Experimenting with Mobile Computing

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Experimenting with Mobile Computing
Peer-to-Peer Systems
Spring 03 COMP 190 290
Lecture 4 7DS Overview

• Prof. Maria Papadopouli
• http//www.cs.unc.edu/maria/courses/spr03

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Outline

• Overview on wireless data access
• Overview of 7DS (last lecture)
• Simulations Analysis on 7DS
• Information dissemination
• Message relaying
• Bandwidth sharing
• Discussion of research projects

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Challenges for providing secure services in ad
hoc networks

• Off-line nature
• Not being able to connect to a trustee entity
  continuously
• Limited resources to run complex cryptographic
  protocols
• Thin clients
• Mobility dynamic environment (new members may
  join frequently)

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Types of attacks in ad hoc networks

• Basic mechanisms
• MAC layer
• Routing mechanisms
• Malicious users agree to forward messages but
  fail to do so
• False routing information messages
• Selfishness service enforcement issues

• Security mechanisms
• Distributed trusted server under the control of
  malicious party
• Public key maliciously replaced

Also, concern for applications in p2p systems
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Avoiding misbehaving nodes

• watchdog identifying the misbehaving nodes
• pathrater define best route to avoid these nodes
  
• The problem of selfishness does not seem to be
  castigated on the contrary, by the combination
  of the watchdog and the pathrater, the
  misbehaving nodes will not be bothered by the
  transit traffic while still enjoying the
  possibility to send and to receive packets.

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Preventing DoS attacks
Host R
Host Q
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Electronic checks

• E-checks are cryptographically bound to the
  transaction which prevents the forgery by another
  host that overhears the exchange of an e-check
• Public-key credential based architecture
• Each host has a public key
• Credentials are short-lived and frequently
  refreshed
• Bank can limit the amount of micropayment a host
  can sent
• Host downloads the credentials from Internet

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Electronic check payment
Host Q
Host R
verify R is known to the bank authorized for
7ds
receive e-check verify it is genuine store
e-check
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Token-based payment
Host Q
Host R
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E-checks vs token-based

• Cannot reuse the received e-checks
• Periodically acquire their credentials from the
  trustee entity (bank)
• Credentials expire

• Token-based
• Reuse the tokens during their communication with
  multiple hosts
• Need of a tamper-proof hardware
• Increase cost

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References on micropayment

• Offline micropayment without trusted hardware
  (Blaze et al)
• www.terminodes.org (EPFL)

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Outline

• Overview on wireless data access
• Overview of 7DS (last lecture)
• Simulations Analysis on 7DS
• Information dissemination
• Message relaying
• Bandwidth sharing
• Discussion of research projects

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Simulation environment
pause time 50 s mobile user speed 0 .. 1.5
m/s host density 5 .. 25 hosts/km2 wireless
coverage 230 m (H), 115 m (M), 57.5 m
(L) ns-2 with CMU mobility, wireless
extension randway model
querier
wireless coverage
dataholder
randway model
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Mobility models

• User mobility
• Randway
• Random direction
• Boundless simulation area
• Gauss-Markov
• with history of previous move

• Group mobility
• Pursue mobility
• Nomadic community mobility
• Reference point group mobility model

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Exponential correlated random model for mobility
b(t) position at time t

• r adjusts the rate of change from the old to the
  new
• r random Gaussian variable with variance s
• Both for individual and group mobility
• Not easy to force a mobility pattern by
  selecting these parameters

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Reference point group mobility model

• Each group has a logical center
• The centers motion defines the entire groups
  motion behavior, including location, speed,
  direction, acceleration
• The group trajectory is determined by providing a
  path for the center
• Nodes are uniformly distributed within the
  geographic scope of the group
• A node is randomly placed in the neighborhood of
  its reference point at each step

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User mobility in a wireless infrastructure
Problem Estimate residency probability
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Mobility models in a wireless infrastructure

• Ignorant model
• No information available from movement history
• Assigns equal residence probability to all the
  zones
• IID
• Relative frequency of locations estimate
  residency probability
• Markov model
• Carries more information about the order of
  cell-visits

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References on mobility pattern

• LeZi-Update An information-theoretic approach to
  track mobile users in PCS networks by Das,
  Mobicom99
• Group mobility model for ad hoc wireless networks
  by Hong et al.

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Simulation environment
pause time 50 s mobile user speed 0 .. 1.5
m/s host density 5 .. 25 hosts/km2 wireless
coverage 230 m (H), 115 m (M), 57.5 m
(L) ns-2 with CMU mobility, wireless
extension
querier
wireless coverage
1m/s
pause
mobile host
data holder
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Simulation environment
pause time 50 s mobile user speed 0 .. 1.5
m/s host density 5 .. 25 hosts/km2 wireless
coverage 230 m (H), 115 m (M), 57.5 m
(L) ns-2 with CMU mobility, wireless
extension
wireless coverage
v1
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Dataholders () after 25 min
high transmission power
P2P
Mobile Info Server
Fixed Info Server
2
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Scaling properties of data dissemination

R
If cooperative host density transmission power
are fixed, data dissemination remains the same
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Scaling properties of data dissemination
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Average delay (s) vs. dataholders ()
Fixed Info Server
one server in 2x2 high transmission power
4 servers in 2x2 medium transmission power
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Average Delay (s) vs Dataholders ()Peer-to-Peer
schemes
high transmission power
medium transmission power
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Scaling properties of data dissemination (contd)
L
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Epidemic model

• Carrier is infected, hosts are susceptible
• Transmit to any give host with probability
  hao(h) in interval h
• Pure birth process
• Ttime until data has spread among all mobiles
• ET1/a S

N-1
i1
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Modeling Fixed Info Server as diffusion-controlle
d process

• trapping model with particles C and T (traps)
• particles C perform random walk in 2D space
• particles T static, randomly distributed in space
  of infinite capacity
• particles T absorb C when C step onto them

querier ? particle C
fixed info server ? trap
trapping ? receiving data

• survival probability fn at long times n
• log (fn) ? -A?n

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Fixed Info Serversimulation and analytical
results
high transmission power
Probability a host will acquire data by time t
follows 1-e-a?t
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Outline

• Introduction on wireless data access
• Overview of 7DS
• Performance analysis on 7DS
• Information dissemination
• Message relaying
• Network connection sharing
• Conclusions
• Future work

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Message relaying with 7DS
WAN
Gateway
WLAN
Message relaying
Host B
Host A
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Message relaying

• Take advantage of host mobility to increase
  throughput
• Hosts buffer messages forward them to a gateway
• Hosts forward their own messages to cooperative
  relay hosts
• Restrict number of times hosts forwards

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Messages () relayed after 25 min (average
number of buffered messages 5)
2
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Outline

• Introduction on wireless data access
• Motivation
• Overview of 7DS
• Performance analysis on 7DS
• Information dissemination
• Message relaying
• Network connection sharing
• Conclusions
• Future work

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Network Connection Sharing
Hosts A B dual-homed They act as gateways to
WAN for hosts C D
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Network connection sharingprotocol
WAN
Host E

• C sends request for gateway
• B A respond advertising their bandwidth in WAN
  link
• 4. C selects least loaded gateway (eg A)
• 5. A ? C admission control

thin wireless WAN links
Host A
Host D
WLAN
Host B
Host C
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Network connection sharing summary
Client
Gateway

• Requests for network connection
• Gateway selection
• Load balancing criteria

• Advertisement of gateway availability
• Admission control using Measured sum Jamin et
  al
• u? ? vr
• v measured load
• r (peak) rate requested
• uutilization target
• ?bandwidth of WAN link

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Gateway selection mechanism

• Load balancing criteria
• Reduction of the maximum difference in
• the average load over an interval t across the
  gateways
• maxiLi(t)-miniLi(t)/b
• Li(t) average traffic measured at gateway i
  over interval t
• Greedy algorithm Choose the least loaded gateway

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Benefits using network connection sharing

• Statistical multiplexing for bursty traffic
• Increase bandwidth utilization of the WAN links
• 80 bandwidth utilization for Pareto traffic
• Load balancing across gateways
• For shared data applications
• Reduction of replicated data
• Increase quality of service

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Network connection sharing
Pareto exponential 312 s(ON), 325s
(OFF) Pareto, shape par. 1.2 Flows 64kb/s,
0.6 s int., avg hold time 5 min Perfect load
balancing 0
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Conclusions

• Dominant parameters
• density of cooperative hosts
• wireless coverage density of cooperative hosts
  their mobility
• For fixed cooperative hosts density
  transmission power
• scale area performance
  same
• For fixed wireless coverage density
• Density of cooperative host ?
  performance ?

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Conclusions (contd)

• Probability a host will acquire data by time t
  in
• Fixed Info Server 1-e-a?t
• Peer-to-Peer 1-e-at
• Message relaying is beneficial
• Probability a message will reach the Internet ?
• Utilization of available throughput ?
• by taking advantage of host mobility

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Summary of contributions in video on demand

• Novel multimedia retrieval scheduling algorithms
• In multi-disk environments
• adapt to bandwidth changes
• maximize data retrieval for all streams
• using replication and multi-resolution
• In single-disk environments
• allocate disk bandwidth in a fair manner

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7DS Implementation

• Cache manager (3k lines)
• GUI server (2k lines)
• HTTP client methods (24k lines)
• Proxy server (1k lines)
• UDP multicast unicast (1k)
• Web client server (2k)
• Jar files used (xerces, xml,lucene, html parcer)

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References on resource sharing in mobile ad hoc
networks

• Effects of power conservation, wireless coverage
  cooperation on data dissemination among
  wireless devices , ACM MobiHoc 2001
• Performance analysis of 7DS a data dissemination
  prefetching tool for mobile users, IEEE
  Sarnoff 2001, best paper/poster award
• 7DS in mobile ad hoc networks, Globecom 2000
• Design implementation of a P2P data
  dissemination prefetching tool for mobile
  users, Metro 2001
• Network connection sharing in ad hoc wireless
  network among collaborative hosts, Nossdav 1999
• Resource sharing in mobile networks, Ph.D.
  Thesis, October 2002

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Reference on information discovery in wide area
network

• with also P. Castro, B. Greenstein, R. Muntz
  (UCLA), C. Bisdikian, P. Kermani (IBM),
  Locating Application Data Across Service
  Discovery Domains,
• ACM MOBICOM 2001

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Location Based Services

• Video games
• Zonemaster unwiredfactory
• Various location-based services
• www.wavemarket.com
• Maps points of interests, driving directions,
  routing, real-time traffic
• ismap, signalsoftcorp, navicom, brillianttec,
  televoke
• More info www.pulver.com/lbs