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First Look at the Deep Impact
DTN Experiment (DINET)
Scott Burleigh
Jet Propulsion Laboratory
California Institute of Technology
Copyright 2008 California Institute of Technology
Government sponsorship acknowledged.
SB-1
20 November 2008
DINET Summary
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The purpose of the DINET project is to demonstrate NASA’s implementation
of the IRTF-conformant open Delay-Tolerant Networking protocols
(Interplanetary Overlay Network – “ION”) in flight and ground software
functioning at Technology Readiness Level 7 or 8, making it ready for use
by space flight projects.
Plan:
– Upload ION software to the Deep Impact “flyby” spacecraft during inactive cruise
period, while the spacecraft is en route to encounter comet Hartley 2.
– Use the DI (now “EPOXI”) spacecraft as a DTN router for image bundles flowing
from one lab machine to another, over interplanetary links.
– Use the Deep Space Network tracking stations: eight tracking passes of 4 hours
each, separated by intervals of 2 to 5 days. Uplink at 250 bytes/sec, downlink at
either 110 or 20,000 bytes/sec.
– On the last four passes, induce data loss by randomly discarding 1/32 of all
received packets, thus forcing the exercise of LTP retransmission.
– One-way signal propagation delay is initially 81 seconds, drops to 49 seconds by
the end of the four-week exercise.
– Use AMS publish/subscribe over BP/LTP to send about 300 small images
through this network, via the spacecraft. Track statistics, display on reception.
SB-2
20 November 2008
The DINET Stack
image publisher/receiver
AMS
messaging
load/go utility
for network administration
Remote AMS
compression
BP forwarding
Convergence layer adapter
LTP retransmission
Link service adapter
admin
programs,
rfx system,
clocks
CCSDS space
packets
CCSDS TM/TC
X-band R/F
SB-3
20 November 2008
Experiment 1: Send images from nodes 12 to node 8 via nodes 6, 3, 7 (the Deep Impact spacecraft), 2, 4. Also send images from
nodes 20 to node 8 via nodes 10, 5, 7 (the Deep Impact spacecraft), 2, 4.
Deep Impact
DSOT
DINET EOC in PTL
NOTE: Deep Impact
science spacecraft
is functioning as a
router (infrastructure).
EVRs
stot
Experiment
database
Load/Go
Load/Go
Load/Go
16
“Earth”
7
2
client
server
4
8
3
client
server
6
12
“Mars”
image files
5
bundles
log msgs
space links
TCP
BRS
LTP/UDP
client
server
10
“Phobos”
20
image files
SB-4
20 November 2008
Experiment 2: Send Load/Go directive loads from node 16 to node 12 via nodes 4, 2, 7, 3, 6. Also from 16 to 20 via 4, 2, 7, 5, 10.
Deep Impact
EVRs
DSOT
DINET EOC in PTL
stot
Experiment
database
Load/Go
Load/Go
Load/Go
16
“Earth”
7
2
client
server
4
8
3
client
server
6
12
“Mars”
image files
5
bundles
log msgs
space links
TCP
BRS
LTP/UDP
client
server
10
“Phobos”
20
image files
SB-5
20 November 2008
Experiment 3: Omit a contact between 7 and 5 and repeat, forcing images from 20 to travel via 10, 6, 3, 7, 2, 4 and forcing directive
loads to 20 to travel via 4, 2, 7, 3, 6, 10.
Deep Impact
EVRs
DSOT
DINET EOC in PTL
stot
Experiment
database
Load/Go
Load/Go
Load/Go
16
“Earth”
7
2
client
server
4
8
3
client
server
6
12
“Mars”
image files
X
NOTE: spacecraft
is temporarily
unable to function
as a router.
5
bundles
log msgs
space links
TCP
BRS
LTP/UDP
client
server
10
“Phobos”
20
image files
SB-6
20 November 2008
Experiment 4: manually route traffic between nodes 12 and 20 (both remote) via the orbiter, without Earth in the loop from node 20 to
12 and 12 to 20.
Deep Impact
EVRs
DSOT
DINET EOC in PTL
stot
Experiment
database
Load/Go
Load/Go
Load/Go
16
“Earth”
7
2
client
server
4
8
3
client
server
6
12
“Mars”
image files
5
bundles
log msgs
space links
TCP
BRS
LTP/UDP
client
server
10
“Phobos”
20
image files
SB-7
20 November 2008
Configuration Parameters
•
Convergence-layer protocols:
– “Bundle Relay Service” (TCP) through firewall between DSOT and EOC nodes.
– LTP (“red”) on all other links, over CCSDS TM/TC (flight) or UDP/IP (ground).
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•
•
•
•
•
•
Images sent at priorities 0, 1. Network mgt traffic, custody signals, critical
images sent at priority 2.
Custody transfer on all application bundles.
All bundles are CBHE-compressed.
Bundle status reports are sent only on destruction of custodial bundles.
Time-to-live is 10 days for all image bundles.
Max bundle size is 64 KB. Max LTP segment size is 739 bytes.
Contact Graph Routing used to compute routes dynamically.
– Intermittent cross-link between nodes 6 and 10 enables alternative paths.
SB-8
20 November 2008
Results
•
Moved 292 images (about 14.5 MB) through the network.
–
•
•
•
Still working on performance analysis, metrics.
No data loss other than nominal time-to-live expiration.
No data corruption anywhere in the network.
Aside from the uploading of spacecraft clock drift corrections, the network
operated continuously without operator intervention throughout the four
weeks of the experiment.
SB-9
20 November 2008
Key Findings
•
The protocols work well.
– Signal propagation delays of 49 to 89 seconds were tolerated.
– End-to-end latencies on the order of days were tolerated.
– Station handovers and transient failures in DSN uplink service were handled
automatically and invisibly.
– Protocol overhead was minimal.
– Dynamic route computation was generally successful.
•
The software is highly stable.
– No software failures in four weeks of continuous operation on VxWorks, Solaris,
and Linux platforms.
– No effect on the operation of other flight software.
– No leakage of memory or non-volatile storage space.
•
Clock synchronization and OWLT estimation errors of several seconds had
no noticeable effect on network operation.
SB-10
20 November 2008
Problems
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Several bugs in Contact Graph Routing resulted in some under-utilization of
network capacity. Revisions in progress.
Spacecraft clock drifted more rapidly than expected – over 1 second per
day. Revised clock correction deltas had to be uploaded to ION for each
tracking pass.
Various other minor bugs, plus one DSN hardware problem revealed when
processing high volume of uplink traffic.
SB-11
20 November 2008
Conclusion
• Fully automatic operation of a DTN over deep space links is
feasible.
SB-12
20 November 2008