Download - Routing in Mobile Ad Hoc Networks (from Ad Hoc Networking by Charles Perkins)

Routing in Mobile Ad Hoc Networks

(from Ad Hoc Networking by Charles Perkins)

Thanks to Prof. Yu at Cleveland State Univ.

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Contents

Network layer solutions in infrastructure-less networks

What is MANET: Mobile ad hoc (multihop) networks?

Routing algorithms for MANETs Flooding Dynamic Source Routing (DSR) Ad-hoc On-Demand Distance Vector (AODV)

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Mobile Ad Hoc Networks (MANETs)

A collection of mobile hosts may form a temporary network without the aid of any established infrastructure or centralized administration (base station)

Routes between nodes may potentially contain multiple hops, thus also called multihop networks

Better space utilization than direct communication Requires a longer latency as for a single communication pair

due to more number of hops But the overall performance improves because simultaneous

transmissions are allowed as long as they are separated in space

Better energy utilization than direct communication

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Mobile Ad Hoc Networks

A

C

B

E E

C

B

A

Multihop communication May need to traverse

multiple links to reach a destination

C is not reachable from A but can communicate via B

Major difficulty Mobility causes route

changes

A graph can be used to model a MANET

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Why Ad Hoc Networks ?

AdvantagesEase of

deploymentSpeed of

deploymentDecreased

dependence on infrastructure

Applications Personal area

networking cell phone, laptop, ear

phone, wrist watch Military environments

soldiers, tanks, planes Civilian environments

taxi cab network meeting rooms sports stadiums boats, small aircraft

Emergency operations search-and-rescue policing and fire

fighting

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Characteristics of MANET

Characteristics Dynamic topology Broadcast transmission: overhearing is possible Bidirectional connection Limited resources: bandwidth, CPU, battery Higher link error rate

Assumptions Moderate movement with respect to packet

transmission latency Promiscuous receive mode Enough number of mobile hosts: no network partitioning

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Hierarchical architecture

Internet

AP AP

AP

APRouter

Router

Router

Router/Gateway

Router/Gateway

Mobile Path

Handoff

Router

Router

AP/Router

Router

Router

Wireless Mesh Backhaul

New Route

Old Route

802.11b/g(2.4GHz)

802.11a(5GHz)

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Contents




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Flooding of Control Packets

How to reduce the scope of the route request flood ?LAR (Localized Area Routing)Query localization

How to reduce redundant broadcasts ?The Broadcast Storm Problem

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Flooding for Data Delivery

Sender S broadcasts data packet P to all its neighbors

Each node receiving P forwards P to its 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

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B

A

S E

F

H

J

D

C

G

IK

Represents transmission of packet P

Z

YBroadcast transmission

M

N

L

12


B

A

S E

F

H

J

D

C

G

IK

• Node H receives packet P from two neighbors: potential for collision

Z

Y

M

N

L

13


B

A

S E

F

H

J

D

C

G

IK

• 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

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B

A

S E

F

H

J

D

C

G

IK

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

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B

A

S E

F

H

J

D

C

G

IK

Z

Y

• Node D does not forward packet P, because node D is the intended destination of packet P

M

N

L

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B

A

S E

F

H

J

D

C

G

IK

• 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

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B

A

S E

F

H

J

D

C

G

IK

• 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

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Flooding for Data Delivery: Advantages & Disadvantages

Advantages

Simplicity

May be more efficient than other protocols when rate of information transmission is low enough

Potentially higher reliability of data delivery Because packets may be delivered to the destination

on multiple paths

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Flooding for Data Delivery: Advantages & DisadvantagesDisadvantages

Potentially, very high overhead Data packets may be delivered to too many nodes who do

not need to receive them

Potentially lower reliability of data delivery Flooding uses broadcasting -- hard to implement reliable

broadcast delivery without significantly increasing overhead

• Broadcasting in IEEE 802.11 MAC is unreliable (no ACK) In our example, nodes J and K may transmit to node D

simultaneously, resulting in loss of the packet • in this case, destination would not receive the packet at

all

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Dynamic Source Routing (DSR)

Source determines the entire path to the destination when it has data packet to sendOn-demand or reactive protocolsNo periodic advertisements

Three control packets: RREQ, RREP, RERR

Procedures(1) Route discovery procedure (2) Route maintenance procedure(3) DSR Optimizations (Route cache)

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Assumption:

Cooperative nodes Moderate speed of node Relatively small network diameter (5-10 hops) Detectable packet error Unidirectional or bidirectional link A single IP address Promiscuous mode (optional)

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A

B

C

E

D

G

H

F

A

A

A-B

A-C

A-C-E

A-C-E

A-C-E

A-B-D

A-B-D-GA-B-D-G

A-B-D-G

Route Discovery

Route Request (RREQ)

Route Reply (RREP)

Initiator ID

Initiator seq#

Target ID

Partial route

RREQ FORMATRREQ FORMAT

- Each node appends own identifier when forwarding RREQ- RREQ is broadcast, while RREP is unicast

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Route Discovery: at source A

A needs to send to G

Lookup Cache for route A to G

Route

found?

Start Route Discovery Protocol

Continue normal

processing

Route Discovery finished

Packet in

buffer?

Send packet to next-hop

done

Buffer

packet

no

Write route in packet header

yes

yes

no

wai

t

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Route Discovery: At an intermediate node

Accept route

request packet

<src,id> in recently

seen requests

list?

Discard

route reque

st

yes

no

Host’s address already

in partial route

Discard

route reque

st

yes

Store <src,id> in

list

Broadcast packet

Send route reply

packet

done

myAddr=target

noAppend myAddr to

partial route

no

yes

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Route Reply in DSR

Route Reply can be sent by reversing the route in Route Request (RREQ) only if links are guaranteed to be bi-directional To ensure this, RREQ should be forwarded only if it

received on a link that is known to be bi-directional

If unidirectional (asymmetric) links are allowed, then RREP may need a route discovery for A from node G Unless node G already knows a route to node A If a route discovery is initiated by G for a route to A,

then the Route Reply is piggybacked on the Route Request from G.

If IEEE 802.11 MAC is used to send data, then links have to be bi-directional (since Ack is used)

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Now, Route is Discovered

Node A on receiving RREP, caches the route included in the RREP

When node A sends a data packet to G, the entire route is included in the packet header hence the name source routing

Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded

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A

B

C

E

D

G

H

F

Route Maintenance

G

RERRRERR

Route Cache (A)G: A, B, D, G G: A, C, E, H, GF: C, E, F

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Additional feature #1: Caching Overheard Routes

A B C D E

V W X Y Z

Node A CacheE: A, B, C, D, E

Node C CacheE: C, D, EE: C, D, E A: C, B, AE: C, D, E A: C, B, A Z: C, X, Y, Z V: C, X, W, V

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Route Caching: Beware!

When sending a RREQ Send first with TTL=1 hoping that nearby hosts

have an entry: non-propagating route request If no reply for some time, send RREQ with maximum

TTL This is called “Expand Ring Search”

Beware Stale caches can adversely affect performance With passage of time and host mobility, cached

routes may become invalid A sender host may try several stale routes

(obtained from local cache, or replied from cache by other nodes), before finding a good route

Adverse impact on TCP

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Additional feature #2: RREP with Cached Routes

A

B

C

E

D

H

F

G

RERRRERR

Route Cache (A)G: A, B, D, G F: C, E, F

RREQ

RREQ

Route Cache (C)G: C, E, H, G

RREPG:A,C,E,H,G

Route Cache (C)G: C, E, D, G

RREQ(! D-G)

RREQ(! D-G) RREQ

(! D-G)

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A

B

C

E

D

G

H

F

Additional feature #3: Packet Salvage

G

RERRRERR

Route Cache (D)G: D, E, H, G

Caution: No double salvage allowed !!!Caution: No double salvage allowed !!!

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A Summary of DSRA Summary of DSR

Entirely on-demand, potentially zero control message overheadTrivially loop-free with source routingConceptually supports unidirectional links as well as bidirectional links

High packet delays/jitters associated with on-demand routingSpace overhead in packets and route cachesPromiscuous mode operations consume excessive amount of powerNetwork partition problem - Message explosion problem (large number of route request packets) Use exponential backoff to limit the rate at which new

route discoveries may be initiated for the same target

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Contents




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Ad Hoc On-Demand Distance Vector Routing (AODV)

DSR vs DSDV DSR typically outperforms DSDV because periodic

control messages are not needed A main drawback of DSR is the large packet header

(entire route information)

AODV = DSR + DSDV Routes are discovered only when there are packets (on-

demand) as in DSR Nodes main distance and next node vectors as in DSDV

to reduce the header size

* Explained very well in C.E.Perkins’ Ad Hoc Networking book

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A

B

C

E

D

G

H

F

A

A

A-B

A-C

A-C-E

A-C-E

A-C-E

A-B-D

A-B-D-GA-B-D-G

A-B-D-G

Route Discovery in DSR


Route Reply (RREP)

Initiator ID

Initiator seq#

Target ID

Partial route


- Each node appends own identifier when forwarding RREQ

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A

B

C

E

D

G

H

F

A, G, 1

A, G, 1

A, G, 2

A, G, 2

A, G, 3

A, G, 3

A, G, 3

A, G, 3

G, A, 3G, A, 2

G, A, 1

Route Discovery in AODV


Route Reply (RREP)

Initiator ID

Target ID

Hop count


- Each node remembers where the packet came from (reverse path) during RREQ.- It set up forward path during RREP.

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Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

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

Those paths are not included in the header of each data packetbut stored in each node as a routing table or DV

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Link State Routing

LSR Each node periodically floods status of its links Each node re-broadcasts link state information received

from its neighbor Each node keeps track of link state information received

from other nodes Each node uses above information to determine next

hop to each destination

Direct application of LSR in a MANET is difficult due to a large control overhead Due to mobility, broadcast period cannot be reduced as

in wired network

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Routing in MANET (Unicast)

Many protocols have been proposed Some have been invented specifically for MANET Others are adapted from previously proposed protocols for

wired networks No single protocol works well in all environments

some attempts made to develop adaptive protocols

Proactive protocols (or called “Table-based”) Determine routes independent of traffic pattern Traditional LS and DV routing protocols are proactive

Reactive protocols (or called “On-demand”) Maintain routes only if needed (DSR)

Hybrid protocols

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Reactive Routing Protocols

Flooding Data packets are flooded to the destination But also to all other nodes => much overhead but more

reliable DSR (Dynamic Source Routing)

First, small control packet is flooded to discover a route Then, unicast data packets along the discovered route But each data packet includes the information on the entire

path in its packet header => larger packet size AODV (Ad-hoc On-demand Distance Vector)

Route discovery as in DSR but each node keeps a routing table as in proactive algorithm to reduce the packet size

Reactive + Proactive

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Trade-Off between Proactive and Reactive Protocols

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

Download - Routing in Mobile Ad Hoc Networks (from Ad Hoc Networking by Charles Perkins)

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