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
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, 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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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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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
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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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
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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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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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
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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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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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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