Re: [dtn] custody transfer I-D

[Carlo Caini <[e-mail]>]

Dear Scott,
   I can see your point. If you do not know who will be the next custodian you cannot set the retramssion timers accurately. However, I would say that this is a small price to pay for having a much more powerful and flexible (optional!) feature. The idea of sending something with the request that following nodes takes custody, if for them is possible and if they are willing (no mandatory!), withouth knowing which they are, which is perfectly possible in all DTN networks is in my opinion of great value. I fully agree with Marius on this.
There is the probelm of timer settings, true. But nodes that takes custody have taken the responsability to keep the bundle until either a new custodian takes custody or lifetime expires. In other words, if custodians have declared themselves ready to keep bundles in custody until lifetime expires they should be able and willing to do this! 
That said, it is not difficult to imagine some heursitic rule to set retransmission timers in the absence of any hints (the worst case). For example, as 3 is often used as a magic number (TCP), a custodian could simply attempt 3 retranmissions at interval equals to the time left to lifetime expiring divided by 4. You can select a minimum waiting time to avoid multiple reTx when a bundle is close to expire. Better rules than my example can be deivsed. Anyway 3 is better than no retranmissions at all and you do not flood the network if the minimum waiting time is selected.
Concerning end-to-end retransmissions, cited by a few others, they are not alternative to custodians, but maybe complementary. End-to-end retranmission can work fine if the delay is short and the the topology is stable, two conditions that are difficult to find toghether in DTNs. For example (I have to credit Stephen Farrell for it) in military networks the source can be no more existent (blown up or out of range) after a while. Another example: spy drones. I suppose that they are not happy to keep in their memory photographs of enemy targetes for a long period of time to be ready to retranmit them if necessary, for obvous reasons.  Drones, would prefer to rely on custodians for possible retranmissions (maybe a plane flying in a safer area). More generally, everytime information is as a hot patato, custodians are useful. 
Yours,
   Carlo 
________________________________________
Da: dtn [[e-mail]] per conto di Burleigh, Scott C (312B) [[e-mail]]
Inviato: venerdì 23 giugno 2017 17:28
A: [e-mail]
Oggetto: Re: [dtn] custody transfer I-D

Marius, that is certainly true.  But in that sort of scenario – where the sender forwards the bundle to a non-custodian and leaves custody to downstream nodes beyond that neighbor – I don’t think the current custodian can reasonably continue to be responsible for moving the bundle to the next custodian.

The current custodian doesn’t know who the next custodian will be or when the bundle will reach that custodian, so it can’t know when its responsibility to forward the bundle demands that it try again.  So the current custodian will either retransmit prematurely (unnecessarily increasing the traffic load on the network) or retransmit too late (increasing the drain on its own storage resources, delaying bundle arrival, and risking bundle loss due to TTL expiration).

In which case maybe we just say “Too bad, life is harsh” and live with sub-optimal network performance, and in some instances maybe this is unavoidable.  Not my preference, though.

Scott

From: dtn [mailto:[e-mail]] On Behalf Of Marius Feldmann
Sent: Friday, June 23, 2017 8:15 AM
To: [e-mail]
Subject: Re: [dtn] custody transfer I-D


Scott, right - I did not state that it is common in all opportunistic ad-hoc networks but that it is a popular case in these networks (at least if we assume the networks I see from a research perspective).

Though this is fine for me, we have to be aware that we exclude scenarios in which the senders' direct neighbors forward bundles and leave custody to the next hops on the path, e.g. because the direct neighbors are just a sort of gateways or relays.

Marius

On 23.06.2017 16:57, Burleigh, Scott C (312B) wrote:
Marius, I will suggest that you can still do custody transfer along these lines in an opportunistic ad-hoc network, so long as every node that receives the bundle takes custody.  The topology can be discovered dynamically, no problem.  It’s only when you don’t know who the next custodian will be – you have discovered a neighbor and will forward the bundle to that neighbor, but that neighbor won’t take custody and you don’t know who will – that there are problems.

You can still use DTN for vehicular networks, disaster response, etc., absolutely – but all nodes need to take responsibility in order for this to work.  Otherwise the current custodian has no way of knowing (in the general case) when to decide that forwarding has failed and the bundle has to be forwarded again.

Scott

From: dtn [mailto:[e-mail]] On Behalf Of Marius Feldmann
Sent: Friday, June 23, 2017 7:46 AM
To: [e-mail]<mailto:[e-mail]>
Subject: Re: [dtn] custody transfer I-D


Yes, for sure this could be also realized as a separate mechanism (e.g. extension block described in a separate document). To keep things simple, in my opinion that should be the way to go.

The sketched scenario should be quite popular in opportunistic ad-hoc networks in which the topology is not known in advance.

If we do not go for a solution, custody transfer is removed from all use cases mentioned in [1] lacking apriori topological knowledge (e.g. Vehicular Delay Tolerant Network, Disaster Response, ...).

Marius

[1] https://trac.ietf.org/trac/dtn/wiki/DtnUseCases

On 23.06.2017 16:18, Burleigh, Scott C (312B) wrote:
You’re right, Marius, this approach excludes the option of saying “Please, anybody on the path, take custody of part or all of this bundle.”  If we need that capability, then we would need a different – or additional – protocol to make that happen.

That is by design, though.  If you don’t know who’s going to take custody then you don’t know when custody will be taken, and in that case you don’t have any way of knowing that custody has not been taken, so there is no efficient way of triggering retransmission of the bundle from the current custodian.  So I think requiring that capability excludes the efficient discharge of custodial responsibility.

I have not yet heard of use cases that require custody transfer – including custodial retransmission – in operational scenarios where the current custodian forwards the bundle without knowing who the next custodian should be.  Are there any?

Scott

From: dtn [mailto:[e-mail]] On Behalf Of Marius Feldmann
Sent: Friday, June 23, 2017 1:02 AM
To: [e-mail]<mailto:[e-mail]>
Subject: Re: [dtn] custody transfer I-D


Hi Scott,

with your approach we can state that a specific node on the path should take the custody for a specific bundle (not just fragments created from it). Thus, it renders it possible to define well-known nodes that guarantee to take the original bundle (even if it has been fragmented between the sender and this well-known node) and confirm custody for it. As far as I get it, that is the basic point.

Let us assume we only have the proposed option available. How do we transfer custody to an unknown node on the path? Thus, to transmit a bundle with the requirement "Please, anybody on the path, take custody for this bundle." or maybe even an extended variant: "Please, anybody on the path, take custody for the whole or partial data encapsulated in this bundle.".

I guess we need - at least additionally to the approach proposed by you - protocol support to cover this case.

Marius

On 23.06.2017 02:04, Burleigh, Scott C (312B) wrote:
Hi.  A few minutes ago I posted an Internet Draft (https://datatracker.ietf.org/doc/draft-burleigh-dtn-bibect/) presenting an idea I had a couple of months ago for cleanly breaking the Custody Transfer procedures out of BP and into a separate document.

In a nutshell, I suggest that we standardize the Bundle-in-Bundle Encapsulation (BIBE) convergence-layer protocol and build Custody Transfer into it, making BIBE a reliable CL.

I am confident that this sounds insane to most people who are reading this email.  But I think I can actually make a fairly strong case for it.

I’ve been claiming for some time that reliable convergence-layer protocols (TCP, LTP) are the best way to provide end-to-end delivery reliability in DTN.  Custody transfer is not as good because (a) there are no partial NAKs, so the only option on any data loss, no matter how small, is to re-send the entire bundle (which may be hundreds of megabytes); (b) there are no negative ACKs that indicate data loss (custody refusal actually indicates successful data arrival, just at an incapable forwarding point), so recovery from data loss happens only when a timer expires at the current custodian; (c) but it is in the general case impossible to set the timeout value for that timer because no node is ever required to take custody.  You never know (in the general case) who the next custodian will be, so you have no idea what the round-trip time to the next custodian is.

At the same time, Keith Scott has been saying that some important use cases need custody transfer instead of reliable CLAs because no suitable reliable convergence-layer protocol exists: the forward path is unidirectional, the return path is very different, and delay-tolerant hop-by-hop forwarding is needed in one or both.

Suppose we are both exactly right.  Let’s make custodial retransmission a property of a (now reliable) convergence-layer protocol that performs delay-tolerant hop-by-hop forwarding, because the CL’s protocol data units are bundles.  Like BIBE.

In the specification I just posted, BIBE CT works in much the same way that CT works in RFC 5050, only a little simpler.  The outbound bundle forms the payload of an encapsulating bundle destined for the next custodian, which might – but would not have to – be the next BP node on the end-to-end path.  On arrival of the encapsulating bundle at the destination node, the CLA at that node extracts the payload (the original bundle) and decides whether or not to accept custody.  It sends a custody signal back to the sending CLA, either accepting or refusing custody, and on acceptance it passes the payload bundle up to the BPA for processing as usual (forwarding, delivery, etc.).  The sending CLA receives and processes custody signals, destroys its copy of the cited original bundle upon custody acceptance, and re-encapsulates and re-transmits the original bundle upon either custody refusal or timer expiration prior to receipt of a responding custody signal.

I think this formulation offers a lot of advantages:

  *   The problem of custodial bundle fragmentation by a non-custodial forwarding node goes away: no node other than the next custodian ever sees the encapsulated bundle, therefore cannot fragment it.  The encapsulating (BIBE) bundle might get fragmented, absolutely, but it gets reassembled at the destination (the next custodian) before any CT processing occurs.  So all of the complexity of fragmentary custody transfer disappears.
  *   Custody transfer suddenly becomes compatible with multi-point delivery.  If you use bundle multicast as prototyped in ION, then each copy of the bundle that is forwarded through the multicast tree is (naturally) conveyed using a point-to-point convergence-layer transfer – which could easily be a BIBE transfer with CT requested.
  *   Looking out a little further, knowing the identity of the next custodian means that CT can take advantage of bundle delivery time estimation mechanisms (which we prototyped a few years ago) to compute custodial retransmission timeout intervals.  So CT becomes more accurate and efficient as well.
  *   The relationship of CT to the rest of BP becomes an extremely clean and simple interface, which can easily be added on to any BP implementation.  Implementation of CT becomes simple and self-contained.
  *   Building CT into BIBE gives us a single CL protocol that can provide cross-domain security solutions, provide reliable disruption-tolerant forwarding over unidirectional links, or both.  And yet the protocol is extremely simple, only 13 pages.

It’s radical, but I don’t think it’s wrong.

Scott






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