[Sis-dtn] Positive reception claim vs. Negative reception claim in LTP Report Segment preparation and processing

Mon Apr 4 10:09:16 UTC 2022

Hi Cheol, Felix,
The efficiency of positive vs. negative claims is highly dependent upon the behaviour of the underlying link. If a link has long periods of successful communication punctuated by brief (complete) fading events, then NACK may be better. If a link is more erratic, then the calculations become a bit harder and are highly dependent on the ratio and duration of successful vs lost packets/frames.

In most “reliable” space links, fading is pretty intermittent (until your elevation reduces), so ACK/NACK should be pretty similar. I think Ka/optical might upset this balance though… We’ll see.

Thanks,
Jeremy

From: SIS-DTN <sis-dtn-bounces at mailman.ccsds.org> On Behalf Of Felix Flentge via SIS-DTN
Sent: Monday, April 4, 2022 12:08 PM
To: 구철회 <chkoo at kari.re.kr>
Cc: sis-dtn at mailman.ccsds.org
Subject: Re: [Sis-dtn] Positive reception claim vs. Negative reception claim in LTP Report Segment preparation and processing

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>>
To:        <Felix.Flentge at esa.int<mailto:Felix.Flentge at esa.int>>
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>>
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>
________________________________

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

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보낸사람 : <Felix.Flentge at esa.int<mailto:Felix.Flentge at esa.int>>
받는사람 : "구철회" <chkoo at kari.re.kr<mailto:chkoo at kari.re.kr>>
참조 : "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>>
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>>
To:        "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>>
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>>
________________________________

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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