Re: [dtn-security] Including fragment offset in the correlator doesn't prevent all fragment collisions.
Amy Alford <aloomis@sarn.org> Thu, 21 March 2013 01:12 UTC
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Date: Wed, 20 Mar 2013 21:11:51 -0400
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From: Amy Alford <aloomis@sarn.org>
To: Elwyn Davies <elwynd@folly.org.uk>
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Subject: Re: [dtn-security] Including fragment offset in the correlator doesn't prevent all fragment collisions.
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A metadata block that tied the fragments together (and was unique to that fragmentation event: perhaps EID and time) would help, as long as reassembly is handled by the destination (so that it can take advantage of this info). - Amy On Wed, Mar 20, 2013 at 8:56 PM, Elwyn Davies <elwynd@folly.org.uk> wrote: > 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 <plovell@mac.com> wrote: > > Amy Alford <aloomis@sarn.org> 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 > > > > > > _______________________________________________ > > dtn-security mailing list > > dtn-security@irtf.org > > https://www.irtf.org/mailman/listinfo/dtn-security > >
- Re: [dtn-security] Including fragment offset in t… Stephen Farrell
- Re: [dtn-security] Including fragment offset in t… Amy Alford
- [dtn-security] Including fragment offset in the c… Amy Alford
- Re: [dtn-security] Including fragment offset in t… Peter Lovell
- Re: [dtn-security] Including fragment offset in t… Ivancic, William D. (GRC-RHN0)
- Re: [dtn-security] Including fragment offset in t… Elwyn Davies
- Re: [dtn-security] Including fragment offset in t… Amy Alford
- Re: [dtn-security] Including fragment offset in t… Ivancic, William D. (GRC-RHN0)
- Re: [dtn-security] Including fragment offset in t… Amy Alford
- Re: [dtn-security] Including fragment offset in t… Amy Alford
- [dtn-security] Re(2): Including fragment offset i… Peter Lovell
- [dtn-security] Re(4): Including fragment offset i… Peter Lovell
- Re: [dtn-security] Re(4): Including fragment offs… Amy Alford
- Re: [dtn-security] Re(4): Including fragment offs… Elwyn Davies
- Re: [dtn-security] Re(4): Including fragment offs… Amy Alford
- Re: [dtn-security] Re(4): Including fragment offs… Burleigh, Scott C (313B)
- Re: [dtn-security] Re(4): Including fragment offs… Amy Alford
- Re: [dtn-security] Re(4): Including fragment offs… Burleigh, Scott C (313B)
- Re: [dtn-security] Re(4): Including fragment offs… l.wood
- Re: [dtn-security] Re(4): Including fragment offs… Amy Alford
- Re: [dtn-security] Re(4): Including fragment offs… Burleigh, Scott C (313B)
- Re: [dtn-security] Re(4): Including fragment offs… Burleigh, Scott C (313B)
- Re: [dtn-security] Re(4): Including fragment offs… Amy Alford
- Re: [dtn-security] Re(4): Including fragment offs… Burleigh, Scott C (313B)