[Sis-dtn] Telecon Notes 20210224

Wed Apr 7 20:14:41 UTC 2021

Dear Scott, 
    Thank you for your informative remarks. I fully agree with the possible removal of intermediate CPs. We too have not implemented them in the LTP version we are developing.
I agree also on session timer, which would protect also against possible stalls, should one of the two node stop operating (e.g. if the sender crashes after having sent an intemediate RAS but before the CP following the re-tx segments, the receiver should stall forever). There is another case, trivial to calculate, which could be used as ceiling value: in a block there are bundles, and bundles have a lifetime. The highest lifetime of the bundles contained in a block could be safely used as a ceiling for the session deadline. However, in this case, it could be the BP to tell LTP to cancel an ongoing session. 
Concerning block lenght, I agree with you on the max lenght constraint. However, to know in advance the lenght of a block could be still advantageous becouse this information could be used to reserve the right amount of space in RAM for the Rx session buffer, not a single byte more than necessary. This way, the total amount of RAM avaiulable for LTP at Rx side, could be pooled among all on going sessions, without putting any hard limit on the max number of concurrent Rx sessions. For example, given a total amount of RAM available, we could use this to fit 10 large blocks, or 100 small blocks, or whatever intermediate combination. This could add flexibility when speed at Rx is necessary (i.e. RAM compulsory). This would be not alternative to fragmenting very large bundles which would be most likely necessary anyway (we can neither use all RAM for a giant block, nor discover after having saturated all mememory available that we are sorry but we cannot accomodate any more bytes!). A possible solution could be to insert the block lenght in an extension, as an option, to be activated when there are no problems of bandwidth available, as in optical links.
Yours,
   Carlo
________________________________________
Da: Burleigh, Scott C (US 312B) [scott.c.burleigh at jpl.nasa.gov]
Inviato: mercoledì 7 aprile 2021 19:37
A: Carlo Caini; Felix.Flentge at esa.int
Cc: sis-dtn at mailman.ccsds.org
Oggetto: RE: [Sis-dtn] Telecon Notes 20210224

A couple of remarks:

As you have guessed, ION doesn't insert intermediate CPs because they would have introduced yet more complexity for limited advantage.  I definitely would not argue against removing them from the protocol.

Session timer is a good idea, though not really trivial to implement.  The right way to compute the timer interval, I think, would be to consult the contact plan and use techniques similar to the ones developed by Nikolaos Bezirgiannidis for bundle delivery time estimation.  This would be more accurate than a user-provided guess, and both ends of the session could make this computation independently; there would be no need for signaling.

Signaling block length is certainly attractive, but I can't think of a clean way to do it.  The first segment of the block is just as likely to be lost in transmission as the last.  Block length could be included in every Nth data segment, to minimize the risk of losing it, but that adds transmission overhead that increases with decreasing values of N.  I think a better approach might be simply to set a block size limit by management; when a bundle is to be inserted into a block whose remaining capacity is less than the size of the bundle, fragment the bundle.  (We already have to do that for the UDP convergence-layer adapter anyway.)

Scott

-----Original Message-----
From: SIS-DTN <sis-dtn-bounces at mailman.ccsds.org> On Behalf Of Carlo Caini
Sent: Friday, February 26, 2021 2:31 AM
To: Felix.Flentge at esa.int
Cc: sis-dtn at mailman.ccsds.org
Subject: [EXTERNAL] Re: [Sis-dtn] Telecon Notes 20210224

Dear Felix,
    intermediate CPs can be useful, in theory, if radiation time >> RTT, as interpreted by Tomaso. Assuming as a rule of thumb that the radiation time of one bundle does not exceeds 1s, intermediate CPs have no advantages starting from Moon communications and on. You could still have some advantages on shorter RTTs, as with LEOs.
However you cannot release anything at Rx side, until the whole block has been received, which means that in the best case you can save only the time elapsed between the intermeiate CP (let me assume for the sake of similicity we have only one at a half of the block) and the last. This is equal to a half radiation time, and this only under the conditions that you have lasses only on the first half of the block and not on the second one. This bacause in the latter case you should wait for the RS triggered by the final CP to start retx of losses, thus the first intermediate CP would be useless in this case. This is why I think these intermediate CPs are not implemented in ION, but of course I am not sure...

Concerning the session timer, I happy to know that we share the same vision. By the way, I suppose it should be easy to implement. This time could also be "advertised" to the Rx side, by means  of a segment extension, inserted on the first segment (or on all). On the same line, I think that it could be useful to let the Rx know the dimension of the block asap. This could allow the Rx to cancel a too long (in size) session in a proactive way, without waiting to have received too many segments. To know the lenght in advance could also help to reserve to the Rx session buffer the right amount of space (provided that there is sufficient space), etc. Last, it could also help when the last segment, that flagged as a CP, is lost. In this case the session stalls for an RTO, because the receiver do nothing until the retranmitted CP arrives (after one RTO, as said, i.e. 6 minutes from Earth to Mars in the best case). By knowing in advance the block lenght, it would be strightforward to force the transmission of a gratuitous RS after a much shorter inter-segment timer has elapsed (it could be be set equal to the expected radiation time of the block).
Yours,
  Carlo

________________________________________
Da: Felix.Flentge at esa.int [Felix.Flentge at esa.int]
Inviato: venerdì 26 febbraio 2021 10:05
A: Carlo Caini
Cc: sis-dtn at mailman.ccsds.org; Tomaso.deCola at dlr.de
Oggetto: RE: [Sis-dtn] Telecon Notes 20210224

Hi Carlo,

I assume there could be advantages of intermediate checkpoints in cases where the duration of the session >> RTT (could reduce overall latency, on-board memory could be freed).

I certainly like the idea of a blocklifetime. It could maybe even be an optional parameter of the Transmission.request (which might need some changes anyway because of the removal of mixed sessions).

Regards,
Felix

From:        "Carlo Caini" <carlo.caini at unibo.it>
To:        "Felix.Flentge at esa.int" <Felix.Flentge at esa.int>, "Tomaso.deCola at dlr.de" <Tomaso.deCola at dlr.de>
Cc:        "sis-dtn at mailman.ccsds.org" <sis-dtn at mailman.ccsds.org>
Date:        26/02/2021 09:01
Subject:        RE: [Sis-dtn] Telecon Notes 20210224
________________________________

Dear Felix,
   I agree with you that the text is difficult to understand, but I would ascribe the problem to the protocol, which is maybe too complex. In particular, 1) the possibility of inserting CP before of the final segment of a block and 2) the possibility of sending multiple RS in response to the same CP, make the the protocol most likely more complex than necessary, and in turn, also difficult to describe.
My interpretation of the text (I am not sue too) is that at the first Tx attempt, the oustanding CPs are 1) the compulsory CP that flags the latest segment; 2) any other gratuitous CP that may be inserted before the latest segment. Each of these CP triggers one or more RS, depending on the number of claims; these RSs, one recived, trigger data ReTx and new CPs, belonging to the first Re-Tx cycle and so on. It is diffiucult to explain and to implement.
Now, if you remove the possibility of inserting intermediate CPs (actually, they are never inserted by ION), things start to improve. By the way, with a propagation delay that is dominant on the "radiation time", in my opinion there is not any advantage in using intermediate CPs. Let us assume that the RTT is 6 minutes and that the radiationtime is 1s. What is the advantage of receiving an intermediate CP at, say 500ms before the last segment flagged as CP? It would just generate an RS 500ms before the last, thus reTx could start 500ms before, which seems of little help, considering the 3600 s RTT.
The possibility of spreading claims on multiple RS is manageable (ION does), but it makes once again maybe more complex than necesary the algorithm, as multiple RSs means multiple CPs (the "oustanding ones" in my interpretation) for each Re-Tx cycle.
Last comment, I agree once again with you that many conditions could suggest the closing of an ongoing sessions. To those you mentioned I would add the duration of a session. I mean that in my opinion a session should have a time limit. Suppose that an RS is followed by an RAS (RS ACK), but not by any retranmissions for whatever reason (the Tx is lazy or worse...). On the Rx side a buffer plenty of data would be kept forever as the RS was confirmed by the RAS...
A second reson for a time limit, is that a block should have a lifetime, as bundles. Otherwise is pefectly possible, in the presence of a scheduled intermittent link, that a session stay frozen for hours and that during this time the content of the block, i.e. a bundle, expires. Is a non-sense that, at the next, contact LTP carry on a session whose content is to be discarded as soon as passed to BP. This could be easily avoided by associating a blocklifetime (<=bundle_lifetime)  to every session.
Yours,
  carlo

________________________________________
Da: SIS-DTN [sis-dtn-bounces at mailman.ccsds.org] per conto di Felix.Flentge at esa.int [Felix.Flentge at esa.int]
Inviato: venerdì 26 febbraio 2021 08:08
A: Tomaso.deCola at dlr.de
Cc: sis-dtn at mailman.ccsds.org
Oggetto: Re: [Sis-dtn] Telecon Notes 20210224

Hi Tomaso,

maybe I am just confused but I have some difficulties to understand what this text actually means (in particular,  what is meant by all outstanding CP segments?). It also seems dependent on the sender's strategy when to send CPs.

Actually, looking at 6.13 I am wondering whether we need this at all. 6.13 allows to cancel a session with RLEXC if the number of transmission problems exceeds an established limit. Maybe this could also cover re-transmission cycle limits?

Thinking about this, I believe we need to leave some room for implementation-specific limits which could come in a lot of different flavours (absolute re-transmitted amount of data, relative re-transmitted amount of data, number of queued segments for re-transmission, ....). It just seems strange that for one of these 'implementation-specific' limits we have a specific error code (RXMTCYCEXC).

Regards,
Felix

From:        <Tomaso.deCola at dlr.de>
To:        <kscott at mitre.org>, <sis-dtn at mailman.ccsds.org>, <Felix.Flentge at esa.int>
Date:        25/02/2021 14:17
Subject:        RE: Telecon Notes 20210224
________________________________

Hi All,

Concerning the point on the retransmission cycle limit, this is first appearing on RFC 5326 at the end of section 6.2.2 (beginning of page 34) and copied here below for your convenience:

There may be other implementation-specific limits that may cause an

  LTP implementation to initiate session-cancellation procedures.  One

  such limit is the maximum number of retransmission-cycles seen.  A

  retransmission cycle at the LTP Sender comprises the two related

  events: the transmission of all outstanding CP segments from the

  sender, and the reception of all RS segments issued from the receiver

  in response to those CP segments.  A similar definition would apply

  at the LTP Receiver but relate to the reception of the CP segments

  and transmission of all RS segments in response.  Note that the

  retransmitted CP and RS segments remain part of their original

  retransmission-cycle.  Also, a single CP segment may cause multiple

  RS segments to be generated if a reception report would not fit in a

  single data link-MTU-sized RS segment; all RS segments that are part

  of a reception report belong to the same retransmission cycle to

  which the CP segment belongs.  In the presence of severe channel

  error conditions, many retransmission cycles may elapse before red-

  part transmission is deemed successful; an implementation may

  therefore impose a retransmission-cycle limit to shield itself from a

  resource-crunch situation.  If an LTP sender notices the

  retransmission-cycle limit being exceeded, it SHOULD initiate the

  Cancel Session procedure (Section 6.19<https://urldefense.us/v3/__https://tools.ietf.org/html/rfc5326*section-6.19__;Iw!!PvBDto6Hs4WbVuu7!ch8g7EEz_e1eMBetbP7PTJqPefb5u01juhSStaKt4RA44_D6-QcvmVew1_M1m83-sy402NXl$ >), queuing a CS segment with

  reason-code RXMTCYCEXC and sending a transmission-session

  cancellation notice (Section 7.5<https://urldefense.us/v3/__https://tools.ietf.org/html/rfc5326*section-7.5__;Iw!!PvBDto6Hs4WbVuu7!ch8g7EEz_e1eMBetbP7PTJqPefb5u01juhSStaKt4RA44_D6-QcvmVew1_M1m83-s1rm8pEy$ >) to the client service.

@Felix.Flentge at esa.int<mailto:Felix.Flentge at esa.int>: which aspects did you want to make normative?
Best Regards,

Tomaso

From: SIS-DTN <sis-dtn-bounces at mailman.ccsds.org> On Behalf Of Dr. Keith L Scott
Sent: Mittwoch, 24. Februar 2021 17:52
To: sis-dtn at mailman.ccsds.org
Subject: [Sis-dtn] Telecon Notes 20210224

SIS-DTN Telecon

LTP

Took some notes in the SharePoint List here:

https://urldefense.us/v3/__https://cwe.ccsds.org/fm/wiki/Lists/SISDTN*20LTP*20Update*202021/AllItems.aspx__;JSUl!!PvBDto6Hs4WbVuu7!ch8g7EEz_e1eMBetbP7PTJqPefb5u01juhSStaKt4RA44_D6-QcvmVew1_M1m83-s4ojz1VX$

What testing / experimentation to drive out issues?

• LTP Extension Header vs. small Green Blocks to signal adaptive coding and modulation.

• ESA looking to use LTP for optical; no experimentation expected for 8-12 months

ReTx cycle limit & Discretionary Checkpoints

People to come up with draft clarifying language and we’ll review.  Consult the RFC5326 text to see if we want to ‘normativeize’ it.  Keith to collect text from RFC5326 to provide context.

BPv7

Keith to build a list of items like the LTP list for discussion at next meeting.

BPSec

In Security WG processing; we need to notify them of status of BPSec doc in IETF.

People need to review current draft and we’ll work any proposed changes to the SEA-SEC WG ASAP:

https://urldefense.us/v3/__https://tools.ietf.org/html/draft-ietf-dtn-bpsec-27__;!!PvBDto6Hs4WbVuu7!ch8g7EEz_e1eMBetbP7PTJqPefb5u01juhSStaKt4RA44_D6-QcvmVew1_M1m83-s9rlVfia$

Default security context from IETF is probably not what we want in CCSDS; we might need to define our own set.

ESA to start an activity on security.

NM Books

Links to I-Ds

NM books going through IETF, about to become WG documents, changes as a result of reviews may be coming over the next 8-12 months.

ICPA Need Dates for NM updated to 2028,  we can prioritize the other efforts above these.

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