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Mobile Adhoc Network: Routing Protocol:AODVCSE 6806 : Wireless and Mobile Communication Networks

Anika AnwarId: 1014052024

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What is a Mobile Ad Hoc Network (MANET)?Collection of mobile nodes Located in such a manner that the

interconnections between nodes are capable of changing on continuous basis

Self-forming, self-maintained and self-healingNodes change location and configure itself

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Routing ProtocolsProactive protocols

◦ Traditional distributed shortest-path protocols◦ Maintain routes between every host pair at all times◦ Based on periodic updates; High routing overhead◦ Example: DSDV (destination sequenced distance

vector)

Reactive protocols◦ Determine route if and when needed◦ Source initiates route discovery◦ Example: AODV (Ad hoc On-Demand Distance

Vector)

Hybrid protocols◦ Adaptive; Combination of proactive and reactive◦ Example : ZRP (zone routing protocol)

tseng:5

Ad hoc routing protocols

AD-HOC MOBILE ROUTING PROTOCOLS

ON-DEMAND-DRIVEN REACTIVE

HYBRIDDSDV

OLSR

TABLE DRIVEN/ PROACTIVE

DSR

AODV

ZRP

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Protocol Trade-offs Proactive protocols

◦ Always maintain routes◦ Little or no delay for route determination◦ Consume bandwidth to keep routes up-to-date◦ Maintain routes which may never be used

Reactive protocols◦ Lower overhead since routes are determined on demand◦ Significant delay in route determination◦ Employ flooding (global search)◦ Control traffic may be bursty

Which approach achieves a better trade-off depends on the traffic and mobility patterns

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MANET Reactive protocolAODV (Ad hoc On-Demand Distance

Vector )Uses Distance Vector routing protocol

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Distance Vector routing protocol

What it does:◦ Computes best paths to all destinations◦ Fully distributed and Updates are performed periodically◦ Using as only information the distances from self to all

destinationsHow it works

◦ Uses Bellman–Ford algorithm, Ford–Fulkerson algorithm, or DUAL FSM

◦ Can faces Count to infinity problemDo not have knowledge of the entire path to a

destination. Instead they use two methods:◦ Direction in which a packet should be forwarded.◦ Distance from its destination

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AODV Protocol The Ad hoc On-Demand Distance Vector protocol is both an on-

demand and a table-driven protocol.

Dynamic, self-starting, multi-hop routing between mobile nodes

A route is maintained only when it is used and hence old and expired routes are never used.

AODV maintains only one route between a source-destination pair.

AODV avoids the “counting to infinity” problem by using destination sequence numbers. This makes AODV loop free.

The packet size in AODV is uniform and supports multicasting and unicasting within a uniform framework.

Each route has a lifetime after which the route expires if it is not used.

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AODV ProtocolAODV defines 3 message types:

◦ Route Requests (RREQs) ◦ Route Replies (RREPs) ◦ Route Errors (RERRs)

RREQ messages are used to initiate the route finding process.

RREP messages are used to finalize the routesRERR messages are used to notify the network

of a link breakage in an active route. Route table information must be kept for all

routes even short-lived routes.

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AODV Routing Table

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Route Request Type 1 J Join flag : reserved for multicast. R Repair flag : reserved for multicast. G Gratuitous RREP flag D Destination only flag U Unknown sequence number Reserved Sent as 0 Hop Count: The number of hops from the Originator

IP Address to the node handling the request.

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Route Reply

Type 2 R Repair flag; used for multicast. A Acknowledgment required Reserved Sent as 0 Prefix Size: If nonzero, the 5-bit Prefix Size specifies that

the indicated next hop may be used for any nodes with the same routing prefix

Hop Count : The number of hops from the Originator IP Address to the Destination IP Address.

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Route Request and Route Reply Route Request (RREQ) includes the last known

sequence number for the destination

An intermediate node may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender

Intermediate nodes that forward the RREP, also record the next hop to destination

A routing table entry maintaining a reverse path is purged after a timeout interval

A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval

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Maintaining Sequence Numbers

Keep AODV loop-free and avoiding the “counting to infinity” problem.

Nodes originating RREQ messages must increment their own sequence number before transmitting the RREQ.

Before a destination node originates a RREP in response to a RREQ, it MUST update its own sequence number

Forwarding nodes update their stored sequence number when forwarding RREP when:◦ The sequence number in the routing table is invalid◦ The sequence number in the RREP message is greater than the

stored number◦ The sequence numbers are identical, but the route is marked

as inactive◦ The sequence numbers are the same, but the hop count is

smaller for the RREP message.

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AODV OperationRoute Requests (RREQ) are broadcasted it to its

neighbors with initial TTL of 1.

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

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Route Requests in AODV

B

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S E

F

H

J

D

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G

IK

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Y

Represents a node that has received RREQ for D from S

M

N

L

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Route Requests in AODV

B

A

S E

F

H

J

D

C

G

IK

Represents transmission of RREQ

Z

YBroadcast transmission

M

N

L

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Route Requests in AODV

B

A

S E

F

H

J

D

C

G

IK

Represents links on Reverse Path

Z

Y

M

N

L

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Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

• Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

Z

Y

M

N

L

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Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

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Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

• Node D does not forward RREQ, because node D is the intended target of the RREQ

M

N

L

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Forward Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

Forward links are setup when RREP travels alongthe reverse path

Represents a link on the forward path

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Link Failure A neighbor of node X is considered active for a

routing table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry

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

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Route Error When node X is unable to forward packet P (from

node S to node D) on link (X,Y), it generates a RERR message

Node X increments the destination sequence number for D cached at node X

The incremented sequence number N is included in the RERR

When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N

When node D receives the route request with destination sequence number N, node D will set its sequence number to N, unless it is already larger than N

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Performance of AODVAODV does not retransmit data packets that

are lost and hence does not guarantee packet delivery.

However, the packet delivery percentage is close to 100 with relatively small number of nodes.

The packet delivery percentage drops with increased mobility.

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Control OverheadsThe overhead packets in AODV are due to

RREQ, RREP and RERR messages.

AODV needs much less number of overhead packets compared to DSDV.

The number of overhead packets increases with increased mobility, since this gives rise to frequent link breaks and route discovery.

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Latency in Route DiscoveryThe route discovery latency in AODV is low

compared to DSR and DSDV.

The latency is almost constant even with increased mobility if the concentration of the nodes remain similar.

The average path length for discovered routes is also quite low.

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AODV: SummaryRoutes need not be included in packet headers

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

Sequence numbers are used to avoid old/broken routes

Sequence numbers prevent formation of routing loops

Unused routes expire even if topology does not change