A Delay-Tolerant Network Architecture for Challenged Internets

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