[Sis-csi] Green book thoughts

[Scott, Keith L.]

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.
 
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.
 
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.
 
        --keith


________________________________

	From: sis-csi-bounces at mailman.ccsds.org
[mailto:sis-csi-bounces at mailman.ccsds.org] On Behalf Of
L.Wood at surrey.ac.uk
	Sent: Wednesday, April 19, 2006 6:16 AM
	To: adrian.j.hooke at jpl.nasa.gov
	Cc: sis-csi at mailman.ccsds.org
	Subject: RE: [Sis-csi] Green book thoughts
	
	

	>At 04:33 PM 4/18/2006, Lloyd wrote:
	>> You can use IP -- and UDP -- and be enhanced.
	>
	> Enhanced? Seems like you've got a custom, non-standard
application running
	> over UDP (which doesn't *do* anything, so presumably that
custom
	> application has its own custom reliability built into it?)
running over IP
	> which "routes" you from a single processor on the spacecraft
over a single
	> link to a single processor located directly in the ground
station.
	
	UDP provides per-packet checksums, multiplexing and
identification via ports, and a standard widespread sockets interface
convention for building on top of.
	
	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.)
	
	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).
	
	ftp://ftp-eng.cisco.com/lwood/cleo/README.html
	
	L.
	
	
<http://www.ee.surrey.ac.uk/Personal/L.Wood/><L.Wood at surrey.ac.uk>
	

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