Disruption Tolerant Networks Aruna Balasubramanian University of Massachusetts Amherst 1

What? Termed coined by DARPA Fundamentally different way of looking at networks Internet Wired LAN Wireless LAN 2 Cell tower Infrastructure = Cell tower, LAN, Access point

DTNs: No contemporaneous end-to- end path need to exist 3

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 4

Infrastructure is difficult to deploy Wild-life tracking TurtleNet project, UMass Deployed in Amherst ZebraNet project, Princeton Deployed in Mpala, Kenya 5

Infrastructure expensive to deploy Providing Internet connectivity to developing regions KioskNet in Waterloo, Digital Gangetic Project in India 6

Even when infrastructure is available Provide a cheaper alternate to cellular data plans DieselNet project, UMassCarTel project, MIT 7

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 8

Traditional routing i Source Destination 9

Routing in DTNs Post office model Store and forward i XZY i i 10

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 11

Key idea in DTN routing: Replication i XZY i i W i Naïve replication using flooding wastes resources and can hurt performance 12

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. 13

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  Replicate in the order of marginal utility of replication.  The first packet replicated is one whose replication decreases the delivery delay by most 14

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 15

Link and transport layer challenges X ZY Link layer challenges: similar to any other network, except in handling handoffs during mobility Transport layer challenges: TCP, UDP are end-to-end protocols. But there is no end-to-end connectivity OSI Stack 16

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 17

Application layer challenges Motivation: Using cheaper connectivity using DTNs, even when infrastructure is available Internet 18 Shift focus from multihop to single hop connectivity

Challenges in deploying applications Clearly VoIP is not possible. How about , FTP? How about Web search? 19 KioskNet DieselNet

Challenge in deploying , FTP (1) Connection establishment takes a long time Average time to connect ~ 13 sec Short contact durations. In DieselNet~25 sec Possible Solution: Shorten the connection cycle by optimizing for the mobile environment. 20

Challenge in deploying , 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. Possible solution: Make TCP differentiate between congestion and bad channel quality. Decrease rate only for congestion. 21

How about web search? Retrieving web…. Retrieving images… Retrieving…. 22

Web search challenges 23

Adapting web search to mobile networks (Our work) Queries from mobile Store query Interface Google, Yahoo, Live, Ask, …. Google, Yahoo, Live, Ask, …. Snippets Prefetch Store web pages Store web pages Web pages returned to mobile Thedu proxy Thedu Client 24

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 25

Power Management Motivation: To have perpetual battery-operated network systems Example: If GPS is on, battery life is 3 hours 26

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 27

Using low power devices when possible Turducken 40W 2W 0.04W Sensor PDA Laptop Simple computation and storage Download Web Pages Very complex computation Send/Rec v Mail Power Needs 28

Power management using programming languages EON: Energy-aware programming language Tight link between program and runtime  Explicit data flow and energy preferences Measure energy harvesting and consumption Automatically conserve energy as needed  execute an alternate implementation  adjust fine grained timers 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: My website: DieselNet, TurtleNet: MIT’s CarTel: Waterloo’s KiokNet: blizzard.cs.uwaterloo.ca/tetherless/index.php/Kio skNet: blizzard.cs.uwaterloo.ca/tetherless/index.php/Kio skNet 36