Inter Planetary Network (IPN)
By Charles B Shah
cbshah@cse.buffalo.edu
Contents
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Introduction
Challenges
Architecture of IPN
Communication Suite
Transport Layer Issues
Protocol: TP Planet
Protocol: RCP Planet
Network Layer Issues
Appendix
References
Introduction
Imagine ???
If I say a “Hi” to you and you hear it after
9 Hours !!!!!
Some Fast Facts
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Time taken by light
Earth
Earth
Earth
Earth
Earth
–
–
–
–
–
Jupiter
Saturn
Pluto
Voyager1
Voyager2
: 32.7 min
: 76.7 min
: 5.5 hours
: 13 hours
: 10.4 hours.
Objectives
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Time-Insensitive Scientific data delivery
Time-Sensitive scientific data delivery
Mission Status Telemetry
Command and Control
Challenges
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Extremely long propagation delay
Asymmetrical forward and reverse Link capacities
High link error rates for radio-frequency (RF)
communication channels
Intermittent link connectivity
Lack of fixed communication infrastructure
Effects of planetary distances on the signal strength
and the protocol design
Power, mass, size, and cost constraints for
communication hardware and protocol design
Backward compatibility requirement due to high cost
involved in deployment and launching processes.
Architecture
Architecture (contd …)
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InterPlanetary Backbone Network
Communication among Earth, outer-space planets,
moons, satellites, relay stations, etc.
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InterPlanetary External Network
Space crafts flying in groups in deep space between
planets, clusters of sensor nodes, and groups of
space stations.
Architecture (contd …)
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Planetary Network
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Planetary Satellite Network
Satellites circling the planets provides relay services,
communication & navigation services to surface
elements. Includes links between orbiting satellites &
links between satellite and surface elements.
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Planetary Surface Network
Links between high power surface elements (rovers,
landers, etc). Surface elements that cannot directly
talk to satellites, organized in an ad hoc manner.
Communication Protocol Suite
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Current Space / Ground protocol used by
CCSDS (Consultative Committee for Space
Data Systems ).
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Each component of the IPN may have to run
different set of protocols to suite the environment.
CCSDS protocol consists of 8 Layers
Used for the Mars Exploration mission
communications.
CCSDS Protocol
CCSDS Protocol
Protocol Layers
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1.
2.
3.
4.
5.
6.
7.
8.
Space
Space
Space
Space
Space
Space
Space
Space
Wireless Frequency and Modulation
Channel Coding
Link
Networking
end-to-end Security
end-to-end Reliability
File transfer
Application
CCSDS Protocol Limitation
Although the current protocol is viable, there is a
need to make the protocol stack adaptable to
different environmental changes allowing integration
of highly optimized regional network protocols.
This leads to the proposed Protocol by Delay Tolerant
Networking Research Group (DTNRG).
DTNRG Protocol Stack
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The protocol replies on a middleware layer called
bundle layer that resides between the application and
the lower layers.
The bundle layer resolves the intermittent
connectivity, long or variable delay, asymmetric data
rates, high error rates by using a store and forward
mechanism similar to email.
It uses per-hop error control which increases the
probability of data transmissions.
DTNRG Protocol
Transport Layer Issues
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InterPlanetary Backbone poses the most challenging
problems for reliable data and multimedia transport.
The transport layer functionalities are necessary for
reliable transfer and timely delivery of multimedia
information.
Most important challenges for the backbone
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Very long propagation delay
High link error rates
Blackouts
Bandwidth Asymmetry
Why not use current protocols??
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Due to the window-based mechanism, there is a high
performance degradation.
In slow start phase of TCP protocols, the congestion
window size (W) is incremented by 1 for every ACK
received until the slow start threshold (Wss).
For = 20 and RTT = 20 min, the slow start algo
cannot utilize the link for 120 min in deep space.
TCP protocols are designed for wired links, assuming
negligible bit error rates while space links have
considerable bit error rates.
Why not use current protocols??
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Even protocols for satellite links could not be applied
for IPN, as the satellite links have RTT of the order of
500ms and also the packet loss due to the blackout
conditions may also mislead the congestion control
mechanisms.
TCP is expected to respond to Network State. The
higher RTT is experienced, the older information
about link conditions is received at the source. This
might not lead to correct action.
TCP uses retransmission which calls for higher buffer
size (1.2GB for RTT = 20min, for 1MB/s
TP Planet
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Developed for the end-points are backbone nodes
such as the relay satellites orbiting around the
planets or the ground stations which are capable of
direct deep space communications.
It runs on top of Internet Protocol (IP) layer and
does not require any specific modification to the
lower layers in the current TCP/IP protocol suite.
The structure of the protocol consists of two
Algorithms: Initial State and Steady State
TP Planet
Initial State (TP Planet)
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Composed of 2 parts – Immediate Startup and Follow-Up.
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Immediate start up ( 0 ≤ t ≤ RTT )
 Divides actual RTT into equal intervals of size T
 During Immediate Start, it emulates slow start and
congestion avoidance algorithms of current TCP
protocols by treating intervals of T as RTTs of the
emulated connection.
 Along with data packets, it transmits low priority NIL
segments to probe the link resources when t ≤ RTT
 The number of data packets sent during each interval T
is maxed to ssthreshe
Immediate Startup
Immediate Startup
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cwnd is increased till ssthreshe and after that remains
constant at cwnd as there is still no feedback on the link
condition.
During Emulated Slow Start, cwnd + cwndn ≤ ssthreshe
During Emulated Congestion Avoidance cwnd = ssthreshe
and cwndn is increased till ssthreshe
Follow Up
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Follow Up Phase ( RTT ≤ t ≤ 2.RTT )
 The packets are received at the other end
 To save scarce resources one ACK is send for several
packets by a delayed SACK ( Selective ACK)
 Each NIL segment received indicates that the link is not
utilized completely, so it counts the total no of NIL
received in one period T and sends this information as
NIL ACK.
 The sender has cwnd = ssthreshe for RTT ≤ t ≤ RTT + T
and later it changes the cwnd based on the information
in NIL ACKs.
 Source also transmits NIX packets for congestion
monitoring
Steady State ( t ≥ 2.RTT )
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Congestion Control
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Low and high priority NIX segments of 40 bytes
Sent at same rate as Data packets, so they experience same
packet loss rate due to space link errors.
Low priority NIX get discarded first.
Sink counts number of received low (Nlow) and high priority
(Nhigh) NIX segments in a window of Tw
Received NIX are not acknowledged, instead reception
statistics within a window Tw is carried by NIX ACKs.
Steady State
Steady State
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Congestion Control (contd…)
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Let Φ = Nlow /Nhigh
Source infers that a congestion exists if Φ < 1.
Let Φd , Φi be preset rate decrease and increase thresholds
If Φ < Φd : Congestion is experienced along the path.
Source goes to Decrease Rate state where transmission S is
decreased multiplicatively.
If Φd ≤ Φ ≤ Φi : the rate S is kept unchanged until further
feedback is recieved.
If Φ > Φi : No congestion is experienced. Consequently, it
increases data transmission rate S additively.
Steady State
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Blackout
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Link outages due to loss of line-of-sight by orbital obscuration
lead to burst packet losses & decrease in the throughput.
If the source does not receive any type of ACK ( data or NIX )
for a certain period Tw, it infers Blackout.
During Blackout, source keeps sending low and high priority
NIX segments without changing the transmission rate.
Similar action is taken by the sink and it sends NIX ACKs with
(Nlow, Nhigh) as (0,0) called ZERO NIX ACKs.
Since RTT is very high, the effect of blackout on performance
changes with it relative location of blackout wrt sink.
Blackout
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Let blackout occur at t = t0 and let L be the duration of the
blackout. Let the blackout occur at x seconds from sink.
For rtt = RTT/2 , there are 2 cases: L < 2x and L ≥ 2x.
Case when L < 2x :
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After rtt – x from t0 i.e. at t1 = t0 + rtt – x, the source detects
the period without ACKs. If this duration is > Tw, the source
enters Blackout state.
Now, source does not send any new data packets, but keeps
sending low and high priority NIX segments with same rate.
At t2 = t1 + L, source receives normal ACKs for a duration of
2x – L. Source infers that Blackout is over and enters either
hold, increase or decrease based on info received in ACKs
At t3 = t2 + 2x – L, the source receives ZERO ACKs transmitted
by sink, now source remains in Hold state.
Blackout ( contd … )
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Case when L ≥ 2x.
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Source detects no ACKs and goes to Blackout state at
t1= t0 + rtt – x.
At t2 = t1 + L, source receives ZERO NIX ACKs for a duration of
2x and leaves Blackout state.
At t3 = t2 + 2x, ZERO NIX ACK period is over and transits to a
state depending on the info in the ACKs
Consequently, the Blackout State reduces the throughput
degradation due to blackout conditions and improves the
link utilization for duration of L or 2x in the cases L < 2x and
L ≥ 2x, respectively.
Delayed SACK
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If the data packets are 1KB, SACK packets are of 40B, i.e.
the ratio of the traffic in the forward and reverse links is
25:1
However, the ratio in case of space links is of the of
1000:1, and hence even a single SACK can cause
congestion in the reverse link.
Therefore, TP Planet sink maintains a delayed-SACK factor,
d, and sends one packet every d packets received.
RCP Planet
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Transport Layer protocol for multimedia traffic
Multimedia does not require 100% reliability but has
strict req on bounded jitter, minimum b/w
Challenges to multimedia traffic in IPN:
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Bounded Jitter
Minimum Bandwidth
Smooth Traffic ( maintain steady rate )
Error Control
Non Suitable methods for IPN
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Store and forward
Use multiple paths
RCP Planet
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RCP planet is a rate control scheme
Two States: Initial State and Steady State
Uses Tornado codes to recover from packet losses
Uses rate probing mechanism ( probing sequence )
Uses new rate control mechanism
Handles Blackout state as in TP Planet
RCP Planet
Network Layer Issues
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Naming and Addressing
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Factors influencing Naming and Addressing
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What objects are named
Whether a name can be used directly by a data router
The method by which the name/object binding are managed.
DNS not suitable for the foll reasons:
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If an object on remote planet wants to resolve earth based
name it could query the DNS server on earth, but long RTT
would hamper the performance
It could maintain a secondary server locally, however updates
will dominate the communication channel
It could have static name resolution, but that would not allow
scalability.
Network Layer Issues
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Compatible with IPv4 and IPv6
Proposed Network Layer Protocol is ( SCPS-NP ),
Space Communication Protocol Standards – Network
Protocol.
Open Issues:
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Distribution of topology information.
Path Calculation
Interaction with transport layer protocols.
Efforts:
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Epidemic Routing
Sensor Web Project
Appendix
An Example of using bundling for deep space communication.
Appendix
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DSN ( Deep Space Networks)
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http://deepspace.jpl.nasa.gov/dsn/
The DSN currently consists of three deep-space
communications facilities placed approximately 120 degrees
apart around the world: at Goldstone, in California's Mojave
Desert; near Madrid, Spain; and near Canberra, Australia.
One 34-meter (111-foot) diameter High Efficiency antenna.
One 34-meter Beam Waveguide antenna.
One 26-meter (85-foot) antenna.
One 70-meter (230-foot) antenna.
Appendix
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Doppler Effect/ Shift used to Trace/Probe the satellite
in transit.
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The apparent change in wavelength of sound or light caused
by the motion of the source, observer or both.
If you have ever had a motorcycle speed up from behind
you, only to fly past you on the freeway, you probably
noticed how the engine sound seemed to get higher in pitch
as it approached you, only to drop down lower once it had
passed. This change in pitch is an example of a Doppler
shift.
Appendix
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Some more facts:
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The Cassini spacecraft is carrying two Motorola Tracking,
Telemetry, and Control (TT&C) deep space transponders
which provide the only communications link between the
spacecraft and the numerous terrestrial tracking stations
that comprise NASA's Deep Space Network.
Mars Mission Cost = $400 Million
ESA Mars mission cost = 150 million Euros
Cassini mission cost = $3 billion
References
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O.B. Akan, J. Fang, I.F. Akyildiz, TP-Planet: a reliable transport protocol
for InterPlaNetary Internet, IEEE Journal on Selected Areas in
Communications.
I. F. Akyildiz, O. B. Akan, C. Chen, J. Fang, andW. Su, “InterPlaNetary
Internet: State-of-the-art and research challenges ”
R.C. Durst, P.D. Feighery, K.L. Scott, “Why not use the standard
internet suite for the interplanetary internet”
O.B. Akan, J. Fang, “Performance of Multimedia Rate Control Protocols
in InterPlaNetary Internet”
http://www.planetary.org/html/news/articlearchive/headlines/2001/cas
shuygfix.html
http://deepspace.jpl.nasa.gov/dsn/
http://www.gdds.com/press1997/1008cassini.html