Re: [dtn-interest] What's the actual effect of using custody transfer?

["Ivancic, William D. (GRC-RHN0)" <[e-mail]>]

If you read the CCSDS CFDP Blue Book, I think you get a feel for Custody Transfer.  I suspect this may be the original operational concept.
http://public.ccsds.org/publications/archive/720x1g3.pdf

Or better yet
http://public.ccsds.org/publications/archive/727x0b4.pdf
See section 6.7 Store and Forward Overlay Operations


One of the items missing from DTN Bundling is what is expected of a Bundle Forwarding Agent.  So, to the best of my knowledge, there is nothing that states that a node will do its best to forward a bundle whether or not custody transfer is requested.  Furthermore, it is probably deployment and policy specific as to whether or not a source node clears its buffer once the bundle is forwarded.  I seem to recall that DTN2 cleared the bundle as soon as it know the forwarding node received the bundle regardless of custody transfer settings.

Remember, taking custody does not guarantee delivery.  It is still a leap of faith without end-to-end confirmation.

Also, I doubt you would want to do custody transfer if you are flooding.

Personally, I suspect custody transfer does little more than give you a warm fuzzy feeling.  Either the forwarding node is going to do it's best to forward the bundle or not.  Also, I do not believe there is anything documented that indicates the a custody bundle has higher priority than a non-custody bundle.

This paper shows some tests related to custody vs non-custody transfer.
http://arc.aiaa.org/doi/pdf/10.2514/6.2012-15228


Will



From: Martin Galvan <[e-mail]<mailto:[e-mail]>>
Date: Wednesday, May 14, 2014 11:20 AM
To: "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Subject: [dtn-interest] What's the actual effect of using custody transfer?

Hi everybody! I'm reading a bit on DTN and I have a doubt about what custody transfer actually does. From what I've read, a node that accepts custody of a bundle promises it won't delete the bundle until it can forward the bundle to either its destination or another node that accepts its custody. However, I don't see what would be the difference between doing that and not using custody transfer. Isn't the whole purpose of a store-and-forward mechanism to do just that?

For example, let's assume we have three nodes, A, B and C. There's no end-to-end path from A to C, and A is directly connected to B.

A----->B     C

If I understood custody transfer right, if A wants to send C a bundle using custody transfer, it will request B to accept custody of the bundle; if B accepts it will store the bundle until a link to C can be established. Once that happens, B will then forward the bundle to C and delete it once it's reached its destination.

What would happen if A didn't want to use custody transfer?

Thanks a lot!