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Disruption Tolerant Networks Aruna Balasubramanian University of Massachusetts Amherst. What?. Termed coined by DARPA Fundamentally different way of looking at networks. Wireless LAN. Internet. Cell tower. Wired LAN.

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  1. Disruption Tolerant Networks Aruna Balasubramanian University of Massachusetts Amherst

  2. What? • Termed coined by DARPA • Fundamentally different way of looking at networks Wireless LAN Internet Cell tower Wired LAN End to End connectivity between device and Internet gateway

  3. Primary characteristics of DTNs: No contemporaneous end-to-end path need to exist

  4. Traditional networks Destination i Source Intermediate

  5. i i i Disruption Tolerant Networks • Post office model • Store and forward X Z Y

  6. Why bother? • Can be adapted to scenarios other than inter-planetary communication • To enable network access, when infrastructure is • difficult to deploy • expensive to deploy • available, but a DTN can still improve performance • Some real life examples where DTNs are being used…..

  7. Infrastructure is difficult to deploy • Wild-life tracking TurtleNet project, UMass Deployed in Amherst ZebraNet project, Princeton Deployed in Mpala, Kenya

  8. Infrastructure expensive to deploy • Providing Internet connectivity to developing regions. E.g., Email KioskNet in Waterloo, Digital Gangetic Project in India

  9. Even when infrastructure is available • Provide a cheaper alternate to cellular data plans. Google from the bus without a 3G plan!!! DieselNet project, UMass CarTel project, MIT

  10. Outline • Why are DTNs useful • Application layer: How are the applications really implemented? • Routing layer • DTN stack • Power management • Lessons learnt from our deployments efforts

  11. Single hop case • When node meets an Internet gateway it sends/receives data. Internet

  12. How to implement email in single hop? • Email protocols today (IMAP/POP) cannot work in the presence of disruptions • Solution: Use a gateway

  13. How about web search? <your favorite search engine> Retrieving web…. Retrieving images… Retrieving….

  14. Web search challenges Frequent disruptions may mean you keep retyping the query

  15. Adapting web search to mobile networks (Thedu) Google, Yahoo, Live , Ask, …. Queries from mobile Store query Interface Thedu Client Snippets Prefetch Web pages returned to mobile Store web pages Thedu proxy

  16. Other web search apps based on DTNs • The TEK search engine: Collects all possible data for a search query and returns it in email format • RuralCafe • Caches search queries to perform local search • Send search responses in the form of sessions

  17. Outline • Why are DTNs useful • Application layer • Routing layer: How can we support multihop? • DTN stack • Power management • Lessons learnt from our deployments efforts

  18. Routing challenges Wired/Mesh/MANETs • End-to-end path exists • Known topology • Low feedback delay • Retries possible DTNs • No end-to-end path • Uncertain topology • Feedback delayed/nonexistent Primary challenge: finding a path to the destination under extreme uncertainty

  19. i i Post office model may not always work! X Y Z

  20. i i i i Key idea in DTN routing: Replication X Z Y W Naïve replication using flooding wastes resources and can hurt performance

  21. Efficient replication • When two nodes X and Y meet, what packets should be replicated? • Heuristics • Random replication: X randomly select packets in the buffer and transfer to Y • Maximum replication count: Set a replication threshold for each packet • Meeting frequency: X will send a packet to Y, if Y has a higher probability of meeting the destination.

  22. More replication-based heuristics • Utility-based routing (Our work) • Each packet is given a utility, based on the routing metric. • For example, if the routing metric is to minimize delays, the utility is the expected delivery delay • The first packet replicated is one whose replication decreases the delivery delay by most

  23. Outline • Why are DTNs useful • Application layer • Routing layer • DTN stack: Can we simply use the wireless stack? • Power management • Lessons learnt from our deployments efforts

  24. Do we need a new link layer? • 802.11 is successful link layer protocol for wireless networks. • Average time to connect ~ 13 sec • Average duration of AP meeting in DieselNet is 25 sec In DTNs, the timeouts and retries and significantly reduced. DHCP overhead is reduced by caching

  25. DTN2 stack • The DTN stack has two additional components • A bundle protocol: For store and forward • A convergence layer: To determine sending rate • Being standardized by IETF, implemented by BBN Receiver Sender

  26. Outline • Why are DTNs useful • Application layer • Routing layer • The DTN stack • Power management • Lessons learnt from our deployments efforts

  27. Power Management • Motivation: To have perpetual battery-operated network systems • Example: If GPS is on, battery life is 3 hours

  28. Key idea for power management: Energy Harvesting • Use solar cells to scavenge energy • Challenges • Amount of energy harvested depends on size of the cell • Variable energy harvested per node • Seasonal, unpredictable Take away: Smart power management scheme needed even with energy harvesting

  29. Outline • Why are DTNs useful • Routing layer challenges • Link and transport layer challenges • Application layer challenges • Power management challenges • Lessons learnt from our deployments efforts

  30. UMass DieselNet

  31. Details • 40 buses, 26-node mesh testbed • Our lab pays $1600 per month for 3G connection on buses; no monthly cost for WiFi • Roughly 50GB of data is downloaded from the bus using WiFi

  32. DieselNet Advantages • Very useful for research: Evaluation is a lot more believable; forced to think practical • Useful for the community. Example: bus tracking project, pothole patrol

  33. Challenges in outdoor deployment • Difficult to fix broken parts • Cannot predict the quality of information collected, because • Many buses may be broken • Maybe running different versions • Bomb scare!!

  34. Take Aways • DTNs useful in various environments • Protocols that work well in wired and even wireless networks do not work well in DTNs • Rethink all four layers of the OSI stack, as well as power management

  35. Resources • DTN research group: http://www.dtnrg.org/ • DieselNet, TurtleNet: http://prisms.cs.umass.edu/dome/ • MIT’s CarTel: http://cartel.csail.mit.edu/ • Waterloo’s KiokNet: blizzard.cs.uwaterloo.ca/tetherless/index.php/KioskNet • My website: www.cs.umass.edu/~arunab

  36. Challenge in deploying Email, FTP (2) • TCP throughput very low in the mobile setting • Starts sending 1 packet per window • Increases packets by 1 per window if not losses • If a single packet is lost, the window size is halved. • TCP thinks losses are due to congestion, and another node is sending • Even if the bandwidth is 1Mbps, TCP only uses a small portion of the bandwidth Possible solution: Make TCP differentiate between congestion and bad channel quality. Decrease rate only for congestion .

  37. Link and transport layer challenges OSI Stack X Z Y Transport layer challenges: TCP, UDP are end-to-end protocols. But there is no end-to-end connectivity Link layer challenges: similar to any other network, except in handling handoffs during mobility

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