disruption tolerant networks aruna balasubramanian university of massachusetts amherst 1

Disruption Tolerant Networks Aruna Balasubramanian

University of Massachusetts Amherst

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What? Termed coined by DARPA

Fundamentally different way of looking at networks

Internet

Wired LAN

Wireless LAN

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Cell tower

Infrastructure = Cell tower, LAN, Access point

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

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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

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Infrastructure is difficult to deploy Wild-life tracking

TurtleNet project, UMassDeployed in Amherst

ZebraNet project, PrincetonDeployed in Mpala, Kenya

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Infrastructure expensive to deploy Providing Internet connectivity to developing

regions

KioskNet in Waterloo, Digital Gangetic Project

in India

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Even when infrastructure is available Provide a cheaper alternate to cellular data

plans

DieselNet project, UMass CarTel project, MIT

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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

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Traditional routing

i

Source

Destination

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Routing in DTNs

Post office model Store and forward

iX Z

Y

i

i

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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

Primary challenge: finding a path to the destination under extreme uncertainty

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Key idea in DTN routing: Replication

iX

Z

Y

i

i

W

i

Naïve replication using flooding wastes resources and can hurt performance

Naïve replication using flooding wastes resources and can hurt performance

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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.

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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

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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

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Application layer challenges Motivation: Using cheaper connectivity using

DTNs, even when infrastructure is available

Internet

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Shift focus from multihop to single hop connectivity

Challenges in deploying applications Clearly VoIP is not

possible.

How about Email, FTP?

How about Web search?

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KioskNet

DieselNet

Challenge in deploying Email, 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.

Possible Solution: Shorten the connection cycle by optimizing for the mobile

environment.20

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 bandwidthPossible 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.

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How about web search?<your favorite search engine>

Retrievin

g web…

.

Retrievin

g images…

Retrieving….

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Web search challenges

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Adapting web search to mobile networks (Our work)

Queries from mobile

Store queryStore query Interface

Google, Yahoo, Live ,

Ask, ….

Google, Yahoo, Live ,

Ask, ….

SnippetsSnippets Prefetch Store

web pages Store

web pagesWeb pages returned to

mobile

Thedu proxy

Thedu ClientThedu Client

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Power Management

Motivation: To have perpetual battery-operated network systems

Example: If GPS is on, battery life

is 3 hours

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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

Take away: Smart power management scheme needed even with energy harvesting

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Using low power devices when possible

Turducken

40W

2W

0.04WSensor

PDA

Laptop

Simple computationand storage

Download Web Pages

Very complex computation

Send/RecvMail

Po

wer

Nee

ds

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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

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UMass DieselNet

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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

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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

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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!!

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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

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Resources DTN research group: http://www.dtnrg.org/

My website: www.cs.umass.edu/~arunab

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

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disruption tolerant networks aruna balasubramanian university of massachusetts amherst 1

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