DSDV Destination Sequenced Distance Vector Protocol

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DSDVDestination Sequenced Distance Vector
Protocol

• Seminar presentation
• for PG MANET-WLAN
• December 2, 2003

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Classification of Routing Protocols for MANETS
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Ad-hoc Networks

• Ad-hoc network
• A collection of wireless mobile hosts forming a
  temporary network without the aid of any
  established infrastructure or centralized
  administration.
• Significant differences to existing wired
  networks
• Wireless
• Self-starting
• No administrator
• Cannot assume, that every computer is within
  communication range of every other computer
• Possibly quite dynamic topology of
  interconnections

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Distance-Vector routing

• Each node maintains a routing table containing
• list of all available destinations
• number distance to each each destination
• next hop to reach a destination
• The succession of next hops leads to a
  destination
• Each node periodically broadcasts its current
  estimate of the shortest distance to each
  available destination to all of its neighbors
• Typical representative Distributed Bellman-Ford
  (DBF)

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Bellman-Ford Algorithm
Bellmann-Ford (G,w,s) Initialize-Single-Source
(G,s) for i ? 1 to VG-1 do for each
edge (u,v) EG do Relax
(u,v,w) for each edge (u,v) EG do
if dv gt duw(u,v) then return
FALSE return TRUE
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Initialize-Single-Source Relax
Initialize Single Source (G,s) for each vertex
v VG do dv ? v ? NIL
ds ? 0
Relax (u,v,w) if dv gt du w(u,v)
then dv ? du w(u,v) v ?
u
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Bellman-Ford Algorithm
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Bellman-Ford Routing

• Computationally efficient
• Easy to implement
• Problem Can cause loops
• Problem Counting to infintiy
• Modifications elminate the problem of loops but
  need some internodal coordination mechanisms
  which imply few topological changes
• Not designed to handle rapid topological changes

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DSDV

• Design goals
• Keep the simplicity of Bellman-Ford
• Avoid the looping problem
• Remain compatible in cases where a base station
  is available
• Idea
• modify the conventional Bellman-Ford routing
  algorithm
• Approach
• Model each host as a router
• Tag each routing table entry with a sequence
  number

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The Routing Table

• All available destinations
• Next hop for each destination
• Number of hops to each available destination
• A sequence number for each route table entry,
  originated by the destination station

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Transmitting Route Information

• Routing information is transmitted by broadcast
• Updates are transmitted periodically or
  immediately when any significant topology change
  is available
• Sequence numbers are assigned by destination
  (even numbers)
• If a broken link is detected metric and
  updated odd sequence number are assigned by
  detecting host
• Full dump all information from the transmitting
  node
• Incremental dump all information that has
  changed since the last full dump
• Full dump if incremental dump exceeds one NPDU
  (network protocol data unit)

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Selection of Routes

• If new routing information is received
• Any route with a more recent sequence number is
  used
• If the new route has equal seqence number but
  better metric, this route is chosen
• Newly recorded routes are scheduled for immediate
  advertisement

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Receiving Fluctuating Routes

• What might happen
• MH9 broadcasts update information to MH
  Collections I and II
• MH2 transmits new routing information to MH4
• MH4 new sequence number ? routing table update ?
  broadcast update
• MH6 transmits new routing information to MH4,
  same sequence number, better metric
• MH4 same seq.no., better metric ? update routing
  table ? broadcast update

MH9
Mobile Host Collection I
Mobile Host Collection II
MH6
MH2
MH4
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Damping Fluctuation

• Causes for Fluctuation
• Many hosts with irregular updates
• Different propagation speed
• Different transmission intervals
• Broadcasts are asynchronous events
• Solution Keep a route settling time table in
  each node with a time to wait for a route with a
  better metric before advertising the update
  message.
• Settling time Calculated by maintaining a
  running weighted average over the most recent
  updates of the routes for each destination.

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Stale Entries

• Stale entries are defined to be entries that have
  not been updated the last few update periods
• Stale entries are deleted at the same time when
  routing updates are applied to the routing table
• Any route using that host as a next hop is
  deleted, included the route indicating that host
  as the actual destination

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Example of DSDV in operation
MH3
MH4
MH5
MH2
MH8
MH6
MH7
MH1
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Example of DSDV in operation
MH3
MH4
MH5
MH2
MH8
MH6
MH7
MH1
MH4 advertised table
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Example of DSDV in operation
MH3
MH4
MH5
MH2
MH8
MH6
MH7
MH1
MH1

• Update triggered by MH1 , broadcasted to MH7 and
  MH8
• On detection of broken link Immediate
  incremental update triggered by MH2 with odd
  sequence number and infinite metric
• Updates are propagated through the network

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Example of DSDV in operation
MH3
MH4
MH5
MH2
MH8
MH6
MH7
MH1
MH4 advertised table (updated)
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Evaluation of DSDV Performance

• Each node maintains two tables
• The need of bandwidth and the size of tables grow
  simultaneously with mobility and number of nodes
• ? overhead for maintaining and updating tables
  will increase
• ? heavy routing overhead will degrade the
  performance of the network

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Simulation results

• Simulation by Broch, Maltz, Johnson, Hu, Jetcheva
• DSDV fails to converge if nodes dont pause for
  at least 300 seconds
• Packet delivery ratio is in the range of 70-92
  at higher rate of mobility
• Packet loss is mainly caused by stale routing
  entries
• Routing overhead is approximately constant,
  regardless of movement rate or traffic load
• Nearly optimal path can be selected in routing
  procedure

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Stability and Scalability

• DSDV requires a full dump update periodically
• ? DSDV is not efficient in route updating
• DSDV limits the number of nodes that can join the
  network
• Whenever topology of a network changes, DSDV is
  unstable until update packets propagate through
  the network

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Conclusion

• DSDV is effective for creating ad-hoc networks
  for small populations of mobile nodes
• DSDV is a fairly brute force approach, because
  connectivity information needs periodical update
  througout the whole network

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Current Status

• DSDV is a well-known routing algorithm for ad hoc
  network routing
• No standard specifications or commercial
  implementations available
• Many improved protocols based on DSDV have been
  developed
• Example AODV Ad-hoc On-Demand Distance Vector
  Routing

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AODV

• AODV is based on the DSDV algorithm
• Distance vector
• Sequence numbers
• Creation of routes on a demand basis
• Nodes that are not on a selected path do not
  maintain routing information or participate in
  routing table exchanges!
• Goal Minimize broadcast overhead and
  transmission latency

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AODV
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AODV

• Path discovery Process
• Source node initiates path discovery process by
  broadcasting RREQ
• Neighbors forward RREQ
• RREQ is forwarded until either destination or
  intermediate node with a fresh enough route to
  it is located
• Destination or intermediate node responds by
  unicasting RREP along the reverse path
• Local connectivity checked on a regular basis by
  listening to retransmission or sending hello
  messages
• Link failure
• Failure notification message (RREP with infinite
  metric) is passed upstream to the source, erasing
  that part of the route

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AODV Simulation results

• Simulation by Broch, Maltz, Johnson, Hu, Jetcheva
• Implementation without Hello mechanism
• Delivery of over 95 regardless of mobility rate
• Routing overhead drops as mobility rate drops
• Not optimal path, up to 4 or more hops longer
  paths
• Requires up to 5 times the overhead of DSR

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Comparison DSDV and AODV

• DSDV
• Table driven, proactive
• Best performance when node mobility rate and
  movement speed are low
• Approximately constant overhead, regardless of
  movement rate or traffic load
• AODV
• On demand
• Good performance at all mobility rates
• Still requires transmission of many routing
  overhead packets

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References

• Charles E. Perkins and P. Bhagwat, Highly
  Dynamic Destination Sequenced Distance Vector
  Routing (DSDV) for Mobile Computers, ACM
  SIGCOMM94, 1994
• Guoyou He, Destination-Sequenced Distance Vector
  (DSDV) Protocol
• Josh Broch, David A. Maltz, David B. Johnson,
  Yih-Chun Hu, Jorjeta Jetcheva, A Performance
  Comparison of Multi-Hop Wireless Ad Hoc Network
  Routing Protocols, MobiCom98, 1998
• Charles E. Perkins, Elizabeth M. Royer, Ad-hoc
  On-Demand Distance Vector Routing, 1999
• Elizabeth M. Royer, Chai-Keong Toh, A Review of
  Current Routing Protocols for Ad Hoc Mobile
  Wireless Networks, IEEE Personal Communications,
  April 1999
• Andrew S. Tanenbaum, Computer Networks, 3rd
  Edition, Prentice Hall, 1996
• T.H. Cormen, C.E. Leiserson, R.L. Rivest,
  Introduction to Algorithms, MIT Press, 1990