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

From: <[email-masked]>
Date: Fri, 1 Apr 2022 07:42:19 -0700

Right, I think it's important to recognize that all of these behaviors are
potentially valuable and need to be considered for inclusion in the new
Compressed Bundle Report or "custody transfer" (or whatever) mechanism we
are contemplating.

Scott

From: [email-masked] <[email-masked]> 
Sent: Thursday, March 31, 2022 11:28 PM
To: [email-masked]
Cc: 'Sipos, Brian J.' <[email-masked]>; [email-masked]; 'Dr. Keith L
Scott' <[email-masked]>; 'Sanchez Net, Marc(JPL-332H)[JPL Employee]'
<[email-masked]>; 'Chaoua, Rachid(GSFC-4500)'
<[email-masked]>; [email-masked]
Subject: RE: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs

I think we should distinguish between the signalling, the re-transmission
and (maybe) custody: 

1) CBR (or whatever better name we can come up with) 
If a node implement CBR, it just means that it is able to send compressed
status reports if requested by a specific extension block. This is about
defining an extension block and administrative records. 

2) CBR-R (CBR-based re-transmission) 
It would mean the a node receiving CBR status reports implements a mechanism
to re-send bundles for which it did not receive a reception report (this is
basically Keith's 'SHALL NOT release resources part'). This is about
defining node behaviour and there could be different variants
(sequence-based, timer-based, command-based, ...) 

3) CBR-C (CBR-based Custody Transfer) 
Not sure, whether we would require this. The difference to 2) could be that
a receiving node would also be able to send a custody signal (if requested
in the extension block) in addition to the reception signal. The meaning of
the custody signal could be beyond the reception signal, eg: 
- the bundle is sitting safely in bundle storage 
- the receiving node is implementing CBR-R and will add/replace the CBR
extension block by a new one where it will request custody for the next hop 
- based on local information, the receiving node will be able to forward the
bundle towards its destination (there is/will be a contact to a next hop) 
- ... 

Regards, 
Felix 

From:        <[email-masked] <mailto:[email-masked]> > 
To:        "'Dr. Keith L Scott'" <[email-masked] <mailto:[email-masked]>
>, <[email-masked] <mailto:[email-masked]> > 
Cc:        "'Sipos, Brian J.'" <[email-masked]
<mailto:[email-masked]> >, <[email-masked] <mailto:[email-masked]> >,
"'dtn'" <[email-masked] <mailto:[email-masked]> >, "'Sanchez
Net, Marc\(JPL-332H\)[JPL Employee]'" <[email-masked]
<mailto:[email-masked]> >, "'Chaoua, Rachid\(GSFC-4500\)'"
<[email-masked] <mailto:[email-masked]> > 
Date:        31/03/2022 19:05 
Subject:        RE: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs 

  _____  

Fine with me.  I wanted not to over-constrain the idea at this early stage
of definition, but if that sort of mandate is okay with everybody I am for
it.

Scott

From: Dr. Keith L Scott <[email-masked] <mailto:[email-masked]> > 
Sent: Thursday, March 31, 2022 9:25 AM
To: [email-masked] <mailto:[email-masked]> ;
[email-masked] <mailto:[email-masked]> 
Cc: 'Sipos, Brian J.' <[email-masked]
<mailto:[email-masked]> >; [email-masked] <mailto:[email-masked]> ;
'dtn' <[email-masked] <mailto:[email-masked]> >; 'Sanchez Net,
Marc(JPL-332H)[JPL Employee]' <[email-masked]
<mailto:[email-masked]> >; 'Chaoua, Rachid(GSFC-4500)'
<[email-masked] <mailto:[email-masked]> >
Subject: Re: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs

I think I agree, with the possible extension that if #3 is postulating some
sort of custody signal the "If.does not release resources" can be
strengthened a bit if we so choose.  That is, if we define how the extra
signaling works, we have the opportunity to define / constrain behaviors of
implementations that implement that signaling.  So if we define some sort of
BP-layer signaling for 'custody' (using compressed bundle reports or
otherwise), we might say that nodes implementing that signaling SHALL NOT
release resources until (receipt of a positive custody signal OR
appropriate rules to address the case that no other node in the system
implements the signaling, etc.).

                                --keith

From: " <mailto:[email-masked]> [email-masked]" <
<mailto:[email-masked]> [email-masked]>
Date: Thursday, March 31, 2022 at 11:19 AM
To: Keith Scott < <mailto:[email-masked]> [email-masked]>, Felix Flentge
< <mailto:[email-masked]> [email-masked]>
Cc: "'Sipos, Brian J.'" < <mailto:[email-masked]>
[email-masked]>, " <mailto:[email-masked]> [email-masked]" <
<mailto:[email-masked]> [email-masked]>, 'dtn' < <mailto:[email-masked]>
[email-masked]>, "'Sanchez Net, Marc(JPL-332H)[JPL Employee]'" <
<mailto:[email-masked]> [email-masked]>,
"'Chaoua, Rachid(GSFC-4500)'" < <mailto:[email-masked]>
[email-masked]>
Subject: RE: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs

Sorry, I just realized that the "if not" clause in #1 below could be
ambiguous.  What I meant there was "if the sending node does not try again."

Scott

From: [email-masked] <mailto:[email-masked]>
<[email-masked] <mailto:[email-masked]> > 
Sent: Thursday, March 31, 2022 8:10 AM
To: 'Dr. Keith L Scott' <[email-masked] <mailto:[email-masked]> >;
[email-masked] <mailto:[email-masked]> 
Cc: 'Sipos, Brian J.' <[email-masked]
<mailto:[email-masked]> >; [email-masked] <mailto:[email-masked]> ;
'dtn' <[email-masked] <mailto:[email-masked]> >; 'Sanchez Net,
Marc(JPL-332H)[JPL Employee]' <[email-masked]
<mailto:[email-masked]> >; 'Chaoua, Rachid(GSFC-4500)'
<[email-masked] <mailto:[email-masked]> >
Subject: RE: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs

I think we are converging on a few principles:

1.        If all of the CLAs (usually only one, but there could be parallel
transmissions) report to the BPA that forwarding failed, then the sending
node may or may not try again, possibly on a different path and/or relying
on different CLA support; if not, it may or may not release the
retransmission resources occupied by the bundle.  This is a matter for
implementation decision, which might vary depending on the size, QoS,
time-to-live, etc. of the bundle.

2.        If successful forwarding is reported from the convergence layer,
then at that time the sending node may or may not release retransmission
resources occupied by the bundle.  This is again a matter for implementation
decision, which might vary depending on the size, QoS, time-to-live, etc. of
the bundle.

3.        If, following the reporting of successful or unsuccessful
forwarding results from the convergence layer, the sending node does not
release the retransmission resources occupied by the bundle, then that
release of resources may (or may not) instead occur upon reception of a
Compressed Bundle Report (or whatever we decide to call it; effectively, a
custody signal) from the receiving node or from some other node further
downstream, in the event that the bundle was flagged for this behavior.  The
absence of such an acknowledgment may or may not result in either automatic
(timeout-driven) or managed re-forwarding of the bundle.  Again these are
implementation matters.

4.        If all else fails, expiration of the bundle's time-to-live will
eventually and unconditionally result in the release of the retransmission
resources occupied by the bundle.

What we've currently got in the BPv7 specification already supports all of
the above except #3; I think that new flagging/signaling mechanism is the
augmentation that we need to focus on.

Scott

From: Dr. Keith L Scott <[email-masked] <mailto:[email-masked]> > 
Sent: Thursday, March 31, 2022 5:24 AM
To: [email-masked] <mailto:[email-masked]> ;
[email-masked] <mailto:[email-masked]> 
Cc: 'Sipos, Brian J.' <[email-masked]
<mailto:[email-masked]> >; [email-masked] <mailto:[email-masked]> ; dtn
<[email-masked] <mailto:[email-masked]> >; 'Sanchez Net,
Marc(JPL-332H)[JPL Employee]' <[email-masked]
<mailto:[email-masked]> >; 'Chaoua, Rachid(GSFC-4500)'
<[email-masked] <mailto:[email-masked]> >
Subject: Re: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs

I do think the kernel of the issue under discussion is whether the rover is
willing to release resources based on a reliable CLA confirming receipt, or
if it (the rover) should wait for some signaling from the BP layer.  The
former implies an assumption that successful receipt by the (reliable) CLA
--> "custody transfer" to the receiving node and that the receiving node
will hold onto the bundle until it can be reliably transferred (via a
reliable CLA) to the next hop.  Without the assumption that every node takes
custody, BP is a best-effort service and I think that's too loose a service
for what we want in space.

The way I read RFC9171, a bundle node that attempts to send a bundle that
results in one or more of the CLAs reporting failure to forward MAY attempt
to retransmit the bundle (or, presumably, it MAY not).  [The text after step
5 in section 5.4].  Depending on settings, some sort of signal SHOULD /
could be generated noting the deletion of the bundle, but it would be lost
regardless.

So, even if forwarding is attempted over a reliable CLA (e.g. TCPCLv4), if
that fails for some reason, bundles can be lost.  Even if the space
community modifies the processing rules to mandate that bundles that fail
CLA forwarding ARE retransmitted, such a change can only apply to the
portion of the path that we control.  If we end up buying commercial DTN
routers for use in the ground, they could very well stick to the MAY clause
and drop bundles for which the initial CLA transmission failed (I think).

To signaling (or at least being able to signal) all sorts of interesting
events per Felix's email: yeah, we should be able to do that.

                                --keith

 

From: Felix Flentge < <mailto:[email-masked]> [email-masked]>
Date: Thursday, March 31, 2022 at 4:37 AM
To: < <mailto:[email-masked]> [email-masked]>
Cc: "'Sipos, Brian J.'" < <mailto:[email-masked]>
[email-masked]>, < <mailto:[email-masked]> [email-masked]>, dtn <
<mailto:[email-masked]> [email-masked]>, Keith Scott <
<mailto:[email-masked]> [email-masked]>, "'Sanchez Net,
Marc(JPL-332H)[JPL Employee]'" < <mailto:[email-masked]>
[email-masked]>, "'Chaoua, Rachid(GSFC-4500)'" <
<mailto:[email-masked]> [email-masked]>
Subject: Re: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs

Well, 

I would say all a reliable CLA would do is to confirm to the forwarding CLA
that it has received all data that has been forwarded. I wouldn't expect the
CLA to check whether this data is a bundle or something else. 
This data will be provided to the BPA. Now: 
a) if it is corrupted, the bundle node may not be able to do anything with
it and just delete it. 
b) the bundle node might be able to read correct bundles --> in this case we
might want to report reception 

After this: 
c) the bundle node may forward the bundle --> we might want to report
forwarding 
d) the bundle node delivers the bundle --> we might want to report delivery 
e) the bundle node deletes the bundle (expired, security checks fail, no
storage, ...) --> we might want to report deletion 

(The Compressed Bundle Reporting mechanism is about making these reporting
more efficient.) 

The report receiving node may do something wrt to the reports received
(delete data, re-transmit data, ...). This might be application specific or
we might want to specify some behaviour (which could be a re-transmission
mechanism). 

Having said this, I agree that re-transmission at the convergence layer is
the preferred mechanism if possible (eg, for bi-directional links) and that
bundle re-transmission should only happen in exceptional (whatever this
means) cases, e..g. if we have reliable convergence layers but maybe loose a
bundle at the end of a contact. 

Regards, 
Felix 

From:        < <mailto:[email-masked]> [email-masked]> 
To:        "'Chaoua, Rachid\(GSFC-4500\)'" < <mailto:[email-masked]>
[email-masked]>, "'Sanchez Net,  Marc\(JPL-332H\)[JPL Employee]'" <
<mailto:[email-masked]> [email-masked]>, <
<mailto:[email-masked]> [email-masked]>, "'Dr. Keith L Scott'"
< <mailto:[email-masked]> [email-masked]> 
Cc:        "'Sipos, Brian J.'" < <mailto:[email-masked]>
[email-masked]>,  <mailto:[email-masked]> [email-masked] 
Date:        30/03/2022 19:52 
Subject:        Re: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs 
Sent by:        "dtn" < <mailto:[email-masked]> [email-masked]> 

  _____  

Hi, Rachid.  Confirming that the next BP node in the end-to-end path - the
next bundle hop (bop) - has received a bundle is precisely what a reliable
convergence-layer protocol does.  Now, it may be the case that the received
bundle will not be forwarded any further:

*	The bundle may be malformed or otherwise corrupted, therefore
deleted. 
*	The bundle may be too large to fit into available storage, therefore
dropped.

In a well-managed network these errors will be rare, but I agree that for
critical transmissions they need to be reported.  That is why I fully
endorse Felix's concept of adding to BP a mechanism for sending aggregated
bundle acknowledgments.  But that mechanism is not a substitute for a
technically sound retransmission protocol at the convergence layer, for
reasons we've already discussed at length; it's an augmentation. 

I also agree that reliance on BIBE ARQ will work only on nodes that support
BIBE ARQ.  Similarly, reliance on custody transfer - aggregated bundle
acknowledgments at the bundle layer - will work only on nodes that support
custody transfer.  This is a deficiency that is common to all protocols that
I know of.

Scott

From: Chaoua, Rachid (GSFC-4500) < <mailto:[email-masked]>
[email-masked]> 
Sent: Wednesday, March 30, 2022 9:39 AM
To:  <mailto:[email-masked]> [email-masked]; Sanchez Net, Marc
(JPL-332H)[JPL Employee] < <mailto:[email-masked]>
[email-masked]>;  <mailto:[email-masked]>
[email-masked]; 'Dr. Keith L Scott' < <mailto:[email-masked]>
[email-masked]>
Cc: 'Sipos, Brian J.' < <mailto:[email-masked]>
[email-masked]>;  <mailto:[email-masked]> [email-masked]
Subject: RE: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs

Marc's statement "the rover unequivocally knows that a complete data product
has made it to the orbiter" reflects the sentiments I've heard within the
Near Space Network when discussing DTN and Bundle Protocol. It is a core
requirement for GSFC to confirm the next hop (bop) has custody of a data
payload prior to that data being released from the source (or previous bop).
This is requirement for certain data flow types not all. 

Regarding the statement (the LTP part), "Why are people in MRN opposed to
using LTP". Our (NSN/GSFC) stance (captured in the Draft LunaNet
Interoperability Specification,
<https://esc.gsfc.nasa.gov/static-files/Draft%20LunaNet%20Interoperability%2
0Specification%20Final.pdf>
https://esc.gsfc.nasa.gov/static-files/Draft%20LunaNet%20Interoperability%20
Specification%20Final.pdf, p.10) is that we will initially support various
CLAs: TCPCL, UDPCL, LTPCL, or Encapsulation (future CLAs will be assessed
and supportability determined by the NSN). Since we are planning on using
various service providers, we are leaving the CLs used up to them.
Regardless of the CL implemented, there is a need for a custody transfer
option at the bundle layer. Relying on BIBE CL only works on nodes that
support BIBE CL.  That gives me some concern. 

Cheers!

From: dtn < <mailto:[email-masked]> [email-masked]> On Behalf Of
<mailto:[email-masked]> [email-masked]
Sent: Tuesday, March 29, 2022 7:22 PM
To: Sanchez Net, Marc (JPL-332H)[JPL Employee] <
<mailto:[email-masked]> [email-masked]>;
<mailto:[email-masked]> [email-masked]; 'Dr. Keith L Scott' <
<mailto:[email-masked]> [email-masked]>
Cc: 'Sipos, Brian J.' < <mailto:[email-masked]>
[email-masked]>;  <mailto:[email-masked]> [email-masked]
Subject: Re: [dtn] [EXTERNAL] Re: [Sis-dtn] [EXT] Re: Bundle custody
transfer and reliable CLs

Marc, I certainly agree that this is a serious problem, but I don't agree
that it motivates development of a reliability mechanism above the CLA.  (I
believe a number of other requirements do that.)  Prox-1 may be technically
a reliable CLA, but since it routinely misses data it's apparently not the
right one to use for this environment.  Why are people in MRN opposed to
using LTP, which is a CCSDS standard that has been in continuous operational
use in low Earth orbit (on ISS) since 2016?

Sorry, this discussion is likely not of general interest to DTN WG; we
should take it to another thread.

Scott

From: Sanchez Net, Marc (US 332H) < <mailto:[email-masked]>
[email-masked]> 
Sent: Tuesday, March 29, 2022 1:09 PM
To:  <mailto:[email-masked]> [email-masked];
<mailto:[email-masked]> [email-masked]; 'Dr. Keith L Scott' <
<mailto:[email-masked]> [email-masked]>
Cc: 'Sipos, Brian J.' < <mailto:[email-masked]>
[email-masked]>;  <mailto:[email-masked]> [email-masked]
Subject: RE: [EXTERNAL] Re: [Sis-dtn] [dtn] [EXT] Re: Bundle custody
transfer and reliable CLs

I also agree that having reliability mechanisms above the CLA is necessary.
For example, the Mars Relay Network (MRN) uses the Prox-1 protocol on the
proximity links, which has a built-in Go-Back-N ARQ mechanism, so it is
technically a reliable CLA. Yet, they routinely miss data on the return
direction and both the relay orbiter and rover missions have accountability
mechanisms that look for missing data.

When I talk to people in the MRN, their biggest desire from DTN is to avoid
the problem above. In their words, they want a mechanism so that the rover
unequivocally knows that a complete data product has made it to the orbiter.

----------------------------------------------------------------------------
-------------------------------------------

Marc Sanchez Net

Telecommunications Engineer

Jet Propulsion Laboratory

Office:  <tel:(818)%20393-5840> (818) 354-1650 | Email:
<mailto:[email-masked]> [email-masked]

----------------------------------------------------------------------------
-------------------------------------------

From: SIS-DTN < <mailto:[email-masked]>
[email-masked]> On Behalf Of sburleig.sb--- via SIS-DTN
Sent: Monday, March 28, 2022 11:44 AM
To:  <mailto:[email-masked]> [email-masked]; 'Dr. Keith L
Scott' < <mailto:[email-masked]> [email-masked]>
Cc: 'Sipos, Brian J.' < <mailto:[email-masked]>
[email-masked]>;  <mailto:[email-masked]>
[email-masked];  <mailto:[email-masked]> [email-masked]
Subject: [EXTERNAL] Re: [Sis-dtn] [dtn] [EXT] Re: Bundle custody transfer
and reliable CLs

I agree with everything Felix says here (except perhaps abandonment of the
term "custodial", which seems to have a lot of resonance in the DTN
community).

It is true that computed retransmission timeout intervals will not
necessarily be absolutely accurate even in BIBE/CT, but at least there are
ways to limit the error.  Methods for bundle delivery time estimation based
on contact plan analysis were studied in detail and published almost 10
years ago: given known source and destination node ID and a bundle
origination time and bundle size, we can estimate the time of arrival of the
original bundle and consequently the time of arrival of its acknowledgment
bundle, even over asymmetric paths.

Certainly we don't have the final answer to BP node congestion.  QoS may
well play a role, and we are starting to think about that in DTN WG.  BPv7
does support the concept of "paring" excessive traffic by imposing lifetime
overrides that result in immediate bundle expiration, a somewhat manageable
mechanism for dropping bundles as necessary.

But I don't think we need to give up on flow control altogether.  When TCP
is used at the convergence layer, a receiving node that is facing congestion
can simply stop reading on the socket and let TCP flow control
back-propagate.  When LTP is used at the convergence layer, a receiving node
that is facing congestion can begin canceling sessions; this will result in
CL protocol failure at the sender, bouncing the affected bundles back up to
the BPA where reforwarding procedures can be initiated (admittedly an
implementation matter, at least for now).  Neither entails bundle reception
that requires the receiving BPA to make a decision.  Personally, I think
building on these kinds of capabilities to address congestion at the
convergence layer would be more powerful than demanding more functionality
from custody transfer.

Scott

From:  <mailto:[email-masked]> [email-masked] <
<mailto:[email-masked]> [email-masked]> 
Sent: Monday, March 28, 2022 7:54 AM
To: Dr. Keith L Scott < <mailto:[email-masked]> [email-masked]>
Cc: 'Sipos, Brian J.' < <mailto:[email-masked]>
[email-masked]>;  <mailto:[email-masked]> [email-masked]; Mehmet Adalier
< <mailto:[email-masked]> [email-masked]>;
<mailto:[email-masked]> [email-masked];
<mailto:[email-masked]> [email-masked]
Subject: Re: [Sis-dtn] [dtn] [EXT] Re: Bundle custody transfer and reliable
CLs

Yes, I do think there is a need for reliability mechanisms above the CLA
Layer. BIBE as a generic reliable CLA allows to address the case of
reliability for uni-directional links (although in this case the setting of
the timers may already get difficult) but it is currently not addressing: 
- non-timer based re-transmission (which might be very specific to a certain
node and deployment and therefore might be better addressed as part of the
BPA / AA) 
- forwarding a bundle without explicitly addressing a (singleton) next hop
(eg, an opportunistic downlink where some signal about reception or delivery
is uplinked at some later time). 

I agree that we somehow can consider all received bundles as 'custodial' as
a node should attempt to store and forward them. I also think that the
typical use of custody transfer for BPv6 has been probably more to be
informed about the reception (or deletion) of a bundle then whatever
(additional ???) responsibility would be taken by the receiving node. So,
maybe we should abandon the term 'custody' altogether. However, there are
certainly situations where a sending node would like to get information
about reception/delivery/deletion/forwarding of a bundle at another node in
an operationally viable and efficient way (so, not using the current Bundle
Status Reports). 

Regards, 
Felix 

From:        "Dr. Keith L Scott" < <mailto:[email-masked]>
[email-masked]> 
To:        "Mehmet Adalier" < <mailto:[email-masked]>
[email-masked]>, " <mailto:[email-masked]>
[email-masked]" < <mailto:[email-masked]>
[email-masked]>, "'Sipos, Brian J.'" <
<mailto:[email-masked]> [email-masked]>, "
<mailto:[email-masked]> [email-masked]" <
<mailto:[email-masked]> [email-masked]>, "
<mailto:[email-masked]> [email-masked]" < <mailto:[email-masked]> [email-masked]>, "
<mailto:[email-masked]> [email-masked]" <
<mailto:[email-masked]> [email-masked]> 
Date:        28/03/2022 15:11 
Subject:        Re: [Sis-dtn] [dtn] [EXT] Re: Bundle custody transfer and
reliable CLs 

  _____  

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: " <mailto:[email-masked]>
[email-masked]" <
<mailto:[email-masked]>
[email-masked]> on behalf of "
<mailto:[email-masked]> [email-masked]" <
<mailto:[email-masked]> [email-masked]>
Reply-To: Mehmet Adalier < <mailto:[email-masked]>
[email-masked]>
Date: Friday, March 25, 2022 at 1:21 PM
To: " <mailto:[email-masked]> [email-masked]" <
<mailto:[email-masked]> [email-masked]>, "'Sipos, Brian J.'" <
<mailto:[email-masked]> [email-masked]>, Felix Flentge <
<mailto:[email-masked]> [email-masked]>, "
<mailto:[email-masked]> [email-masked]" < <mailto:[email-masked]> [email-masked]>, "
<mailto:[email-masked]> [email-masked]" <
<mailto:[email-masked]> [email-masked]>
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 < <mailto:[email-masked]>
[email-masked]> on behalf of "sburleig.sb--- via SIS-DTN"
< <mailto:[email-masked]> [email-masked]>
Reply-To: < <mailto:[email-masked]> [email-masked]>
Date: Friday, March 25, 2022 at 10:07 AM
To: "'Sipos, Brian J.'" < <mailto:[email-masked]>
[email-masked]>, < <mailto:[email-masked]>
[email-masked]>, < <mailto:[email-masked]> [email-masked]>, <
<mailto:[email-masked]> [email-masked]>
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 < <mailto:[email-masked]> [email-masked]> On Behalf Of
Sipos, Brian J.
Sent: Friday, March 25, 2022 4:46 AM
To:  <mailto:[email-masked]> [email-masked];
<mailto:[email-masked]> [email-masked];  <mailto:[email-masked]>
[email-masked]
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 < <mailto:[email-masked]> [email-masked]> On Behalf Of
<mailto:[email-masked]> [email-masked]
Sent: Friday, March 25, 2022 6:11 AM
To:  <mailto:[email-masked]> [email-masked];  <mailto:[email-masked]>
[email-masked]
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" < <mailto:[email-masked]>
[email-masked]> 
To:        < <mailto:[email-masked]> [email-masked]>, "'Sipos,
Brian J.'" < <mailto:[email-masked]> [email-masked]>, <
<mailto:[email-masked]> [email-masked]> 
Date:        25/03/2022 01:16 
Subject:        Re: [dtn] Bundle custody transfer and reliable CLs 
Sent by:        "dtn" < <mailto:[email-masked]> [email-masked]> 

  _____  

"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 < <mailto:[email-masked]> [email-masked]> on behalf of
< <mailto:[email-masked]> [email-masked]>
Date: Thursday, March 24, 2022 at 6:04 PM
To: "'Sipos, Brian J.'" < <mailto:[email-masked]>
[email-masked]>, < <mailto:[email-masked]> [email-masked]>
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 < <mailto:[email-masked]> [email-masked]> On Behalf Of
Sipos, Brian J.
Sent: Thursday, March 24, 2022 1:39 PM
To:  <mailto:[email-masked]> [email-masked]
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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---

From: Lloyd W <[email-masked]>
Date: Sun, 17 Apr 2022 22:24:48 +1000

Anyone who has followed Bundle Protocol development for, 
oh, twenty years, will find the continued lack of agreement
on what custody transfer is and does to be quite revealing.

Lloyd Wood
http://lloydwood.users.sf.net/Personal/L.Wood/dtn/?

> On 25 Mar 2022, at 09:03, [email-masked] wrote:
> 
> 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.