AdHoc Routing

8/2/2019 AdHoc Routing

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Module I

Routing in mobile ad hoc networks

Prof. JP Hubaux

Mobile Networks

http://mobnet.epfl.ch


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The classical solution for mobile networks

2nd generation (GSM, IS-41,) and 3rdgeneration (UMTS,)deployed soon

Huge, expensive fixed infrastructure

License for a share of the spectrum

Operational responsibility: network operators (telcos, ISPs)


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The new paradigm: ad hoc networks

Terminal and node merge

Everything is potentially mobile Initial applications: communication in the battlefield (Packet

Radio Networks, in the 70s)

The network is self-organizedwhen it is run by the usersthemselves

Similar trend at the application layer: peer-to-peer(e.g., Napster Gnutella)


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Application examples of ad hoc networks

Sensor networks

Hybrid cellular / ad hoc networks (multi-hop cellularnetworks)

Cars

Assisted driving (adaptive cruise control,)

Collision avoidance

Optimization of traffic flows

Crisis networks (e.g., rescue operations after majordisaster)

Military networks


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Upper bound for the throughput of ad hocnetworks

If we have:

- identical randomly located nodes

- each capable of transmitting bits/s

Then the throughput ( ) obtainable by each node

for a destination is

( )log

n

W

n

randomly chosenW

nn n

Ref: P. Gupta, P. Kumar, The Capacity of Wireless Networks

IEEE Transactions on Information Theory, March 2000


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Intuition behind the upper bound

N nodes (users)

O(N) users O(N) users

Cut set ~ N

O(N) transmissions from left to right

over

O( ) transmission links

mean

O( ) capacity per attempted transmission

N

1N

Ways to improve scalability:

Directional antennas

Locality of the traffic

Hybrid system


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Routing in ad hoc networks

Peculiarities

Node mobilityHigh rate of link failure Traditional routing approaches are not well suited

Assumptions

Multihop communication

Symmetric links (in most cases)Omnidirectional antennas (in most cases)All nodes have equal capabilities and responsibilities

Figures of merit

Latency of route discoveryOverhead (bandwidth, energy, processing power)

Security

Current status of research:

Many, many proposalsOptimal solution depends on deployment scenario: mobility

patterns, radio model, traffic characteristics,


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Brief reminder : Link-state protocols

Example: OSPF

May consume a lot of resources to update the routes

Techniques to alleviate the problem : limit thepropagation of information

Does not seem to be well suited to cope with mobility


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Distance vector routing (1/2)

0 1 5

1 0 1 3

5 1 0 7

3 7 0

A

B

D

C

1

3

1

5 7

A

B

C

D

A B C D

1 0 1 3

Distance

vector

Distance

vector of B

2 1 2 4

+ Distance from A to B =

Cost to dest.

via B

Take the min

0 1 2,B 4,B

(1 row stored in each node)


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Distance vector routing (2/2)

Even if the updates are asynchronous, the routingtables converge

The algorithm is often called Bellman-Ford

Problem: undesirable behaviour when links go upand down (e.g., count to infinity problem)


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Routing protocols for wireless ad hoc networks

Response time,

bandwidthEnergy

Mobile ad hoc networks Sensor networks

Proactive

protocols

Reactive

protocols

Destination-Sequenced

Distance-Vector (DSDV)

Optimized Link-

State Routing

(OLSR)

Ad Hoc On-Demand

Distance-Vector

(AODV)

Dynamic

Source

Routing

(DSR)

Geography-

based routing

Cluster-based

(or hierarchical)

routing

Geodesic

packetforwarding


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Dynamic source routing (DSR)

Reactive routing protocol

2 phases, operating both on demand:

Route discovery Used only when source S attempts to to send a packet to

destination D Based on flooding of Route Requests (RREQ)

Route maintenance makes S able to detect, while using a source route to D, if it can

no longer use its route (because a link along that route no

longer works)


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DSR: Route discovery (1)

E G

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A

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DS

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E G

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(S)


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E G

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(S,A)

(S,E)


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E G

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(S,E,G)

(S,B,C)


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E G

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(S,E,G,J)

(S,A,F,H)


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E G

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A

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DS

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Q(S,A,F,H,K)


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E G

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A

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DS

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(S,A,F,H,K,P)


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E G

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A

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DS

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RREP(S,E,G,J,D)


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DSR: Route Discovery (9)

Route reply by reversing the route (as illustrated)works only if all the links along the route arebidirectional

If unidirectional links are allowed, then RREP may

need a route discovery from D to S Note: IEEE 802.11 assumes that links are

bidirectional


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DSR: Data delivery

E G

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A

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DS

K

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DATA(S,E,G,J,D)


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DSR: Route maintenance (1)

E G

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DS

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DATA(S,E,G,J,D)

X


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DSR: Route maintenance (2)

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RERR(G-J)

When receiving the Route Error message (RERR),

S removes the broken link from its cache.

It then tries another route stored in its cache; if none,

it initializes a new route discovery


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DSR: Optimization of route discovery: routecaching

Principle: each node caches a new route it learns byany means

ExamplesWhen node S finds route (S, E, G, J, D) to D, it also learns

route (S, E, G) to node G

In the same way, node E learns the route to DSame phenomenon when transmitting route replies

Moreover, routes can be overheard by nodes in theneighbourhood

However, route caching has its downside: stalecaches can severely hamper the performance of thenetwork


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DSR: Strengths

Routes are set up and maintained only betweennodes who need to communicate

Route caching canfurther reduce the effort of routediscovery

A single route discovery may provide several routesto the destination


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DSR: Weaknesses

Route requests tend to flood the network andgenerally reach all the nodes of the network

Because of source routing, the packet header sizegrows with the route lengh

Risk of many collisions between route requests byneighboring nodes need for random delays beforeforwarding RREQ

Similar problem for the RREP (Route Reply stormproblem), in case links are not bidirectional

Note: Location-aided routingmay help reducing thenumber of useless control messages


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Ad Hoc On-Demand Distance VectorRouting (AODV)

As it is based on source routing, DSR includessource routes in data packet headers

Large packet headers in DSR risk of poorperformance if the number of hops is high

AODV uses a route discovery mechanism similar toDSR, but it maintains routing tables at the nodes

AODV agesthe routes and maintains a hop count

AODV assumes that all links are bi-directional

AODV R di ( )


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AODV : Route discovery (1)

E G

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AODV R di (2)


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Note: if one of the intermediate nodes (e.g., A)

knows a route to D, it responds immediately to S: Route Request (RREQ)

AODV R di (3)


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E G

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A

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DS

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: represents a link on the reverse path

AODV R t di (4)


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AODV R t di (5)


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AODV R t di (6)


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M

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AODV R t di (7)


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AODV : Route replyand setup of the forwardpath

M

D

K

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E G

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A

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: Link over which the RREP is transmitted

: Forward path

Route reply in AODV


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Route reply in AODV

In case it knows a path more recent than the onepreviously known to sender S, an intermediate nodemay also send a route reply (RREP)

The freshness of a path is assessed by means of

destination sequence numbers Both reverse and forward paths are purged at the

expiration of appropriately chosen timeout intervals

AODV : Data delivery


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AODV : Data delivery

M

D

K

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E G

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A

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Data

The route is not included in the packet header

AODV : Route maintenance (1)


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M

D

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A

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Data

X



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M

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RERR(G-J)

When receiving the Route Error message (RERR),

S removes the broken link from its cache.

It then initializes a new route discovery.

AODV: Destination sequence numbers


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AODV: Destination sequence numbers

If the destination responds to RREP, it places itscurrent sequence number in the packet

If an intermediatenode responds, it places its recordof the destinations sequence number in the packet

Purpose of sequence numbers:

Avoid using stale information about routes

Avoid loops (no source routing!)

AODV : Avoiding the usage of stale routing


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AODV : Avoiding the usage of stale routingtables

S DA

B

DSN(D) = 5

: Forward path

S

D

A

B

DSN(D) = 5

DSN(D) = 8

1.2.

S

D

A

B

DSN(D) = 5

DSN(D) = 8

3.

RREQ

S

D

A

B

DSN(D) = 5

DSN(D) = 8

RREP

4.

AODV : Avoiding loops


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AODV : Avoiding loops

A B S D

C

: Forward path

Assume there is a route between A and D; link S-D breaks; assume A is not aware of this, e.g. because

RERR sent by S is lost

Assume now S wants to send to D. It performs a RREQ, which can be received by A via path S-C-A

Node A will reply since it knows a route to D via node B

This would result in a loop (S-C-A-B-S) The presence of sequence numbers will let S discover that the routing information from A is outdated

Principle: when S discovers that link S-D is broken, it increments its local value of DSN(D). In this way,

the new local value will be greater than the one stored by A.

X

AODV (unicast) : Conclusion


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AODV (unicast) : Conclusion

Nodes maintain routing information only for routesthat are in active use

Unused routes expire even when the topology doesnot change

Each node maintains at most one next-hop perdestination

Many comparisons with DSR (via simulation) havebeen performed no clear conclusion so far

Geodesic Packet Forwarding


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Geodesic Packet Forwarding

L. Blazevic, S. Giordano, J.-Y. Le Boudec (IP4)

AP -geographical anchor pointAGPF (Anchored Geodesic Packet Forwarding) - sourcerouting with anchors

S

AP1

D

AP2

S has anchored path {AP1,AP2}from S: packets are forwarded in direction of AP1from A: packets are forwarded in direction of AP2from B: packets are forwarded in direction of Ds position

from X : use of Terminode Local Routing (TLR)

A

B

X

TLR area of X

Other (Swiss) proposals


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Other (Swiss) proposals

Last Encounter Routing H. Dubois-Ferrire, M. Grossglauser, M. Vetterli (EPFL, IP1 & 7)

Principle: Nodes exchange information about their previousencounters

No explicit location service, no transmission overhead to toupdate the state

Ongoing work: prediction, based on declared mobility

Face routing

F. Kuhn, R. Wattenhofer, A. Zollinger (ETHZ, IP9)

Principle: exploit the geometric properties of the connectivitygraph

Worst-case optimal

Conclusion on routing


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Conclusion on routing

DSR and AODV are the mainstream proposals

Both have been extensively studied (by simulation)

No clear superiority of one wrt the other

Scalability is still an open issue

Other very promising proposals

References


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References

Ch. Perkins: Ad Hoc Networking, Addison Wesley,2001

www.mics.org: IP1, IP4, IP7 and IP9

AdHoc Routing

Documents

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