01-11-2016, 03:52 PM
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Introduction: DSDV vs. AODV
DSDV broadcasts every change in the network to every node
❒ When two neighbors enter communication range of each other
❍ This results in a network wide
broadcast
❒ Similarly when two nodes drift apart from each other’s range –> link breakage
❍ Also results in a network wide
broadcast
❒ Local movements have global effects
In AODV such broadcasts are
not necessary
❒ If a link breakage does not affect on going transmission -> no global broadcast occurs
❒ Only affected nodes are informed
❒ Local movements of nodes
have local effects
❒ AODV reduces the network
wide broadcasts to the extent
possible
❒ Significant reduction in control overhead as compared to DSDV
Ad Hoc On Demand DistanceVector
(AODV) Routing (1)
❒ Reactive or on Demand
❒ Descendant of DSDV
❒ Uses bi-directional links
❒ Route discovery cycle used for route
finding
❒ Maintenance of active routes
❒ Sequence numbers used for loop prevention
and as route freshness criteria
❒ Provides unicast and multicast
communication
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Ad Hoc On Demand DistanceVector
(AODV) Routing (2)
❒ Whenever routes are not used -> get expired -> Discarded
❍ Reduces stale routes
❍ Reduces need for route maintenance
❒ Minimizes number of active routes between an
active source and destination
❒ Can determine multiple routes between a source and a destination, but implements only a single route, because
❍ Difficult to manage multiple routes between same source/destination pair
❍ If one route breaks, its difficult to know whether other
route is available
❍ Lot of book-keeping involved
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AODV Properties (1)
1. AODV discovers routes as and when
necessary
❒ Does not maintain routes from every node to
every other
2. Routes are maintained just as long as
necessary
3. Every node maintains its monotonically
increasing sequence number -> increases
every time the node notices change in the
neighborhood topology
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AODV Properties (2)
❒ AODV utilizes routing tables to store routing
information
1. A Routing table for unicast routes
2. A Routing table for multicast routes
❒ The route table stores: <destination addr, next-hop addr,
destination sequence number, life_time>
❒ For each destination, a node maintains a list of
precursor nodes, to route through them
❍ Precusor nodes help in route maintenance (more later)
❒ Life-time updated every time the route is used
❍ If route not used within its life time -> it expires
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AODV – Route Discovery (1)
❒ When a node wishes to send a packet to some
destination –
❍ It checks its routing table to determine if it has a
current route to the destination
• If Yes, forwards the packet to next hop node
• If No, it initiates a route discovery process
❒ Route discovery process begins with the creation
of a Route Request (RREQ) packet -> source node
creates it
❒ The packet contains – source node’s IP address,
source node’s current sequence number,
destination IP address, destination sequence
number
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AODV – Route Discovery (2)
❒ Packet also contains broadcast ID number
❍ Broadcast ID gets incremented each time a source node
uses RREQ
❍ Broadcast ID and source IP address form a unique
identifier for the RREQ
❒ Broadcasting is done via Flooding