A Delay-Tolerant Network Architecture for Challenged Internets

1. A Delay-Tolerant Network Architecture for Challenged Internets Author: Kevin Fall Paper Presentation: Vinay Goel

2. Internet Service Model • TCP/IP based • End to end IPC using concatenation of potentially dissimilar link layer technologies • Packet-switched model of service • A number of key assumptions…

3. Key Assumptions • Assumptions • End to end path exists b/w data source and its peer(s) • Maximum RTT b/w any node pairs is not excessive • End to end packet drop probability is low • A class of challenged networks violate one or more of the assumptions

4. Examples of challenged networks • Terrestrial Mobile Networks • Unexpectedly partitioned due to node mobility, changes in signal strength etc. • Exotic Media Networks • High latencies with predictable interruption, suffer outage due to environmental conditions etc. • Military Ad-hoc Networks • Hostile environments • Mobility, environmental factors, or intentional jamming may cause disconnection • Data traffic competing for bandwidth

5. Characteristics of these networks • Path and Link characteristics • High latency, low data rate • Disconnection • Long queuing times • Network Architectures • Interoperability considerations • Security • End system characteristics • Limited longevity • Low duty cycle operation • Limited resources

6. Adapt Internet to these environments? • Link-repair approaches • Engineer problem links to appear more similar to the types of links for which TCP/IP was designed • “fool” the internet protocols: strive to maintain end-to-end reliability etc. • Attach these networks to the edge of the Internet • Use of a special proxy agent • Provides access to and from challenged networks from the Internet • No support for using such networks for data transit

7. Link repair approaches • In-network entities (“middle boxes”) • Performance Enhancing Proxies (PEPs) & protocol boosters • Contain state necessary for connection violating the Internet fate sharing principles • Confound end-to-end diagnostics and reliability, increase system complexity if mobility is frequent • Pose a significant challenge for end-to-end security mechanisms

8. Application Layer proxies • Provide specialized Internet-to-”special network” name mapping & protocol translation • Used at the edge of special networks • Disadvantage: their specificity • Either respond to a specialized set of commands or act as raw data conduits • Limit the ability to re-use proxies for different applications • Fail to take advantage of special resources (storage, processing capabilities etc.)

9. Electronic Mail • Asynchronous message delivery system • Provides an abstraction that comes close to addressing many problems • Flexible naming, asynchronous message-based operation etc. • Falls short • Lack of dynamic routing • Weakly defined delivery semantics • lack of consistent API

10. What’s the most desirable framework? • A network service and API providing non-interactive messaging • System should combine some overlay routing capability (such as in P2P systems) with delay-tolerant and disconnection-tolerant properties of e-mail

11. Delay Tolerant Message Based Overlay Architecture • Based on abstraction of message switching • Message aggregates known as “bundles” • Routers that handle them are called “bundle forwarders” or DTN gateways • Architecture provides a store-and-forward gateway function between various network architectures

13. Regions and DTN gateways • Two nodes are in the same region if they can communicate without using DTN gateways • DTN gateway • Point through which data must pass in order to gain entry to a region • Can serve as a basis for both translation and well as a point to enforce policy and control

14. Name Tuples • Identifiers for objects or groups of objects • DTN name tuple {Region Name, Entity Name} • First portion is a globally unique, hierarchically structured region name • Interpreted by DTN gateways to find the path(s) to one or more DTN gateways at the edge of the specified region • Second portion identifies a name resolvable within the specified region • Need not be unique outside the region

15. Name resolution • Only region identifier is used for routing a message that is in transit across a collection of regions • Entity name information is locally interpreted in the destination region • Form of late binding

16. A Postal Class of Service • Priority based resource allocation • Adopt a subset of the types of services provided by US Postal Service • Attractive characteristics • Low, ordinary and high priority delivery • Return receipt, delivery records

17. Path Selection and Scheduling • Architecture targeted at networks where an end-to-end path can’t be assumed to exist • Routes are comprised of a cascade of time-dependent contacts (communication opportunities) • Particular details of path selection and scheduling - heavily influenced by region-specific routing protocols and algorithms

18. Custody Transfer and Reliability • Custody transfer: acknowledged delivery of a message from one DTN hop to the next and corresponding passing of reliable delivery responsibility. • End hosts do not ordinarily need to keep a copy of data that has been custodially transferred to a DTN next hop • Custody transfer can be viewed as a performance optimization for end-to-end reliability that involves endpoint movement

19. Convergence Layers and Retransmission • Facilities provided by transport protocols in use within the regions may vary significantly • Bundle forwarding assumes underlying reliable delivery capability with message boundaries when performing custody transfer • Transport protocols lacking these features must be augmented • Include transport-protocol-specific convergence layers

20. Time Synchronization • Coarse Level • Identifying message fragments • Purging messages that have exceeded their source specified lifetimes • Stringent constraints • Scheduling, path selection • Congestion management

21. Security • Verifiable access to the carriage of traffic at a particular class of service • Avoid carrying traffic potentially long distances that is later found to be prohibited • Each message includes an immutable “postage stamp” containing • Verifiable identity of sender, an approval, class of service etc. • Credentials checked at each DTN hop by routers; use of public key cryptography

22. Congestion and Flow Control • Flow control: limiting the sending rate of a DTN node to its next (DTN) hop • Attempt to take advantage of underlying protocols’ mechanisms • Congestion control: handling of contention for the persistent storage of a DTN gateway • Shared priority queue for allocating custody storage

23. Application Interface • Applications must be careful not to expect timely responses • Must be capable of operating in a region where a request/response RTT may exceed the longevity of the client and server processes • Structured to continue operating in the face of reboots or network partitioning as much as possible

24. Conclusion • Design embraces notion of message switching with in-network storage & retransmission, late-binding of names & routing tolerant of network partitions • Puts forth several design decisions worthy of consideration

25. Questions?