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A Delay-Tolerant Network Architecture for Challenged Internets

A Delay-Tolerant Network Architecture for Challenged Internets. Kevin Fall. Challenged Networks. Terrestrial mobile networks Unexpected partitions due to node mobility or RF interference Periodic, predictable partitions e.g. Commuter bus acting as store and forward switch.

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A Delay-Tolerant Network Architecture for Challenged Internets

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  1. A Delay-Tolerant Network Architecture for Challenged Internets Kevin Fall Anshul Kantawala

  2. Challenged Networks • Terrestrial mobile networks • Unexpected partitions due to node mobility or RF interference • Periodic, predictable partitions • e.g. Commuter bus acting as store and forward switch Anshul Kantawala

  3. Challenged Networks (cont.) • Exotic Media Networks • Near-Earth satellites, very long-distance radio (deep space) etc. • High latencies with predictable interruption • Outage due to environmental conditions • Predictably available store and forward network service – e.g. low-earth orbiting satellites Anshul Kantawala

  4. Challenged Networks (cont.) • Military Ad-Hoc Networks • Operate in hostile environments • mobile nodes, environmental factors or intentional jamming cause disconnections • Data traffic may be pre-empted by higher priority voice traffic • Strong infrastructure protection requirements Anshul Kantawala

  5. Challenged Networks (cont.) • Sensor networks • Limited end-node power, memory and CPU capability • Thousands or millions of nodes per network • Communication scheduled to conserve power • Interfaced to other networks using proxy nodes Anshul Kantawala

  6. Current Solutions • Link-repair approach • Engineer problem links to appear similar to regular links • Use proxy agents • Attach challenged networks at edges using proxy agents • Does not provide a general way to use these networks for data transit Anshul Kantawala

  7. Characteristics of Challenged Networks • Path and Link characteristics • Network architectures • End System characteristics Anshul Kantawala

  8. Path and Link characteristics • High latency, low data rate • e.g. 10 kbps, 1-2 second latencies • Asymmetric data rates • e.g. remote instruments – large return channel, small uplink for device control • Protocols should be terse and dynamic control functions performed open-loop or hop-by-hop Anshul Kantawala

  9. Path and Link characteristics • Disconnection • Non-faulty disconnections • Motion • Predictable: satellite passes, bus acts as router • Random: motion of nodes/routers, interference • Low-duty-cycle operation • Routing subsystem should not treat predictable disconnections as faults and can use this information to pre-schedule messages Anshul Kantawala

  10. Path and Link characteristics • Long queueing times • Conventional networks rarely greater than a second • Challenged network could be hours or days due to disconnection Anshul Kantawala

  11. Network Architectures • Interoperability considerations • Networks may use application-specific framing formats, data packet size restrictions, limited node addressing and naming etc. • Security • End-to-end approach not attractive • Require end-to-end exchanges of keys • Undesirable to carry traffic to destination before authentication/access control check Anshul Kantawala

  12. End System Characteristics • Limited longevity • Round-trip time may exceed node’s lifetime making ACK-based policies useless • Low duty cycle operation • Disconnection affects routing protocols • Limited resources • Affects ability to store and retransmit data due to limited memory Anshul Kantawala

  13. Can we use TCP/IP? • Transport layer (TCP) • High latency and moderate to high loss rates severely limit TCP’s performance • Network layer (IP) • Performance affected by loss of fragments • Routing • High latency will cause current routing protocols to incorrectly label links as non-operational Anshul Kantawala

  14. Proxies and Protocol Boosters • Proxies and protocol boosters are inherently fragile • Increase system complexity if mobility is frequent • May require both directions to flow through the proxy – fail for asymmetric routing • Application proxies have limited re-use abilities and may fail to take advantage of special resources of the proxy node Anshul Kantawala

  15. Delay Tolerant Message-Oriented Overlay Architecture Anshul Kantawala

  16. Abstraction • Message switching • Use message aggregates or “bundles” • Allows network’s path selection and scheduling functions a-priori knowledge of the size and performance requirements of data transfers • Overlay architecture • DTN will operate over existing protocol stacks and provide a gateway when a node touches two or more dissimilar networks Anshul Kantawala

  17. Regions and DTN Gateways • DTN gateways are interconnection points between dissimilar network protocol and addressing families called regions • e.g. Internet-like, Ad-hoc, Mobile etc. • DTN gateways • Perform reliable message routing • Perform security checks • Store messages for reliable delivery • Resolve globally-significant name tuples to locally-resolvable names for internal destined traffic Anshul Kantawala

  18. Name Tuples • Two variable length portions • Region name • Globally-unique hierarchically structured region name • Used by DTN gateways for forwarding messages • Entity name • Resolvable within the specified region, need not be unique outside it • E.g. { internet.icann.int, http://www.ietf.org/ } Anshul Kantawala

  19. Class of Service • Similar to the Postal service • Delivery priority: low, ordinary, high • Notifications of mailing, delivery to receiver and route taken • Reliable delivery using custody transfer at each routing hop Anshul Kantawala

  20. Path Selection and Scheduling • End-to-end path routing path cannot be assumed to exist • Can solve a multicommodity flow optimization problem using approximate algorithms, since the protocol is message based Anshul Kantawala

  21. Custody Transfer • Two types of message nodes • Persistent (P) and non-persistant (NP) • P nodes assumed to contain persistent memory storage and participate in custody transfer • Custody Transfer • Acknowledged delivery of message from one DTN hop to the next and passing of reliability delivery responsibility Anshul Kantawala

  22. Custody Transfer (cont.) • Advantages • Relieves potentially resource-poor end nodes from maintaining end-to-end connection states • Useful for overcoming high loss rates along the delivery path • As reliable as typical end-to-end reliability Anshul Kantawala

  23. Protocol Translation and Convergence Layers • Bundle forwarding function assumes underlying reliable delivery capability with message boundaries • Convergence layer augments underlying network protocols appropriately Anshul Kantawala

  24. Time Synchronization • Need for time synchronization • Provide a mechanism to deliver pre-programmed control instructions to be executed at future points in time • Use for scheduling, path selection and to remove expired pending messages • Propose time synchronization on the order of 1 ms Anshul Kantawala

  25. Security • Each message contains • Identity of sender • Requested class of service (CoS) • Use public key cryptography • First DTN router verifies user and validates CoS request • Re-signs message using its key • Core routers need only cache keys of their neighbours Anshul Kantawala

  26. Congestion and Flow Control • Flow control is hop-by-hop • Uses underlying protocols mechanisms if they exist • Congestion control • Refers to contention of persistent storage at a DTN forwarder • Current approach uses a priority queue • Priority inversion and head-of-line blocking can occur Anshul Kantawala

  27. Application Interface • Applications must be able to operate in a regime where request/response time may exceed the longevity of the client and server processes • Application interface is non-blocking • Also has registration and callback functions between bundle-based applications and the local forwarding agent Anshul Kantawala

  28. Implementation Anshul Kantawala

  29. Implementation (cont.) • Prototype DTN system under Linux • Application interface • Rudimentary bundle forwarding across scheduled and “always on” connections • Detection of new and lost contacts • Two convergence layers • TCP/IP • Bundle-based proxy to the Berkeley mote network Anshul Kantawala

  30. Conclusion • DTN architecture attempts to provide interoperable communications between and among challenged networks • Design uses message switching with in-network retransmission, late-binding of names and routing tolerant of network partitioning Anshul Kantawala

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