05 MANETs BÀI GIẢNG MẠNG MÁY TÍNH NÂNG CAO

Route Discovery in DSRDestination D on receiving the first RREQ, Route Reply RREP sends a g RREP is sent on a route obtained by reversing route appended to received RREQ the g RREP inc

Mobile Ad Hoc Networks

Formed by wireless hosts which may be mobile

Mobile Ad Hoc Networks

Mobile Ad Hoc Networks (MANET)

Mobility causes route changes

g  

3

Why Ad Hoc Networks ?

Limited wireless transmission range

Broadcast nature of the wireless medium

g  

g  

i Hidden terminal problem (see next

slide)

Packet losses due to transmission errors

Mobility-induced route changes

Mobility-induced packet losses

Battery constraints

Potentially frequent network partitions

Ease of snooping on wireless transmissions

Hidden Terminal Problem

Unicast Routing

in Mobile Ad Hoc Networks

11

Why is Routing in MANET different ?

Unicast Routing Protocols

Many protocols have been proposed

g  

Some have been invented specifically for MANET

g  

Others are adapted from previously proposed

protocols for wired networks

from X to Y will be found only when X attempts to send to Y

Overhead of route discovery/maintenance

g  

i Reactive protocols may have lower overhead since

routes

are determined only if needed

iProactive protocols can (but not necessarily) result in higher overhead due to continuous route updating

Which approach achieves a better trade-off depends

g  

Overview of Unicast Routing Protocols

16

Flooding for Data Delivery

Sender S broadcasts data packet P to all its

neighbors

g  

Each node receiving P forwards P to its neighbors

g  

Sequence numbers used to avoid the possibility of

forwarding the same packet more than once

g  

Packet P reaches destination D provided that

reachable from sender S

D is

g  

Node D does not forward the packet

Flooding for Data Delivery

Y

Z

F B

Represents a node that has received packet P

Represents that connected nodes are within each

Flooding for Data Delivery

Y Broadcast transmission

Z

F B

Flooding for Data Delivery

Y

Z

F B

Flooding for Data Delivery

Y

Z

F B

Flooding for Data Delivery

Y

Z

F B

transmissions may collide => Packet P may not be delivered to node D at all,

despite the use of flooding 22

Flooding for Data Delivery

Y

Z

F B

Flooding for Data Delivery

Y

Z

F B

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)

24

Flooding for Data Delivery

Y

Z

F B

•  Flooding may deliver

(in the worst case , all

packets to too many nodes nodes reachable from sender may receive the packet)

25

Flooding for Data Delivery: Advantages

Simplicity

g  

May be more efficient than other protocols when rate

of information transmission is low enough that the

overhead of explicit route discovery/maintenance

incurred by other protocols is relatively higher

g  

i this scenario may occur, for instance, when nodes

transmit

changes occur between consecutive packet transmissions

Potentially higher reliability of data delivery

g  

iBecause packets may be delivered to the destination on

multiple paths

26

Flooding for Data Delivery: Disadvantages

Potentially, very high overhead

g  

i Data packets may be delivered to too many nodes who

do

not need to receive them

Potentially lower reliability of data delivery

g  

i Flooding uses broadcasting hard to implement

reliable

broadcast delivery without significantly increasing overhead

–  Broadcasting in IEEE 802.11 MAC is unreliable

 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

i

27

Flooding of Control Packets

Many protocols perform (potentially limited) flooding

of control packets, instead of data packets

28

g  

Dynamic Source Routing (DSR) [Johnson96]

When node S wants to send a packet to node D, butdoes not know a route to D, node S initiates a routediscovery

Route Discovery in DSR

Y Broadcast transmission

F B

Represents transmission of RREQ

[X,Y] Represents list of identifiers appended to RREQ 31

Route Discovery in DSR

Y

Z [S,E]

F B

but does not forward

forwarded RREQ once

33

F B

F B

Route Discovery in DSR

Destination D on receiving the first RREQ,

Route Reply (RREP)

sends a

g  

RREP is sent on a route obtained by reversing

route appended to received RREQ

the

g  

RREP includes the route from S to

was received by node D

D on which RREQ

g  

36

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

g  

i 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, thenRREP may need a route discovery for S from node D

Route Reply is piggybacked on the Route Request from D.

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

g  

38

Dynamic Source Routing (DSR)

Node S on receiving RREP, caches the route

included in the RREP

g  

When node S sends a data packet to D, the entire

route is included in the packet header

g  

i hence the name source

routing

Intermediate nodes use the source route included in

a packet to determine to whom a packet should be

forwarded

g  

39

Data Delivery in DSR

Y

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

E

F C

Z S

When to Perform a Route Discovery

When node S wants to send data to node D, but does

g  

not know a valid route node D

41

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,

S also learns route [S,E,F] to node F

When node K receives Route Request [S,C,G]

route [E,F,J,D] to node D

A node may also learn a route when it overhears

Data packets

g  

42

Use of Route Caching

When node S learns that a route to node D is broken,

it uses another route from its local cache, if such a

ican speed up route discovery

reduce propagation of route requests

Use of Route Caching

B

C [C,S]

Use of Route Caching:

Can Speed up Route Discovery

B

C [C,S]

RREP

RREQ

Z 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

Use of Route Caching:

Reduce Propagation of Route

B

C [C,S]

RREQ

Z

Assume that there is no link between D and Z.

Route Reply (RREP) from node K limits flooding of RREQ.

In general, the reduction may be less dramatic. 46

Route Error (RERR)

Y

F B

Route Caching: Beware!

Stale caches can adversely affect performance

g  

With passage of time and host mobility, cached

routes may become invalid

g  

A sender host may try several stale routes (obtained

g  

from local cache, or replied from cache

nodes), before finding a good route

by other

48

Dynamic Source Routing: Advantages

Routes maintained only between nodes who need

communicate

to

g  

ireduces overhead of route maintenance

Route caching can further reduce route discovery

Dynamic Source Routing: Disadvantages

Packet header size grows with route length due to

source routing

g  

Flood of route requests may potentially reach all

nodes in the network

Dynamic Source Routing: Disadvantages

An intermediate node may send Route Reply usingstale cached route, thus polluting other caches

Broadcast Storm Problem [Ni99Mobicom]

When node A broadcasts a route query, nodes B and

and C both forward to their neighbors

and C transmit at about the same time since they

g  

D

Broadcast Storm Problem

Redundancy: A given node may receive the same

g  

route request from too many nodes, when one

would have sufficed

Solutions for Broadcast Storm

Probabilistic scheme: On receiving a route request forthe first time, a node will re-broadcast (forward) the

request with probability p

g  

Also, re-broadcasts by different nodes should be

staggered by using a collision avoidance technique

a random delay when channel is idle)

Solutions for Broadcast Storms

Counter-Based Scheme: If node E hears more than k

neighbors broadcasting a given route request, before

it can itself forward it, then node E will not forward therequest

g  

Intuition: k neighbors together have probably already

g  

E’sforwarded the request to all of neighbors

Solutions for Broadcast Storms

Distance-Based Scheme: If node E hears RREQ

broadcasted by some node Z within physical distance

d, then E will not re-broadcast the request

g  

Intuition: Z and E are too close, so transmission

areas covered by Z and E are not very different

g  

i  if E re-broadcasts the request, not many nodes who

have

not already heard the request from Z will hear the request

E <d

Z

56

Summary: Broadcast Storm Problem

Flooding is used in many protocols,

Redundancy may be reduced by selectively

re-broadcasting packets from only a subset of the nodes

57

g  

Ad Hoc On-Demand Distance Vector Routing

AODV attempts to improve on DSR by maintaining

routing tables at the nodes, so that data packets donot have to contain routes

g  

AODV retains the desirable feature of DSR that

routes are maintained only between nodes which

need to communicate

g  

58

Route Requests (RREQ) are forwarded in a mannersimilar to DSR

g  

When a node re-broadcasts a Route Request, it sets

up a reverse path pointing towards the source

g  

iAODV assumes symmetric (bi-directional) links

When the intended destination receives a Route

Request, it replies by sending a Route Reply

g  

Route Reply travels along the reverse path set-up

when Route Request is forwarded

g  

59

Route Requests in AODV

Y

Z

F B

Route Requests in AODV

Y Broadcast transmission

Z

F B

Route Requests in AODV

Y

Z

F B

Reverse Path Setup in AODV

Y

Z

F B

it again, because node

from G and H, but does not forward

C has already forwarded RREQ once

63

Reverse Path Setup in AODV

Y

Z

F B

Reverse Path Setup in AODV

Y

Z

F B

Route Reply in AODV

Y

Z

F B

Route Reply in AODV

An intermediate node (not the destination) may also

To determine whether the path known to an

intermediate node is more recent,

destination sequence numbers are used

assigned a higher destination sequence number An

intermediate node which knows a route, but with a smaller sequence number, cannot send Route Reply 67

Forward Path Setup in AODV

Y

Z

F B

Data Delivery in AODV

Routing table entries used to forward data packet.

Route is not included in packet

header.

69

A routing table entry maintaining a reverse path is

purged after a timeout interval

purged if not used for a active_route_timeout interval

i if no is data being sent using a particular routing table entry, that entry will be deleted from the routing table (even if the route may actually still be valid)

70

Link Failure Reporting

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

g  

When the next hop link in a routing table entry

breaks, all active neighbors are informed

g  

Link failures are propagated by means of Route Errormessages, which also update destination sequencenumbers

71

g  

Route Error

When node X is unable to forward packet P (from

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

Destination Sequence Number

Continuing from the previous slide …

g  

When node D receives the route

destination sequence number N,

request withnode D will set

g  

itssequence number to N, unless it is already larger

than N

73

Link Failure Detection

Hello messages: Neighboring nodes periodically

exchange hello message

Alternatively, failure to receive several

acknowledgement may be used as an

link failure

MAC-levelindication of

g  

74

Why Sequence Numbers in AODV

To avoid using old/broken routes

iAssume that A does not know about

RERR sent by C is lost

failure of link C-D because

i Now C performs a route discovery for D Node A

receives

the RREQ (say, via path C-E-A)

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

i Results in a loop (for instance,

C-E-A-B-C )

Why Sequence Numbers in AODV

Optimization: Expanding Ring Search

Route Requests are initially sent with small Live (TTL) field, to limit their propagation

Time-to-g  

i DSR

also

includes a similar optimization

If no Route Reply is received, then larger TTL tried

g  

77

Summary: AODV

Routes need not be included in packet headers

g  

Nodes maintain routing tables containing entries

for routes that are in active use

Link Reversal Algorithm [Gafni81]

D

80

Link Reversal Algorithm

Links are bi-directional

But algorithm imposes logical directions on them

81

Link Reversal Algorithm

Link (G,D) broke D

Any node, other than the destination, that has no outgoing links reverses all its incoming links.

82

Node G has no outgoing links

Link Reversal Algorithm

Represents a link that was

Link Reversal Algorithm

Represents a link that was

Link Reversal Algorithm

Represents a link that was

Link Reversal Algorithm

Represents a link that was reversed recently

D

Now all nodes (other than destination D) have an outgoing link

86

Link Reversal Algorithm

Link Reversal Algorithm

Attempts to keep link reversals local to where

failure occurred

the

g  

iBut this is not guaranteed

When the first packet is sent to a destination,

destination oriented DAG is constructed

Link Reversal Algorithm

The previous algorithm is called a full reversal

method since when a node reverses links, it reverses

all its incoming links

g  

Partial reversal method [Gafni81]: A node reversesincoming links from only those neighbors who havenot themselves reversed links “previously”

g  

i If all neighbors have reversed links, then the node

reverses

all its incoming links

i “Previously” at node X means since the last link

reversal

done by node

X

89

Partial Reversal Method

Link (G,D) broke D

Node G has no outgoing links

90

Partial Reversal Method

Represents a link that was reversed recently

Represents a node that has reversed links D

Now nodes E and F have no outgoing links

91

Partial Reversal Method

Represents a link that was

reversed recently

D

Nodes E and F

do not reverse links from node G

Partial Reversal Method

Represents a link that was

Partial Reversal Method

Represents a link that was reversed recently

D

Now all nodes (except destination D) have outgoing links

94

Partial Reversal Method

Link Reversal Methods: Advantages

Link reversal methods attempt to limit updates to

routing tables at nodes in the vicinity of a broken link

Link Reversal Methods: Disadvantage

Need a mechanism to detect link failure

Link Reversal in a Partitioned Network

Full Reversal in a Partitioned Network

Full Reversal in a Partitioned Network

Full Reversal in a Partitioned Network

Full Reversal in a Partitioned Network

Full Reversal in a Partitioned Network

In the partition disconnected from destination D, link reversals continue, until the partitions merge

Need a mechanism to minimize this wasteful activity

D

Similar scenario can occur with partial reversal method too

103

Temporally-Ordered Routing Algorithm

(TORA) [Park97Infocom]

TORA modifies the partial link reversal method

able to detect partitions

to be

g  

When a partition is detected, all

are informed, and link reversals

nodes

in that

in the partitionpartition cease

g  

104

Partition Detection in TORA

B

DAG for destination

A

D C

E

D

F

105

Partition Detection in TORA

B A

C E

modified partial reversal method F

Node A has no outgoing

Partition Detection in TORA

B A

C E

modified partial reversal method F

Node B has no outgoing links

107

Partition Detection in TORA

B A

C E

D

F

Node B has no outgoing links

108