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color=#0000ff size=2>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.</FONT></SPAN></DIV>
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<DIV dir=ltr align=left><SPAN class=554092813-19042006><FONT face=Arial
color=#0000ff size=2>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 Saratoga.</FONT></SPAN></DIV>
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<DIV dir=ltr align=left><SPAN class=554092813-19042006><FONT face=Arial
color=#0000ff size=2>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.</FONT></SPAN></DIV>
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<DIV dir=ltr align=left><SPAN
class=554092813-19042006> <FONT
face=Arial color=#0000ff size=2>--keith</FONT></SPAN></DIV><BR>
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<FONT face=Tahoma size=2><B>From:</B> sis-csi-bounces@mailman.ccsds.org
[mailto:sis-csi-bounces@mailman.ccsds.org] <B>On Behalf Of
</B>L.Wood@surrey.ac.uk<BR><B>Sent:</B> Wednesday, April 19, 2006 6:16
AM<BR><B>To:</B> adrian.j.hooke@jpl.nasa.gov<BR><B>Cc:</B>
sis-csi@mailman.ccsds.org<BR><B>Subject:</B> RE: [Sis-csi] Green book
thoughts<BR></FONT><BR></DIV>
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<P><FONT size=2>>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><BR></FONT></P></BLOCKQUOTE></BODY></HTML>