[Sls-slp] My comments on SDLS requirements on TM, TC, AOS / Presence of a parameter

[Gian.Paolo.Calzolari at esa.int]

Dear Greg,
        thank you for your comments that will be discussed in the next 
days.

I would like just to report about the usage of the zero value to indicate 
absence of a given parameter.

You suggest to delete the two managed "Presence of Space Data Link 
Security Header" and "Presence of Space Data Link Security Trailer" as the 
length parameter covers the presence/absence of the field.
We discussed extensively the matter and even acknowledging that a zero 
value for the length can be used by an implementation to show absence, we 
finally agreed not to use this option for the following reasons:
- As stated in the chapter introduction, the managed parameters are 
defined in an abstract sense and are not intended to imply any particular 
implementation of a management system. As a consequence presenting two 
parameters instead of one is much more logical and explanatory to show 
that the given parameter can be omitted. Of course this is not preventing 
implementing the couple of parameters with a single "variable" in a real 
system.
- most of the times, using a zero value for the length imposes to describe 
a non continuous range for the allowed values (e.g. 0 and 2 to 4 octets). 
Conversely the use of two parameters really shows only the allowed values. 
As well stating only "Integer" for allowed values would be incorrect.

If needed, we can add a note to state that implementers may combine the 
two parameters in a single variable, but I do suggest keeping the two 
parameters for clarity and readability.

Best regards

Gian Paolo



From:
"Kazz, Greg J (313B)" <greg.j.kazz at jpl.nasa.gov>
To:
"Gian.Paolo.Calzolari at esa.int" <Gian.Paolo.Calzolari at esa.int>, "Marjorie 
de Lande Long" <marjorie at delandelong.com>
Cc:
"sls-slp at mailman.ccsds.org" <sls-slp at mailman.ccsds.org>, Moury Gilles 
<Gilles.Moury at cnes.fr>
Date:
16/01/2013 00:39
Subject:
[Sls-slp] My comments on SDLS requirements on TM, TC, AOS
Sent by:
sls-slp-bounces at mailman.ccsds.org



G.P. and Marjorie,

Attached please find my green lines (Word editing feature) to the 
documents you supplied us in Dec. 2012.
As you will see most of my edits are editorial in nature, except for the 
issue regarding what should the link layer protocol do with the contents 
of the frame when the SDLS authentication function returns a 
authentication verification failure. In this case, I do not think there 
should be an option as I stated in my notes, the link layer protocol 
should not pass the data on to the user, but rather dispose of it and let 
the user know that allow the user to know optionally that it was an error 
detected by security fct. But we can discuss this further before and 
during the Spring meeting.

Best regards,

Greg

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