[Sls-slp] Packet channel multiplexing no longer needed? : Results of Discussion with Takahiro Yamada on Revising SPP Blue Book

Wed Jan 31 23:38:42 UTC 2018

All,

Here is a follow on to the telecon that occurred today between myself, Gian-Paolo, Stefan Veit, and Ed Greenberg.

We are searching for the right approach to revise the SPP.
We are still in the brainstorming part of development. All inquiries and ideas are most invited!
As we discussed in the Haag in Nov. 2017, the Space Packet provides a duality of functionality: It is both an Application Layer (AP) Data Structure and Format as well as a data link layer transport mechanism i.e. it is transported within the frame data field of CCSDS transfer frames.
We all seem to agree that there should be no mention of multiple subnetworks and routing in the future SPP. These things are handled much better by DTN, IP, etc.

We go back to the basic structure of what we shall describe in the SPP Blue Book: (This is where I would like to get consensus on first)

  1.  In the Application Layer, there are multiple users (data sources) including the SPP Packet Assembly Function (out of strings it makes packets) that request the transfer of Space Packets out of the Application Layer by supplying the APID, APID_Qualifier (optional), QoS Rqmt (optional) – as currently defined by the SPP. This is accomplished by generating the packet.request primitive in SPP. So multiple users are producing packets with different APIDs under management control and outside the purview of CCSDS.
  2.  In the Application Layer, those packets provided by those multiple users and SPP Packet Assembly Function, shall be multiplexed to produce a single packet stream in the appropriate order set by management whose algorithm is not specified by CCSDS – as currently defined by SPP. (SPP talks about a single Queue, but use of the term “stream” is better because it is implementation agnostic)
  3.  NOW HERE COMES SOME CONTENTION – Do we want the SPP to have the capability of forwarding data (not routing data) on-board (mostly envisioned for on-board the spacecraft) within a single closed subnetwork in an A-B-A configuration ? If the answer is yes (I think it is yes), then APID_Qualifier needs to be well and unambiguously defined for SPP. I can supply a use case from the ISS. For Telecommand, Space Station uses Logical Data Path – Endpoint as the APID_Qualifier (this field occurs in the packet 2nd Header, which is currently non-compliant with SPP because there is currently no standard packet 2nd header defined) to forward commands to user modules on-board the ISS. In this case, Logical Data Path is short circuited to be a Logical Data Path Endpoint. OR Gian Paolo contents that we should retire the concept of Logical Data Path completely, and only leave a historical note in SPP about its previous existence?

We may need to hold open the possibility of Logical Data Path Endpoint or something similar until we get clarification from all agencies and the SIS area about their desires for eventually modifying the SPP packet 2nd header in order to accomplish data forwarding on-board a spacecraft. Does this data forwarding function take place in the application layer ?

  1.  In the Data Link Layer, the transfer across the space link of those Space Packets is requested. Here an additional multiplexing step may take place, packets of multiple PVNs are multiplexed into the stream, before packets are placed into the transfer frame data fields of frames. Each single packet stream is then injected into the TF data field of a specific VC frame by sending it to the instance of the Packet Processing Function for that VC that transports packets as defined in the appropriate SDLP Packet Processing Function. The most generic way of expressing this primitive is to use Encapsulation.request (Data Unit, SDLP_Channel, PVN, EPI). Of course, one could also accomplish this transfer by using VCP.request (only for AOS or TM) or MAPP.request (TC) or (USLP) or Packet Service in Prox-1.

Makes sense ?

Thanks!

Greg
Chairman SLP WG

From: "Gian.Paolo.Calzolari at esa.int" <Gian.Paolo.Calzolari at esa.int>
Date: Wednesday, January 31, 2018 at 6:30 AM
To: John Pietras <john.pietras at gst.com>
Cc: Greg Kazz <greg.j.kazz at jpl.nasa.gov>, "sls-slp at mailman.ccsds.org" <sls-slp at mailman.ccsds.org>
Subject: Packet channel multiplexing no longer needed? : [Sls-slp] Results of Discussion with Takahiro Yamada on Revising SPP Blue Book

John,
        I think that Packet channel multiplexing is still needed and surely implemented by space agencies.
The point is that those implementation are not exposed and I find this correct.
Let's say that we should keep the same approach we have had in the past; i.e. SPP book shall mention that Packet channel multiplexing is to be performed but leaving the implementation to users as loong as there no for a commen service (either SLS or CSTS).
If you think this is the same that happened with the TC standard and FSP.
The TC Standard mentioned that TC Frames were multiplexed over VCs/MAPs but gave no definition.
When SLE FSP was done it was realised that the TC Frame multiplexing required to be defined and in fact you have relevant clauses and an Annex in FSP.

The same for TM frame, they are multiplexed over a Master Channel and their multiplexing is mentioned but not defined (specially because that multiplexing is normally done on board a spacecrat :o).

Regards

Gian Paolo

From:        John Pietras <john.pietras at gst.com>
To:        "Gian.Paolo.Calzolari at esa.int" <Gian.Paolo.Calzolari at esa.int>, "Kazz, Greg J (312B)" <greg.j.kazz at jpl.nasa.gov>
Cc:        "sls-slp at mailman.ccsds.org" <sls-slp at mailman.ccsds.org>, SLS-SLP <sls-slp-bounces at mailman.ccsds.org>
Date:        30/01/2018 19:06
Subject:        Re: [Sls-slp] Results of Discussion with Takahiro Yamada on Revising SPP Blue Book
Sent by:        "SLS-SLP" <sls-slp-bounces at mailman.ccsds.org>
________________________________

All,

In an earlier email in this thread, Greg commented that he understood that many missions multiplex different “packet channels” into the same VC, but that he did not see this happening in an interoperable way. I think that Greg has hit on the important factor here – what is exposed in an interoperable, cross-supported way?

Back when we wrote the original AOS book, we had the notion that “cross support” users would be able to access individual packet channels via standard intefaces. This notion was formalized in the Cross Support Reference Model, Part 1 – Space Link Extension Services Blue Book (colloquially known as the “SLE Ref Model”), which envisioned two “flavors” of Forward Space Packet (TC and AOS) and one Return Space Packet service. Those services would have allowed multiple FSP instances to carry packete channels all destined for the same VC.

As it turned out, only the TC flavor of the Forward Space Packet service was ever realized.  Different instances of the (TC) FSP service are indeed capable of carrying different packet channels destined for the same VC. The FSP service uses MAP, so the necessary packet channel multiplexing into VCs is accomplished (indirectly) through MAP Multiplexing.

But had the AOS FSP been realized, it *would* have required the packet channel muxing that currently resides in the SPP. But since it wasn’t realized, the main use case (that I know of) disappears, and perhaps packet channel multiplexing is indeed no longer needed at a cross-support interoperability level, which I agree with Greg is the important factor in considering whether to retain the specification in the SPP (or migrate it back into one or more of the SDLP books).

Best regards,

John

From: Gian.Paolo.Calzolari at esa.int [mailto:Gian.Paolo.Calzolari at esa.int]
Sent: Tuesday, January 30, 2018 8:55 AM
To: Kazz, Greg J (312B)
Cc: John Pietras; sls-slp at mailman.ccsds.org; SLS-SLP
Subject: Re: [Sls-slp] Results of Discussion with Takahiro Yamada on Revising SPP Blue Book

Greg,
       what we have to be sure it captured in our CCSDS  books is the following situation (e.g. for a TM/AOS link)

  *   There multiple sources generating packet contents and asking creation of packets
  *   Those packets are created with different APIDs depending on the source
  *   Those different APIDs shall be multiplexed to produce a single packet stream:
  *   This single packet stream is then injected in in a VC
  *
It is then correct that "The current AOS SDLP version i.e., CCSDS 732.0-B-3 Sept 2015 is agnostic about “packet channels”  and “Individual CCSDS packets from multiple sources” (implying multiple APIDs). It is really only concerned about multiplexing of packets with different PVNs onto VCs." In fact AOS would only see that single stream.

All the rest shall be done above the SDLP.

We shall check also what we lost with respect to CCSDS 103.0-B-2 (now silver) where there was a nice picture for which I attach a detail.
Another document to be checked for losses is CCSDS 203.0-B-2 and even FSP that mention APID multiplexing if I remember well.

Finally we shall verify all fits within the encapsulation service.

Ciao

Gian Paolo
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