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Ad-Hoc Networks:Routing Algorithms

Advanced Algorithms & Data StructuresLecture Theme 05

Robin Pomplun

Summer Semester 2006

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Overview

Mobile Ad Hoc Networks Position-unaware Routing Algorithms

Proactive Protocols

Destination-Sequenced Distance Vector Protocol

Reactive Protocols

Dynamic Source Routing Protocol

Ad-Hoc On-Demand Routing Protocol

Hybrid Protocols

Zone Routing Protocol

Research

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Mobile Ad Hoc Networks

Definition:A mobile ad hoc network is an autonomous system of mobile routers

connected by wireless links the union of which form an arbitrary graph.

The routers are free to move randomly and organize themselves arbitrarily;

thus, the network's wireless topology may change rapidly and unpredictably.

Source: Internet Engineering Task Force (IETF)

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An Example

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Networking without an Infrastructure

If all the wireless nodes are within the transmission range of each other,

routing is easy. Every node can listen to all transmissions.

However, this is not true in most situations, due to short transmission range.

Hence, most ad hoc networks are multi-hop.

A message from a source node must then go through intermediate nodes to

reach its destination.

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An Example

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Position-unaware Routing

We will assume that each node is capable of running fairly complicated

algorithms locally.

Each node has an address and each node keeps track of the neighbours

addresses.

However, when a message has to be sent from a source to a destination, thesource node is not aware of the current position of the destination.

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Position-unaware Routing

Position-unaware Routing Protocols can be classified based on the way a

protocol tries to find a route to a destination:

Proactive Routing Protocol

Reactive Routing Protocol

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Proactive Routing

Entire network topology is known to all nodes and maintained in a routingtable

Since each node knows the complete topology, a node can immediatelyfind the best route to a destination.

Routing messages are exchanged among the nodes periodically to update

their routing tables

Routing Table Advertising

Protocols:

Destination-Sequenced Distance Vector (DSDV) Fisheye State Routing (FSR)

...

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Destination-Sequenced Distance Vector Protocol

Packets are transmitted between the nodes using route tables stored at each

node.

Each route table lists all available destinations and the number of hops to

each destination.

For each destination, a node knows which of its neighbours leads to theshortest path to the destination.

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Routing Table Entries

The destinations address

Next Hop to Destination

The number of hops to the destination

The sequence number of the information received from that destination.

This is the original sequence number assigned by the destination.

k

l

m

n

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How the local Routing Table is Used

13

23

8i x

k

l

m

Consider a node i. Suppose, i needs to send a message to node x.

i can look up the best route to x from its routing table and forwards the

message to the neighbour along the best route.

The neighbour in turn checks the best route from its own table and forwards the

message to its appropriate neighbour. The routing progresses this way.

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Routing Table Advertising

The DSDV protocol requires each mobile node to advertise its own route

table to all of its current neighbours.

Each mobile node agrees to forward route advertising messages from other

mobile nodes.

This forwarding is necessary to send the advertisement messages all over

the network.

In other words, route advertisement messages help mobile nodes to get anoverall picture of the topology of the network.

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Responding to Topology Changes

It is necessary to avoid excessive control traffic (route update

information). Otherwise, the bandwidth will be taken up by control

traffic.

The solution is to broadcast two types of updates:

Full Dump / Incremental Dump

A full dump carries complete routing tables. A node broadcasts a fulldump infrequently.

An incremental dump carries minor changes in the routing table. This

information contains changes since the last full dump.

When the size of an incremental dump becomes too large, a full dump is

preferred.

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Responding to Topology Changes

When a node i receives incremental dump or full dump from another

node j, the following actions are taken :

The sequence number of the current dump from j is compared with

previous dumps from j

If the sequence number is new, the route table at i is updated with

this new information. (Reason for Sequence number: Loops)

Node i now broadcasts its new route table as an incremental or a full

dump.

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An Example of Route Update

At the start, each node gets route

updates only from its neighbours.

For n4, the distances to the other

nodes are :

n5=1, n3=1, n2=

n1 =

All nodes broadcast with a sequence

number 1

n1

n2

n3

n4 n5

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After this, nodes forward messages

that they have received earlier.

The message that n2 sent to n3 is now

forwarded by n3

For n4, the distances are now :

n5=1, n3=1, n2=2, n1=

All messages have sequence number 1

n1

n2

n3

n4 n5

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Finally, after second round of

forwarding, n4 gets the following

distances :

n5=1, n3=1, n2=2, n1=3

n1

n2

n3

n4 n5

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Suppose n5 has moved to its new

location.

Also, n5 receives a new message from

n1 with a sequence number 2

This message is forwarded by n5 ton4

Two distances to n1 in n4

Distance 3 with sequence number 1,

and Distance 2 with sequence number 2

Since the latter message has a more

recent sequence number, n4 will

update the distance to n1 as 2

n1

n2

n3

n4

n5

n5

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How good is DSDV?

DSDV is an efficient protocol for route discovery. Whenever a route to a

new destination is required, it already exists at the source.

Hence, latency for route discovery is very low.

However, DSDV needs to send a lot of control messages. These messages

are important for maintaining the network topology at each node.

This may generate high volume of traffic for high-density and highly

mobile networks.

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Performance Evaluation

Performance Evaluation with real networks is to expensive

Freely available Simulation Tool (e.g. Network Simulator ns-2)

Simulation is done on packet level (trace file)

Generate random mobility

Random Waypoint Model

Generate Traffic (TCP, UDP)

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Trace File (ns-2)

s -t 0.500000000 -Hs 0 -Hd -2 -Ni 0 -Nx 16.77 -Ny 16.77 -Nz 0.00 -Ne -1.000000 -NlAGT -Nw --- -Ma 0 -Md 0 -Ms 0 -Mt 0 -Is 0.0 -Id 1.0 -It tcp -Il 40 -If 2 -Ii 0 -Iv32 -Pn tcp -Ps 0 -Pa 0 -Pf 0 -Po 0

r -t 0.500000000 -Hs 0 -Hd -2 -Ni 0 -Nx 16.77 -Ny 16.77 -Nz 0.00 -Ne -1.000000 -Nl

RTR -Nw --- -Ma 0 -Md 0 -Ms 0 -Mt 0 -Is 0.0 -Id 1.0 -It tcp -Il 40 -If 2 -Ii 0 -Iv32 -Pn tcp -Ps 0 -Pa 0 -Pf 0 -Po 0

s -t 0.506923068 -Hs 0 -Hd -1 -Ni 0 -Nx 16.79 -Ny 16.79 -Nz 0.00 -Ne -1.000000 -NlRTR -Nw --- -Ma 0 -Md 0 -Ms 0 -Mt 0 -Is 0.255 -Id 1.255 -It DSR -Il 32 -If 0 -Ii1 -Iv 32 -P dsr -Ph 1 -Pq 1 -Ps 1 -Pp 0 -Pn 1 -Pl 0 -Pe 0->0 -Pw 0 -Pm 0 -Pc 0 -Pb

0->0s -t 0.508398068 -Hs 0 -Hd -1 -Ni 0 -Nx 16.80 -Ny 16.80 -Nz 0.00 -Ne -1.000000 -Nl

MAC -Nw --- -Ma 0 -Md 0 -Ms ffff0008 -Mt 0 -Is 0.255 -Id 1.255 -It DSR -Il 32-If 0 -Ii 1 -Iv 32 -P dsr -Ph 1 -Pq 1 -Ps 1 -Pp 0 -Pn 1 -Pl 0 -Pe 0->0 -Pw 0 -Pm 0 -Pc 0 -Pb 0->0

r -t 0.508526679 -Hs 1 -Hd -1 -Ni 1 -Nx 146.78 -Ny 146.06 -Nz 0.00 -Ne -1.000000 -Nl MAC -Nw --- -Ma 0 -Md 0 -Ms ffff0008 -Mt 0 -Is 0.255 -Id 1.255 -It DSR -Il32 -If 0 -Ii 1 -Iv 32 -P dsr -Ph 1 -Pq 1 -Ps 1 -Pp 0 -Pn 1 -Pl 0 -Pe 0->0 -Pw 0 -Pm0 -Pc 0 -Pb 0->0

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Performance Evaluation

Performance Evaluation with real networks is to expensive

Freely available Simulation Tool (e.g. Network Simulator ns-2)

Simulation is done on packet level (trace file)

Generate random mobility

Random Waypoint Model

Generate Traffic (TCP, UDP)

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Evaluation

Packet Delivery Ratio:

The ratio of the data packets delivered to the destinations to the number of sent packets

Average End-End-Delay (Latency):

This includes all possible delays caused by buffering during route discovery, queuing at theinterface queue, retransmission delays, propagation and transfer times.

Routing Overhead

Number of all packets which are needed to do Route / Topology Maintenance

Throughput (in Bits/sec or Packets/sec)

Number of Bits (Packets) per second which are received in the network

Scenario:

Number of Nodes: 50

Transmission Range: 250m

Plane: 500x500m

Simulation Time: 200 (start: 100)

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Evaluation

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Reactive Routing

In a reactive protocol, a route is discovered only on-demand, when it is

necessary.

These protocols generate much less control traffic at the cost of latency,i.e., it usually takes more time to find a route compared to a proactive

protocol.

Protocols:

Dynamic Source Routing (DSR)

Ad Hoc On-Demand Distance-Vector (AODV)

...

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Dynamic Source Routing

Each node maintains a route cache to remember routes that it has learnt

about.

A node may store multiple routes to a destination in its route cache.

A node can react to changes in network topology much more rapidly by

taking advantage of cached routes.

For example, if one route to a destination is broken, the source node can

choose another route to the destination from its route cache.

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Route Discovery

The DSR protocol has two important mechanisms through which the

protocol operates.

Route Discovery: A node A wishing to send a packet to node E obtains

a route to E Route Request

Route Reply

Route Maintenance: When A is using a discovered route to E, A may

detect that the route is broken. In such cases, A may use an alternate

route to E (if it is known), or start another route discovery phase to E.

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Route Discovery

Node A is trying to discover a route to node E.

A broadcasts a route request message to its neighbours. This message is

received by all nodes within the transmission range of A.

Each route request message contains the source and target of the routediscovery.

Also, each route request is stamped with an unique ID assigned by the source.

A

B

C

D

A

AB

ABC

E

ABCD

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Route Discovery

Every node that receives a route request message, does one of the following:

Check the unique ID of route request; already received: discard RReq

A node like C first searches its route cache to see whether it has a storedroute to E. If it has such a route, C sends that route to A. (Route Reply)

If there is no such route in its route cache, C broadcasts the route request

message to its neighbours. C attaches its own ID to the route requestmessage

A

B

C

D

A

AB

ABCE

ABCD

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Example

SD

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Route Maintenance

The DSR protocol has two important mechanisms through which the

protocol operates.

Route Discovery: A node A wishing to send a packet to node E obtainsa route to E

Route Request

Route Reply

Route Maintenance: When A is using a discovered route to E, A may

detect that the route is broken. In such cases, A may use an alternate

route to E (if it is known), or start another route discovery phase to E.

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Route Error

A

B

C

D

A node like C tries to forward the message and waits for acknowledgment. C

will retransmit the message a fixed number of times if no acknowledgment

arrives.

After that, C will initiate a route error message.

In this example, C will initiate a route error message back to A indicating

that the link to D is currently broken.

A will remove this route from its route cache and try another route to E, if it

has one. Or, A may start a new route discovery.

E

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Caching Overheard Routing Information

DSR extensively takes advantage of existing knowledge of the network

topology.

Each node gathers information about the network topology by overhearing

other nodes transmissions.

AB

C

D

E

P

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Route Request Hop Limit

Sometime it is not good to propagate a route request message throughout the

network.

In case D is in the neighbourhood of S, the route request message from Sshould not propagate too far away.

If D is near S, propagating the route request message too far will result in too

many unnecessary route reply messages in future.

S

D

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Restricted Propagation of Route Request

A better strategy is to propagate route request messages with increasing hop

count.

Initially, send the route request to a distance of 2 hops. If no route reply isreceived after sometime, send the route request to a distance of 4 hops and so

on.

This reduces network congestion by reducing the number of route reply

messages.

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Non-uniform Packet Size in DSR

When a source node A sends a packet to a destination node E, A should

send the entire route to E along with the packet.

This is necessary for the intermediate nodes to forward the packet.

Usually all media support packets of uniform size. If a packet is large, it

has to be split into smaller packets.

This may cause problems in the wireless medium as packets that are split

into smaller parts may not arrive in correct order.

Intermediate nodes may not be able to forward packets correctly.

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Performance

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Summary DSDV & DSR

DSDV

Advantages:

Low Latency (all information saved in the routing tables)

Disadvantages:

High Overhead (local movements have global effect)

Independent of traffic (no traffic, same overhead)

DSR

Advantages:

On-Demand (only routing overhead if traffic)

Disadvantages: High Latency (no route in cache, route discovery)

Non-Uniform Packet Size (in case of long routes very bad)

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Summary DSDV & DSR

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

Table-Driven

Routing Tables (for saving information about topology)

On-Demand

Route Discovery

Expanding Ring Search (Route Request Type)

Forward Path Setup (Saving Route Information in RTables)

Route Maintenance (Methods to repair broken links)

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Routing Tables

Entries:

Destination IP

Next Hop IP

Destination Sequence Number

Life Time

List of Precursors

Hop Count Number

Sequence Number:

The Seq. Number is monotonically increased each time the node learns of achange in the topology.

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Example (Routing Table)

E

B

F

D

G

A

2E64GC

2A, E84GD

1E102BB

Hop CountPrecursorsLifetimeDest. SeqNo.Next Hop IPDest. IP

C

Routing Table of Node F:

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Route Discovery

Like DSR, we use two types of messages, route request (RREQ) and route

reply (RREP):

Route Request Messages (RREQ) Route Request Processing (RREQ)

Reverse Route Entry

Expanding Ring Search (RREQ)

Responding to Route Request Messages (RREP)

Forward Path Setup

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Route Request Message

When node S wants to send a message to node D, S searches its routing

table for a valid route to D.

If there is no valid route, S initiates a RREQ message with the following

components :

The IP addresses of S and D

The current sequence number of S and the last known sequence number

of D

A broadcast ID from S. This broadcast ID is incremented each time S

initiates a RREQ message.

Hop count

The pair of the source S forms a unique

identifier for the RREQ.

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Example (Routing Request Message)

304FD


S

B

F

E

G

A D

Routing Table of Node S:

Routing Request Message:

15

Broadcast

ID

047DS

Hop CountDest. Seq.

No.

Source Seq.

No.

Dest. IPSource IP

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Route Request Processing

Suppose a node P receives the RREQ from S. P first checks whether it has

received this RREQ before.

Each node stores the pairs for all the recent

RREQs it has received. (for a specific amount of time)

S

D

i Q

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Processing a RREQ Message

If P has seen this RREQ from S already, P discards the RREQ. Otherwise,P processes the RREQ :

P sets up a reverse route entry in its routing table for the source S.

This entry contains the IP address and current sequence number of S,number of hops to S and the address of the neighbour from whom P got

the RREQ.

S DP

Q

R R E

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Reverse Route Entry

2107FS


S

F

E

G

A D

Routing Table of Node G:


15

Broadcast

ID

247DS

Hop countDest. Seq.

No.

Source Seq.

No.

Dest. IPSource IP

R di t RREQ M

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Responding to a RREQ Message

P can respond to the RREQ from S if P has an unexpired entry for D in itsrouting table.

Moreover, the sequence number from D that P has, must not be less thanthe sequence number of D that was in the RREQ from S.

This ensures that there is no loop in the route.

If P satisfies both of these requirements, it sends a RREP message back toS.

If P cannot reply to the RREQ from S,

P increments the hop-count of the RREQ and broadcasts it to itsneighbours.

S DP

Q

R di t RREQ M

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Responding to a RREQ Message

1X106DD


S

F

E

G

A D



15

Broadcast

ID

247DS

Hop CountDest. Seq.

No.

Source Seq.

No.

Dest. IPSource IP

X

Forward Path Setup (Sending a RREP)

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Forward Path Setup (Sending a RREP)

A RREP message has several fields :

The IP address of both source and destination

If the destination is sending the RREP, it sends its current sequence

number, a lifetime for the route and sets the hop-count to 0

If an intermediate node is responding, it sends the last known sequence

number from the destination, sets the hop-count equal to distance from

the destination and a lifetime for the route.

RREPS

D

M

N

Responding to a RREP Message

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Responding to a RREP Message

1X46DD


S

F

EA D


Routing Reply Message:

4

Lifetime

167DS

Hop CountDest. Seq.

No.

Source Seq.

No.

Dest. IPSource IP

G X

Forward Path Setup

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Forward Path Setup

A node (here Node M) sends a RREP back to a neighbour from whom it

received the RREQ.

When an intermediate node (here Node N) receives a RREP, it sets up aforward path to the destination in its route table.

This contains the IP addresses of the neighbour and the destination, hop-

count to the destination and a lifetime for the route.

RREPS

D

M

N

Forward Path Setup

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Forward Path Setup

1G107SS

2S46GD

Hop CountPrecursorsLifetimeDest.Seq. NoNext Hop IPDest. IP

S

F

E

G

A D


Routing Reply Message:

4

Lifetime

167DS

Hop CountDest. Seq.

No.

Source Seq.

No.

Dest. IPSource IP

Handling more than one RREP

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Handling more than one RREP

An intermediate node P may receive more than one RREP for the same

RREQ.

P forwards the first RREP it receives and forwards a second RREP lateronly if :

The later RREP contains a greater sequence number for the destination,

(RREP for later RREQ)

The hop-count to the destination is smaller in the later RREP Otherwise, it does not forward the later RREPs. This reduces the

number of RREPs propagating towards the source.

RREPS

DM

P

Expanding Ring Search

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Expanding Ring Search

For route discovery, a source node broadcasts a RREQ across the network.

This may create a lot of messages in a large network.

A source node uses an expanding ring search strategy. With a ring diameter

K, a RREQ dies after its hop-count exceeds K.

If a RREQ fails, the source node increases the value of K incrementally.

Example (Expanding Ring Search)

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Example (Expanding Ring Search)

S

D


0

Hop

Count

3

Broadcast

Id

145DS

Time

To Live

Dest. Seq.

No.

Source

Seq. No.

Dest. IPSource IP

Route Maintenance

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Route Maintenance

Once a route has been established between two nodes S and D, it is

maintained as long as S (source node) needs the route.

If S moves during an active session, it can reinitiate route discovery to

establish a new route to D.

When D or an intermediate node moves, a route error (RERR) message issent to S.

SD

M

N

Route Error

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Route Error

If S moves during an active session, it can reinitiate route discovery to

establish a new route to D.

When D or an intermediate node moves, a route error (RERR) message issent to S.

Example:

The link from node 3 to D is broken as 3 has moved away to a position 3. Node 2 sends a RERR message to 1 and 1 sends the message in turn to S.

S initiates a route discovery if it still needs the route to D.

1 2

3

S D

RERR RERR3

Updating Routing Tables

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p g g

Suppose neighbours 4 and 5 route through 2 to reach D. Node 2 broadcasts

RERR to all such neighbours.

Each neighbour marks its route table entry to D as invalid by setting the

distance to infinity.

Each neighbour in turn propagates the RERR message.

1 2

3

S D

RERR RERR3

4 5

Example (Routing Error)

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p ( g )

E

B

F

D

G

A

2E64GC

2A, E84GD

1E102BB


C


G

Overview

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AODV 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.

The overhead packets in AODV are due to RREQ, RREP and RERR

messages.

AODV needs much less number of overhead packets compared toDSDV.

The number of overhead packets increases with increased mobility, since

this gives rise to frequent link breaks and route discovery.

The route discovery latency in AODV is low compared to DSR and

DSDV.

AODV Evaluation

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AODV vs. DSR & DSDV

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Hybrid Routing Protocol

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Proactive Protocols:

Low Latency (Routing Tables map whole Network Topology)

High Overload (Updating of Routing Tables)

Reactive Protocols

Low Overload (On-Demand Protocol)

High Latency (Route Discovery needed)

Idea:

Mix reactive and proactive parts together

Introduce Routing Zones for Zone Routing Protocol

Routing Zones

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S

LK

G

H

I

J

AB

CD

E

Routing Zone for Source

Node S

Routing Zone radius = 2 Peripheral Nodes:

G, H, J, I, K

Basic Strategy of ZRP

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Intrazone Routing:

First, the packet is sent within the routing zone of the source node to

reach the destination node or peripheral nodes.

Interzone Routing:

Then the packet is sent from the peripheral nodes towards the destination

node.

S

D

Intrazone - Routing

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Each node collects information about all the nodes in its routing zone

proactively. This strategy is similar to a proactive protocol like DSDV.

Each node maintains a route table for its routing zone, so that it can find a

route to any node in the routing zone from this table.

Zone Notification Message (k-Hop Hello Message) to maintain Routing

Table for Routing Zone

Interzone - Routing

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The Interzone routing discovers routes to the destination reactively.

Consider a source (S) and a destination (D). If D is within the routing zone of

S, the routing is completed in the Intrazone routing phase.

Otherwise, S sends the packet to the peripheral nodes of its zone through

bordercasting.

Bordercasting: S maintains complete routing table for its zone and routes the

packet to the peripheral nodes by consulting this routing table.

S

D

Interzone Route Discovery

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S sends a route request (RREQ) message to the peripheral nodes of its zone

through bordercasting.

Each peripheral node P executes the same algorithm.

First, P checks whether the destination D is within its routing zone and if

so, sends the packet to D.

Otherwise, P sends the packet to the peripheral nodes of its routing zone

through bordercasting.

Example (Interzone Routing)

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S

D

B

H

A

C

Evaluation

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When the radius of the routing zone is 1, the behaviour of ZRP is like a

pure reactive protocol, for example, like DSR.

When the radius of the routing zone is infinity (or the diameter of thenetwork), ZRP behaves like a pure proactive protocol, for example, like

DSDV.

The optimal zone radius depends on node mobility and route query rates

/ number of traffic connections.

Research

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DSR with non-uniform packet size (intermediate nodes as routers)

Mobility and Protocols (Traffic?)

modeling of mobility Freeway / Manhattan / Pedestrian Scenario

distribution of mobility

every node has its own mobility environment

which metric decides on mobility? (link duration?)

what to do with information about mobility?

change routing zone radius (ZRP)

Network Scenarios

large networks

different mobility scenarios wireless plus connection point to internet through wired network

References

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Charles E. Perkins (Hrsg.). Ad Hoc Networking, Addison-Wesley 2000.

D. B. Johnson, D. A. Maltz, and Y. Hu. The dynamic source routing protocol formobile ad hoc networks (DSR). Technical report, IETF MANET Working Group,2004.

Ch. E. Perkins, E. M. Belding-Royer, and S. R. Das. Ad hoc on-demand distancevector (AODV) routing. RFC Experimental 3561, Internet Engineering Task Force,July 2003.

Ch. E. Perkins and P. Bhagwat. Highly dynamic destination-sequenced distance-vector routing (DSDV) for mobile computers. InACM Conference on

Communications Architectures, Protocols and Applications, SIGCOMM '94,London, UK, pages 234-244. ACM, ACM, August 1994.

Z. J. Haas, M. R. Pearlman, and P. Samar. The Zone Routing Protocol (ZRP) for adhoc networks. Internet-draft, IETF MANET Working Group, July 2002.

VINT. The network simulator - ns-2., http://www.isi.edu/nsnam/ns/

Lecture: Mobile Computing, Amitava Datta, http://electures.informatik.uni-freiburg.de:8484/catalog/course.do?courseId=mcws200304

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