AODV - Ad hoc On-Demand Distance Vector Routing Protocols

AODV : Ad Hoc On-Demand Distance
Vector Protocol
By
Mr. Darwin Nesakumar A, M.E., (P.hD)
Assistant Professor
Department of ECE
R. M. K. Engineering College
“I measure the progress of a community by the degree of progress which women have achieved”.-
Dr.B.R.Ambedkar, P.hD
Welcome

Agenda
 Review of previous session
 AODV : Ad Hoc On-Demand Distance Vector Protocol
 AODV Examples
 Quizzes
Monday, 20 July 2020 AODV Protocol

Review of previous session
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IMPORTANT POINTS TO BE REMEMBER
MANETs
• Multi Hop Relaying Network
• No fixed infrastructure
• No Pre existing Infrastructure
• Dynamic network topology
• Nodes in the network are mobile in general
• No central Controller ( To serve as server)
• The wireless hosts in such networks
communicate with each other without
existing of fixed infrastructure and without
central controller
• Decentralized Network
• Cooperative Communications
• Heterogeneous in nature
• Device to Device Communication
• A MANET can be connected to other fixed
network or to the internet
• Limited bandwidth and power
• No peer – peer communication
• Multipath Propagation
• Proactive / Reactive Protocols
• CSMA/CA Protocol to avoid hidden
terminal problem

Issues in MANETs
• Computation, storage, and communications
capabilities and interoperability
• Identification of neighbouring devices and
their corresponding attributes
(Characteristics).
• Battery capacity
• Limitations imposed by Mobility
• Dynamically changing topologies/routes
• Lack of mobility awareness by
system/applications
• Limitations of the Mobile Computer
• Short battery lifetime
• Packet loss due to transmission errors
• Mobility-induced route changes
• Mobility-induced packet losses
• Variable capacity links
• Frequent disconnections/partitions
• Limited communication bandwidth
• Broadcast nature of the communications
• Hidden terminal problem
• Limited wireless transmission range
• Ease of snooping on wireless transmissions
(security hazard)
• Quality of Service

Applications of MANETs
Tactical Network - Military Communications, Telecommunication Network
Sensor Networks - Environmental / Earth Sensing -Air Pollution Monitoring, Forest Fire detection,
Water Quality Monitoring, Natural Disaster Monitoring, Industrial Monitoring - Machine health
monitoring, Data Logging, Health care monitoring - Implanted devices, Wearable devices,
Environment-embedded systems, Waste water monitoring, Area Monitoring
Emergency Services - Search and rescue -Crowd control, Commando operations, Disaster recovery
Sensors used to sense the natural disasters, Replacement of a fixed infrastructure in case of
earthquakes, hurricanes, fire, Emergency operations -108 Ambulance, Using Google Maps, KAVALAN
App, Policing and fire fighting
Location Based Services - Automatic call forwarding, Google Maps – SWIGY, OLA, UBER, OYO,
AMAZON, FLIPCART & all online based service applications, WhatsApp, Face Book, We Chat, Arokya
Setu APP – COVID 19

Applications of MANETs
Home and enterprise Networking- Home/office wireless networking(WLAN), e.g., shared
whiteboard, application, Use PDA (Personal Digital Assistant) to print anywhere – Bus, Super Market,
Personal area network (PAN) ,Cell phone, laptop, ear phone, SMART wrist watch, Mobile Hotspot
Educational Applications - Set up virtual classrooms or conference rooms, Google MEET, Zoom
Communications, Microsoft Meeting, My Meeting, Set up ad hoc communication during
conferences, meetings, or lectures
Commercial applications- E-commerce - Electronic payments from anywhere(Ex. In taxi) – Google
PAY, Phone Pay, PayTM, Business, Dynamic access to customer files stored in a central location.
Mobile – AADHAAR, Vehicular services -Transmission of news ,road conditions ,weather, Local ad hoc
network with nearby vehicles for road/accident guidance, Civilian environments - Taxi cab network,
Meeting rooms, Sports stadiums, Boats, small aircraft
Entertainment - Multiuser games – PUBG, Robotic pets, Outdoor internet access, Online Games -
Cricket – Tennis – Foot Ball, Online FUN Quizzes – Multiple Participants, Online Movies – NETFLIX,
AMAZON PRIME etc.

Classification of routing protocols
1. Routing information update mechanism - Proactive, Reactive, Hybrid
2. Usage of temporal information (e.g. cached routes): - Past temporal , Future temporal
3. Usage of topology information - Flat, Hierarchical
4. Usage of specific resources - Power-aware routing, Geographical information assisted routing

Routing Protocols
Routing Protocols
Reactive ProtocolHybrid Protocol Proactive Protocol

ROUTING PROTOCOLS
Proactive
Table-Driven Routing Protocol
• Continuously evaluate the routes
• Attempt to maintain consistent, up-to-
date routing information
• When a route is needed, one may be
ready immediately
• When the network topology changes,
the protocol responds by propagating
updates throughout the network to
maintain a consistent view
• Each node maintains a routing table which
stores - next hop, cost metric towards each
destination, a sequence number that is
created by the destination itself
• Each node periodically forwards routing
table to neighbors
• Each node increments and appends its
sequence number when sending its
local routing table
• Each route is tagged with a sequence
number; routes with greater sequence
numbers are preferred
• Each node advertises a monotonically
increasing even sequence number for itself
• When a node decides that a route is broken,
it increments the sequence number of the
route and advertises it with infinite metric
• Destination advertises new sequence
number .
• Example DSDV (Destination Sequenced
Distance Vector)

ROUTING PROTOCOLS
Proactive
Table-Driven Routing Protocol
• Maintain routes between every host pair at
all times
• Based on periodic updates; High routing
overhead
• Traditional distributed shortest-path
protocols
• Determine routes independent of traffic
pattern
• Traditional link-state and distance-vector
routing protocols are proactive
• Route from each node to every other node
in the network
• Periodic route-update packets
• Large routing tables
• Routes are ready to use instantaneously

Reactive
On-Demand Routing Protocol
• On-demand style: create routes only
when it is desired by the source node
• The route is discovered only when it is
required/needed
• Process of route discovery occurs by
flooding the route request packets
throughout the mobile network.
 Route Maintenance:
This phase performs the maintenance
work of the route as the topology in
the mobile ad-hoc network is dynamic
in nature
 Route Discovery:
This phase determines the most
optimal path for the transmission of
data packets between the source and
the destination mobile nodes
 Route discovery invokes a route-
determination procedure
• The procedure is terminated when
• A route has been found
• No route is found after all
route permutations are
examined
• Longer delay: sometimes a route may
not be ready for use immediately when
data packets come
• Example AODV
ROUTING PROTOCOLS

• Flood is propagated outwards from the
source
• Pure flooding = every node transmits the
request only once
• Source floods the network with a Route
Request packet when a route is not
available to the required destination
• Destination send the Route reply to Route
Request
• Reply uses reversed path of Route Request
• Sets up the forward path
On-Demand Routing Protocol
• Determine route if and when needed
• Source initiates route discovery
• Maintain routes only if needed
• Example: DSR (dynamic source routing)
• Routes from Source to Destination only
• Routes constructed when needed, higher
connection setup delay
• Route update when necessary
• Small or No routing tables
Reactive
ROUTING PROTOCOLS

Ad-Hoc On-Demand Distance
Vector (AODV) Routing Protocol
(On-Demand Routing Protocol)

Outline
• Introduction to AODV
• Properties
• Route discovery
• Route maintenance
• Summary

Outline
• Introduction to AODV
• Types of AODV nodes
• Basic functions of a node
• How to find Route?
• Flooding technique

AODV Nodes
AODV Nodes
Intermediate NodeDestination Node Source Node

AODV Nodes
Source Node
• Initiate a path discovery process by broadcasting a RREQ packet to its neighbors.
(RREQ – Route Request)
• Reinitiate the path discovery process when it receives a failure notification message from
nodes along the path to the destination.

Intermediate Node
• Reply to the RREQ packet by unicasting a RREP packet back to the source, only when
they have a route to the destination whose seq.no# is greater or equal to the seq.no#
contained in the RREQ packet. (RREP- Route Reply)
• Store in their routing tables the address of the neighbor from which the RREQ is
received, thereby establishing a reverse path back to the source.
• Discard additional copies of the same RREQ later received (Broadcast ID and source IP
address uniquely specify the RREQ packet).
• Send failure notification messages to the source (route erase ) when moving away.

Destination Node
• Respond to the RREQ packet by unicasting a route reply (RREP) packet back to the
neighbor it has first received a RREQ packet. This packet is routed along the already
established reverse path.
• Reinitiate the path discovery process when it moves.
• Reinitiate the path discovery process when it receives a failure notification message
from nodes along the path to the destination.

Question
What are the types of AODV nodes?

Answer for the Question
Source, Intermediate, Destination Nodes

Question
In which of the following the Source will send the packets
to their neighbours?
Multicast
Unicast
Broadcast

Broadcast

Question
In which of the following the Destination will send the
Packets to their neighbours?
Multicast
Unicast
Broadcast

Unicast

Question
In which of the following the Intermediate node will use
to send as reply?
RREP
RREQ

RREP

Basic Functions of a Node
• Maintain route information in its route table.
• RREQ consists of
– Destination Address and Seq.#
– Source node’s sequence number and IP address
– # hops to the source and the IP’s of neighboring nodes
– Broadcast_ID
– Expiration time for reverse path (RREP) entry
• Information obtained through RREQ & RREP messages is kept with other routing
information in the route table.
• Update the Seq. # to eliminate unused routes
• Manage aging process of routes with old seq. #

Question
What are available in RREQ packet?

– Destination Address and Seq.#
– Source node’s sequence number and IP address
– # hops to the source and the IP’s of neighboring nodes
– Broadcast_ID
– Expiration time for reverse path entry

Question
AODV uses single hop transmission
TRUE
FALSE

FALSE

Question
Can a destination node responds with route reply
YES
NO

YES

How to find Route?
• Sender S broadcasts data packet P to all its all neighbors
• Each node receiving P, then forwards it to its all neighbors
• Sequence numbers used to avoid the possibility of forwarding the same packet more than
once.
• Packet P reaches destination D provided that D is reachable from sender S
• Node D does not forward the packet

Flooding for Data Delivery
B
A
S E
F
H
J
D
C
G
I
K
Represents that connected nodes are within each other’s transmission
range
Z
Y
Represents a node that has received packet P
M
N
L

B
A
S E
F
H
J
D
C
G
I
K
Represents transmission of packet P
Represents a node that receives packet P for the first time
Z
Y
Broadcast transmission
M
N
L

B
A
S E
F
H
J
D
C
G
I
K
Node H receives packet P from two neighbors: potential for collision
Z
Y
M
N
L

B
A
S E
F
H
J
D
C
G
I
K
Node C receives packet P from G and H, but does not forward it again, because node C has
already forwarded packet P once
Z
Y
M
N
L

B
A
S E
F
H
J
D
C
G
I
K
Z
Y
M
Nodes J and K both broadcast packet P to node D, Since nodes J and K are hidden from each
other, their transmissions may collide => Packet P may not be delivered to node D at all,
despite the use of flooding
N
L

B
A
S E
F
H
J
D
C
G
I
K
Z
Y
Node D does not forward packet P, because node D is the intended destination of packet P
M
N
L

B
A
S E
F
H
J
D
C
G
I
K
Flooding completed
Nodes unreachable from S do not receive packet P (e.g., node Z)
Nodes for which all paths from S go through the destination D also do not receive packet P
(example: node N)
Z
Y
M
N
L

B
A
S E
F
H
J
D
C
G
I
K
Flooding may deliver packets to too many nodes (in the worst case, all nodes reachable from
sender may receive the packet)
Z
Y
M
N
L

Question
Flooding uses
Multicast
Unicast
Broadcast

Question
If a Node C receives packet P from two nodes at the same
time node C will do the following
Discard
Receive

Discard

Outline
• Properties
• General Properties
• Fields of Routing Table
• Basic Message Set

General Properties
• Loop Free Routing
• Two Operation Phases
• Route Establishment
• Route Request  RouteRequest Packet, flooded by the Source node
• Route Reply  RouteReply Packet, returned to source node by Destination or
Intermediate node
• Route Maintenance
• Route Reconstruction
• Route Deletion

Question
AODV is a loop free protocol
TRUE
FALSE

TRUE

AODV Properties
1. AODV discovers routes as and when necessary
2. 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.
4. The route table stores: <destination addr, next-hop addr, destination sequence number,
life_time>
5. For each destination, a node maintains a list of precursor nodes, to route through them
Precursor nodes help in route maintenance.
Life-time updated every time the route is used
If route not used within its life time -> it expires

Fields of Routing Table in AODV
• Destination IP address
• Destination Sequence Number – To determine an up-to-date path to the destination - A node
updates its path information only if the DestSeqNum of the current packet received is greater
than the last DestSeqNum stored at the node.
• Source Sequence Number
• Hop Count (needed to reach destination)
• Next Hop
• Lifetime (route expiration or deletion time)
• Source IP address

Question
What is the purpose of having destination sequence number
To up-to-date the route
To up-to-date memory
To up-to-date lifetime of nodes

To up-to-date the route

Basic Message set of AODV
Route Request : “I need a route”
Route Reply : “Route advertisement”
Route Error : “Withdraw route”
Hello : “Link status monitoring”

Outline
• Route discovery
• Route Discovery Basics
• Route Discovery Example -1
• Route Discovery Flow Chart

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
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

AODV – Route Discovery (2)
Sender S broadcasts a control packet P to all its neighbors
Each node receiving P forwards P to its neighbors
Sequence numbers help to avoid the possibility of forwarding the same packet more than once
Packet P reaches destination D provided that D is reachable from sender S Node D does not
forward the packet
Once an intermediate node receives a RREQ, the node sets up a reverse route entry (path) for the
source node in its route table
Reverse route entry consists of <Source IP address, Source seq. number, number of hops to
source node, IP address of node from which RREQ was received>
Using the reverse route a node can send a RREP (Route Reply packet) to the source
• Reverse route entry also contains – life time field
RREQ -> Seq. number of intermediate nodes /Destination nodes should be greater than the Seq.
number in RREQ (for loop prevention), If this condition is not satisfied, then node increments the
hop count in RREQ and broadcasts to its neighbors

Question
Broadcasting is done via Flooding
YES
NO

Outline
• Route discovery
• Route Discovery Basics
• Route Discovery Flow Chart

AODV - Route Discovery
Route Request
• Source Node broadcast RouteRequest
packets.
• A node broadcast only once
• Every nodes have their own ID ( IP
address)
• It has TTL (Time To Live) for each node
to send respond back the nodes which
sends the RouteRequest
S
B
E
D
A
C
F
G
RouteRequest

S
B
E
D
A
C
F
G
RouteRequest
Reverse Path Setup
Reverse Path Setup
• Each neighbor will check if it received
this request before or if its ID is in the
list.
• If yes it will drop the packet.
• If not it will append its ID to the packet
and rebroadcast the packet again.

S
B
E
D
A
C
F
G
RouteRequest
Reverse Path Setup
RouteRequest Dropped
Route Request Dropped
• If anyone of the packets are received
duplicate packets (Same ID) will drop the
packets
• The packet coming from Node C to E and
from C to F will be dropped at E and F
respectively because E and F already got
a RouteRequest from the same source.

S
B
E
D
A
C
F
G
RouteRequest
Reverse Path Setup
RouteReply
Route Reply
• D has received the packet from the E
but not from F yet. So the RouteReply
is sent from D to E
• In the next hop, D receives the packet
from F, hence D again sends the route
reply to F

S
B
E
D
A
C
F
G
Reverse Path Setup
RouteReply
Forward Route Setup
Forward Route Setup
• If the RouteReply is sent, the Forward
Route Setup is established between E
and D, this process continues till the
Forward Route Setup is established with
S.
• We may get many routes but the
shortest distance route will be
considered for the data transmission

S
B
E
D
A
C
F
G
Reverse Path Setup
RouteReply
Forward Route Setup
Process
Route Request (RREQ)
Reverse Path Setup (RREP)
Route Reply
Forward Path Setup

Represents Route Request (Forward
direction only, Only once, Broadcasting, Node’s
Id appended)
Represents Reverse Path ( Opposite
direction to Route Reply)
Represents a link on the forward path
(Opposite direction to Route Reply, Forward
links are setup when RREP travels along)
Represents Route Reply (Starts from
Destination, Opposite direction to Route
Request)
S
B
E
D
A
C
F
G
Reverse Path Setup
RouteReply
Forward Route Setup

AODV – Route Discovery - Example
1. Node S needs a route to D
S
DC
B
A

AODV – Route Discovery
2. Creates a Route Request (RREQ)
Enters D’s IP addr, seq#, S’s IP addr, seq#, hopcount (=0)
S
DC
B
A

2. Creates a Route Request (RREQ)
Enters D’s IP addr, seq#, S’s IP addr, seq#, hopcount (=0)
3. Node S broadcasts RREQ to neighbors
S
DC
A
B
RREQ

4. Node A receives RREQ
Makes a reverse route entry for S
dest=S, nexthop=S, hopcount=1
It has no routes to D, so it rebroadcasts RREQ
S
DC
A
B
RREQ

4. Node A receives RREQ
Makes a reverse route entery for S
dest=S, nexthop=S, hopcount=1
It has no routes to D, so it rebroadcasts RREQ
S
DC
B
A
RREQ

5. Node C receives RREQ
Makes a reverse route entry for S
dest=S, nexthop=A, hopcount=2
It has a route to D, and the seq# for route to D is >= D’s seq# in
RREQ
S
DC
B
A
RREQ

Route Reply in AODV - Comments
An intermediate node (not the destination) may also send a Route Reply (RREP) provided
that it knows a more recent path than the one previously known to sender S
To determine whether the path known to an intermediate node is more recent, destination
sequence numbers are used
A new Route Request by node S for a destination is assigned a higher destination
sequence number. An intermediate node which knows a route, but with a
smaller sequence number, cannot send Route Reply

5. Node C receives RREQ (Cont.)
C creates a Route Reply (RREP)
Enters D’s IP addr, seq#, S’s IP addr, hopcount to D (=1)
Unicasts RREP to A
S
DC
B
A
RREQ

5. Node C receives RREQ (Cont.)
C creates a Route Reply (RREP)
Enters D’s IP addr, seq#, S’s IP addr, hopcount to D (=1)
Unicasts RREP to A
S
DC
B
A
RREP

AODV – Forward Path Setup
5. Node A receives RREP
Makes a forward route entry to D
dest=D, nexthop=C, hopcount=2
Unicasts RREP to S
S
DC
A
RREP
B
RREP

Forward Path Setup (1)
When a node determines that it has a current route to respond to RREQ i.e. has a path to
destination – It creates RREP (Route Reply)
RREP contains <IP address of source and destination>
If RREP is being sent by destination
• The RREP will also contain the <current sqn # of destination, hop-count=0, life-time>
If RREP is sent by an intermediate node
• RREP will contain its record of the <destination sequence number, hop-count=its
distance to destination, its value of the life-time>

Forward Path Setup (2)
When an intermediate node receives the RREP, it sets up a forward path entry to the
destination in its route table
Forward path entry contains
<IP Address of destination, IP address of node from which the entry arrived, hop-count to
destination, life-time> To obtain its distance to destination i.e. hop-count, a
node increments its distance by 1
If route is not used within the life time, its deleted
After processing the RREP, the node forwards it towards the source

AODV – Forward Path Setup
5. Node S receives RREP
dest=D, nexthop =A, hopcount = 3
S
DC
A
B
RREP

Receipt of Multiple RREP
A node may receive multiple RREP for a given destination from more than one neighbor
The node only forwards the first RREP it receives
May forward another RREP if that has greater destination sequence number or a smaller
hop count
Rest are discarded -> reduces the number of RREP propagating towards the source
Source can begin data transmission upon receiving the first RREP

AODV – Data Delivery
5. Node S receives RREP
dest=D, nexthop =A, hopcount = 3
Sends data packet on route to D
S
DC
B
A

Example 3: Route Requests in AODV
B
A
S E
F
H
J
D
C
G
I
K
Y
Z
Represents a node that has received RREQ for D from S
M
N
L

B
A
S E
F
H
J
D
C
G
I
K
Represents transmission of RREQ
Z
Y
Broadcast transmission
M
N
L
Route Requests in AODV

Route Requests in AODV
B
A
S E
F
H
J
D
C
G
I
K
Represents links on Reverse Path
Y
Z
M
N
L

Reverse Path Setup in AODV
B
A
S E
F
H
J
D
C
G
I
K
Node C receives RREQ from G and H, but does not forward it again,
because node C has already forwarded RREQ once
Y
Z
M
N
L

B
A
S E
F
H
J
D
C
G
I
K
Y
Z
M
N
L

B
A
S E
F
H
J
D
C
G
I
K
Y
Z
Node D does not forward RREQ, because node D is the intended
target of the RREQ
M
N
L

Route Reply in AODV
B
A
S E
F
H
J
D
C
G
I
K
Y
Z
Represents links on path taken by RREP
M
N
L

Forward Path Setup in AODV
B
A
S E
F
H
J
D
C
G
Y
Z
M L
K
I N
Forward links are setup when RREP travels along the
reverse path
Represents a link on the forward path

Data Delivery in AODV
B
A
S E
F
H
J
D
C
G
I
K
Z
Y
M
N
L
Routing table entries used to forward data packet.
Route is not included in packet header.
DATA

Route Discovery Flow Chart
Existence
of
active
route
Yes No
A source wishing to transmit
Check the route table
Forward packet
to next hop
Initiate a Route
Discovery Process
Create RREQ
packet
Broadcast RREQ &
Set the timer for a reply

Check its record to see if
it had seen it.
Intermediate node receiving
the RREQ
Record the RREQ
Information & broadcast
Setup reverse route entry
Discard the packet
Unicast a RREP
Increment hop count
and broadcast
Did it see
the RREQ
before?
NoYes
Reverse
path
expired
Yes
No

Outline
• Route maintenance
• Time Outs
• Link Failure Reporting
• Link Failure Detection
• Route Maintenance Example

Timeouts
A routing table entry maintaining a reverse path is purged after a timeout interval
timeout should be long enough to allow RREP to come back
A routing table entry maintaining a forward path is purged if not used for a
active_route_timeout interval
if no is data being sent using a particular routing table entry, that entry will be deleted
from the routing table (even if the route may actually still be valid)

Link Failure Reporting
A neighbor of node X is considered active for a routing table entry if the neighbor sent a packet
within active_route_timeoutinterval and was forwarded using that entry
If a source node moves, a new route discovery process is initiated
If intermediate nodes or the destination move -> The next hop links break resulting in link failures
Routing tables are updated for the link filaures
All active neighbors are informed by RERR message
Neighboring nodes periodically exchange hello message
When the next hop link in a routing table entry breaks, all active neighbors are informed
Link failures are propagated by means of Route Error (RERR) messages, which also update
destination sequence numbers

Link Failure Detection
Hellomessages: Neighboring nodes periodically exchange hello message
Absence of hello message is used as an indication of link failure
Alternatively, failure to receive several MAC-level acknowledgements may be used as an
indication of link failure

AODV – Route Maintenance
• When a node detects a link failure, it sends
special RouteReply with infinity distance
• RouteReply is propagated to source node
• Source node initiates a new RouteRequest
• When a link is broken  due to movement
of nodes or any other reason
• The node that discover the failure link will
send RouteError to the Source
• When the source gets the RouteError
Packet it will delete the path from the cache
• And will find another route in its cache, if it
didn’t find any route it will run
RouteRequest again
S
B
E
D
A
C
F
G
RouteReply
RouteReply

Route Maintenance - RERR
RERR is initiated by the node upstream (closer to the source) of the break
Its propagated to all the affected destinations
RERR lists all the nodes affected by the link failure -> Nodes that were using the link to
route messages (precursor nodes)
When a node receives an RERR, it marks its route to the destination as invalid ->
Setting distance to the destination as infinity in the route table
When a source node receives an RRER, it can reinitiate the route discovery

AODV – Route Maintenance- Example
1. Link between C and D breaks
2. Node C invalidates route to D in route table
3. Node C creates Route Error message Lists all destinations that are now
unreachable Sends to upstream neighbors
S
DC
B
A
X
RERR

4. Node A receives RERR
Checks whether C is its next hop on route to D Deletes route to D
(makes distance -> infinity) Forwards RERR to S
S
DC
A
RERR
B
RERR

5. Node S receives RERR
Checks whether A is its next hop on route to D Deletes route to D
Rediscovers route if still needed
S
DC
A
B
RERR

AODV Summary
• Protocol objectives – To send the packets from Source to Receiver on-demand through a
single route
• Path Discovery – To discover the Path from source and destination
• Route Request – Source/intermediate nodes to neighboring nodes
• Reverse Path Setup – If a node receives a packet (No duplicate packet) it sends the
reverse path set up to the nodes from which it had received the route request (Towards
Source)
• Forward Path Setup – Towards Destination (Starts from destination to source)
• Route Reply – From destination towards source
• Route Error – Link Failure

AODV Summary
• Route Table Management
• Path Maintenance
• Local Connectivity Management
• Hop by Hop basis
• No need to include the full path in the data packet
• Update Neighborhood information through periodic beacons
• Source Node broadcast RouteRequest packet

AODV Summary
• Each intermediate node gets a RouteRequest do the following steps:
• Establish a reverse link to node it received the RouteRequest from
• If request received before  discard
• If route to destination is available and up-to-date  return RouteReply using the
reverse link
• Otherwise  rebroadcast the RouteRequest
• Destination node respond with RouteReply using the reverse link

AODV Summary
1. Reactive or On-demand routing protocol
2. In AODV, the source node and the intermediate nodes store the next-hop information
corresponding to each flow for data packet transmission
3. AODV supports multicasting and unicasting within a uniform framework
4. Each route has a lifetime after which the route expires if it is not used
5. AODV maintains only one route between a source-destination pair
6. Routing table size is minimized by only including next hop information, not the entire route to
a destination node.
7. Sequence numbers for both destination and source are used.
8. Managing the sequence number is the key to efficient routing and route maintenance

AODV Summary
• DSR includes source routes in packet headers
• Resulting large headers can sometimes degrade performance
• particularly when data contents of a packet are small
• AODV attempts to improve on DSR by maintaining routing tables at the nodes, so that data
packets do not have to contain routes
• AODV retains the desirable feature of DSR that routes are maintained only between nodes
which need to communicate
 Route Requests (RREQ) are forwarded in a manner similar to DSR
 When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the
source
 AODV assumes symmetric (bi-directional) links
 When the intended destination receives a Route Request, it replies by sending a Route Reply
(RREP)
 Route Reply travels along the reverse path set-up when Route Request is forwarded

AODV: Summary
Routes need not be included in packet headers (DSR does it)
Nodes maintain routing tables containing entries only for routes that are in active
use
At most one next-hop per destination maintained at each node
DSR may maintain several routes for a single destination
Unused routes expire even if topology does not change
Reactive/On – demand
Sequence numbers used for route freshness and loop prevention Route discovery
cycle
Optimizations can be used to reduce overhead and increase scalability

AODV Optimizations
Route Requests are initially sent with small Time-to-Live (TTL) field, to limit their propagation
DSR also includes a similar optimization
If no Route Reply is received, then larger TTL tried
Expanding Ring Search
Prevents flooding of network during route discovery
Control Time to Live (TTL) of RREQ to search incrementally larger areas of network
Advantages: Less overhead when successful
Disadvantages: Longer delay if route not found immediately
Local Repairs
Repair breaks in active routes locally instead of notifying source
Use small TTL because destination probably hasn’t moved far
If first repair attempt is unsuccessful, send RERR to source
Advantage: repair links with less overhead, delay and packet loss
Disadvantages: longer delay and greater packet loss when unsuccessful

Advantages & Disadvantages of AODV
Advantages
Routes are established on demand and destination sequence numbers are used to find the latest route to the
destination
The connection setup delay is less
Disadvantages
Intermediate nodes can lead to inconsistent routes if the source sequence number is very old
Multiple RouteReply packets in response to a single RouteRequest packet can lead to heavy control overhead.
Periodic beaconing leads to unnecessary bandwidth consumption.

Challenges of routing protocols in ad hoc networks
Movement of nodes:
 Path breaks
 Partitioning of a network
 Inability to use protocols developed for fixed network
 Bandwidth is a scarce resource:
 Inability to have full information about topology
 Control overhead must be minimized
 Shared broadcast radio channel:
 Nodes compete for sending packets
 Collisions
 Erroneous transmission medium:
 Loss of routing packets

Challenges of routing protocols in ad hoc networks
Issues
• Frequent route changes
• amount of data transferred between route changes may be much smaller than traditional networks
• Route changes may be related to host movement
• Low bandwidth links
Goal of routing protocols
• Decrease routing-related overhead
• Find short routes
• Find “stable” routes (despite mobility)

Trade-Off
• Latency of route discovery
• Proactive protocols may have lower latency since routes are maintained at all times
• Reactive protocols may have higher latency because a route from X to Y will be found only
when X attempts to send to Y
• Overhead of route discovery/maintenance
• Reactive protocols may have lower overhead since routes are determined only if needed
• Proactive protocols can (but not necessarily) result in higher overhead due to continuous
route updating
• Which approach achieves a better trade-off depends on the traffic and mobility patterns

AODV - Ad hoc On-Demand Distance Vector Routing Protocols

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AODV - Ad hoc On-Demand Distance Vector Routing Protocols