragmentation and multi-path, but perhaps not.
>         
>         Can you describe a bundle scenario that exemplifies the issue
>         you see, so we can think about it.
>         
>         Thanks.....Peter
>         
> 
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