Ad hoc On-demand Distance Vector (AODV) Routing Protocol by Ashok Panwar

Presented By:-
Ashok Panwar
Technical Officer in ECIL (NPCIL)
Tarapur, Mumbai

 AODV is a packet routing protocol designed for use in
mobile ad hoc networks (MANET)
 Intended for networks that may contain thousands of
nodes
 One of a class of demand-driven protocols
• The route discovery mechanism is invoked only if a route to a
destination is not known
 UDP is the transport layer protocol
 Source, destination and next hop are addressed using IP
addressing
 Each node maintains a routing table that contains
information about reaching destination nodes.
• Each entry is keyed to a destination node.
Presented By:-
Ashok Panwar
Technical Officer in ECIL
2

 Destination IP address
 Destination Sequence Number
 Valid Destination Sequence Number Flag
 Other state and routing flags
 Network Interface
 Hop Count (needed to reach destination)
 Next Hop
 Precursor List
 Lifetime (route expiration or deletion time)
Presented By:-
Ashok Panwar
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 Routing table size is minimized by only including next
hop information, not the entire route to a destination
node.
 Sequence numbers for both destination and source are
used.
 Managing the sequence number is the key to efficient
routing and route maintenance
• Sequence numbers are used to indicate the relative freshness of
routing information
• Updated by an originating node, e.g., at initiation of route
discovery or a route reply.
• Observed by other nodes to determine freshness.
Presented By:-
Ashok Panwar
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The basic message set consists of:
• RREQ – Route request
• RREP – Route reply
• RERR – Route error
• HELLO – For link status monitoring
Presented By:-
Ashok Panwar
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 RREQ Messages
• While communication routes between nodes are valid,
AODV does not play any role.
• A RREQ message is broadcasted when a node needs
to discover a route to a destination.
• As a RREQ propagates through the network,
intermediate nodes use it to update their routing tables
(in the direction of the source node).
• The RREQ also contains the most recent sequence
number for the destination.
• A valid destination route must have a sequence
number at least as great as that contained in the
RREQ.
Presented By:-
Ashok Panwar
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B?
B?
B?
B?
B?
B? B?
B
A
Presented By:-
Ashok Panwar
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RREP Messages
• When a RREQ reaches a destination node, the
destination route is made available by unicasting a
RREP back to the source route.
• A node generates a RREP if:
 It is itself the destination.
 It has an active route to the destination. Ex: an intermediate
node may also respond with an RREP if it has a “fresh enough”
route to the destination.
• As the RREP propagates back to the source node,
intermediate nodes update their routing tables (in the
direction of the destination node).
Presented By:-
Ashok Panwar
8

B
A
A
A
A
A
A
A
Presented By:-
Ashok Panwar
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RERR Messages
• This message is broadcast for broken links
• Generated directly by a node or passed on when
received from another node
Presented By:-
Ashok Panwar
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 Hello Messages
• Hello Message = RREP with TTL = 1
• This message is used for broadcasting connectivity
information.
 Ex: If a neighbor node does not receive any packets (Hello
messages or otherwise) for more than
ALLOWED_HELLO_LOSS * HELLO_INTERVAL mseconds, the
node will assume that the link to this neighbor is currently lost.
• A node should use Hello messages only if it is part of
an active route.
Presented By:-
Ashok Panwar
11

A
B D
F
C
G
E
RREQ
RREQ
RREQ
RREQ
RREQ
RREQ
RREQ
RREQ
RREQ
RREP
RREP
RREP
Source
Destination
Presented By:-
Ashok Panwar
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 One method that AODV handle congestion is:
• If the source node receives no RREP from the
destination, it may broadcast another RREQ, up to a
maximum of RREQ_RETRIES.
• For each additional attempt that a source node tried to broadcast
RREQ, the waiting time for the RREP is multiplied by 2.
 DSR is not capable of handling congestion.
Presented By:-
Ashok Panwar
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 Other possible methods to improve AODV
congestion handling:
• A route may predict when congestion is about to occur
and try to avoid it by reduce the transmission rate.
• Schedule the requests so that it will not overload the
network.
Presented By:-
Ashok Panwar
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 There are two phases
• Route Discovery.
• Route Maintenance.
 Each node maintains a routing table with knowledge about the
network.
 AODV deals with route table management.
 Route information maintained even for short lived routes – reverse
pointers.
Presented By:-
Ashok Panwar
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 Destination IP Address
 Destination Sequence Number
 Valid Destination Sequence Number flag
 Other state and routing flags (e.g., valid, invalid, repairable, being
repaired)
 Network Interface
 Hop Count (number of hops needed to reach destination)
 Next Hop
 List of Precursors
 Lifetime (expiration or deletion time of the route)
 DSR maintains additional table entries, causing a larger memory
overhead
Presented By:-
Ashok Panwar
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 Broadcast RREQ messages.
 Intermediate nodes update their routing table
 Forward the RREQ if it is not the destination.
 Maintain back-pointer to the originator.
 Destination generates RREQ message.
 RREQ sent back to source using the reverse pointer set
up
by the intermediate nodes.
 RREQ reaches destination, communication starts.
Presented By:-
Ashok Panwar
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 @Originator
 If a route to the destination is available, start sending data.
 Else generate a RREQ packet. Increment the RREQID by
1.Increment the sequence number by 1.Destination IP address
,currently available sequence number included.
 @Intermediate Node
 Generate route reply, if a 'fresh enough' route is a valid route entry
for the destination whose associated sequence number is at least as
great as that contained in the RREQ. Change the sequence number
of the destination node if stale, increment the hop count by 1 and
forward.
 @Destination 1.Increment sequence number of the destination.
2.Generate a RREQ message and sent back to Originator.
Presented By:-
Ashok Panwar
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Presented By:-
Ashok Panwar
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 Hello messages broadcast by active nodes periodically
HELLO_INTERVAL.
 No hello message from a neighbor in DELETE_PERIOD,link failure
identified.
 A local route repair to that next hop initiated.
 After a timeout ,error propagated both to originator and destination.
 Entries based on the node invalidated.
Presented By:-
Ashok Panwar
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 Each originating node maintains a monotonically increasing
sequence number.
 Used by other nodes to determine the freshness of the
information.
 Every nodes routing table contains the latest information
available about the sequence number for the IP address of
the destination node for which the routing information is
maintained.
• Updated whenever a node receives new information about the
sequence number from RREQ, RREP, or RERR messages
received related to that destination.
Presented By:-
Ashok Panwar
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 AODV depends on each node in the network to own and
maintain its destination sequence number.
 A destination node increments its own sequence number
immediately before it originates a route discovery
 A destination node increments its own sequence number
immediately before it originates a RREP in response to a
RREQ
 The node treats its sequence number as an unsigned
number when incrementing accomplishing sequence
number rollover.
 Destination information is assured by comparing the
sequence number of the incoming AODV message with
its sequence number for that destination.
Presented By:-
Ashok Panwar
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• Message is broadcasted when:
i. A node detects that a link with adjacent neighbor is
broken (destination no longer reachable).
ii. If it gets a data packet destined to a node for which
it does not have an active route and is not
repairing.
iii. If it receives a RERR from a neighbor for one or
more active routes.
Presented By:-
Ashok Panwar
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• Build Affected Destination Listing
i. List unreachable destinations containing unreachable
neighbor & destination using unreachable as next hop
ii. Only one unreachable destination, which node already
has.
iii. List of nodes where RERR is next hop
• Update information
• Transmit RERR for each item listed
Presented By:-
Ashok Panwar
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• Destination Sequence #
- Update sequence # for case i and ii
- Copy sequence # for case iii
• Invalidate route entry
• Update Lifetime field as (currtime +
DELETE_PERIOD)
• Only now may route entry be deleted
Presented By:-
Ashok Panwar
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• Unicast
- Send RERR to single recipient
• Unicast iteritive
- Send RERR to a number of recipients individually
• Broadcast
- Notify multiple recipients simultaniously
- Broadcast via 255.255.255.255 TTL = 1
Presented By:-
Ashok Panwar
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• Unicast
 A node detects that a link with adjacent neighbor is
broken (destination no longer reachable).
 If it gets a data packet destined to a node for which it
does not have an active route and is not repairing.
 If it receives a RERR from a neighbor for one or more
active routes.
• Unicast iteritive
• Broadcast
Presented By:-
Ashok Panwar
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 All nodes directly using the broken link
get a RERR packet.
 Then those nodes sends the RERR
packet to their predecessor nodes.
 This is continued all the way to the
source nodes.
Presented By:-
Ashok Panwar
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Upon completion of sending the RERR
packet through the source node will no
longer use the link.
• AODV uses loop free-routes.
• There are only a finite number of nodes in a ad-
hoc network.
Presented By:-
Ashok Panwar
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DSR does not remove the path as well.
• In DSR nodes in the network can still think the
broken link is still valid.
• This can lead to having to search for new paths
multiple times.
Presented By:-
Ashok Panwar
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The source node can restart the discovery
process if a path is still needed.
The source node or any node on the path
can rebuild the route by sending out a
RREQ.
Presented By:-
Ashok Panwar
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Ad hoc On-demand Distance Vector (AODV) Routing Protocol by Ashok Panwar

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