Adhoc OnDemand Distance Vector Routing

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Ad-hoc On-Demand Distance Vector Routing

• Presented by Qifeng Lu

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• Ad-hoc On-Demand Distance Vector Routing, Charles
  E. Perkins, Elizabeth M. Royer. Proceedings of
  IEEE WMCSA'99,New Orleans, LA, Feb. 1999

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Outline

• Introduction
• Ad-hoc Network routing constraints
• Destination Sequenced Distance Vector (DSDV)
  Routing
• The Ad-hoc On-Demand Distance Vector (AODV)
  Algorithm
• Simulation and Results
• Current Status and Future Work
• Conclusion

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Constraints in Ad-hoc Network Routing

• No infrastructure support
• No specialized routers
• Also no fixed routers (physically)
• Frequent topology change
• Problems due to wireless media
• Bandwidth, range of communication, collisions due
  to broadcasting, asymmetric one-way links

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Destination Sequenced Distance-Vector Routing
(DSDV)

• A variant of the distance vector routing method
• Not efficient for large ad-hoc networks
• Uses periodic advertisements
• Global dissemination of connectivity information
• Nodes need to maintain a complete list of routes.

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What is AODV?

• An improved algorithm of DSDV
• A Source Initiated (reactive) routing protocol
• Main goals
• Quick adaptation under dynamic link conditions
• Lower transmission latency
• Low network utilization (less broadcast)
• Loop-free property (How using destination
  sequence )
• Scalable to large network

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AODV

• Features
• Routes are created only when required
• Each node doesnt maintain all routes to other
  nodes (only active routes)
• Use of sequence numbers at each destination to
  maintain freshness of routing information
• Reduces periodic broadcast
• Paths generated are loop-free
• Uses symmetric links (if a link is not symmetric
  it is not up)
• Works both on wired media and wireless media

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AODV Algorithm

• Primary Objectives
• To broadcast discovery packets only when
  necessary
• To distinguish between local connectivity
  management and general topology maintenance
• To disseminate information about changes in local
  connectivity to neighboring mobile nodes that may
  need the information

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AODV Algorithm

• Path Discovery
• Reverse Path Setup
• Forward Path Setup
• Route Table Management
• Path Maintenance
• Local Connectivity Management

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Path Discovery

• Initiated when no route to reach destination node
• Each node has 2 counters
• Source node sends a route request (RREQ)

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Details of RREQ

• ltsource_addr, broadcast_idgt is unique
• Broadcast_id is incremented for new RREQ
• If the neighboring node doesnt reply with a
  RREP, hop_cnt is incremented.
• RREQ from same node with same broadcast_id will
  not be broadcasted more than once.

For example, node A wants to contact node D, but
there is no active path to reach node D.
A
B
D
C
B wont rebroadcast this RREQ
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RREP

• Route Reply

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Reverse Path Setup

• Source_sequence_ To maintain freshness
  information about the reverse route to the source
• Dest_sequence_ how fresh a route to the
  destination must be in order to be accepted by
  the source
• Every node will record the neighbors address
  where first copy of RREQ is from
• These entries will be maintained for long enough
  for RREQ to traverse and produce a RREP to the
  sender

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Reverse Path Setup

• Reverse Path
• Broadcast route request (RREQ) lt source_addr,
  source_sequence- , broadcast_id, dest_addr,
  dest_sequence_, hop_cnt gt
• RREQ uniquely identified by ltsource_addr ,
  broadcast_idgt
• Route reply (RREP) if neighbor is the target, or
  knows a higher dest_sequence_
• Otherwise setup a pointer to the neighbor from
  whom RREQ was received
• Maintain reverse path entries based on timeouts

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Example
Assume same dest_sequence_ for all nodes
Source
Source
A
A
B
B
C
C
D
E
D
E
Destination
Destination
Reverse Path Formation
Network Layout
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Forward Path Setup

• RREQ arrives at a node that has current route to
  the destination ( larger/same sequence number)
• unicast request reply (RREP)ltsource_addr,
  dest_addr, dest_sequence_, hop_cnt,lifetimegt to
  neighbor
• RREP travels back to the source along reverse
  path
• each upstream node updates dest_sequence_, sets
  up a forward pointer to the neighbor who transmit
  the RREP

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Forward Path Setup
Nodes A sends B a RREQ with dest_sequence_ 100
and B has a route entry to reach D with
dest_sequene_ 99. B can NOT use its recorded
route to respond to the RREQ. B will rebroadcast
the RREQ. However, for node C, C can respond with
its recorded route.
100
100 or gt 100
99
E
C will send node A RREP ltAs address, Ds
address, 100, 1, lifetimegt

• If C can supply a route to destination D, a
  reverse path has been connected from D to A.
• RREP traverses from D to A, C (intermediate
  nodes) will setup a forward pointer to D and
  updates lifetime and records the latest
  dest_sequence_
• Nodes that are not on the path determined by the
  RREP will timeout after 3 seconds and will delete
  the reverse pointers (e.g. reverse pointer from E
  to A)

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Example
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More Example
100
101
E
RREP Paths from H to A
B
A
H
D
100
C
G
F
101
Path with largest last dest_sequence_ or
smallest hop_cnt with same dest_sequence_ will
be chosen
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Route Table Management

• Route request expiration time to purge reverse
  path routing entries from nodes are not on the
  path from source to destination
• Route caching timeout when the route is
  considered to be invalid

What is active?
A
B
C

• B is an active neighbor of A if A sends/receives
  gt1 packets from B within active_timeout period
• The route entry on node A (A??B) is active if A
  and B communicates
• The path A? C is active if route entries along
  node A to node C are active

Note like DSDV, all routes in routing table are
tagged with destination sequence numbers ?
loop-free
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Route Table Management (cont.)

• Each route table entry contains
• Destination
• Next Hope
• Number of hops
• Sequence number for the destination
• Active neighbors for this route
• Expiration time for the route table entry
• When a route entry is used to transmit data from
  source to destination, Timeout for each entry is
  current time active_route_timeout
• When a new route is available, route table will
  be updated only if new route has larger
  dest_sequence_ or same dest_sequence_ but with
  smaller hop_cnt to the destination.

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Route Table Management (cont.)

• Route request expiration timer purges reverse
  paths that do not lie on active route
• Active neighbor relays a packet within
  active_route_timeout
• Route cache timer purges inactive routes
• New routes preferred if higher destination
  sequence number or lower metric

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Path Maintenance

• If source node moves during an active session ?
  it can redo Path discovery
• If destination or intermediate nodes move ? a
  special RREP is sent to affected source nodes
  (Upon link breakage, affected node propagates an
  unsolicited RREP ltdest_sequence_1, 8gt to all
  upstream nodes)
• Source may restart route discovery process

• If link between C and D is broken, a special
  RREP will sent to B and E by C. (C will
  increment its dest_sequence_ by 1 and set hop
  count to infinity)
• Then B and E will send this RREP to its active
  neighbors F and G. At the end, all active source
  nodes are notified.

E
F
G

• When source find out a link to destination is
  broken, and like to rebuild the path to
  destination, source will send a RREQ with last
  dest_squence_ 1

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Local Connectivity Management

• 2 ways for a node to find its neighbors
• Receiving broadcast from its neighbors
• Send its neighbors hello messages containing its
  identity and sequence number
• Sequence number is not changed for hello message
• Nodes can not rebroadcast hello messages (TTL1)
• Local Connectivity has changed if
• Receiving a broadcast or a hello from a new node
• Failing to receive allowed_hello_loss consecutive
  hello message from a node was in the neighborhood
• Neighbors only communicate when heard each
  others hello message Ensure the link is
  bidirectional