[Sis-dtn] R: Special use of sequence number in bundle creation time stamp

Wed Aug 9 15:50:19 UTC 2023

I fully agree with Scott on his comments and suggestions. More generally, I would avoid, whenever possible, the use of a parameter designed to a specific end, such as timestamp sequence number here, to different purposes. However, appealing it may be on a short term, it is looking for troubles on the long term.
Specifically, the timestamp sequence number is not the same as an hupothetical bundle sequence number, as pointed out by Scott. Moreover, if we assume that a source generates bundles to a give destination to specific rate, e.g. one bundle per second, the sequence number increment between two consecutive bundles of this flow may be larger than one. For example, the first sequence number of the bundle flow could be 1 and the second 5. It is enough that the same source generates in the meantime other 3 bundles to different destinations.
Yours,
   Carlo

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Da: SIS-DTN <sis-dtn-bounces at mailman.ccsds.org> per conto di sburleig.sb--- via SIS-DTN <sis-dtn at mailman.ccsds.org>
Inviato: mercoledì 9 agosto 2023 17:31
A: 'Felix Flentge' <Felix.Flentge at esa.int>; 'Dr. Keith L Scott via SIS-DTN' <sis-dtn at mailman.ccsds.org>
Cc: 'Singh, Somendra {Simon} (GSFC-5820)' <simon.singh at nasa.gov>; 'Wilmot, Jonathan J. (GSFC-580.0)[VANTAGE SYSTEMS INC]' <jonathan.j.wilmot at nasa.gov>
Oggetto: Re: [Sis-dtn] Special use of sequence number in bundle creation time stamp

Hi, Felix.  One thing to keep in mind is that the purpose of the sequence number in the bundle creation timestamp is to distinguish among bundles that have the same bundle creation time.  This can occur when bundles are created at a very high rate or when bundle creation time is zero due to the absence of an accurate clock.

I would suggest that using bundle creation timestamp sequence numbers for other purposes makes it more difficult to use those numbers for their original purpose.

If sequence numbering of bundles in these various ways is important, I think it would be better to encode those numbers (and the flags describing their significance) in an immutable extension block, i.e., an extension block that is one of the additional targets of a block integrity block covering the primary block.

Scott

From: SIS-DTN <sis-dtn-bounces at mailman.ccsds.org> On Behalf Of Felix Flentge via SIS-DTN
Sent: Wednesday, August 9, 2023 7:43 AM
To: Dr. Keith L Scott via SIS-DTN <sis-dtn at mailman.ccsds.org>
Cc: Singh, Somendra {Simon} (GSFC-5820) <simon.singh at nasa.gov>; Wilmot, Jonathan J. (GSFC-580.0)[VANTAGE SYSTEMS INC] <jonathan.j.wilmot at nasa.gov>
Subject: [Sis-dtn] Special use of sequence number in bundle creation time stamp

Dear All,

I think we have had some initial exchange on the use of the sequence number in the bundle creation timestamp some time ago. Meanwhile, we had a number of side meetings with Jonathan, Simon and others to further discuss the topic of ‘flows’, compressed bundle reporting and network management.

We have arrived at a point where we think that it makes sense to allow using the sequence number in the primary header for identifying gaps in sequences of bundles or to even allow for in-sequence delivery.

The basic idea is that the sequence number does not get reset to zero but is incrementing by one until a specified maximum. Further we might use different, individual sequence numbers per source node ID or per destination endpoint ID. Such a behaviour might be governed by policy or even indicated in the Primary Block’s Processing Control Flags, like

  1.  Use two bits to specify whether the creation timestamp sequence number is used in special way:

  *   00 – single sequence number - no specific usage
  *   01 – an individual sequence number is used per source node id
  *   10 – an individual sequence number is used per destination endpoint id
  *   11 – reserved (until we find a good usage; maybe individual sequence number per next hop)

In case 10 with different destination endpoint IDs, the BPA has to guarantee that the same values can only appear with different bundle creation times!

  1.  Use of two bits to indicate the maximum length of the sequence number (00 above) or individual sequence numbers (01 and 10 above):

  *   00 - sequence numbers are assigned somehow (reset to zero, randomly, ..)
  *   01 - increasing sequence number up to 2^16-1
  *   10 - increasing sequence number up to 2^32-1
  *   11 - increasing sequence number up to 2^64-1

My main questions:

  1.  Does it make sense to use flags in the primary header for this purpose or is it sufficient to document that sequence numbers can be used in that way and leave it up to policy to specify certain behaviours? I have a slight preference for using the BPCF as it seems more formal and could ease interoperability (and the sequence number is in the primary header, so why not indicate it there).
  2.  If we want to use BPCF, what is the way to specify this? Should we / do we need to go IETF or could CCSDS be sufficient (to start with)? Ideally we would use some of the reserved first 16 bits (e.g. Bit 7 – 10)
  3.  No matter whether we want to use BCPF or not, I think something should end up in the CCSDS BB as soon as possible. So, maybe there is a chance for a late RID (if it does not delay the overall publication process). We should start discussing (maybe tomorrow).

Regards,

Felix

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