[Css-csts] FW: Monitored Data CSTS RID "Generation Time for GET"

Wed Nov 19 16:37:26 UTC 2014

CSTSWG colleagues --
You have already received Mr. Pecchioli's reply (copied below) to my questions regarding his suggestion to add a Generation Time parameter to the GET Return. He states that for troubleshooting purposes it may be necessary to have the measurements timestamped to within an accuracy of a few milliseconds. He also goes on to say that if multiple parameter values are queried via a single GET invocation, then each value may have to have an individual timestamp. Here are my thoughts on the matter.

Regarding the need for a timestamp with accuracy to within an few milliseconds, if we agree that the MD-CSTS should be applicable to his troubleshooting use case, then we can add an appropriate generationTime parameter. 

However, I don't believe that his assertion that each queried parameter has its own timestamp in the Return necessarily holds true. We said in our meeting last week that the assumption is that each parameter value that is returned is captured at the time that the GET Return is generated. In other words, every value in the GET Return would have been sampled no more than some few (TBD) milliseconds before the time that the GET Return was created. I would make that requirement explicit in the procedure specification (although at this point I'm not exactly sure what the best way would be).  A follow-on question is, should the MD-CSTS specification set the specific limit, or simply say that it is an implementation parameter that must be documented in the PICS Proforma (and then add it to the PICS Proforma)?

If we do decide to add the generationTime parameter to the GET Return, should that be only an MD-CSTS extension of the Cyclic Report procedure, or should it be a general capability of the CSTS SFW Cyclic Report procedure?

Finally, I have been thinking about the statement that we made that the MD-CSTS shouldn't require a second Red Book review. However, we are considering adding this new capability, and we have already agreed to modify the Cyclic Report procedure to allow on-change-only reporting. These are significant technical changes, and I don't know that we can justify the "no Red-2 Agency review" claim. 

I welcome your comments on Mr. Pecchioli's reply and my proposed response. If you have comments, I would like to ask you to provide them (please copy the CSTSWG)  by 1 December so that we can come to a final resolution of this RID in a timely manner. Thank you.

Best regards,
John

-----Original Message-----
From: Mauro.Pecchioli at esa.int [mailto:Mauro.Pecchioli at esa.int] 
Sent: Wednesday, November 19, 2014 4:47 AM
To: John Pietras
Cc: CCSDS_CSTSWG (css-csts at mailman.ccsds.org)
Subject: Re: Monitored Data CSTS RID "Generation Time for GET"

Dear Mr. Pietras,

thanks for contacting me. The main reason why I raised the suggestion to add the Generation Time in the GetReturn is to harmonise the definition of this PDU with the TransferData PDU and avoid unnecessary differences in the associated processing of parameter data on the receiving side. However, you raise an important point about the 'time-stamping' accuracy of parameter samples. When using this data for standard monitoring (e.g. range checking, live visualisation, etc.), I believe that an accuracy in the order of few seconds is adequate. But I assume that the parameter data may also be useful in the context of troubleshooting analyses implying correlation with spacecraft telemetry data, in which case I believe that an equivalent accuracy in the time-stamping is required. It should be noted that we typically require an accuracy of a few milliseconds for the spacecraft telemetry parameter samples time-stamping. This may imply that each individual CSTS parameter sample is associated to its own generation time, rather than the complete PDU and all the contained parameter data.

I hope this helps

Best regards

M. Pecchioli

From:       John Pietras <john.pietras at gst.com>
To:         "mauro.pecchioli at esa.int" <mauro.pecchioli at esa.int>,
Cc:         "CCSDS_CSTSWG (css-csts at mailman.ccsds.org)"
            <css-csts at mailman.ccsds.org>
Date:       17/11/2014 21:15
Subject:    Monitored Data CSTS RID "Generation Time for GET"

Mr. Pecchioli,
Hello, I’m John Pietras. I’m the book editor for the CSTS Monitored Data Service Red Book. Thank you for reviewing the book.

One of your RIDs, titled “Generation time for GET”, stated:
“I suggest to add the generationTime parameter also for the GET Return PDU (similarly to the TRANSFER-DATA PDU). This is useful for the receiving side to know when the parameter value has been sampled.”

The CSTS Working Group discussed this RID and decided that we need a bit more information about your intentions for the recommended changes before we could decide on how to proceed.

Here is a little bit more background information that will hopefully help us come to a mutual understanding.

In the RID, you use the TRANSFER-DATA as an example of an invocation carrying a generationTime. However, there is a difference between the generation time of a TRANSFER-DATA invocation and a GET return, in that (in some cases,
anyway) a TRANSFER-DATA invocation may be stored in a Recording Buffer, and so the generation time is disjoint from the actual delivery time. In contrast, the GET return is always sent in immediate response to the GET invocation, so the generation time of the parameters can be assumed to be the time of receipt, minus some time for transmission through the underlying communications network. Furthermore, since the GET returns are all delivered in-sequence by the underlying TCP connection, there is no question as to the sequencing of the returns.

Therefore, adding a generationTime parameter to the GET return would be needed only if the generation time that can be estimated from the receipt time (i.e., normally within a second or two) is insufficiently accurate for an Agency’s (or Mission’s) purposes. Is there is a requirement to know the generation time with greater accuracy than within 2 seconds of the receipt time of the return?

The CSTS WG  is considering extending the GET return set of parameters to include a single generationTime parameter that applies to all values carried in the return . Note that this would be the generation time of the GET return itself; it would provide no indication of the actual freshness of each of the parameters being reported. The assumption is that all values being reported are current values.

If we were to add a generationTime parameter to the GET return, it would have to be with the understanding that all values being returned in the GET return are current as of the time that the GET return is generated and sent. That is, it would not be possible (without major reworking of not only the MD-CSTS but the CSTS Framework as a whole) to have measurements taken at various times sitting and waiting to be retrieved via the GET operation. If a measurements is “stale” by any appreciable amount of time, it negates the usefulness of having the generationTime in the GET return.

So, to summarize:
      1.       To your understanding, is it sufficient to know that the time
      of a measurement to within 2 seconds of its true generation time? If
      so, then no change to the MD service would be required.
      2.       If it is not sufficient, we could put a single generationTime
      parameter in the GET return. This generationTime would apply to all
      values in the return, which implies that all values contained in the
      return are sampled essentially instantaneously at generationTime. Would
      that suit the purposes for which you envision the generationTime
      parameter?

Thank you in advance for your help in refining and resolving this RID.

Best regards,
John Pietras
Principal Engineer
GST, Inc.
7855 Walker Drive, Suite 200
Greenbelt, MD 20770
240-542-1155

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