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

Gian.Paolo.Calzolari at esa.int Gian.Paolo.Calzolari at esa.int
Thu Feb 1 14:43:14 UTC 2018


Dear All,
        in the attached file you find my considerations over Greg's input 
and Ed's comments (marked red).

I converted Ed's e-mail below to a Word File to be able to use WinWord 
comments etc.
If required I can provide the original WinWord file, while I attach here 
the pdf for easier reading.

Comments are welcome.

Regards

Gian Paolo







From:   "Greenberg, Edward (312B)" <edward.greenberg at jpl.nasa.gov>
To:     "Kazz, Greg J (312B)" <greg.j.kazz at jpl.nasa.gov>, 
"Gian.Paolo.Calzolari at esa.int" <Gian.Paolo.Calzolari at esa.int>, John 
Pietras <john.pietras at gst.com>
Cc:     "sls-slp at mailman.ccsds.org" <sls-slp at mailman.ccsds.org>
Date:   01/02/2018 07:17
Subject:        Re: [Sls-slp] Packet channel multiplexing no longer 
needed? : Results of Discussion with Takahiro Yamada on Revising SPP Blue 
Book
Sent by:        "SLS-SLP" <sls-slp-bounces at mailman.ccsds.org>



My comments are in red in the text below. 
 
From: SLS-SLP [mailto:sls-slp-bounces at mailman.ccsds.org] On Behalf Of 
Kazz, Greg J (312B)
Sent: Wednesday, January 31, 2018 3:39 PM
To: Gian.Paolo.Calzolari at esa.int; John Pietras <john.pietras at gst.com>
Cc: sls-slp at mailman.ccsds.org
Subject: Re: [Sls-slp] Packet channel multiplexing no longer needed? : 
Results of Discussion with Takahiro Yamada on Revising SPP Blue Book
 
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.  APID is already in the packet thus it need 
not be supplied externally.
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).  There is a packet stream to each VC SAP. 
If there is only one packet stream then the transfer protocol must include 
the VCID along with the QoS desired.  I don’t understand APID_Qualifier.
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?  I’m not quite sure what you mean by data forwarding.   The 
Logical Data Path concept that was defined in the otiginal Source Packet 
sped was for a single link.  That could be a local network addressing 
scheme or a remote addressing scheme across a single link.  This later 
scheme is what is used almost totally by non-manned missions.  The use of 
a standard set of labels within the secondary header is a fine use of the 
current  packet format for multiple things but the current users use the 
secondary header flag for non-standard secondary headers.
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 ?   We 
may not need to modify the current format if we set a few constraints(i.e. 
we could define a single APID in the packet header as a flag signaling the 
inclusion of standard labels in the secondary header.  These standard 
labels could provide the true APID, APID_Qualifier( whatever this is), QoS 
Rqmt, time of generation, time to live, priority for delivery, global 
source and destination addresses, packet security parameters. Etc.  This 
methodology would have little effect on today’s usage of the Packet Format 
and Link layer Packet service.
4.      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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