mobile ad hoc networks: routing, mac and transport issues material in this slide set are from a...

Mobile Ad Hoc Networks:Routing, MAC and Transport Issues

Material in this slide set are from a tutorial by Prof. Nitin Vaidya

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Mobile-ad hoc network

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Mobile Ad Hoc Networks

• Networks formed by wireless hosts which may be mobile• Without (necessarily) using a pre-existing infrastructure• Routes between nodes may potentially contain multiple hops

• Roles of node and endsystems are merged

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CommunicationNetwork

end system

node

Mobile Ad Hoc Networks

• May need to traverse multiple links to reach a destination

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Mobile Ad Hoc Networks (MANET)

• Mobility causes route changes

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Many Applications

• Personal area networking– Connect cell phone, laptop, wrist watch

• Military environments– soldiers, ships

• Civilian environments– taxi cab network

• Emergency operations– search-and-rescue– policing and fire fighting

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Why is Routing in MANET different ?

• Host mobility– link failure/repair due to mobility may have different

characteristics than those due to other causes

• Rate of link failure/repair may be high when nodes move fast

• New performance criteria may be used– route stability despite mobility– energy consumption

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Classification

• Many protocols have been proposed

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Ad hoc routing protocols

Reactiveprotocols

Proactiveprotocols

Hybrid

• 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)

• Determine route if and when needed

• Source initiates route discovery

• Example: DSR (dynamic source routing)

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

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Flooding of Control Packets

• Many ad-hoc routing protocols perform (potentially limited) flooding of control packets– The control packets are used to discover routes– Discovered routes are subsequently used to send data

packet(s)– Overhead of control packet flooding is amortized over data

packets transmitted between consecutive control packet floods

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Dynamic Source Routing (DSR)

• When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery

• Source node S floods Route Request (RREQ)

• Each node appends own identifier when forwarding RREQ

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Route Discovery in DSR

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B

A

S EF

H

J

D

C

G

IK

Z

Y

Represents a node that has received RREQ for D from S

M

N

L

Route Discovery in DSR

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B

A

S EF

H

J

D

C

G

IK

Represents transmission of RREQ

Z

YBroadcast transmission

M

N

L

[S]

[X,Y] Represents list of identifiers appended to RREQ

Route Discovery in DSR

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B

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S EF

H

J

D

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Y

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N

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[S,E]

[S,C]

Route Discovery in DSR

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B

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S EF

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

[S,C,G]

[S,E,F]

Route Discovery in DSR

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B

A

S EF

H

J

D

C

G

IK

Z

Y

M

• Nodes J and K both broadcast RREQ to node D

• Since nodes J and K are hidden from each other, their transmissions may collide

N

L

[S,C,G,K]

[S,E,F,J]

Route Discovery in DSR

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B

A

S EF

H

J

D

C

G

IK

Z

Y

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

M

N

L

[S,E,F,J,M]

Route Discovery in DSR

• Destination D on receiving the first RREQ, sends a Route Reply (RREP)

• RREP is sent on a route obtained by reversing the route appended to received RREQ

• RREP includes the route from S to D on which RREQ was received by node D

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Route Reply in DSR

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S EF

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RREP [S,E,F,J,D]

Represents RREP control message

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

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Dynamic Source Routing (DSR)

• Node S on receiving RREP, caches the route included in the RREP

• When node S sends a data packet to D, 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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Data Delivery in DSR

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S EF

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D

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Y

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N

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DATA [S,E,F,J,D]

Packet header size grows with route length

DSR Optimization: Route Caching

• Each node caches a new route it learns by any means• When node S finds route [S,E,F,J,D] to node D, node S also

learns route [S,E,F] to node F• When node K receives Route Request [S,C,G] destined for

node, node K learns route [K,G,C,S] to node S• When node F forwards Route Reply RREP [S,E,F,J,D], node

F learns route [F,J,D] to node D• When node E forwards Data [S,E,F,J,D] it learns route

[E,F,J,D] to node D• A node may also learn a route when it overhears Data

packets

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Use of Route Caching

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S EF

H

J

D

C

G

IK

[P,Q,R] Represents cached route at a node

M

N

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[S,E,F,J,D][E,F,J,D]

[C,S]

[G,C,S]

[F,J,D],[F,E,S]

[J,F,E,S]

Z

Use of Route Caching:Can Speed up Route Discovery

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B

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S EF

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J

D

C

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IK

Z

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[S,E,F,J,D][E,F,J,D]

[C,S]

[G,C,S]

[F,J,D],[F,E,S]

[J,F,E,S]

RREQ

• When node Z sends a route request for node C, node K sends back a route reply [Z,K,G,C] to node Z using a locally cached route

[K,G,C,S]RREP

Use of Route Caching:Can Reduce Propagation of Route Requests

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B

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S EF

H

J

D

C

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IK

Z

Y

M

N

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[S,E,F,J,D][E,F,J,D]

[C,S]

[G,C,S]

[F,J,D],[F,E,S]

[J,F,E,S]

RREQ

• Assume that there is no link between D and Z. Route Reply (RREP) from node K limits flooding of RREQ.

[K,G,C,S]RREP

Dynamic Source Routing

Advantages• Routes maintained only

between nodes who need to communicate– reduces overhead of route

maintenance

• Route caching reduces route discovery overhead

• A single route discovery may yield many routes to the destination

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Disadvantages• Packet header size grows

with route length due to source routing

• Flood of route requests• Route Reply Storm problem

– Reply storm may be eased by preventing a node from sending RREP if it hears another RREP with a shorter route

Ad Hoc On-Demand Distance Vector Routing (AODV)

• 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

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AODV

• 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

• Route Reply travels along the reverse path set-up when Route Request is forwarded

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Route Requests in AODV

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B

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S EF

H

J

D

C

G

IK

Z

Y

• Represents a node that has received RREQ for D from S

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N

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Route Requests in AODV

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B

A

S EF

H

J

D

C

G

IK

• Represents transmission of RREQ

Z

YBroadcast transmission

M

N

L

Route Requests in AODV

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B

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S EF

H

J

D

C

G

IK

• Represents links on Reverse Path

Z

Y

M

N

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Reverse Path Setup in AODV

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B

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S EF

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

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Reverse Path Setup in AODV

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S EF

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Reverse Path Setup in AODV

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B

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S EF

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D

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Y

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

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N

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Route Reply in AODV

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B

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S EF

H

J

D

C

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IK

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Represents links on path taken by RREP

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N

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Route Reply in AODV

• 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

• The likelihood that an intermediate node will send a Route Reply when using AODV not as high as DSR– 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

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Forward Path Setup in AODV

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S EF

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• Forward links are setup when RREP travels along the reverse path

Represents a link on the forward path

Data Delivery in AODV

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B

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S EF

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J

D

C

G

IK

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Y

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• Routing table entries used to forward data packet.

• Route is not included in packet header.

DATA

Summary: AODV

• Routes 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

• Unused routes expire even if topology does not change

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Link State Routing

• 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

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Optimized Link State Routing (OLSR)

• The overhead of flooding link state information is reduced by requiring fewer nodes to forward the information

• A broadcast from node X is only forwarded by its multipoint relays

• Multipoint relays of node X are its neighbors such that each two-hop neighbor of X is a one-hop neighbor of at least one multipoint relay of X– Each node transmits its neighbor list in periodic beacons,

so that all nodes can know their 2-hop neighbors, in order to choose the multipoint relays

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Optimized Link State Routing (OLSR)

• Nodes C and E are multipoint relays of node A

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A

B F

C

D

E H

GK

J

Node that has broadcast state information from A

Optimized Link State Routing (OLSR)

• Nodes C and E forward information received from A

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A

B F

C

D

E H

GK

J

• Node that has broadcast state information from A

Optimized Link State Routing (OLSR)

• Nodes H and A are multipoint relays for node E• Node H forwards information received from E

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A

B F

C

D

E H

GK

J

• Node that has broadcast state information from A

Advantages• Routing table always

available• Suitable for dense

networks where frequent communication is required between a (relative) large number of nodes.

Disadvantages• Flooding (even if

limited)• Unsuitable for sensor

networks • Routes used by OLSR

only include multipoint relays as intermediate nodes

OLSR

Destination-Sequenced Distance-Vector (DSDV)

• Each node maintains a routing table which stores– next hop towards each destination– a cost metric for the path to each destination– a destination sequence number that is created by the

destination itself– Sequence numbers used to avoid formation of loops

• Each node periodically forwards the routing table to its neighbors– Each node increments and appends its sequence number

when sending its local routing table– This sequence number will be attached to route entries

created for this node47

Destination-Sequenced Distance-Vector (DSDV)

• Assume that node X receives routing information from Y about a route to node Z

• Let S(X) and S(Y) denote the destination sequence number for node Z as stored at node X, and as sent by node Y with its routing table to node X, respectively

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X Y Z

Destination-Sequenced Distance-Vector (DSDV)

• Node X takes the following steps:

– If S(X) > S(Y), then X ignores the routing information received from Y

– If S(X) = S(Y), and cost of going through Y is smaller than the route known to X, then X sets Y as the next hop to Z

– If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y)

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X Y Z

Spanning Tree Protocol

A routing algorithm for mobile self-organizing networks

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Spanning Tree Protocol

• Adaptation of Perlman’s algorithm for bridges

• Assumption– Substrate has broadcast operation

(e.g., 802.11b)

• Components– Builds a rooted tree topology– All node agree on a common root– Each node selects a parent – Each node broadcasts beacon

messages periodically– Unicast and multicast forwarding

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1

6

2

9

5

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Spanning Tree Protocol

• Assumption– Wireless channel can be asymmetric– Outgoing broadcast packet can be received by adjacent

nodes– Each node has a unique Identifier (an integer number)

• Components– A shared tree topology– Periodic beacon messages– Configurable topology policy (parent selection algorithm)

Tree-based topology

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8

6

7

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Advantages– Topology is always maintained– Easy to do broadcasting (loop-free)– A shared tree is guaranteed given that

there’s no partition

Disadvantages– No redundancy– Message may not travel on most

efficient path– Difficult to do unicast

Spanning Tree Protocol: Beacon Message

• Periodically each node sends a beacon to its adjacent nodes– Used as in IEEE 802.1b:

Used to select parents in spanning tree

– Probes link quality

Overlay Hash4 bytes

SenderID

Physical Address

Core ID

Ancestor ID

Hop Count

Metric

Sequence Number

Adjacency Table

4 bytes

variable

4 bytes

4 bytes

2 bytes

2 bytes

8 bytes

4 bytes +5 bytes per

entry

Size

4 bytesID 1

Link Quality

ID 2

1 byte

4 bytes

1 byte

. . .

Adjacency TableEntry

Beacon Message

Link Quality

4 bytes

Spanning Tree Protocol

ID Link QualityLQ/RLQ

Time Stamp

476 9/10 1098900558838

167 2/10 1098900559750

250 8/10 1098900558724

•Adjacency Table–Record the link state of adjacent nodes–Being updated by beacon message received from others

•Link Quality–Measured by counting the number of beacons from the sender during a specified period

•Functions–Eliminate asymmetric links–Eliminate bad quality links–Used in topology algorithm

• LQ = %beacon received from this node

• RLQ= %beacon delivered to this node BiLQ= Min(LQ, RLQ)

Root and parent selection

• Root selection– The node with minimum ID is the root

• Parent selection– Choose the node that broadcasts the smaller root ID– When two nodes broadcast the same root ID

• Minimum Hop Algorithm: choose the node that has the minimum hop count to root

• Path Quality Algorithm: choose the node that has the best path to root

Path Quality = Product of the BiLQs of all links on the path

Minimum Hop vs. Path Quality

• Minimum Hop– Widely used in current ad-hoc routing protocols– Has good performance in simulator, e.g., GlomoSim– Tends to select distant links that has worse link quality in

real 802.11 network

• Path Quality– Tends to have longer path– Favors the path with lower loss rate– Doesn’t show better performance in simulator– Does show large improvement in real 802.11 network

Routing in SPT

• Multicast– Sends along the tree edges

• Unicast– Maintains routing table– Routing table can be updated passively or actively

data data

Dest ID NextHop ID

Time Stamp

476 271 1098900558838

167 123 1098900559750

250 4 1098900558724

rreq

Updating Routing Table

Passive updating(Learning)

Active updating (Route Request)

data data2

3

4

5

2 2

2 4

2 4 2 3

data:5 data:5

1

2

3

4

5

rreq rreq

rrep5 5

5 35 4

Create a path Invalidate a path

data:5

1

2

3

4

5

rreq

5 4 5 3

1

Simulation Result

• Platform: GlomoSim• 10 unicast data streams• Compared to AODV

Mobility Experiment

• 6 PDAs are fixed at location, and 1 moves around• Using UDP adapter• Sender sends out 10 packets per second by unicast

to the moving node, and no ACKs.• Randomized logical address

A B

Sender

C D E F90ft 90ft 90ft 90ft 90ft

50ft

90ft 90ft

M

Receiver

Mobility Experiment

Minimum Hop

Path Quality