Inter Planetary Network (IPN)

By Charles B Shahcbshah@cse.buffalo.edu

ContentsIntroductionChallenges Architecture of IPN Communication SuiteTransport Layer IssuesProtocol: TP PlanetProtocol: RCP PlanetNetwork Layer IssuesAppendixReferences

IntroductionImagine ???If I say a Hi to you and you hear it after9 Hours !!!!!

Some Fast Facts Time taken by light Earth Jupiter : 32.7 minEarth Saturn : 76.7 minEarth Pluto : 5.5 hoursEarth Voyager1 : 13 hoursEarth Voyager2 : 10.4 hours.

ObjectivesTime-Insensitive Scientific data deliveryTime-Sensitive scientific data deliveryMission Status TelemetryCommand and Control

ChallengesExtremely long propagation delayAsymmetrical forward and reverse Link capacitiesHigh link error rates for radio-frequency (RF) communication channelsIntermittent link connectivityLack of fixed communication infrastructureEffects of planetary distances on the signal strength and the protocol designPower, mass, size, and cost constraints for communication hardware and protocol designBackward compatibility requirement due to high cost involved in deployment and launching processes.

Architecture

Architecture (contd )InterPlanetary Backbone NetworkCommunication 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 NetworkPlanetary Satellite NetworkSatellites 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 SuiteCurrent 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 LayersUsed for the Mars Exploration mission communications.

CCSDS Protocol

CCSDS ProtocolProtocol LayersSpace Wireless Frequency and ModulationSpace Channel CodingSpace LinkSpace NetworkingSpace end-to-end SecuritySpace end-to-end ReliabilitySpace File transferSpace Application

CCSDS Protocol LimitationAlthough 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 StackThe 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 IssuesInterPlanetary 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 backboneVery long propagation delayHigh link error ratesBlackoutsBandwidth 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 PlanetDeveloped 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 TDuring 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 RTTThe number of data packets sent during each interval T is maxed to ssthreshe

Immediate Startup

Immediate Startupcwnd 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 ssthresheDuring 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 endTo 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 ControlLow and high priority NIX segments of 40 bytesSent 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 StateCongestion Control (contd)Let = Nlow /NhighSource infers that a congestion exists if < 1.Let d , i be preset rate decrease and increase thresholdsIf < d : Congestion is experienced along the path. Source goes to Decrease Rate state where transmission S is decreased multiplicatively.If d i : the rate S is kept unchanged until further feedback is recieved.If > i : No congestion is experienced. Consequently, it increases data transmission rate S additively.

Steady StateBlackoutLink 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.

BlackoutLet 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 ACKsAt 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 SACKIf the data packets are 1KB, SACK packets are of 40B, i.e. the ratio of the traffic in the forward and reverse links is 25:1However, 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 trafficMultimedia does not require 100% reliability but has strict req on bounded jitter, minimum b/w Challenges to multimedia traffic in IPN:Bounded JitterMinimum BandwidthSmooth Traffic ( maintain steady rate )Error ControlNon Suitable methods for IPN Store and forwardUse multiple paths

RCP PlanetRCP planet is a rate control schemeTwo States: Initial State and Steady StateUses Tornado codes to recover from packet lossesUses rate probing mechanism ( probing sequence )Uses new rate control mechanismHandles Blackout state as in TP Planet

RCP Planet

Network Layer IssuesNaming and AddressingFactors influencing Naming and AddressingWhat objects are namedWhether a name can be used directly by a data routerThe 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 performanceIt could maintain a secondary server locally, however updates will dominate the communication channelIt could have static name resolution, but that would not allow scalability.

Network Layer IssuesCompatible with IPv4 and IPv6 Proposed Network Layer Protocol is ( SCPS-NP ), Space Communication Protocol Standards Network Protocol.Open Issues:Distribution of topology information.Path CalculationInteraction with transport layer protocols.Efforts:Epidemic RoutingSensor Web Project

AppendixAn Example of using bundling for deep space communication.

AppendixDSN ( 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.

AppendixDoppler 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.

AppendixSome 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 MillionESA Mars mission cost = 150 million EurosCassini mission cost = $3 billion

ReferencesO.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 internetO.B. Akan, J. Fang, Performance of Multimedia Rate Control Protocolsin 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