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<title>RE: [Sis-csi] Green book thoughts</title>
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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>I’ve been watching these emails go
back and forth and it sounds to me like the discussions we have previously had
in the process industries (refineries, petro-chem, food and beverage, pharmaceuticals,
etc),.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>In the process industries we use UDP as the
base commerical protocol for our control networks, and some of them go across
satellite links. We use UDP for one primary reason – cost and
availability. We don’t use TCP very often because of the overhead, the
fact that it is stream-oriented, and the lack of determinism caused by
variations of implementations. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>In addition, UDP is simple, doing very
little except for maintaining message boundaries (message in/message out).
We rarely, if ever, use checksums since we run over ethernet, which has proven
over the years to be reliable when it comes to data transmission. Of course,
there are always the arguments that there could be missed messages that aren’t
covered by the DLL checksum, but those haven’t been seen in the field. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>In addition, where we need to ensure a
higher level of reliability, the application layer protocols detect missed
messages and provide for necessary retransmissions. Generally, these
protocols are proprietary to one of the major process control companies. This
is because there was no existing standard protocol that would meet the needs of
a given control system when the control system was initially designed.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>The process industry protocols above UDP
always convert the UDP PDU (the message) into some data structure with
semantics (a Read Request or Response, and Event Notification, etc). They
generally do not create new layers for missed message detection and retransmission.
Instead these functions are integrated into the protocols that runs on top of
UDP. The reason for this is implementation and configuration simplicity,
which ultimately leads to cost management. We, in the process control
industries, do not make our money on protocols. They are a cost to us. Therefore,
they are carefully crafted to be simple and bundled to keep costs low. As
you know, there are significant fixed and variable costs associated with design,
implementation, testing, and deployment of each layer protocol.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
sis-csi-bounces@mailman.ccsds.org [mailto:sis-csi-bounces@mailman.ccsds.org] <b><span
style='font-weight:bold'>On Behalf Of </span></b>Scott, Keith L.<br>
<b><span style='font-weight:bold'>Sent:</span></b> Wednesday, April 19, 2006
8:44 AM<br>
<b><span style='font-weight:bold'>To:</span></b> L.Wood@surrey.ac.uk;
adrian.j.hooke@jpl.nasa.gov<br>
<b><span style='font-weight:bold'>Cc:</span></b> sis-csi@mailman.ccsds.org<br>
<b><span style='font-weight:bold'>Subject:</span></b> RE: [Sis-csi] Green book
thoughts</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>High utilization of limited contacts is
definitely a consideration. For reliable communications in that
environment, one could build reliability on top of UDP, or use TCP PEPs at
either end of the space link, or use DTN. Using PEPs allows the end
systems to use a standard model for reliable communications while still
enabling full use of the space link. DTN is attractive in that the end
systems don't have to be cognizant of the contact schedule.</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>Your points about CFDP are well taken, and
I believe Scott agreed that CFDP over UDP doesn't do congestion control.
As such I think it would fall into the category of 'use with caution so as not
to melt the network.' One benefit of CFDP is that it's designed to be a
standard service for applications, so at least every app wouldn't have to
re-implement it. I think it shares this with <st1:City w:st="on"><st1:place
w:st="on">Saratoga</st1:place></st1:City>.</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>For right now I would like to settle on an
architecture which is encompassing in what it allows and not get too involved
in guidance to mission designers. We WILL have to provide that guidance
soon, but I would prefer to make progress with the current document, since
we're not going to get to the degree of specificity for mission design in the
architecture document anyway. When it comes time to pick the recommended
set of protocols / services for cislunar (starting with the next set of
meetings?) we can figure out how to come to consensus on the set of recommended
protocols / services.</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font><font size=2
color=blue face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:blue'>--keith</span></font><o:p></o:p></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><b><font size=2 face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma;font-weight:bold'>From:</span></font></b><font
size=2 face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'> sis-csi-bounces@mailman.ccsds.org
[mailto:sis-csi-bounces@mailman.ccsds.org] <b><span style='font-weight:bold'>On
Behalf Of </span></b>L.Wood@surrey.ac.uk<br>
<b><span style='font-weight:bold'>Sent:</span></b> Wednesday, April 19, 2006
6:16 AM<br>
<b><span style='font-weight:bold'>To:</span></b> adrian.j.hooke@jpl.nasa.gov<br>
<b><span style='font-weight:bold'>Cc:</span></b> sis-csi@mailman.ccsds.org<br>
<b><span style='font-weight:bold'>Subject:</span></b> RE: [Sis-csi] Green book
thoughts</span></font><o:p></o:p></p>
<p><font size=2 face="Times New Roman"><span style='font-size:10.0pt'><!-- Converted from text/plain format -->>At
04:33 PM 4/18/2006, Lloyd wrote:<br>
>> You can use IP -- and UDP -- and be enhanced.<br>
><br>
> Enhanced? Seems like you've got a custom, non-standard application running<br>
> over UDP (which doesn't *do* anything, so presumably that custom<br>
> application has its own custom reliability built into it?) running over IP<br>
> which "routes" you from a single processor on the spacecraft
over a single<br>
> link to a single processor located directly in the ground station.<br>
<br>
UDP provides per-packet checksums, multiplexing and identification via ports,
and a standard widespread sockets interface convention for building on top of.<br>
<br>
That's why many custom non-IETF-standard applications, including Skype, Real,
and CFDP, use UDP. I presume CFDP also implements its own custom reliability?
Is that somehow a bad thing? (No.)<br>
<br>
The choice of path from one of a network of scheduled processors on the DMC
spacecraft to one of a network of computers in the ground station LAN is
dictated solely by pass utilization and wanting to get the most from the
space/ground link while it's active, rather than introduce a bottleneck
elsewhere with a longer path. That applies no matter what UDP-based protocol
you use; a CFDP implementation was used in exactly the same way (until it was
replaced to increase performance and link utilization).<br>
<br>
<a href="ftp://ftp-eng.cisco.com/lwood/cleo/README.html">ftp://ftp-eng.cisco.com/lwood/cleo/README.html</a><br>
<br>
L.<br>
<br>
<<a href="http://www.ee.surrey.ac.uk/Personal/L.Wood/">http://www.ee.surrey.ac.uk/Personal/L.Wood/</a>><L.Wood@surrey.ac.uk></span></font><o:p></o:p></p>
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