[Sls-slp] Proposed Changes to 732.1-B USLP Space Data Link Protocol

[Kazz, Greg (US 312B)]

Dear SLP WG,

Based upon Joachim’s concerns and my initial responses, I would like to propose the following changes to the CCSDS 732.1-B-3 book.
First,  I will copy my responses to Joachim’s questions, thereafter are my proposals to some relatively simple improvements to the USLP Blue Book to address them.
Please let me know your thoughts concerning this proposed change.

best regards,
Greg
Chair SLP WG

Hello Greg,

We noticed that you were involved with the addition of these two services, maybe even the initiator and author of the document changes? We are creating a USLP implementation and I'm a little confused by these additions.
** Actually, VCA and VCP are traditional link layer services for most of the CCSDS space data link protocols. We originally didn’t have them in USLP, because we thought users could a single default MAP ID and set it to zero, when they planned to emulate VCP or VCA services. However, we found it less ambiguous if we simply added VCP and VCA services to USLP. Happy to clarify below. **

In a related email you wrote that this amounts to more than using the corresponding MAP service and setting the MAP ID to zero, but it isn't clear from the current USLP specification how it is different from just that.

At the very least, there are loose ends in the specification.

1. The MAP ID field will still be present in the primary header. I cannot find any place in the specification that says what this field should be set to for the VCA and CVP services. Should it be set to zero, as your old email may have implied?
** Right, set the MAP ID field to zero in those two cases, since MAP IDs don’t apply. I will request that CCSDS add a statement to both services to make that explicit. **

2. Section 4.2.7.2 says "one VCA_SDU is treated as one TFDF". What does this mean? The VCA_SDU are defined in other sections as variable length packets, shouldn't a VCA_SDU get split into multiple TFDF, in general? Was this meant to apply only to the case when USLP Transfer Frames have variable length? But this section doesn't seem to be only about the variable-length transfer frame case. Note, that the same paragraph says "shall accept Transfer Frame Data Fields (TFDFs) from
the MAP Multiplexing Function. There seems to be inconsistent language here about MAPA and VCA. What exactly is this paragraph trying to say for VCA?


**"VCA_SDUs are variable length octet, delimited, octet aligned data structures, the format of which is unknown to the service provider”, so they are not packets. Packets fall under the VCP or MAPP cases. And you are correct in general, a VCA_SDU does span multiple TFDFs. In any case, the parenthetical  "one VCA_SDU is treated as one TFDF” is misleading and will be removed. But the previous  statement applies to all three cases: The Frame Initialization Procedure shall accept Transfer Frame Data Fields (TFDFs) from

the MAP Multiplexing Function, the VC Packet Processing Function, or a VCA Service User…  and accepts multiple TFDFs. **



3. Is there any difference whatsoever between a virtual channel used for VCA and a virtual channel used for MAPA with a single MAP ID in use?

** In the context of USLP, there is no difference.  It is my understanding that to date, ESA has a long history of using MAP IDs for packet telecommand applications. Other agencies such as NASA, have not implemented MAPs. **

Thanks,

Joachim
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Proposed Changes to USLP (732.1-B-3)


  1.
Introduce normative statements to explicitly state that for VCP and VCA Service, the MAP ID field shall be set to 0.

1.1   Under Section 3.6 VCA Service and under 3.6.2 General, add a new statement, 3.6.3 “The MAP ID field is set to 0.”  NOTE:  "MAP IDs are not used by the VCA Service but the MAP ID appears in the Transfer Frame Header.”
1.2   Under Section 3.4 VCP Service and under 3.4.1 Overview, add a new statement, 3.4.1.1 "The MAP ID field is set to 0.” NOTE:  "MAP IDs are not used by the VCP Service but the MAP ID appears in the Transfer Frame Header.”


  1.
Under Section 4.2.7.2 remove the parenthetical i.e., “(one VCA_SDU is treated as one TFDF)”.


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