Inter Planetary Network (IPN)

  Inter Planetary Network (IPN).ppt (Size: 419 KB / Downloads: 0)
Some Fast Facts
Time taken by light
Earth – Jupiter : 32.7 min
Earth – Saturn : 76.7 min
Earth – Pluto : 5.5 hours
Earth – Voyager1 : 13 hours
Earth – 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 page link error rates for radio-frequency (RF) communication channels
Intermittent page 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 (contd …)
InterPlanetary Backbone Network
Communication among Earth, outer-space planets, moons, satellites, relay stations, etc.
InterPlanetary External Network
Space 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 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
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
Protocol Layers
Space Wireless Frequency and Modulation
Space Channel Coding
Space Link
Space Networking
Space end-to-end Security
Space end-to-end Reliability
Space File transfer
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.
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 page 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 page 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 page 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
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 page link resources when t ≤ RTT
The number of data packets sent during each interval T is maxed to ssthreshe
Immediate Startup
cwnd is increased till ssthreshe and after that remains constant at cwnd as there is still no feedback on the page 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 page 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 page 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
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 page 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 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
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.
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
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.
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.
Some more facts:
The Cassini spacecraft is carrying two Motorola Tracking, Telemetry, and Control (TT&C) deep space transponders which provide the only communications page 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