[Sis-dtn] [dtn] [EXT] Re: Bundle custody transfer and reliable CLs

[Dr. Keith L Scott]

The question remains: do we need some form of reliability above the CLA layer?  As Scott points out, BIBE is ‘just’ a reliable CLA that uses bundles as its transport mechanism.  That allows BIBE to function as a reliable CLA in environments where other CLAs that require bidirectional connectivity cannot, but viewed from the standpoint of the sending and receiving BIBE endpoints, it’s just another reliable CLA.

The not-often-stated assumption here is that bundle nodes that receive bundles via reliable CLAs do in fact take on the responsibilities of what was traditionally referred to as ‘custody transfer’ – i.e. following whatever procedures are necessary to ensure that the received bundle reaches its destination (e.g. store it until a forward path becomes available, attempt retransmission to the next hop until there’s an indication that the next hop has (via a reliable CLA in this context) received the bundle, etc.).  I.e. this approach assumes that receipt of a bundle • accepting ‘custody’ of it.  Without this assumption, the service provided by BP is essentially the same best-effort service that IP provides, which I think is less than what we want, particularly for space missions. So I think that in the BPv7 context, all bundles are considered ‘custodial’.


If we consider cases where there may be congestion (contention for storage space at nodes), this means that when congestion happens at a node, the only course of action available to the node will be to refuse new incoming bundles (presumably because the receiving CLAs stop accepting them).

There are at least two things to consider here:

  1.  What if I have a bundle node that is capable of forwarding but that has quite minimal storage?  By receiving a bundle, this node is committing to storing that bundle until it can be reliably forwarded.  It seems like there could be cases where this is really inefficient, especially if the reliable forwarding is over something like BIBE where the RTT to get an acknowledgement could be high.  In BPv6, such a node could simply forward a custody-requesting bundle without actually taking custody, sort of like a ‘transit node’ in a BIBE tunnel.  In this case, it might be able to achieve a higher throughput at the expense of NOT accepting the storage requirements from the current custodians.
  2.  Even if a congested node has a lot of storage (but still becomes congested), in BPv6 there was the notion that the node could drop (probably lower-priority) non-custodial bundles in favor of (probably higher-priority) custodial bundles.  We don’t currently have any notion of priority in BPv7, but if we ever want to admit the possibility that a bundle node might drop a bundle due to congestion, it seems like the assumption that receipt • ‘custodial’ acceptance constrains us.  In the BPv7 ‘receipt is (custody) acceptance’ model, the node would have to refuse new bundles.

This might be right thing for some CONOPS.  It would impose backpressure (at DTN timescales) on the network, eventually to the bundle sources.  The same thing would happen with BPv6 and custodial bundles, the difference being that a BPv6 node would have the option of dropping non-custodial bundles to accommodate newer (again, presumably higher-priority) bundles.



I’ll readily admit that calculating a (good) retransmission timer value in the case where a node does NOT know if the proximate (or even which) downstream node will take custody is difficult or impossible for some networks.  BIBE still has a bit of this problem, especially if the path to the BIBE destination is long, as the sender won’t necessarily know the path the BIBE bundles will take, but it is at least more constrained than the completely open case.





If we want to have the option of dropping lower-priority bundles that have already been received and are being stored at a node, we’ll need an extension block to mark priority, fine.  We could then create rules that operate at the BP layer to drop bundles when congestion occurs according to their priority markings and address #2 above. I suppose in this case the ‘reliability’ is still at the CLA layer and the decision-making process on whether or not to drop an incoming bundle is at the BP layer.

  *   That might not address #1 above but maybe #1 is sufficiently rare (or non-existent) that we decide to ignore it.
  *   Regardless, this has the disadvantage that the transmitting node would believe that the bundle was accepted (because it would have been, by the receiving CLA) event though it then got dropped by the BP layer at the receiving node.



                                --keith



From: "sis-dtn-bounces at mailman.ccsds.org" <sis-dtn-bounces at mailman.ccsds.org> on behalf of "sis-dtn at mailman.ccsds.org" <sis-dtn at mailman.ccsds.org>
Reply-To: Mehmet Adalier <madalier at antarateknik.com>
Date: Friday, March 25, 2022 at 1:21 PM
To: "sburleig.sb at gmail.com" <sburleig.sb at gmail.com>, "'Sipos, Brian J.'" <Brian.Sipos at jhuapl.edu>, Felix Flentge <Felix.Flentge at esa.int>, "dtn at ietf.org" <dtn at ietf.org>, "sis-dtn at mailman.ccsds.org" <sis-dtn at mailman.ccsds.org>
Subject: Re: [Sis-dtn] [dtn] [EXT] Re: Bundle custody transfer and reliable CLs

I believe Scott’s analysis below succinctly articulates the difference between the two approaches and I agree that they should be kept separate. For my intended use cases, approach 1 (BIBE/BPARQ?) is what I would need to use. I’ll be happy to contribute to this approach and prototype.

Best
mehmet

From: SIS-DTN <sis-dtn-bounces at mailman.ccsds.org> on behalf of "sburleig.sb--- via SIS-DTN" <sis-dtn at mailman.ccsds.org>
Reply-To: <sburleig.sb at gmail.com>
Date: Friday, March 25, 2022 at 10:07 AM
To: "'Sipos, Brian J.'" <Brian.Sipos at jhuapl.edu>, <Felix.Flentge at esa.int>, <dtn at ietf.org>, <sis-dtn at mailman.ccsds.org>
Subject: Re: [Sis-dtn] [dtn] [EXT] Re: Bundle custody transfer and reliable CLs

Hi, Brian.  Actually I think Felix’s analysis is pretty much spot-on.  We really are talking about two different behaviors, which respond to two different sets of requirements.

The “custody transfer” procedures that I proposed in the BIBE draft are very specifically aimed at defining a reliable convergence-layer adapter that happens to use BP as its underlying convergence-layer protocol.  There is a requirement for this capability: Keith Scott has often pointed out that there are scenarios in which reliable transmission between nodes is required but no reliable transmission protocol is available, e.g., when the sender’s communication capability is temporarily unidirectional.  These are not hypothetical; MITRE’s customers must at times operate in such environments, and some space flight missions and other IoT systems could be similarly constrained.  Under these conditions, the mechanism by which NAKs and ACKs are returned to the sender may function at a later time and/or be unrelated to the mechanism by which the sender transmitted the data.  There might be better standardized protocols than BP for supporting these kinds of scenarios, but none leap to mind.

The ”custody transfer” procedures for which Felix proposes requirements are different.  Since there is no need for timeout-triggered retransmission (retransmission is instead triggered by command or by negative acknowledgment), there is no need for accurate estimation of the round-trip time; therefore there is no need for the sender to know which node will issue the responding (positive) custody acknowledgment, exactly as required.  I think of it as a resource management system rather than an ARQ system.  A mechanism very much like BPv6 custody transfer will work fine.

I would propose that we term the latter procedures “custody transfer” and proceed to standardize them, while renaming the former something like “BPARQ”.

I don’t think there’s any need to impose any additional requirements on CL protocols, TCPCL or other, to satisfy the requirements.  These are separate things.  Let’s keep them separate and support them separately and clearly.

Scott

From: dtn <dtn-bounces at ietf.org> On Behalf Of Sipos, Brian J.
Sent: Friday, March 25, 2022 4:46 AM
To: Felix.Flentge at esa.int; dtn at ietf.org; sis-dtn at mailman.ccsds.org
Subject: Re: [dtn] [EXT] Re: Bundle custody transfer and reliable CLs

Felix,
Thank you for this feedback. There is a misinterpretation of what I (and maybe also Scott via BIBE) am suggesting about what happens during reliable reception. The idea isn’t that two different things happen, it’s that it’s the same BP agent custody acceptance criteria just that some CLs can provide intrinsic signaling of that acceptance while other CLs have no means to signal that specific feedback.

While IETF doesn’t (normally) specify internal APIs, the CCSDS documents do and this can help here. Currently 734.2-B-1 does not actually define a BPA—CLA interface API, but it seems like the rough interface looks like:

  *   Send.request to the CLA
  *   Send.response from the CLA
  *   Receive.indication from the CLA

What I am suggesting is that the Receive side could be changed from the asynchronous “I just got this transfer. Here it is, thanks.” to a synchronous “I just got this transfer. Will you accept it?” similar to:

  *   Receive.indication from the CLA
  *   Receive.response to the CLA

The TCPCL and LTPCL already provide the negative response over-the-wire (TCPCL reception can send XFER_REFUSE at any time before the END ACK, and LTPCL can send “Cancel from the block receiver” similarly) there is currently just no specific formal definition of what, from the BPA side, the positive acknowledgement is required to mean. For example “If the transfer is not canceled by the receiver and the final ACK is sent, the transferred bundle SHALL be completely and positively received within the BP agent’s forwarding or delivery queue.”

As Scott mentioned earlier, custody isn’t an anthropomorphization of the BPA, it’s a specific behavior, and it seems like by acknowledging that the bundle was received into the queue for delivery/forwarding the agent has “accepted” it. If the intent of custody is that it’s a more restricted/reserved resource pool (e.g. my forwarding queue is size X but of that only Y (with Y<X) is reserved for bundles over which I have custody) then that’s a local agent management issue, not an over-the-wire signaling issue. The BPA has still positively accepted the bundle and some CLAs can communicate this back to the sending agent synchronously.

Thanks for any further clarification,
Brian S.

From: dtn <dtn-bounces at ietf.org<mailto:dtn-bounces at ietf.org>> On Behalf Of Felix.Flentge at esa.int<mailto:Felix.Flentge at esa.int>
Sent: Friday, March 25, 2022 6:11 AM
To: dtn at ietf.org<mailto:dtn at ietf.org>; sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>
Subject: [EXT] Re: [dtn] Bundle custody transfer and reliable CLs

(cross-posting to CCSDS DTN mailinglist as it seems to be of high relevance to the on-going discussions in the DTN WG).

I think we clearly need to distinguish between (at least) two different types of 'custody transfer' according to where it is implemented:

(a) BPv6 - like custody transfer which has been a function of the BPA / AA Administrative element
(b) BIBE-like custody transfer which is implemented in the CLA

The main differences I see are:
- In (a) the decision whether to take custody or not can be taken on more available information, eg timely availability of a route toward a next hop, available storage, policies (eg, checking Bundle Integrity Blocks), ... In (b), the CLA may accept custody and the BPA would just decide to discard the bundle for whatever reason (of course, there could be interfaces to make such information available to the CLA but this would somehow 'blur' the architecture.
- In (b), the node to take custody needs to be explicitly addressed (it's the BIBE destination) while in (a) any node could take custody.

Requirements regarding custody transfer I would see for space missions are:

1) Assertion of a high probability that a bundle will reach its destination once a hop has accepted custody (which would allow the forwarding node to release storage).
The meaning of 'high probability' does really depend on the mission data return requirements. While for some missions it may be close to 100%, other mission may be ready to accept higher data loss in favour of timeliness (eg, certain types of Earth Observation missions).

2) Forwarding bundles without knowing which node will take custody.
In particular with high data rates and optical direct-to-Earth downlinks we may have situations where the spacecraft may not know the actual next hop is sending to but may want to get a confirmation that the bundle has been received on ground. With high-data rates, bundles might already be prepared and encoded in frames and be sitting in some buffers within optical terminal because the data rates on the on-board buses would not allow to generate and send in real time. Use of optical direct-to-Earth links may be opportunistic and we may not know in advance how much data will go down. So, addressing a specific DTN node in a ground station becomes unpractical (if DTN nodes in ground station would share the same anycast eID it may be possible but BIBE is currently limited to singleton endpoints).

It is clear that these requirements cannot be solved by protocol specification as Scott pointed out below but will also require that implementing nodes conform to certain behaviours. This will not be possible for an open, Internet-like DTN network (where we can only try to take defensive actions). For space missions I would still expect limited, tightly-controlled network for some time to come (maybe becoming 'trusted zones' in a larger network). For these, we should have protocol mechanism which can support above requirements (while being aware that some of these mechanisms will not work in a fully open DTN).

Finally, 'custody transfer' seems to be always related to timer-base re-transmission. However, I think there are other options as well, like:
- command-based re-transmission: an explicit command is sent to a DTN node to re-transmit all bundles for which custody has been requested but no signal has been received
- sequence-based re-transmission: in some situations it might be possible (using additional extension blocks) to detect which bundles have not been received by inspecting the received custody signals and re-send the ones for which no custody signal have been received
Again, this would likely not work in an open DTN but only in (very) limited, controlled networks. But this doesn't make such mechanisms less useful (although I would prefer to have something more generic if possible) and we should address it in the IETF (if there is general interest) and/or the CCSDS (if it is for near-to-medium term space mission use cases only).

Regarding reliable CLs: I currently don't see the point of a reliable CL taking custody because it would be (only) type (b) custody which is basically just a confirmation that the bundle has been received and this can already be assumed by the fact that it is a reliable CL.

Regards,
Felix




From:        "William Ivancic" <ivancic at syzygyengineering.com<mailto:ivancic at syzygyengineering.com>>
To:        <sburleig.sb at gmail.com<mailto:sburleig.sb at gmail.com>>, "'Sipos, Brian J.'" <Brian.Sipos at jhuapl.edu<mailto:Brian.Sipos at jhuapl.edu>>, <dtn at ietf.org<mailto:dtn at ietf.org>>
Date:        25/03/2022 01:16
Subject:        Re: [dtn] Bundle custody transfer and reliable CLs
Sent by:        "dtn" <dtn-bounces at ietf.org<mailto:dtn-bounces at ietf.org>>
________________________________


“Custody” is  a bundle level concept, not transport.  Way back when, “Custody” meant that the node taking “Custody” would try it’s best to ensure the bundle was forwarded either to another “Custody” node or to eventually the destination node.  The idea, as I recall, was this would allow the original bundle source to clear its memory of that bundle.



For space operations, I don’t think the operations people were ever comfortable with this concept.



//Will



From: dtn <dtn-bounces at ietf.org<mailto:dtn-bounces at ietf.org>> on behalf of <sburleig.sb at gmail.com<mailto:sburleig.sb at gmail.com>>
Date: Thursday, March 24, 2022 at 6:04 PM
To: "'Sipos, Brian J.'" <Brian.Sipos at jhuapl.edu<mailto:Brian.Sipos at jhuapl.edu>>, <dtn at ietf.org<mailto:dtn at ietf.org>>
Subject: Re: [dtn] Bundle custody transfer and reliable CLs



Brian, there was some further discussion of custody transfer in this morning’s meeting of the CCSDS Space Internetworking Systems DTN Working Group as well.  A couple of notes:



It’s important to remember that we are talking about state machines here, not people.  Anthropomorphizing the DTN architecture is tempting but treacherous.  BP nodes have no will; they cannot take responsibility; they cannot promise to do anything.  All they do is behave, ideally in a fashion that conforms to the protocol specifications to which they were allegedly developed.



Also, any given node may have been designed with malign intent or implemented with errors.  Stating a requirement in a protocol specification does not ensure its satisfaction; what it does is give a node’s human (maybe eventually AI) network operators a means of assessing the behavior of another node and possibly taking some sort of out-of-band administrative action in defense against that behavior as needed.



You’re right, the term “custody” is not defined for BPv7.  It is still widely used to refer to some behavior that future users of DTN for space flight operations state will be very important, but for which the requirements are not yet clearly established.  It is starting to look like the BP-based ARQ in the most recent BIBE draft, while needed (I think), is not exactly what people mean by “custody transfer.”  So it may become useful to define an additional BP extension (TBD) that we label with this term.



TCPCL enhancements along the lines you propose here may very well be valuable; I don’t know, need to think about them some more.  But I would say the BPv7 specification already contains language (in 5.4, Step 5 and the following two paragraphs, and in 7.2, second bullet point) constraining the sort of convergence-layer reliability that I think is indispensable, regardless of what we call it.



Scott



From: dtn <dtn-bounces at ietf.org<mailto:dtn-bounces at ietf.org>> On Behalf Of Sipos, Brian J.
Sent: Thursday, March 24, 2022 1:39 PM
To: dtn at ietf.org<mailto:dtn at ietf.org>
Subject: [dtn] Bundle custody transfer and reliable CLs



All,

There was some brief discussion during the BIBE presentation about custody transfer concept and CL mechanisms. This is also an open topic in the CCSDS drafting of BPv7 standardization. I would like to add some additional points for thought in how a reliable convergence layer can relate to the concept of custody transfer for agents on either side of the transfer.



Overall, there still seems to be some vagueness about what “custody” means (in the sense of a service level agreement) between peers exchanging bundles. My understanding is that “custody” means the custodial node is willing to make some kind of effort to keep the bundle moving toward its destination until the bundle lifetime expires.



The current RFC 9174 document is silent about what exactly an XFER_ACK segment with END flag set (an END ACK) means from the perspective of the BP agent and what is guaranteed about the transferred bundle. This provides an opportunity for a follow-on clarification of END ACK semantics for the TCPCL entity and for the BP agent. Two potential ways of making the behavior more well-defined:



1.       A network-specific profile of TCPCL could simply mandate that any node accepts custody by sending an END ACK. This would simply be a condition of conformance to the profile in a controlled network. This could be done immediately without any change elsewhere, but needs out-of-band coordination.

2.       One or more (quite simple) extension types can be defined for TCPCL to allow an entity to expose its END ACK behavior (RX) and desire (TX):

a.       A session extension can allow an entity to assert what its sent END ACK means for received transfers. The value in this is to allow the peer entity to adjust behavior depending on the capability (e.g. use BIBE if the next-hop doesn’t take END ACK custody), including possibly refusing to establish a session with (or refusing to send bundles to) a peer that does not take custody via END ACK.

b.      Additionally, a transfer extension can allow a sender to assert its custody desire on a per-bundle basis (signaling that some bundles need custody transfer while others do not). The value in this is to allow the receiving entity to optimize its behavior based on whether or not custody is needed for a bundle; though I don’t know how much benefit this would be.



The possible values enumerated by the session extension would be something like:

•         Custody is not taken at END ACK

•         Custody is taken at END ACK

And if there is a transfer extension defined, a the session extension could indicate:

•         Reception behavior is unconditional

•         Reception behavior can be overridden per-transfer based on the sender’s desire

These changes would all be backward compatible in the sense that a default policy would be in place in the absence of this extension item. And all of this is an independent mechanism from BIBE for a custody transfer to take place; both this mechanism and BIBE have their own costs, benefits, and side effects of such a transfer.



Trying to make almost-there-already capabilities more obvious,

Brian S.

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