DTN/VDTN Architecture VDTN Fragmentation Experimental evaluation - - PDF document

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DTN/VDTN Architecture VDTN Fragmentation Experimental evaluation - - PDF document

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technology from seed Bundles Fragmentation in Vehicular Delay-Tolerant Networks Narcio Magaia, Paulo Rogrio Pereira , Augusto Casaca, (Inesc-ID, Portugal), Joel J. P. C. Rodrigues, Joo A. Dias, Joo N. Isento, (IT, Portugal) Cristina

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Bundles Fragmentation in Vehicular Delay-Tolerant Networks

Naércio Magaia, Paulo Rogério Pereira, Augusto Casaca, (Inesc-ID, Portugal), Joel J. P. C. Rodrigues, João A. Dias, João N. Isento, (IT, Portugal) Cristina Cervelló-Pastor, Javier Gallego (UPC, Spain).

Instituto de Engenharia de Sistemas e Computadores Investigação e Desenvolvimento em Lisboa

27-06-2011 NGI'2011: Bundles Fragmentation in Vehicular Delay-Tolerant Networks 111

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Summary

  • DTN/VDTN Architecture
  • VDTN Fragmentation
  • Experimental evaluation
  • Conclusion

Instituto de Engenharia de Sistemas e Computadores Investigação e Desenvolvimento em Lisboa

27-06-2011 NGI'2011: Bundles Fragmentation in Vehicular Delay-Tolerant Networks 222

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Summary

  • DTN/VDTN Architecture
  • VDTN Fragmentation
  • Experimental evaluation
  • Conclusion

Instituto de Engenharia de Sistemas e Computadores Investigação e Desenvolvimento em Lisboa

NGI'2011: Bundles Fragmentation in Vehicular Delay-Tolerant Networks 27-06-2011 3333

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Introduction

  • Delay/Disruption Tolerant Networks (DTNs) are networks

that may experience frequent and long duration partitions.

  • Examples: military communications in the battlefield, deep

space communications, some forms of ad-hoc sensor/actuator networks, rescue actions in catastrophe

Instituto de Engenharia de Sistemas e Computadores Investigação e Desenvolvimento em Lisboa

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sensor/actuator networks, rescue actions in catastrophe hit areas, wild life monitoring, offering Internet connectivity in developing regions, vehicular networks.

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Bundle Layer

  • Link disruptions are not properly handled by the Internet

protocols, due to high delays and high packet loss.

  • The DTN Research Group, which was chartered as part of the

Internet Research Task Force (IRTF), has proposed an architecture (RFC 4838) and a communication protocol for DTNs (RFC 5050).

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  • “Bundle Layer” added on top of the

transport layer for the end-to-end exchange of messages (called bundles) taking advantage of scheduled, predicted, opportunistic

  • r permanent connectivity.

Application Application Bundle end point Bundle end point Bundle end point Bundle end point Transport Transport Transport Transport Network Network Network Network

DTNs (RFC 5050).

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DTN characteristics

  • Random and predictable node mobility
  • Intermittent/Scheduled/Opportunistic Links
  • Large or variable delays
  • High Error Rates / Low Usable Capacity
  • Limited node uptime (e.g. to save power)

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  • Limited node uptime (e.g. to save power)
  • Link bandwidth/loss/delay asymmetry
  • Heterogeneous Network Architectures
  • End-to-end path may not exist
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Delay-Tolerant Networking Architecture

  • Goals

– Support interoperability across ‘radically heterogeneous’ networks – Tolerate delay and disruption

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  • Acceptable performance in high

loss/delay/error/disconnected environments

  • Decent performance for low loss/delay/errors

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Message Abstraction

  • Network protocol data unit: bundles

– “postal-like” message delivery – origination and useful life time -assumes synchronized clocks – source, destination, and respond-to EIDs (Endpoint Identifiers) – Options: return receipt, “traceroute”-like function, alternative reply-to field, custody transfer – fragmentation capability

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– fragmentation capability – overlay atop TCP/IP or other (link) layers: layer ‘agnostic’

  • Applications send/receive messages

– “Application data units” (ADUs) of possibly-large size – Applications should “bundle” together all data concerning a transaction that is sent asynchronously, to minimize interactions with large RTT – Adaptation to underlying protocols via ‘convergence layer’

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Vehicular Contact Times

  • Contact times between vehicles crossing each other are rather short
  • Results for UDP with packet size 1460 bytes over IEEE 802.11a/g

[6]: Car speed Transferred data Contact Time Goodput 20 Km/h 13.0 MB 38.48 s 2.75 Mbps

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  • For TCP, the performance is very low: average of 80 KB transferred

in a crossing at 60 Km/h (relative speed 120 Km/h), in 4 out of 10 experiments no data is transferred at all.

  • Motivates the study of fragmentation, necessary for large bundles.

40 Km/h 3.6 MB 14.90 s 1.90 Mbps 60 Km/h 1.6 MB 10.83 s 1.15 Mbps

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VDTN

  • Vehicular networks may benefit from DTN concepts

– especially in regions with no communications infrastructure or with few vehicles, such as: rural and mountainous regions. – non real-time applications, such as: traffic information, meteorological information – the use of fixed relay nodes in cross-roads may improve the

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– the use of fixed relay nodes in cross-roads may improve the delivery ratio

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VDTN Architecture

  • Bundle layer below the network layer

– objective is to route large size messages instead of small size IP packets – results in fewer packets processing and routing decisions, which can be translated to less complexity, lower cost and energy savings

BAD layer: Bundle Aggregation and De-aggregation

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  • BAD layer: Bundle Aggregation and De-aggregation

Application Layer Transport Layer Network Layer BAD Layer BSC Layer MAC Layer MAC Layer PHY Layer PHY Layer Data Plane Control Plane Persistent Storage

  • BSC layer: Bundle Signaling Control
  • Persistent storage to avoid data loss
  • Relay nodes only have the lower

layers

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Summary

  • DTN/VDTN Architecture
  • VDTN Fragmentation
  • Experimental evaluation
  • Conclusion

Instituto de Engenharia de Sistemas e Computadores Investigação e Desenvolvimento em Lisboa

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IP fragmentation

  • Fragmentation in IP networks has been considered

harmful

– poor performance or complete communication failure – additional headers, additional bandwidth, additional processing – complex reassembly mechanisms at the receiver

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– loss of fragments requires retransmission of the entire original datagram

  • IPv6 disallows fragmentation inside the network, requiring

that fragmentation is done in the source node

  • HTTP protocol allows byte ranges of objects to be

individually retrieved: useful to resume partial file transfers

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DTN Fragmentation Types

  • Two types of fragmentation
  • Proactive: a node divides a bundle in fragments before

transmission

– used primarily when the contact duration is known (or predicted) in advance, e.g. satellite networks

  • Reactive: nodes in communication fragment a bundle when

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  • Reactive: nodes in communication fragment a bundle when

a contact is broken

– the received part is a fragment – the undelivered part is another fragment – “toilet paper” variant allows fragmentation only at predefined boundaries

  • Fragments reassembled at the destination only.
  • Any fragment loss results in the entire bundle loss.

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Fragmentation Operation

  • Different types of fragmentation:

i) no fragm entation ii) proac tive fragm entation

ac k header

Da ta (4 KB )

ac k header

Da ta (4 KB )

ack header

Da ta

header

D a ta (4 KB ) Da ta (4 KB )

ack

Da ta (2KB )

Instituto de Engenharia de Sistemas e Computadores Investigação e Desenvolvimento em Lisboa

  • ACK used to prevent data loss when the contact is broken

– particularly important when routing uses only a single copy of bundles – not necessary for Epidemic routing

iii) reactive fragmentation

ac k header

Da ta (4 KB )

ac k header

Da ta (4 KB )

ack header (2 KB ) ac k header

Da ta (4 KB )

ac k

Da ta (4 KB )

ac k

Da ta (2KB )

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Summary

  • DTN/VDTN Architecture
  • VDTN Fragmentation
  • Experimental evaluation
  • Conclusion

Instituto de Engenharia de Sistemas e Computadores Investigação e Desenvolvimento em Lisboa

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VDTN laboratory prototype

  • Car made with Lego Mindstorm NXT robot with a PDA
  • Bluetooth 2.0 (3 Mbit/s) used for out-of-band signaling
  • IEEE 802.11b (11 Mbit/s) used for data transmission

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Test parameters

  • A single mobile node going back and forth
  • Contact times vary from 9-21 seconds (average 15

seconds, standard deviation 3.5 seconds)

– corresponds to vehicles crossing at a relative speed of approximately 80 Km/h

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  • Interval between successive contacts varies from 1-27

seconds (average 9 seconds, standard deviation 7 seconds)

  • Each situation tested 20 times. Repeated if necessary.
  • UDP maximum datagram size (in the PDA): 32 KB

– Tested with 4KB, 8KB and 32KB datagrams

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Average number of contacts to transfer a message

  • Instituto de Engenharia de Sistemas e Computadores Investigação e Desenvolvimento em Lisboa
  • Using 32 KB data chunks
  • With fragmentation: average 1.4 contacts to transfer a

900KB message

  • Without fragmentation: almost impossible to transfer a

900KB message

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Message Delivery Latency 4KB data chunk

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  • Almost impossible to transfer a 600KB message without fragmentation
  • Fragmentation is worth being used even for 100 KB messages
  • Proactive fragmentation has a significant overhead

– signaling and headers.

  • Reactive fragmentation performs better, without degradation

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Data overhead

Message Size Data Chunk

100 KB 300 KB 600 KB 900 KB 4 KB 117 KB 347 KB 693 KB 1010 KB 8 KB 108 KB 322 KB 642 KB 962 KB 32 KB 103 KB 306 KB 611 KB 915 KB

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  • Fragmentation results in additional headers, as shown in

the table

  • Each fragment has to be individually announced through

signaling and processed

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Message Delivery Latency 8KB data chunk

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  • Fragmentation overhead is reduced
  • Proactive fragmentation still performs 25-85% worse than

reactive fragmentation

  • Fragmentation greatly outperforms no fragmentation

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Message Delivery Latency 32KB data chunk

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  • Proactive fragmentation overhead now has little significance
  • The difference is smaller than the confidence interval
  • Proactive fragmentation has less processing during contacts
  • Difficult to adjust fragment sizes proactively

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Summary

  • DTN/VDTN Architecture
  • VDTN Fragmentation
  • Experimental evaluation
  • Conclusion

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Conclusions

  • DTN is a new networking paradigm that can overcome

challenging situations.

  • Fragmentation allows the transfer of messages larger than

the typically short contacts between vehicles, resulting in an increased delivery ratio and decreased delivery

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an increased delivery ratio and decreased delivery latency.

  • Reactive fragmentation adapts the fragments size

dynamically to the duration of the contact, resulting in an efficient use of the contact opportunities.

  • Proactive fragmentation requires an a priori definition of

the fragment size, which is difficult to adjust to contacts.

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Future Work

  • Test more complex VDTN scenarios
  • Use contact duration prediction mechanisms to combine

scheduling and fragmentation of bundles to optimize the data transfer phase.

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Thank you! Questions?

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27-06-2011 NGI'2011: Bundles Fragmentation in Vehicular Delay-Tolerant Networks 27 20-06-2011 Título da apresentação 27

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