[Sls-sea-dls] [EXTERNAL] Re: SDLS EP GVCID Issues

Wed Jul 17 21:00:36 UTC 2019

I am just now catching up on email threads in preparation for our teleconference.

With reference to the SDLS EP GVCID, it is indeed a concatenation of the longest defined versions of the respective fields (TFVN + SCID + VCID + MAPID if applicable).  The intention was for every SDLS EP implementation to have a standard, interoperable way to parse these values between Initiator and Recipient, for any combination of SLPs.  It would be possible to add a NOTE to that effect.  Since these values are logical bit fields taken from the Primary Header, I did not foresee them being used in mathematical operations where overflow conditions could be a problem.

As for inherent ambiguity, there is still the problem that TC and TM have the same TFVN ('00').  The solution to this problem was for the PDU itself to specify in the Service Group field (section 5.3.2.2.2.3...) whether the applicable SDLS link is Initiator-to-Recipient ('01') or Recipient-to-Initiator ('10').  With that, each end should have enough information locally to know whether TM or TC is the SLP used on the link.

Craig

From: SLS-SEA-DLS <sls-sea-dls-bounces at mailman.ccsds.org> On Behalf Of David Koisser (external)
Sent: Monday, July 15, 2019 8:26 AM
To: Gian.Paolo.Calzolari at esa.int
Cc: SLS-SEA-DLS <sls-sea-dls-bounces at mailman.ccsds.org>; Lucas, John P. (GSFC-5960) <john.p.lucas at nasa.gov>; Matt Cosby <matt.cosby at goonhilly.org>; "sls-sea-dls at mailman.ccsds.org"@esa.int
Subject: [EXTERNAL] Re: [Sls-sea-dls] SDLS EP GVCID Issues

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<mailto:David.Koisser at esa.int>>
Cc:        "Lucas, John P.\(GSFC-5960\)" <john.p.lucas at nasa.gov<mailto:john.p.lucas at nasa.gov>>, "SLS-SEA-DLS" <sls-sea-dls-bounces at mailman.ccsds.org<mailto:sls-sea-dls-bounces at mailman.ccsds.org>>, "sls-sea-dls at mailman.ccsds.org<mailto:sls-sea-dls at mailman.ccsds.org>", "Kazz, Greg J (313B)" <greg.j.kazz at jpl.nasa.gov<mailto:greg.j.kazz at jpl.nasa.gov>>, "Matt Cosby" <matt.cosby at goonhilly.org<mailto: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<mailto:David.Koisser at esa.int>>
To:        "sls-sea-dls at mailman.ccsds.org"@esa.int<mailto:%22sls-sea-dls at mailman.ccsds.org%22 at esa.int>
Cc:        "Lucas, John P.\(GSFC-5960\)" <john.p.lucas at nasa.gov<mailto: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<mailto: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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