[Css-csts] Propose SpaceLinkDatUnit lower bound be set to 1

[John Pietras]

Members of the CSTS WG,
Last week I identified a discrepancy between the minimum size of the
SpaceLinkDataUnit type definition in the CLTU Blue Book-2 and the rest
of the Blue Books (see below).

I would like to propose that in the next revisions, the minimum size be
set to 1. This request is the result of a practical concern that I have
come across in trying to apply the CLTU service to our admittedly
non-CCSDS-standard application, but I think that the current
greater-than-one minimum size might arise as a more general problem.

As I understand it, the minimum size has been set to the smallest
possible size of the any of the CCSDS space link data units (as defined
in the reference model) that are moved by an SLE transfer service. When
only the RAF and CLTU services were being developed, the minimum size
was 7 octets, driven by the smallest possible telemetry frame. The
minimum size is now 4 octets, because the OCF field is 4 octets. 

Clearly, data type-specific processing is not depending on SLE to
enforce SLDU sizes, since the common SpaceLinkDataUnit allows too-small
(by CCSDS standards) "telemetry frames" and "CLTUs", and too-large OCF
fields (up to 64k). In all cases the legal size for any specific data
type must be enforced by application-level processes. 

There seems to be little advantage to requiring SLE to enforce a minimum
size that is specific to only one CCSDS SLDU (the OCF), and a potential
disadvantage for non-standard uses that transfer smaller data units.
Setting the minimum size to 1 would not diminish, SLE and it might allow
more potential uses of SLE transfer services. 

Thank you in advance for considering this proposal.

Best regards,
John

> -----Original Message-----
> From: John Pietras
> Sent: Thursday, May 12, 2005 4:58 PM
> To: css-csts at mailman.ccsds.org
> Subject: Discrepancy in SpaceLinkDatUnit lower bound
> 
> Members of the CSTS WG,
> Perhaps this is something that has already been recognized, but I will
> raise it in case it has so far otherwise escaped detection.
> 
> There is a discrepancy in the lower bound of the SpaceLinkDataUnit
type
> definition in the various sections A2.1 SLE TRANSFER SERVICE--COMMON
> TYPES:
> - In RAF-B-2, RCF-B-1, FSP-B-1, and ROCF-B-1, the lower bound is 4.
> - In CLTU-B-2, it is 7 (it was also 7 in both RAF-B-1 and CLTU-B-1).
> 
> I assume that the value was lowered to accommodate ROCF, but that the
> change failed to be reflected in CLTU-B-2.
> 
> Best regards,
> John