The document describes the Ad Hoc On-Demand Distance Vector (AODV) routing protocol. AODV is designed for mobile ad hoc networks and uses route discovery and maintenance to dynamically discover and maintain routes. It uses sequence numbers to determine freshness of routing information and broadcasts RREQ, RREP, RERR and HELLO messages for route discovery, maintenance and link status monitoring.

1. Ad hoc On-demandAd hoc On-demand
Distance Vector (AODV)Distance Vector (AODV)
Routing ProtocolRouting Protocol
ECE 695ECE 695
Spring 2006Spring 2006

2. AODV OverviewAODV Overview
 AODV is a packet routing protocol designed forAODV is a packet routing protocol designed for
use in mobile ad hoc networks (MANET)use in mobile ad hoc networks (MANET)
 Intended for networks that may containIntended for networks that may contain
thousands of nodesthousands of nodes
 One of a class ofOne of a class of demand-drivendemand-driven protocolsprotocols
• TheThe route discoveryroute discovery mechanism is invoked only if amechanism is invoked only if a
route to a destination is not knownroute to a destination is not known
 UDPUDP is the transport layer protocolis the transport layer protocol
 Source, destinationSource, destination andand next hopnext hop are addressedare addressed
usingusing IP addressingIP addressing
 Each node maintains aEach node maintains a routing tablerouting table that containsthat contains
information about reaching destination nodes.information about reaching destination nodes.
• Each entry is keyed to a destination node.Each entry is keyed to a destination node.

3. Routing Table FieldsRouting Table Fields
 Destination IP addressDestination IP address
 Destination Sequence NumberDestination Sequence Number
 Valid Destination Sequence Number FlagValid Destination Sequence Number Flag
 Other state and routing flagsOther state and routing flags
 Network InterfaceNetwork Interface
 Hop Count (needed to reach destination)Hop Count (needed to reach destination)
 Next HopNext Hop
 Precursor ListPrecursor List
 Lifetime (route expiration or deletion time)Lifetime (route expiration or deletion time)

4. Overview (continued)Overview (continued)
 Routing table size is minimized by only includingRouting table size is minimized by only including
next hop information, not the entire route to anext hop information, not the entire route to a
destination node.destination node.
 Sequence numbers for both destination andSequence numbers for both destination and
source are used.source are used.
 Managing the sequence number is the key toManaging the sequence number is the key to
efficient routing and route maintenanceefficient routing and route maintenance
• Sequence numbers are used to indicate the relativeSequence numbers are used to indicate the relative
freshnessfreshness of routing informationof routing information
• Updated by an originating node, e.g., at initiation ofUpdated by an originating node, e.g., at initiation of
route discovery or a route reply.route discovery or a route reply.
• Observed by other nodes to determine freshness.Observed by other nodes to determine freshness.

5. Overview (continued)Overview (continued)
 The basic message set consists of:The basic message set consists of:
• RREQ – Route requestRREQ – Route request
• RREP – Route replyRREP – Route reply
• RERR – Route errorRERR – Route error
• HELLO – For link status monitoringHELLO – For link status monitoring

6. AODV Operation – Message TypesAODV Operation – Message Types
 RREQ MessagesRREQ Messages
• While communication routes between nodesWhile communication routes between nodes
are valid, AODV does not play any role.are valid, AODV does not play any role.
• A RREQ message is broadcasted when a nodeA RREQ message is broadcasted when a node
needs to discover a route to a destination.needs to discover a route to a destination.
• As a RREQ propagates through the network,As a RREQ propagates through the network,
intermediate nodes use it to update theirintermediate nodes use it to update their
routing tables (in the direction of the sourcerouting tables (in the direction of the source
node).node).
• The RREQ also contains the most recentThe RREQ also contains the most recent
sequence number for the destination.sequence number for the destination.
• A valid destination route must have a sequenceA valid destination route must have a sequence
number at least as great as that contained innumber at least as great as that contained in
the RREQ.the RREQ.

7. RREQ MessageRREQ Message
B?B?
B?B?
B?B?
B?B?
B?B?
B?B? B?B?
BB
AA

8. AODV Operation – Message TypesAODV Operation – Message Types
 RREP MessagesRREP Messages
• When a RREQ reaches a destination node, theWhen a RREQ reaches a destination node, the
destination route is made available bydestination route is made available by
unicasting a RREP back to the source route.unicasting a RREP back to the source route.
• A node generates a RREP if:A node generates a RREP if:
 It is itself the destination.It is itself the destination.
 It has an active route to the destination. Ex: anIt has an active route to the destination. Ex: an
intermediate node may also respond with an RREP ifintermediate node may also respond with an RREP if
it has a “fresh enough” route to the destination.it has a “fresh enough” route to the destination.
• As the RREP propagates back to the sourceAs the RREP propagates back to the source
node, intermediate nodes update their routingnode, intermediate nodes update their routing
tables (in the direction of the destinationtables (in the direction of the destination
node).node).

9. RREP MessageRREP Message
BB
AA
AA
AA
AA
AA
AA
AA

10. AODV Operation – Message TypesAODV Operation – Message Types
 RERR MessagesRERR Messages
• This message is broadcast for brokenThis message is broadcast for broken
linkslinks
• Generated directly by a node or passedGenerated directly by a node or passed
on when received from another nodeon when received from another node

11. AODV Operation – Message TypesAODV Operation – Message Types
 Hello MessagesHello Messages
• Hello Message = RREP with TTL = 1Hello Message = RREP with TTL = 1
• This message is used for broadcastingThis message is used for broadcasting
connectivity information.connectivity information.
 Ex: If a neighbor node does not receive any packetsEx: If a neighbor node does not receive any packets
(Hello messages or otherwise) for more than(Hello messages or otherwise) for more than
ALLOWED_HELLO_LOSS * HELLO_INTERVALALLOWED_HELLO_LOSS * HELLO_INTERVAL
mseconds, the node will assume that the link to thismseconds, the node will assume that the link to this
neighbor is currently lost.neighbor is currently lost.
• A node should use Hello messages only if it isA node should use Hello messages only if it is
part of an active route.part of an active route.

12. Message routingMessage routing
A
B D
F
C
G
E
RREQ
RREQ
RREQ
RREQ
RREQ
RREQ
RREQ
RREQ
RREQ
RREP
RREP
RREP
Source
Destination

13. Congestion HandlingCongestion Handling
 One method that AODV handle congestionOne method that AODV handle congestion
is:is:
• If the source node receives no RREP from theIf the source node receives no RREP from the
destination, it may broadcast another RREQ,destination, it may broadcast another RREQ,
up to a maximum of RREQ_RETRIES.up to a maximum of RREQ_RETRIES.
• For each additional attempt that a source node tried toFor each additional attempt that a source node tried to
broadcast RREQ, the waiting time for the RREP isbroadcast RREQ, the waiting time for the RREP is
multiplied by 2.multiplied by 2.
 DSR is not capable of handling congestion.DSR is not capable of handling congestion.

14. Congestion HandlingCongestion Handling
 Other possible methods to improve AODVOther possible methods to improve AODV
congestion handling:congestion handling:
• A route may predict when congestion is aboutA route may predict when congestion is about
to occur and try to avoid it by reduce theto occur and try to avoid it by reduce the
transmission rate.transmission rate.
• Schedule the requests so that it will notSchedule the requests so that it will not
overload the network.overload the network.

15. AODV RoutingAODV Routing
 There are two phasesThere are two phases
• Route Discovery.Route Discovery.
• Route Maintenance.Route Maintenance.
 Each node maintains a routing table with knowledge aboutEach node maintains a routing table with knowledge about
the network.the network.
 AODV deals with route table management.AODV deals with route table management.
 Route information maintained even for short lived routes –Route information maintained even for short lived routes –
reverse pointers.reverse pointers.

16. Entries in Routing TableEntries in Routing Table
 Destination IP AddressDestination IP Address
 Destination Sequence NumberDestination Sequence Number
 Valid Destination Sequence Number flagValid Destination Sequence Number flag
 Other state and routing flags (e.g., valid, invalid,Other state and routing flags (e.g., valid, invalid,
repairable, being repaired)repairable, being repaired)
 Network InterfaceNetwork Interface
 Hop Count (number of hops needed to reach destination)Hop Count (number of hops needed to reach destination)
 Next HopNext Hop
 List of PrecursorsList of Precursors
 Lifetime (expiration or deletion time of the route)Lifetime (expiration or deletion time of the route)
 DSR maintains additional table entries, causing a largerDSR maintains additional table entries, causing a larger
memory overheadmemory overhead

17. DiscoveryDiscovery
 Broadcast RREQ messages.Broadcast RREQ messages.
 Intermediate nodes update their routing tableIntermediate nodes update their routing table
 Forward the RREQ if it is not the destination.Forward the RREQ if it is not the destination.
 Maintain back-pointer to the originator.Maintain back-pointer to the originator.
 Destination generates RREQ message.Destination generates RREQ message.
 RREQ sent back to source using the reverseRREQ sent back to source using the reverse
pointer set uppointer set up
by the intermediate nodes.by the intermediate nodes.
 RREQ reaches destination, communication starts.RREQ reaches destination, communication starts.

18. Algorithm for DiscoveryAlgorithm for Discovery
 @Originator@Originator
 If a route to the destination is available, start sendingIf a route to the destination is available, start sending
data.data.
 Else generate a RREQ packet. Increment the RREQID byElse generate a RREQ packet. Increment the RREQID by
1.Increment the sequence number by 1.Destination IP1.Increment the sequence number by 1.Destination IP
address ,currently available sequence number included.address ,currently available sequence number included.
 @Intermediate Node@Intermediate Node
 Generate route reply, if a 'fresh enough' route is a validGenerate route reply, if a 'fresh enough' route is a valid
route entry for the destination whose associated sequenceroute entry for the destination whose associated sequence
number is at least as great as that contained in the RREQ.number is at least as great as that contained in the RREQ.
Change the sequence number of the destination node ifChange the sequence number of the destination node if
stale, increment the hop count by 1 and forward.stale, increment the hop count by 1 and forward.
 @Destination 1.Increment sequence number of the@Destination 1.Increment sequence number of the
destination. 2.Generate a RREQ message and sent back todestination. 2.Generate a RREQ message and sent back to
Originator.Originator.

19. DiscoveryDiscovery

20. MaintenanceMaintenance
 Hello messages broadcast byHello messages broadcast by activeactive nodes periodicallynodes periodically
HELLO_INTERVAL.HELLO_INTERVAL.
 No hello message from a neighbor in DELETE_PERIOD,linkNo hello message from a neighbor in DELETE_PERIOD,link
failure identified.failure identified.
 A local route repair to that next hop initiated.A local route repair to that next hop initiated.
 After a timeout ,error propagated both to originator andAfter a timeout ,error propagated both to originator and
destination.destination.
 Entries based on the node invalidated.Entries based on the node invalidated.

21. Information “Freshness”Information “Freshness”
AssuredAssured
 Each originating node maintains a monotonicallyEach originating node maintains a monotonically
increasing sequence number.increasing sequence number.
 Used by other nodes to determine the freshness ofUsed by other nodes to determine the freshness of
the information.the information.
 Every nodes routing table contains the latestEvery nodes routing table contains the latest
information available about the sequence numberinformation available about the sequence number
for the IP address of the destination node for whichfor the IP address of the destination node for which
the routing information is maintained.the routing information is maintained.
• Updated whenever a node receives new informationUpdated whenever a node receives new information
about the sequence number from RREQ, RREP, orabout the sequence number from RREQ, RREP, or
RERR messages received related to that destination.RERR messages received related to that destination.

22. Information “Freshness”Information “Freshness”
AssuredAssured
 AODV depends on each node in the network to ownAODV depends on each node in the network to own
and maintain its destination sequence number.and maintain its destination sequence number.
 A destination node increments its own sequenceA destination node increments its own sequence
number immediately before it originates a routenumber immediately before it originates a route
discoverydiscovery
 A destination node increments its own sequenceA destination node increments its own sequence
number immediately before it originates a RREPnumber immediately before it originates a RREP
in response to a RREQin response to a RREQ
 The node treats its sequence number as anThe node treats its sequence number as an
unsigned number when incrementingunsigned number when incrementing
accomplishing sequence number rollover.accomplishing sequence number rollover.
 Destination information is assured by comparingDestination information is assured by comparing
the sequence number of the incoming AODVthe sequence number of the incoming AODV
message with its sequence number for thatmessage with its sequence number for that
destination.destination.

23. RERR MessagesRERR Messages
• Message is broadcasted when:Message is broadcasted when:
i.i. A node detects that a link with adjacentA node detects that a link with adjacent
neighbor is broken (destination no longerneighbor is broken (destination no longer
reachable).reachable).
ii.ii. If it gets a data packet destined to a nodeIf it gets a data packet destined to a node
for which it does not have an active routefor which it does not have an active route
and is not repairing.and is not repairing.
iii.iii. If it receives a RERR from a neighbor forIf it receives a RERR from a neighbor for
one or more active routes.one or more active routes.

24. RERR Processing (for aboveRERR Processing (for above
broadcasts)broadcasts)
• Build Affected Destination ListingBuild Affected Destination Listing
i.i. List unreachable destinations containingList unreachable destinations containing
unreachable neighbor & destination usingunreachable neighbor & destination using
unreachable as next hopunreachable as next hop
ii.ii. Only one unreachable destination, which nodeOnly one unreachable destination, which node
already has.already has.
iii.iii. List of nodes where RERR is next hopList of nodes where RERR is next hop
• Update informationUpdate information
• Transmit RERR for each item listedTransmit RERR for each item listed

25. RERR – information updateRERR – information update
• Destination Sequence #Destination Sequence #
- Update sequence # for case i and iiUpdate sequence # for case i and ii
- Copy sequence # for case iiiCopy sequence # for case iii
• Invalidate route entryInvalidate route entry
• Update Lifetime field as (currtime +Update Lifetime field as (currtime +
DELETE_PERIOD)DELETE_PERIOD)
• Only now may route entry be deletedOnly now may route entry be deleted

26. RERR message transmissionRERR message transmission
• UnicastUnicast
- Send RERR to single recipientSend RERR to single recipient
• Unicast iteritiveUnicast iteritive
- Send RERR to a number of recipientsSend RERR to a number of recipients
individuallyindividually
• BroadcastBroadcast
- Notify multiple recipients simultaniouslyNotify multiple recipients simultaniously
- Broadcast via 255.255.255.255 TTL = 1Broadcast via 255.255.255.255 TTL = 1

27. RERR message transmissionRERR message transmission
• UnicastUnicast
 AA node detectsnode detects that a link with adjacentthat a link with adjacent
neighbor is broken (destination no longerneighbor is broken (destination no longer
reachable).reachable).
 If it gets a data packet destined to a nodeIf it gets a data packet destined to a node
for which it does not have an active routefor which it does not have an active route
and is not repairing.and is not repairing.
 If it receives a RERR from a neighbor for oneIf it receives a RERR from a neighbor for one
or more active routes.or more active routes.
• Unicast iteritiveUnicast iteritive
• BroadcastBroadcast

28. How Broken Links are HandledHow Broken Links are Handled
 All nodes directly using the brokenAll nodes directly using the broken
link get a RERR packet.link get a RERR packet.
 Then those nodes sends the RERRThen those nodes sends the RERR
packet to their predecessor nodes.packet to their predecessor nodes.
 This is continued all the way to theThis is continued all the way to the
source nodes.source nodes.

29. How Broken Links are HandledHow Broken Links are Handled
(Cont)(Cont)
 Upon completion of sending theUpon completion of sending the
RERR packet through the sourceRERR packet through the source
node will no longer use the link.node will no longer use the link.
• AODV uses loop free-routes.AODV uses loop free-routes.
• There are only a finite number of nodesThere are only a finite number of nodes
in a ad-hoc network.in a ad-hoc network.

30. How Broken Links are HandledHow Broken Links are Handled
(Cont)(Cont)
 DSR does not remove the path asDSR does not remove the path as
well.well.
• In DSR nodes in the network can stillIn DSR nodes in the network can still
think the broken link is still valid.think the broken link is still valid.
• This can lead to having to search forThis can lead to having to search for
new paths multiple times.new paths multiple times.

31. Reestablishing a LinkReestablishing a Link
 The source node can restart theThe source node can restart the
discovery process if a path is stilldiscovery process if a path is still
needed.needed.
 The source node or any node on theThe source node or any node on the
path can rebuild the route bypath can rebuild the route by
sending out a RREQ.sending out a RREQ.