Dynamic Source Routing in Ad hoc Wireless Networks

1
Dynamic Source Routing in Ad hoc Wireless
Networks
David B. Johnson, David A. Maltz Computer Science
Department Carnegie Mellon University

• Lee, Hanjin

2
Contents

• Introduction
• Assumptions
• Basic Operation
• Optimization
• Performance Evaluation
• Conclusion

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Introduction (1/2)

• What is ad hoc network?
• No fixed wired infrastructure
• Routing
• When two hosts are not within wireless
  transmission range, other hosts between them
  forward packets
• Routing protocols in conventional wired networks
• Distance vector
• Each router broadcasts to each of its neighbor
  routers its view of the distance to all hosts
• Link state
• Each router broadcasts to all other routers in
  the network its view of the status of each of its
  adjacent network link

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Introduction (2/2)

• Dynamic source routing
• The sender of a packet determines the complete
  sequence of nodes through which to forward the
  packet
• Dynamically determines a route based on
• Cached information
• The result of a route discovery
• The advantages of DSR
• No periodic routing advertisement messages
• bandwidth, battery power
• Don't require transmissions between hosts to work
  bidirectionally
• Be able to adapt quickly to changes

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Assumptions

• All hosts participate fully in the protocols of
  the network
• forward packets for other hosts
• Diameter
• The number of hops between one extreme edge of
  the network and the opposite extreme
• In ad hoc network, diameter is small but may
  often be greater than one
• Hosts can move but the speed is moderate
• Promiscuous receive mode

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Basic Operation - Overview

• The sender constructs a source route in the
  packet's header
• If the receiver is the destination host, sends a
  route reply
• Else forward packets
• Each host maintains a route cache
• If no route cache is found, use the route
  discovery
• Hosts monitor the operation of the route and use
  the route maintenance

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Basic Operation Route Discovery (1/2)

• The sender
• Broadcast a route request packet
• The receiver
• ltinitiator address, request idgt
• If same, discard
• This host's address is already listed in the
  route record - loop
• Discard
• This host is the target
• Send a route reply
• Else
• Append this host's address to the route record,
  and re-broadcast

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Basic Operation Route Discovery (2/2)
B
G
D
A
H
E
C
F
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Basic Operation Route Maintenance (1/2)

• No periodic messages
• Monitors the operation of the route and informs
  the sender of any routing errors
• Utilize a hop-by-hop acknowledgement
• At the data link level
• The route error packet
• Contains the addresses of the hosts at both ends
  of the hop in error
• Removed from the route cache
• Send to the sender
• Route cache, reverse the route from the packet in
  error, route discovery
• Passive acknowledgement
• promiscuous mode

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Basic Operation Route Maintenance (2/2)
B
G
D
G
A
Route Cache (A)G A, B, D, G G A, C, E, H,
GF B, C, F
H
E
C
F
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Optimization Full Use of the Route Cache (1/2)

• A tree of routes rooted at this host, to other
  hosts in the ad hoc network
• A hop can add entries to its route cache any time
  it learns a new route
• Intermediate hop
• Promiscuous mode
• If the host has a route cache entry for the
  target, return a route reply without
  re-broadcasting
• Congestion
• Pick a delay period dH(h-1r)
• Delay transmitting the route reply for a period
  of d
• Promiscuously receive all packets, if the length
  of the route on the packet is less than h, do not
  transmit
• Loop
• The host discards the request if the route in its
  reply would contain a loop

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Optimization Full Use of the Route Cache (2/2)

• Specify the maximum number of hops over which the
  packet may be propagated
• Procedure
• To perform a route discovery, send the route
  request with a hop limit of one
• If no route reply is received, send a new route
  request with a hop limit of the maximum value
• Purpose
• Check if the target is currently within wireless
  transmitter range

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Optimization Piggybacking Reflecting shorter
routes

• Piggybacking on route discoveries
• Small data packets can be piggybacked
• Ex) initial SYN packet opening a TCP connection
• Problem
• When replied based on the host's route cache,
  data would be lost
• Construct and send a new packet containing the
  piggybacked data
• Reflecting shorter routes
• Possible if hosts operate their network
  interfaces in promiscuous receive mode
• Send unsolicited route reply packet to the
  original sender

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Optimization Improved Handling of Errors (1/2)

• Use exponential backoff
• When the network becomes partitioned
• To limit the rate at which new route discoveries
  may be initiated from any host for the same
  target
• In order to reduce the overhead
• Eavesdrop on route error packets being sent to
  other hosts

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Optimization Improved Handling of Errors (2/2)

• Different route
• Once the route error packet reaches the original
  sender, retransmits the route error packet back
  to the point of error
• Caching negative information in a host's route
  cache
• Hosts can ignore replies
• Short expiration period must be placed

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Performance Evaluation (1/3)

• The simulation
• The area in which the hosts move - 9m²
• Velocity - 0.30.7m/s
• Transmission range - 3m
• Placed at a random position, randomly choose a
  new location
• 70 of the packets are long packet(1000bytes),
  the remainder are short packets(32bytes)
• Fail with probability of 5, 2 retransmission
  attempts
• The other hosts in range of the sender each have
  a 95 chance of overhearing the packet
• The bandwidth for transmitting data is
  100Kbytes/s

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Performance Evaluation (2/3)

• Each run - 4000s
• Pause time - 0 4000s

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Performance Evaluation (3/3)
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Conclusion

• Dynamic source routing protocol
• Routing packets between wireless mobile hosts in
  an ad hoc network
• Adapts quickly to routing changes
• Route lengths are on average within a factor of
  1.02 of optimal
• Incorporate some additional optimizations
• Future work
• Interconnection of an ad hoc network with a
  wide-area network such as the Internet
• Security