Delay-Tolerant Networks

1. Delay-Tolerant Networks Acknowledgements: Most materials presented in the slides are based on the tutorial slides made by Dr. Ling-Jyh Chen, Dr. Kevin Fall and Dr. Thrasyvoulos Spyropoulos.

2. “Legacy” Networks • Internet, Telephone network • Wired or fixed links • A SUCCESS STORY!

3. Wireless Last Hop Wired Backbone Wireless Networks: Cellular • Cellular Networks: Wired backbone + wireless last link • A SUCCESS STORY for voice/SMS! • Internet? (GPRS): not really (low bandwidth + high price)

4. Wireless Networks: WiFi • 802.11, wimax • Still: only wireless local-loop • Higher bandwidth than cellular: 54Mbps • Much cheaper/KB

5. Wireless Networks: WiFi (2) • Only Partial Coverage: HOTSPOTS • No real “mobile computing”!

6. Disaster Recovery Wireless Networks: Ad-hoc and Sensor Networks • Self-organized: no wired infrastructure • Peer-to-peer: nodes are routers • Examples: sensor nets; disaster recovery, etc. Target Tracking

7. End-to-end path S D node link Wireless NetworksAd Hoc and Sensor Networks (2) • The past approach: “apply the successful and well understood Internet paradigm to ad hoc networks also” • Assume existence of explicit links (strong enough SINR) • Establish end-to-end paths • Mobility might change these paths: re-establish them

8. Wireless NetworksAd Hoc and Sensor Networks (3) • Ad-hoc Networks: A success story? • NOT REALLY! • No real ad hoc application (killer app) out there • except maybe some military networks • Why? Most wireless networks are NOT like the Internet!

9. The “Internet” Assumptions • E2E path doesn’t have really long delay • Reacting to flow control in ½-RTT effective • Reacting to congestion in 1-RTT effective • E2E path doesn’t have really big, small, or asymmetric bandwidth • Re-ordering might happen, but not much • End stations don’t cheat • Links not very lossy (<1%) • Connectivity exists through some path • Even MANET routing usually assumes this

10. More Internet Assumptions • Nodes don’t move around or change addresses • Easy to assign addresses in hierarchy • Thought to be important for scalability • In-network storage is limited • Not appropriate to store things long-term in network • End-to-end principle • Routers are “flakier” than end hosts

11. Non-Internet-Like Networks • Random and predictable node mobility • Military/tactical networks (clusters meet clusters) • Mobile routers w/ disconnections • Big delays, low bandwidth (high cost) • Satellites • Exotic links (deep space comms, underwater acoustics) • Big delays, high bandwidth • Busses, mail trucks, delivery trucks, etc.

12. Challenged Networks • Intermittend/scheduled/opportunistic links • High error rates/low usable capacity • Very large delays • Different network architectures

13. Characteristics 1: Path and Link characteristics • High latency, low data rate • e.g. 10 kbps, 1-2 second latencies • Asymmetric data rates • Disconnection • Non-faulty disconnections • Motion • Low-duty-cycle operation • Routing subsystem should not treat predictable disconnections as faults and can use this information to pre-schedule messages • Long queueing times • Conventional networks rarely greater than a second • Challenged network could be hours or days due to disconnection

14. Characteristics 2: 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

15. Characteristics 3: 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

16. IP Routing May Not Work • E2E path may not exist • Lack of many redundant links • Path may not be discoverable (e.g., fast oscillations) • Traditional routing assumes at least one path exists, fails otherwise • Routing algorithm solves wrong problem • Wireless broadcast media is not an edge in a graph • Objective function does not match requirements • Different traffic types wish to optimize different criteria • Physical properties may be relevant (e.g., power)

17. IP Routing May Not Work • E2E path may not exist • Lack of many redundant links • Path may not be discoverable (e.g., fast oscillations) • Traditional routing assumes at least one path exists, fails otherwise • Routing algorithm solves wrong problem • Wireless broadcast media is not an edge in a graph • Objective function does not match requirements • Different traffic types wish to optimize different criteria • Physical properties may be relevant (e.g., power)

18. Inter-Planetary Internet (IPN)Networking in Space • Existing satellite networks for deep space missions: • Proprietary, not that efficient, one for each mission • NASA/JPL: “Extend the idea of Internet in outer space” • One reusable network for all missions • Gain from experience already acquired

19. Extending the Internet in Space

22. Long Propagation Delays vs.“Chatty” Internet Protocols • Propagation Delay is much larger than transmission time! (minutes around our solar system) • Internet protocols are “chatty” TCP: S: “Hi! You want to talk?” (SYN) 20min R: “Sure! Let’s establish a session” (SYN+ACK) 20min S: “Ok, let’s go for it!” (ACK) 20 min ….. (slow start phase) S: “Can you send me the pic of Mars?” …..

23. TCP chatiness More than 3h for one 1MB pic! transmission time (1MB/128Kbps) = 1min !!!

24. Idea: “Bundles” • Bundle: Application-meaningful message • Contains all necessary info packed inside one “bundle” (atomic message) • Next hop has immediate knowledge of storage and bandwidth requirements • Optional ACKs • Depending on class service • Goal: Avoid chattiness • Minimize number of propagation delays “paid”

25. Intermittent Connectivity • No more links! Now we have “contacts” Contact 1: “Dish A sees earth Sat B from 12:30h to 12h:45h” Contact 2: “Sat B sees rover C on mars from 17:30h to 18:30h”

26. Idea: Store-Carry-and-Forward • Store a bundle for a looong period of time. • Forward when the next contact is available • Hours or even days until appropriate contact. • Postal system: “move packages from one storage place to another (switch intersection), along a path that eventually reaches the destination” • How is this different from Internet routers’ store-and-forward? 1) Persistent storage (hard disk, days) vs memory storage (few ms) 2) Wait for next hop to appear vs. wait for table-lookup and available outgoing routing port

27. Store-Carry-and-Forward (2) 1 12 D 13 S 14 2 16 11 15 3 7 4 5 8 10

28. Store-Carry-and-Forward (3)

29. Store-Carry-and-Forward (4)

30. DTN vs End-to-end Internet Operation

31. Networking in SpaceHeterogeneity • Heterogeneous networks to interconnect • Link delay, asymmetry, error rate, reliability mechanism • Different protocol stack + Different node capabilities Examples: Earth’s Internet: short delays, low error rate, TCP reliability Sensor network at Mars: short delays, high error rate, data aggregation at sink(s) Satellite backbone: long delays, high error rate, LTP (lightweight transport protocol)

32. Boundles: A Store and Forward Overlay

33. What About Retransmission?Custody Transfers • Error rates can be high in wireless links • What if a retransmission is needed? Contact 1: “Dish A sees earth Sat B from 12:30h to 12:45h” Contact 2: “Sat B sees rover C on mars from 17:30h to 18:30h” Contact 3: “Dish A sees Sat B again in one week” It’s better that B takes “custody” of message and retries sending it itself

34. Custody Transfer (2)

35. Custody Transfer (3)Moving the Retransmission Point Closer • Benefits of hop-by-hop vs. end-to-end error control • For paths with many lossy links re-Tx requirements are much higher for end-to-end (linear vs. exponential) E.g. 3 links each with error 1-p: • (hop-by-hop) 3/p extra bandwidth • (end-to-end) 3/(p^3) extra bandwidth • Retransmission overhead is increased by long propagation delays

37. 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 & security checks • Store messages for reliable delivery • Resolve globally-significant name tuples to locally-resolvable names for internal destined traffic • 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/ }

38. Naming

39. Delay Tolerant Networks (DTN) • Kevin Fall (~2002): “maybe these idea is not only useful for deep space networks”

40. DTN: Very Brief History • DTNRG chartered as IRTF research group (end of 2002) • Chair: Kevin Fall (Intel Research Berkeley) • Architecture evolved from deep-space-focused Interplanetary Internet project • Funded by DARPA 1999-2002 • IRTF Group IPNRG retired when DTNRG formed • DTN became a DARPA program in 2004 • 11+ Internet draft • Implementation: simulator (DTNSIM) and Linux codes (DTN2)

41. End-to-end path S D node link Intermittent Connectivity:The Technical Argument Wireless links are not like wires!

42. A B B B Intermittent Connectivity:The Technical Argument • Intermittent Connectivity may appear because of: p • propagationeffects: shadowing, deep fades X

43. A C B C Intermittent Connectivity:The Technical Argument(2) • Intermittent Connectivity may appear because of: • Propagation effects, shadowing, deep fades • Mobility: paths change too fast; huge overhead for maintenance

44. A B C B Intermittent Connectivity:The Technical Argument(2) • Intermittent Connectivity may appear because of: • Propagation effects, shadowing, deep fades • Mobility: paths change too fast; huge overhead for maintenance • Power: nodes shut down to save power or “hide” Save power (e.g. sensor) Low probability of detection (LPD) (e.g. army node)

45. Intermittent ConnectivityThe Economical Argument • Maybe it’s cheaper to not assume connectivity rather than enforce it • Rural areas (countryside, freeways) : • overprovision of base stations? • OR just live with a sparse network and “episodic” connectivity? • Sensor Networks (attached on animals): • Enough Tx power for connectivity? ($100/node) • Very low power nodes? (e.g. RFID, $1/node)

46. End-to-end path X S D X X path disruption! X path disruption! node link Wireless Connectivity: A Different View

47. Applications: Sensor Networks for Habitat Monitoring • ZebraNet (Princeton) • Biologists want to learn animal habits • Size of herds • Mobility patterns (running, sleeping, grazing) • Daily habits (watering) • Attach “tracking collars” on animals • Current technology surprisingly inefficient • Satellite trackers: high energy, low bit rate • GPS trackers: often have to retrieve collar for data • Sensor nodes with wireless radios?

48. Z Z Z Z Z Z Z Z Z Z Applications: Sensor Networks for Habitat Monitoring (2) Herd of zebras (range of few meters) • Increase power for connectivity? • Considerably reduce lifetime of network! (power law) • What about obstacles? • Live with a sparse network (connected clusters) • Use DTN principles to carry traffic towards sink Herd of zebras (range of few meters) base station

49. Vehicular Networks“Drive-Thru Internet” Vehicle-to-roadside (base station, sensors)

50. Vehicular Networks“Drive-Thru Internet” (2) • Asynchronous operation: OK for e-mail! • Web caching; Local information; download news • Enough bandwidth even at high speeds! Internet send email email reply send email email reply write email