Adhoc OnDemand Distance Vector Routing AODV

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Ad-hoc On-Demand Distance Vector Routing (AODV)

• (CS710)Special issues on computer architecture
• 7, September, 2004
• Presented by Dongwook Kim

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Content

• Introduction to ad-hoc networks
• AODV Concept
• AODV Mechanism
• Performance comparison with DSR
• Critique of DSR and AODV
• Simulation
• Conclusions

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Introduction to ad-hoc networks

• Network of mobile wireless nodes
• No infrastructure (e.g., base stations, fixed
  routers, centralized servers)
• Dynamic topology
• Routing infrastructure created dynamically
• Data can be relayed by intermediate nodes
• Limited battery power and transmission range
  resources in the nodes
• Military environments, emergency and rescue
  operations, meeting rooms, etc

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AODV Concept

• Reactive routing
• Pure on-demand route acquisition system
• The routes are created when needed, so called
  on-demand
• A broadcast route discovery mechanism
• RREQ (Route REQest) broadcasting to find a route
• Dynamic establishment of route table entries
• Nodes lie on active paths only maintain routing
  information
• Destination sequence number
• Prevention of routing loops
• Avoidance of old and broken routes
• Maintenance of timer-based states
• A routing table entry is expired if not used
  recently

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AODV Mechanism

• Path discovery
• The source requests using RREQ broadcasting
• ltsource_addr, source_sequence, broadcast_id,
  dest_addr, dest_sequence, hop_cntgt
• Destination number of RREQ is the last known
  number
• The destination replies using RREP (Route Reply)
  unicasting
• ltsource_addr, dest_addr, dest_sequence, hop_cnt,
  lifetimegt
• The sequence number is first incremented if it is
  equal to the number in the request
• RREP contains the current sequence number, hop
  count 0, full lifetime
• Intermediate nodes
• Discard duplicate requests
• Replies if it has an active route with higher
  sequence number
• Otherwise broadcasts the request on all interfaces

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• Path discovery (Contd)
• Intermediate nodes (Contd)
• Setup reverse path
• A node records the address of the neighbor
  sending RREQ
• Keep track of some information
• Destination IP address, Source Ip address,
  Broadcast_id, Expiration time for reverse path
  route entry, Source nodes sequence number
• Setup forward path
• Unicast RREP (Route reply) back to the reverse
  path
• Each node along the path sets up a forward
  pointer to the node from which the RREP came
• Update its routing table entry
• Propagate the first RREP or the RREP that
  contains a greater destination sequence or the
  same sequence with a smaller hop count
• Nodes that are not along the path determined by
  the RREP will timeout and will delete the reverse
  pointers

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• Example

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(No Transcript)
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Forward pointer
RREP ltS, D, 12, 3, lifetimegt
S
E
F
B
C
J
A
G
H
D
K
I
N
timeout
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• Route table management
• Soft-state associated with the entry
• Route request expiration
• Purge of a reverse path
• Route caching timeout
• Active route timeout
• A neighbor is considered active if it originates
  or relays at least one packet to the destination
• Use of route maintenance

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• Path maintenance
• Neighboring nodes with active routes periodically
  exchange hello messages
• If a next hop link in the routing table fails,
  the active neighbors are informed
• The RERR (unsolicited RREP) indicates the
  unreachable destinations
• ltsource_addr, dest_addr, current sequence 1,
  infinity, lifetimegt
• The source performs a new route request when it
  receives a RERR

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• Proof of the Loop-free property

X1
A
Z
X2
S
X3
X1 ? X2, X2 ? X3, X3 ? X1 T1 lt T2, T2 lt T3, T3
lt T1, so T1 T2 T3 Mi is hop , Xi ? Xi1, so
Mi Mi1 1 M1 3, M2 2, M3 3, 4 M1 M2
1, M3 M1 1 (contradiction)
AODV is loop-free!
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Performance comparison with DSR

• Performance Comparison of Two On-demand Routing
  Protocols for Ad Hoc Networks (INFOCOM2000)

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Critique of DSR and AODV

• Common points
• On-demand basis
• RREQ broadcasting, RREP unicasting
• Redundant RREQ is discarded
• Different points
• Routing overhead
• DSR access greater amount of routing info.
• AODV gather only a very limited amount of
  routing info.
• AODV gtgt DSR
• Expiration mechanism about route information
• DSR no explicit mechanism
• AODV timer-based mechanism
• The route deletion activity using RERR
• DSR route back only the source
• AODV reach active nodes using a failed link on
  its route to any destination

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Simulation model

• MAC (Medium Access Control) protocol
• DCF (Distributed Coordination Function) of IEEE
  802.11
• Avoidance of hidden terminal problem
• RTS (Request-To-Send) / CTS (Clear-To-Send) /
  data / ACK exchange for unicast packet
• The Traffic and Mobility Models
• Traffic sources CBR (Constant Bit Rate)
• Random location of S (source) D (destination)
  pairs
• Change of the number of S - D pairs and packet
  sending rate
• Mobility model
• Random waypoint model
• Various pause time ( various mobility)

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(1) Performance metrics

• Packet delivery fraction
• Received throughput (kilobits per second) at the
  destination
• Average end-to-end delay of data packets
• Normalized routing load
• Evaluate the efficiency of the routing protocol

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(1) Simulation results
100 nodes Varying mobility and number of sources
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(1) Summary

• In lower number of sources
• delivery fraction and delay of DSR and AODV is
  similar
• In higher number of sources
• AODV starts outperforming DSR
• DSR is always a lower routing load than AODV
• AODVs routing overhead RREQ
• DSRs routing overhead RREP
• In Routing load, if we use bytes metric instead
  of packets, relative routing load differences
  will be much smaller
• DSR uses larger routing packets and data packets
  due to source routing

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(2) Performance metrics

• Throughout
• Combined received throughput at the destinations
  about offered load
• Offered load is combined sending rate of all data
  sources
• Kilobits per second
• Routing load
• The number of routing packets
• Kilobits per second
• Delay

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(2) Simulation results
100 nodes, zero pause time Varying offered load
10 sources
40 sources
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(2) Summary

• Routing load and MAC overhead
• DSR almost always has lower routing load than
  AODV
• AODVs routing load ? RREQ packets (broadcast)
• Routing table updates
• DSRs routing load ? RREP packets (unicast)
• Potentially many cache replies
• The higher MAC load for DSR
• RERR, RREP, data is unicast packet ? DSR is
  required more control packets
• control packets RTS/CTS/ACK
• In AODV
• RREP, data is only unicast packet

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• Packet Delivery and Choice of Routes
• In AODV
• Good packet delivery and effective choice of
  routes in stressful situations
• Larger number of nodes, sources, higher mobility
• In DSR
• Better performance in less stressful situations
• The use of route caching
• Delay and Choice of Routes
• In AODV
• The destination replies only to the first
  arriving RREQ ? this favors the least congested
  route
• In DSR
• the source receive many replies ? this make it
  difficult to determine the least congested route

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Conclusions

• DSR and AODV both use on-demand route discovery
• DSR
• Source routing, route caches, no timer-based
  activities, aggressive route caching
• AODV
• Routing table, destination sequence number, loop
  prevention and freshness of routes mechanism
• AODV outperforms DSR in more stressful situations