[Sis-dtn] [EXT] Re: Positive reception claim vs. Negative reception claim in LTP Report Segment preparation and processing
Felix.Flentge at esa.int
Felix.Flentge at esa.int
Tue Apr 5 06:09:43 UTC 2022
Hi,
maybe I am overlooking something: LTP already provides asynchronous
reception reports. Whether these are reporting gaps (NAK) or confirm
received parts (ACK) seems largely equivalent (the entries may just vary
by one depending on where we have gaps). When to send these asynchronous
reports is and shall be completely up to the receiver.
Regards,
Felix
From: "sburleig.sb--- via SIS-DTN" <sis-dtn at mailman.ccsds.org>
To: "'구철회'" <chkoo at kari.re.kr>, "'Dr. Keith L Scott'"
<kscott at mitre.org>, "'Carlo Caini'" <carlo.caini at unibo.it>,
<tomaso.decola at dlr.de>, <dstanton at keltik.co.uk>
Cc: sis-dtn at mailman.ccsds.org
Date: 05/04/2022 02:49
Subject: Re: [Sis-dtn] [EXT] Re: Positive reception claim vs.
Negative reception claim in LTP Report Segment preparation and processing
Sent by: "SIS-DTN" <sis-dtn-bounces at mailman.ccsds.org>
Some notes in-line below:
From: 구철회 <chkoo at kari.re.kr>
Sent: Monday, April 4, 2022 4:59 PM
To: sburleig.sb at gmail.com; Dr. Keith L Scott <kscott at mitre.org>; Carlo
Caini <carlo.caini at unibo.it>; tomaso.decola at dlr.de; dstanton at keltik.co.uk
Cc: sis-dtn at mailman.ccsds.org
Subject: RE: [Sis-dtn] [EXT] Re: Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
I think Keith thought it as if the last CP was missing.
That’s not how I read “will send the whole block and then a CP, which will
elicit a RS” but I may have misunderstood.
Immediately NACK RS can save time as amount of the difference between
current time and the last CP’s arrival time. I think the effect can be
negligible when bandwidth is very high and a session block length is
relatively small, so saving time is order of ms. Of course this effect
signifies as big as an RTT when the last CP from a sender is lost during
transmission.
Yes, in my example 9.96 seconds, about 2% of a round trip. I think this
is what Carlo was saying.
But I do like the way of “immediately RS”. However the immediately
discretionary RS NACK can quickly burn out the return link bandwidth when
continuous segment losses are happening while for independent segment loss
it will be reasonable. There should be some optimization and I think this
concept make LTP be expandable in various space environment.
I worry that this would introduce some additional configuration complexity
in a protocol that is already very complex to configure.
Cheol
From: sburleig.sb at gmail.com <sburleig.sb at gmail.com>
Sent: Tuesday, April 5, 2022 4:10 AM
To: Dr. Keith L Scott <kscott at mitre.org>; Carlo Caini <
carlo.caini at unibo.it>; tomaso.decola at dlr.de; 구철회 <chkoo at kari.re.kr>;
dstanton at keltik.co.uk
Cc: sis-dtn at mailman.ccsds.org
Subject: RE: [Sis-dtn] [EXT] Re: Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
Keith, I think this isn’t quite right.
Suppose we’re sending a block that is 1 Megabyte in size, split into
10,000 segments of 100 bytes each. Suppose the OWLT is 240 seconds and
the transmission rate is 800 kbps (so 100,000 bytes per second).
Suppose the 3rd segment is lost. The receiver will detect this loss when
the 4th segment is received. That segment will have left the antenna .04
seconds after transmission began, and it will arrive at the receiver 240
seconds later. The receiver will transmit a report segment immediately;
let’s ignore the processing and radiation time for issuing the report and
say that the report is received at the sender 240 seconds after the
arrival of the 4th segment ? so 480.04 seconds after transmission began.
That’s the time at which retransmission can be initiated, right?
Now suppose there is no further data loss. The EOB CP leaves the antenna
10 seconds after transmission began and arrives at the receiver 240
seconds later. The earlier report is still en route to the sender, so the
lost segment has not yet been retransmitted. The receiver transmits the
CP-triggered report immediately, at 250 seconds after transmission began.
That report is received at the sender 240 seconds after arrival of the
last segment, so 490 seconds after transmission began; that’s the earliest
time at which retransmission can be initiated.
So it seems to me that the gap-triggered RS has reduced closure latency by
9.96 seconds, not by 1 RTT. What have I got wrong?
Scott
From: Dr. Keith L Scott <kscott at mitre.org>
Sent: Monday, April 4, 2022 11:50 AM
To: Carlo Caini <carlo.caini at unibo.it>; tomaso.decola at dlr.de;
sburleig.sb at gmail.com; chkoo at kari.re.kr; dstanton at keltik.co.uk
Cc: sis-dtn at mailman.ccsds.org
Subject: Re: [Sis-dtn] [EXT] Re: Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
I think the 'win' would be (comparing a system that generated proactive
NACKs vs. a system that generated a CP only at the end of the block):
If there are 10,000 LTP segments and the 3rd segment is lost:
The NACK system will nack it “immediately” and elicit a retransmission.
Increased latency: about 1 segment.
The non-NACK (call it CP-only) system will send the whole block and then a
CP, which will elicit a RS and cause the hole to be filled. Adds 1 RTT
(and 1 retransmitted segment) to the block.
So the NACK system can save an RTT. If the loss rate is small (say, the
probability of filling all holes in one CP/RS/ReTX event is high) then
that’s really all it saves, regardless of the number of losses in the
block. If the loss rate is high enough that the expected number of
CP/RS/ReTX cycles is higher, the benefit is greater).
So in general if there are N segments and p(segment loss) is p, the number
of retransmission rounds should be about -log(N)/log(p) + 1
N = 10,000
P = 0.1
# rounds: 5
N = 10,000
P = 0.01
# rounds: 3
So a proactive-NACK implementation could potentially save about 5 RTTs
over a CP-only implementation (sort of a blatent assumption that none of
the losses are ‘too close’ to the end, but hey).
v/r,
--keith
On 4/4/22, 1:57 PM, "Carlo Caini" <carlo.caini at unibo.it> wrote:
Dear all,
on terrestrial applications LTP segment may arrive out of order,
thus NACK could result in unecessary retranmissions; maybe this is not a
problem as it is the same in TCP (fast retransmit cannot tolerate more
than a disorder of 3 TCP segments).
In space, maybe we can assume ordered delivery of LTP segment, thus is
true that the LTP receiver could immediately send a NACK as soon as a gap
is found (i.e. at the first non contigous claim rfeceived), instead of
waiting for the CP, thus saving some time; however, the advantage is
limited. If we call radiation time the time necessary to tranmit "on air"
a block, we could say that the NACK time gain should be on the evarage of
about a half of the radiatrion time. This saved time should however be
compared with the RTT, wich is the minimum time necessary for loss
recovery. RTT in space is usually >> radiation time, thus the advantage
would be negligible. Maybe it could be useful on LEO, where the RTT is
small, but we should also have large blocks and slow links...
Yours,
Carlo
________________________________________
Da: Tomaso.deCola at dlr.de [Tomaso.deCola at dlr.de]
Inviato: lunedi 4 aprile 2022 17:54
A: kscott at mitre.org; sburleig.sb at gmail.com; Carlo Caini;
chkoo at kari.re.kr; dstanton at keltik.co.uk
Cc: sis-dtn at mailman.ccsds.org
Oggetto: RE: [Sis-dtn] [EXT] Re: Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
(I cc again the whole SIS-DTN mailinglist that apparently disappeared
from my initial message)
At first glance I don’t see a space network configuration in which the
LTP segments belonging to a given LTP block could arrived misordered.
Perhaps if LTP is operated over UDP also in space (the current spec does
not prohibit it to the best of my memory) this could happen but I’d say it
is an unlikely configuration.
Tomaso
From: Dr. Keith L Scott <kscott at mitre.org>
Sent: Montag, 4. April 2022 17:43
To: sburleig.sb at gmail.com; Cola, Tomaso de <Tomaso.deCola at dlr.de>;
carlo.caini at unibo.it; chkoo at kari.re.kr; dstanton at keltik.co.uk
Subject: Re: [Sis-dtn] [EXT] Re: Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
But for CCSDS applications (and LTPv2 is a CCSDS thing not an IETF
thing) maybe we make the assumption that segments are not misordered? Or
that the misordering is ‘small’ so that some sort of timer / couter at the
receiver could filter out small anomalies? (e.g. hold off sending a NACK
for 1,000 segments)?
--keith
From: "sburleig.sb at gmail.com<mailto:sburleig.sb at gmail.com>" <
sburleig.sb at gmail.com<mailto:sburleig.sb at gmail.com>>
Date: Monday, April 4, 2022 at 11:39 AM
To: Tomaso de Cola <Tomaso.deCola at dlr.de<mailto:Tomaso.deCola at dlr.de
>>, Keith Scott <kscott at mitre.org<mailto:kscott at mitre.org>>, "
carlo.caini at unibo.it<mailto:carlo.caini at unibo.it>" <
carlo.caini at unibo.it<mailto:carlo.caini at unibo.it>>, Cheol Koo <
chkoo at kari.re.kr<mailto:chkoo at kari.re.kr>>, Dai Stanton <
dstanton at keltik.co.uk<mailto:dstanton at keltik.co.uk>>
Subject: RE: [Sis-dtn] [EXT] Re: Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
Yes, and for good reason: the design of LTP was originally lifted
directly from the CFDP Acknowledged Procedures (and thereupon tweaked a
bit). I think the argument against proactively reporting negative DS
reception claims was that the missing segments might be already en route
but slightly delayed due to transmission over a longer path. In space
flight communications this won’t happen because LTP will be running
directly over the link; when we test LTP on Earth it is somewhat more
likely, as LTP is running directly over UDP/IP and in theory those packets
might travel over multiple different routes.
Scott
From: Tomaso.deCola at dlr.de<mailto:Tomaso.deCola at dlr.de> <
Tomaso.deCola at dlr.de<mailto:Tomaso.deCola at dlr.de>>
Sent: Monday, April 4, 2022 8:31 AM
To: kscott at mitre.org<mailto:kscott at mitre.org>;
sburleig.sb at gmail.com<mailto:sburleig.sb at gmail.com>;
carlo.caini at unibo.it<mailto:carlo.caini at unibo.it>;
chkoo at kari.re.kr<mailto:chkoo at kari.re.kr>;
dstanton at keltik.co.uk<mailto:dstanton at keltik.co.uk>
Subject: RE: [Sis-dtn] [EXT] Re: Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
This looks similar to CFDP-class 2 with retransmission happening in
asynchronous mode, isn’t it?
Regards,
Tomaso
From: SIS-DTN <
sis-dtn-bounces at mailman.ccsds.org<mailto:sis-dtn-bounces at mailman.ccsds.org
>> On Behalf Of Dr. Keith L Scott via SIS-DTN
Sent: Montag, 4. April 2022 17:28
To: sburleig.sb at gmail.com<mailto:sburleig.sb at gmail.com>; 'Carlo Caini'
<carlo.caini at unibo.it<mailto:carlo.caini at unibo.it>>; '"구철회"' <
chkoo at kari.re.kr<mailto:chkoo at kari.re.kr>>; 'Keltik' <
dstanton at keltik.co.uk<mailto:dstanton at keltik.co.uk>>
Cc: sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>
Subject: Re: [Sis-dtn] [EXT] Re: Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
I think maybe a larger opportunity for improvement would be to have a
capability for a receiver to proactively NACK segments WITHOUT having to
receive a checkpoint first. That would allow autonomously sending NACKs
during the block transmission (thereby filling holes quickly as they are
detected) and then relying on the CP/RS/RA exchange to close out the
session.
--keith
From: "
sis-dtn-bounces at mailman.ccsds.org<mailto:sis-dtn-bounces at mailman.ccsds.org>
" <
sis-dtn-bounces at mailman.ccsds.org<mailto:sis-dtn-bounces at mailman.ccsds.org
>> on behalf of "
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>" <
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>>
Reply-To: "sburleig.sb at gmail.com<mailto:sburleig.sb at gmail.com>" <
sburleig.sb at gmail.com<mailto:sburleig.sb at gmail.com>>
Date: Monday, April 4, 2022 at 11:21 AM
To: 'Carlo Caini' <carlo.caini at unibo.it<mailto:carlo.caini at unibo.it>>,
Cheol Koo <chkoo at kari.re.kr<mailto:chkoo at kari.re.kr>>, Dai Stanton <
dstanton at keltik.co.uk<mailto:dstanton at keltik.co.uk>>
Cc: "sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>" <
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>>
Subject: [EXT] Re: [Sis-dtn] Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
Hi, guys. I believe we are actually talking about two distinct things
here.
It is true that positive ACKs are required. Positive acknowledgments
turn off the retransmission timers for checkpoints, report segments, and
cancellation segments.
Separately, the individual "claims" within a report segment might be
either positive or negative. I agree with Carlo, but think Cheol is
correct that negative claims can yield a small overhead advantage. For
any LTP transmission whose scope is from block offset P to Q in which
there are N gaps:
? If one of the gaps begins at P and another of the gaps ends at
Q, then the report must contain either N negative claims or N - 1 positive
claims.
? If no gap begins at P and no gap ends at Q, then the report
must contain either N negative claims or N + 1 positive claims.
? If one of the gaps begins at P or one of the gaps ends at Q,
but not both, then the report must contain either N negative claims or N
positive claims.
I would expect the second of these cases to occur more frequently than
the other two, assuming AOS/LOS events don't occur during the
transmission.
I don't see how either negative or positive claims processing is
simpler or easier, though; the representations are equivalent. Ease of
implementation would depend strictly on the manner in which segment
information is stored and accessed at the sending and receiving ends of
the transmission. I personally found positive claims to be simpler to
work with.
Scott
-----Original Message-----
From: SIS-DTN <
sis-dtn-bounces at mailman.ccsds.org<mailto:sis-dtn-bounces at mailman.ccsds.org
>> On Behalf Of Carlo Caini via SIS-DTN
Sent: Monday, April 4, 2022 5:47 AM
To: "구철회" <chkoo at kari.re.kr<mailto:chkoo at kari.re.kr>>; Keltik <
dstanton at keltik.co.uk<mailto:dstanton at keltik.co.uk>>
Cc: sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>
Subject: Re: [Sis-dtn] Positive reception claim vs. Negative reception
claim in LTP Report Segment preparation and processing
Dear Cheol,
let me consider an LTP block with for example 20 non contigous
losses, i.e. 20 gaps. The corresponding RS would include either 20
positive claims (if you have a gap at the start or at the end of the
block) or 21 cl;aims otherwise.
With NAK claim you would need 20 megative claims. Is that so
different?
You can say that it is easier to resend what has been explicietely
said is missing, true; however, on the rx side it is easier to say what
has been received than what is missing; all things considered, I cannot
see any signifiocant advantage by excplicietely declaring gaps instead of
received chunks.
Yours,
Carlo
________________________________________
Da: SIS-DTN [sis-dtn-bounces at mailman.ccsds.org] per conto di "구철회"
via SIS-DTN [sis-dtn at mailman.ccsds.org]
Inviato: lunedi 4 aprile 2022 14:20
A: Keltik
Cc: sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>
Oggetto: Re: [Sis-dtn] Positive reception claim vs. Negative reception
claim in LTP Report Segment preparation and processing
I think current LTP spec has positive ACK and negative ACK both. So if
it is reversed the result will be the same.
Let me bring below example again. To provide claim inforamtion for
retransmission of block 1000-2999,
<<original-positive ACK>>
lower bound = 0
upper bound = 7000
negative reception claim count = 2
offset = 0 <-- Positive ACK
length = 1000 <-- Positive ACK
offset = 3000 <-- Positive ACK
length = 4000 <-- Positive ACK
* Negative ACKs are hidden in separated Positive ACKs.
<<negative ACK>>
lower bound = 0 <-- Positive ACK
upper bound = 7000 <-- Positive ACK
negative reception claim count = 1
offset = 1000 <-- Negative ACK
length = 2000 <-- Negative ACK
I think the latter case can work too! Or am I missing something?
Cheol
--------- 원본 메일 ---------
보낸사람 : Keltik <dstanton at keltik.co.uk<mailto:dstanton at keltik.co.uk
>>
받는사람 : Vint Cerf <vint at google.com<mailto:vint at google.com>>
참조 : <Felix.Flentge at esa.int<mailto:Felix.Flentge at esa.int>>, <
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>>, "구철회" <
chkoo at kari.re.kr<mailto:chkoo at kari.re.kr>> 받은날짜 : 2022-04-04 (월)
19:53:46 제목 : Re: [Sis-dtn] Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing Scott
Burleigh and I went through this developing CFDP/LTP three decades ago.
Whilst Negative ACKs can be very efficient for bulk data in the
delay/disruption environment, protocol directives such as initiation,
metadata exchange, end of data, end of transaction, pause, resume etc
require positive ACKs. Otherwise the state machines will never close.
Dai
Sent from my iPhone
On 4 Apr 2022, at 11:09, Vint Cerf via SIS-DTN <
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org>> wrote:
a system based solely on negative acks will not work.
v
On Mon, Apr 4, 2022 at 6:08 AM Felix Flentge via SIS-DTN <
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org%3cmailto:sis-dtn at mailman.ccsds.org
>>> wrote:
Ah, yes, of course you are right.
We will look into the negative ACK as part of our LTPv2 prototyping
activity.
Regards,
Felix
From: "구철회" <
chkoo at kari.re.kr<mailto:chkoo at kari.re.kr<mailto:chkoo at kari.re.kr%3cmailto:chkoo at kari.re.kr
>>>
To: <
Felix.Flentge at esa.int<mailto:Felix.Flentge at esa.int<mailto:Felix.Flentge at esa.int%3cmailto:Felix.Flentge at esa.int
>>>
Cc: "
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org><mailto:sis-dtn at mailman.ccsds.org%3cmailto:sis-dtn at mailman.ccsds.org%3e>
" <
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org%3cmailto:sis-dtn at mailman.ccsds.org
>>>
Date: 04/04/2022 11:58
Subject: RE: Re: [Sis-dtn] Positive reception claim vs.
Negative reception claim in LTP Report Segment preparation and processing
Sent by:
chkoo at kari.re.kr<mailto:chkoo at kari.re.kr<mailto:chkoo at kari.re.kr%3cmailto:chkoo at kari.re.kr
>>
________________________________
Hi Felix,
I think current LTP spec quite works well with negative claim also.
Consider below reception claim according to the LTP spec but negative
claim.
lower bound = 0
upper bound = 7000
negative reception claim count = 1
offset = 1000
length = 2000
it means a receiver is requesting block of segements which starts at
1000 and length is 2000, i.e., 1000 ~ 2999, for retransmission.
A sender can safely remove 2 blocks, i.e., 0 - 999 and 3000 - 7000. I
think it is simpler, lower overhead and *importantly* easier to calculate
(acutally no painful for localizing the target segment position).
Cheol
--------- 원본 메일 ---------
보낸사람 : <
Felix.Flentge at esa.int<mailto:Felix.Flentge at esa.int<mailto:Felix.Flentge at esa.int%3cmailto:Felix.Flentge at esa.int
>>>
받는사람 : "구철회" <
chkoo at kari.re.kr<mailto:chkoo at kari.re.kr<mailto:chkoo at kari.re.kr%3cmailto:chkoo at kari.re.kr
>>>
참조 : "
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org><mailto:sis-dtn at mailman.ccsds.org%3cmailto:sis-dtn at mailman.ccsds.org%3e>
" <
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org%3cmailto:sis-dtn at mailman.ccsds.org
>>>
받은날짜 : 2022-04-04 (월) 17:40:24
제목 : Re: [Sis-dtn] Positive reception claim vs. Negative reception
claim in LTP Report Segment preparation and processing Hi Cheol,
interesting question. One thing I can think of is that the positive
claims would allow you to free memory earlier while for negative claims
you need to wait until the end of a session.
Regards,
Felix
From: "구철회 via SIS-DTN" <
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org%3cmailto:sis-dtn at mailman.ccsds.org
>>>
To: "
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org><mailto:sis-dtn at mailman.ccsds.org%3cmailto:sis-dtn at mailman.ccsds.org%3e>
" <
sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org<mailto:sis-dtn at mailman.ccsds.org%3cmailto:sis-dtn at mailman.ccsds.org
>>>
Date: 04/04/2022 10:15
Subject: [Sis-dtn] Positive reception claim vs. Negative
reception claim in LTP Report Segment preparation and processing
Sent by: "SIS-DTN" <
sis-dtn-bounces at mailman.ccsds.org<mailto:sis-dtn-bounces at mailman.ccsds.org<mailto:sis-dtn-bounces at mailman.ccsds.org%3cmailto:sis-dtn-bounces at mailman.ccsds.org
>>>
________________________________
Greetings,
This is Cheol. I am developing an LTP reference implementation. During
reading the LTP specification (RFC-5326), the preparation of reception
claim in Report Segment makes me confusing about why it is positive claim
not negative claim for segments that were not received successfully (i.e.,
NAK).
For reference, CFDP’s NAK PDU has the negative claim structure when it
is requested to report missing PDUs. Does anyone know about the background
of choosing the positive claim for NAK operation in LTP?
I think negative claim is simpler and more efficient in terms of
overhead for sender and receiver both.
I like to listen experts’ opinion on LTP operation and honestly hope
it to be changed in newly coming LTP spec.
Cheol
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