[Sls-sea-dls] SDLS EP GVCID Issues

[David Koisser (external)]

Hi Gian Paolo,

As you said, it needs to be implemented "with some caution". However, I 
think standards should either avoid pitfalls like this entirely (if 
possible) or hint at them *very* clearly. It should be crystal clear how 
to process frames. This is especially true for a security standard. 

I don't think the current version does enough to hint at the inherent 
ambiguity of the GVCID, which took me quite a while to actually notice.

Cheers,
David



From:   Gian Paolo Calzolari/esoc/ESA
To:     "David Koisser (external)" <David.Koisser at esa.int>
Cc:     "Lucas, John P.\(GSFC-5960\)" <john.p.lucas at nasa.gov>, 
"SLS-SEA-DLS" <sls-sea-dls-bounces at mailman.ccsds.org>, 
"sls-sea-dls at mailman.ccsds.org", "Kazz, Greg J (313B)" 
<greg.j.kazz at jpl.nasa.gov>, "Matt Cosby" <matt.cosby at goonhilly.org>
Date:   15/07/2019 11:04
Subject:        Re: [Sls-sea-dls] SDLS EP GVCID Issues


David,
        I am mot sure I fully capture your issue/doubts.
There are indeed different definitions sometimes coming ether from real 
difference (TM and AOS do not have MAPs) or from some different 
terminology for historical reasons.

The general definition is indeed GVCID = TFVN + SCID + VCID & GMAPID = 
TFVN + SCID + VCID + MAPID with the caveat that the MAP Identifies may not 
exist and that the total length depends on the applied standard.
Implementers need to implement it with some caution.

The mapping between USLP and Proximity is due - as far as I remember - 
that Proximity-1 never really defined a GVCID.

Greg & Matt, copied here, may add more comments.

Note also tat there a pending corrigendum (not yet on web but approved) 
for  CCSDS 320.0-M-7    CCSDS Spacecraft Identification Field Code 
Assignment Control Procedures. Magenta Book. Issue 7. November 2017.

Regards

Gian Paolo




From:   "David Koisser (external)" <David.Koisser at esa.int>
To:     "sls-sea-dls at mailman.ccsds.org"@esa.int
Cc:     "Lucas, John P.\(GSFC-5960\)" <john.p.lucas at nasa.gov>
Date:   15-07-19 10:21
Subject:        [Sls-sea-dls] SDLS EP GVCID Issues
Sent by:        "SLS-SEA-DLS" <sls-sea-dls-bounces at mailman.ccsds.org>



Hello everyone, 

We noticed some more issues regarding the GVCID / MAPID. After checking, 
it turns out different standards have very different interpretations. I 
attached screenshots of the regarding sections of the blue books for your 
convenience. 

The SDLS EP GVCID seems to be an aggregate of the longest defined versions 
of the respective fields. To my knowledge SDLS EP seems to be the only and 
first document, which specifically defines a (merged) GVCID. All other 
standards only define them as GVCID = TFVN + SCID + VCID & GMAPID = TFVN + 
SCID + VCID + MAPID (without sizes). In USLP the relation between old and 
new frames is defined, but only for Proximity-1 (see C1.1). I am not sure 
if this is more clearly explained somewhere else. 

In SDLS EP I see two problems. First, there is no explanation or 
justification for the lengths. Second, this may possibly lead to severe 
bugs, e.g. overflowing a TM VCID bigger than what fits in the defined 3 
bits, like 16 bits in USLP or worse, 6 bits in TC (with identical TFVN as 
TM). Improper sanity checks here may even lead to security critical bugs. 

Cheers, 
David 

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[attachment "USLP.PNG" deleted by Gian Paolo Calzolari/esoc/ESA] 
[attachment "TM.PNG" deleted by Gian Paolo Calzolari/esoc/ESA] [attachment 
"TC.PNG" deleted by Gian Paolo Calzolari/esoc/ESA] [attachment 
"Segment.PNG" deleted by Gian Paolo Calzolari/esoc/ESA] [attachment "SDLS 
EP.PNG" deleted by Gian Paolo Calzolari/esoc/ESA] 
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