Re: [dtn] custody transfer I-D

[Stephen Farrell <[e-mail]>]

Hiya.

On 24/06/17 20:32, Burleigh, Scott C (312B) wrote:
> Sure, you can do experiments for anything.  What's the operational
> use case?  Who needs this?

By "experiment" I meant experimental field deployments. And
since that's mostly all that's ever used the BP, I don't think
we are yet in a position to disparage those;-)

> And yes, for broadcast you certainly don't know the identity of the
> next hop, but not for BP multicast -- at least not for BP multicast
> in ION, which is the only one I know anything about -- because the
> multicast messages are propagated through a tree, and over each
> branch of the tree you know the identity of the receiving node.

It's CLs that broadcast/multicast/anycast where I see the problem
arising, i.e. where the BPA is unaware that various lower layer
recipients may see the same bundle and is hence unaware of any of
their identifiers.

If your draft means that such CLs aren't usable, (with custody)
then I think that ought be positively justified/garner consensus,
rather than the onus being the other way about.

We also probably do not want to lose the ability for bundles to
be sent on USB sticks or equivalent and (even though we decided
those are nodes and not CLs), hops like that have the same issues
I think.

Cheers,
S.

PS: Just to be clear: I'm not a huge fan of custody but I do
think (under the current charter) we ought not lose the concept
except via conscious choice that's positively supported.


> -----Original Message----- From: Stephen Farrell
> [mailto:[e-mail]] Sent: Saturday, June 24, 2017
> 11:58 AM To: Burleigh, Scott C (312B)
> <[e-mail]>; Carlo Caini <[e-mail]>;
> [e-mail] Subject: Re: [dtn] custody transfer I-D
> 
> 
> 
> On 24/06/17 19:31, Burleigh, Scott C (312B) wrote:
>> Hi, Carlo.  I completely agree that end-to-end retransmission is
>> not a viable alternative to intra-network retransmission (of which
>> Custody Transfer is one instance) in delay-tolerant networks.
>> 
>> But I disagree that custodial retransmission based on inaccurate 
>> timeout intervals is better than no custodial retransmission at
>> all. I would be open to persuasion on this if, as I asked earlier,
>> there are plausible 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.  If someone can cite a few such cases
>> that are important enough to justify the potential impact of
>> inefficient retransmission on network performance, I would like to
>> hear about them.
> 
> I've not read your draft yet sorry, but I can answer that one. If I
> use a CL that is broadcast (or multicast or anycast) in nature, then
> I don't know the identity of the next hop. And I may have a DTN where
> I want every (or almost every) node to take custody. IIRC we have
> seen that scenario in the past in experiments.
> 
> Cheers, S.
> 
> 
>> 
>> Scott
>> 
>> -----Original Message----- From: Carlo Caini 
>> [mailto:[e-mail]] Sent: Saturday, June 24, 2017 7:52
>> AM To: Burleigh, Scott C (312B) <[e-mail]>; 
>> [e-mail] Subject: RE: [dtn] custody transfer I-D
>> 
>> 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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