Disruption Tolerant Networks Aruna Balasubramanian University of Massachusetts Amherst

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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

Wireless LAN
Internet
Cell tower
Wired LAN
Infrastructure Cell tower, LAN, Access point
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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, UMass Deployed in Amherst
ZebraNet project, Princeton Deployed 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
Destination
i
Source
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Routing in DTNs

• Post office model
• Store and forward

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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
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Key idea in DTN routing Replication
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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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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Link and transport layer challenges
OSI Stack
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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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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Application layer challenges

• Motivation Using cheaper connectivity using
  DTNs, even when infrastructure is available

Shift focus from multihop to single hop
connectivity
Internet
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Challenges in deploying applications

• Clearly VoIP is not possible.
• How about Email, FTP?
• How about Web search?

KioskNet
DieselNet
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Challenge in deploying Email, FTP (1)

• Connection establishment takes a long time
• Average time to connect 13 sec
• Short contact durations. In DieselNet25 sec

Possible Solution Shorten the connection cycle
by optimizing for the mobile environment.
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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 .
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How about web search?
ltyour favorite search enginegt
Retrieving web.
Retrieving images
Retrieving.
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Web search challenges
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Adapting web search to mobile networks (Our work)
Google, Yahoo, Live , Ask, .
Queries from mobile
Store query
Interface
Thedu Client
Snippets
Prefetch
Web pages returned to mobile
Thedu proxy
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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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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
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Using low power devices when possible

• Turducken

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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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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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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/d
  ome/
• MITs CarTel http//cartel.csail.mit.edu/
• Waterloos KiokNet blizzard.cs.uwaterloo.ca/tethe
  rless/index.php/KioskNet