Inter Planetary Network (IPN)

By Charles B Shahcbshah@cse.buffalo.edu

Contents 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 Time taken by light

Earth – Jupiter : 32.7 min

Earth – Saturn : 76.7 minEarth – Pluto : 5.5 hoursEarth – Voyager1 : 13 hoursEarth – Voyager2 : 10.4 hours.

Objectives Time-Insensitive Scientific data delivery Time-Sensitive scientific data delivery Mission Status Telemetry Command and Control

Challenges 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 …)

InterPlanetary Backbone Network

Communication among Earth, outer-space planets, moons, satellites, relay stations, etc.

InterPlanetary External NetworkSpace crafts flying in groups in deep space between planets, clusters of sensor nodes, and groups of space stations.

Architecture (contd …)

Planetary Network 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.

Planetary Surface NetworkLinks 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 Current Space / Ground protocol used

by CCSDS (Consultative Committee for Space Data Systems ). 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

1. Space Wireless Frequency and Modulation2. Space Channel Coding3. Space Link4. Space Networking5. Space end-to-end Security6. Space end-to-end Reliability7. Space File transfer8. Space 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 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 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 Very long propagation delay High link error rates Blackouts Bandwidth Asymmetry

Why not use current protocols??

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?? 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

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) Composed of 2 parts – Immediate Startup and Follow-

Up.

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

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 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 )

Congestion Control 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 Congestion Control (contd…)

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

Blackout 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 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 :

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 … ) Case when L ≥ 2x.

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

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

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:

Bounded Jitter Minimum Bandwidth Smooth Traffic ( maintain steady rate ) Error Control

Non Suitable methods for IPN Store and forward Use multiple paths

RCP Planet

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

Naming and Addressing Factors influencing Naming and Addressing

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:

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 Compatible with IPv4 and IPv6 Proposed Network Layer Protocol is ( SCPS-

NP ), Space Communication Protocol Standards – Network Protocol.

Open Issues: Distribution of topology information. Path Calculation Interaction with transport layer protocols.

Efforts: Epidemic Routing Sensor Web Project

AppendixAn Example of using bundling for deep space communication.

Appendix

DSN ( Deep Space Networks) 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

Doppler Effect/ Shift used to Trace/Probe the satellite in transit.

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

Some more facts: 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 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/casshuygfix.html http://deepspace.jpl.nasa.gov/dsn/

http://www.gdds.com/press1997/1008cassini.html