04/18/23 1

Mobile Ad-hoc Networks: Issues and Challenges

Uyen Trang NguyenDept. of Computer Science & EngineeringYork University (Toronto, Canada)

04/18/23 2

Wireless Networks: TaxonomyWireless Networks

Infrastructure-based Infrastructureless

Cellular Networks

Wireless LANs Ad Hoc NetworksMobile Adhoc Networks

Static NodesMobile Nodes

Sensor Networks

04/18/23 3

MANETs: Introduction No infrastructure (no base stations or access

points) Mobile nodes

Form a network in an ad-hoc manner Act both as hosts and routers Communicate using single or multi-hop wireless links

Topology, locations, connectivity, transmission quality are variable.

04/18/23 4

Operations

S

X

Y

D

S

X

Z

D

04/18/23 5

Applications Civil

Disaster recovery Taxi cabs Communications over water using floats Vehicular ad-hoc network

Defense Battlefield communications Monitoring and planning

04/18/23 6

Challenges Unpredictable mobility Wireless channels: error-prone media Low bandwidth channels Devices: low power, limited resources Maintaining connectivity, states Security

04/18/23 7

Issues to Be Discussed Medium access control Transport layer issues Security Incentives for cooperation

04/18/23 8

MAC for MANETs: Requirements To avoid interference among simultaneous

transmissions But enable as many non-interfering transmission as

possible Maintain fairness among transmissions

No centralized coordinators: fully distributed operations

No clock synchronization: asynchronous operations

04/18/23 9

Carrier Sensing in MANETs Problems

Hidden terminal problem Exposed terminal problem

Sensing range Transmission range

Contention matters only at the receiver’s end

04/18/23 10

Hidden Terminal Problem

A

B

X

No carrier OK to transmit

04/18/23 11

Exposed Terminal Problem

A

B

XY

Presence of carrier holds off transmission

04/18/23 12

MACs Suitable for MANETs MACA [Karn 1990]

Proposes to solve the hidden terminal problem by RTS/CTS dialog

MACAW [Bharghanvan 1994] Increasing reliability by RTS/CTS/DATA/ACK dialog

IEEE 802.11 Distributed Coordination Function (DCF) Also use RTS/CTS/DATA/ACK dialog

04/18/23 13

RTS/CTS dialog (1)

RTS

Defer

Any node hearing this RTS will defer medium access

04/18/23 14

RTS/CTS dialog (2)

RTS

Defer

CTS

Defer

Any node hearing this CTS will defer medium access

04/18/23 15

RTS/CTS/DATA/ACK dialog

Data

Defer

ACK

Defer

04/18/23 16

IEEE 802.11 DCF Uses RTS/CTS exchange to avoid hidden

terminal problem Any node overhearing a CTS cannot transmit for the

duration of the transfer. Any node overhearing an RTS cannot transmit for the

duration of the transfer (to avoid collision with ACK) Uses ACK to achieve reliability CSMA/CA

Contention-based random access Collision detection not possible while transmitting

04/18/23 17

IEEE 802.11 DCF (cont.) Carrier sense in 802.11

Physical carrier sense Virtual carrier sense using Network Allocation Vector

(NAV) RTS/CTS specify duration of subsequent DATA/ACK NAV is updated based on overheard RTS/CTS

Collision avoidance Nodes stay silent when carrier sensed busy

(physical/virtual) Backoff intervals are used to reduce collision

probability

04/18/23 18

Backoff Interval When channel is busy, choose a backoff

interval in the range [0, cw]. Count down the backoff interval when medium

becomes idle. Count down is suspended if medium becomes

busy again. When backoff interval reaches 0, transmit RTS. Binary exponential backoff in 802.11 DCF:

When a node fails to receive CTS, cw is doubled up (up to an upper bound).

When a data transfer completes successfully, cw is reset to cwmin.

04/18/23 19

IEEE 802.11 CSMA/CA – Example

DIFS: DCF inter-frame space SISF: short inter-frame space

04/18/23 20

Disadvantages of IEEE 802.11 DCF

High power consumption Hidden terminal problem not totally solved

(e.g., collision of RTS) Exposed terminal problem not solved Fairness problem among different transmitting

nodes Only providing best-effort service

04/18/23 21

MAC for Multicast: a Challenging Issue Multicast: efficient info

delivery from a source to a set of destinations simultaneously

Uses 802.11 CSMA/CA

Cannot use RTS/CTS exchange

Currently there are no effective MAC protocols for multicast

04/18/23 22

Issues to Be Discussed Medium access control Transport layer issues Security Incentives for cooperation

04/18/23 23

TCP in Wired Networks Receiver sends ACKs for packets received correctly

Sender times out on unacknowledged packets, retransmits Sender adjusts congestion window

04/18/23 24

TCP in MANETs

TCP is designed for wired networks

Low bit error rate Loss mainly caused

by congestion Routes relatively

fixed

TCP in MANETs

High bit error rate Unreliable wireless

channels Route changes due

to node mobility

04/18/23 25

Consequences TCP sender misinterprets losses as congestion

retransmits unacknowledged segments Why retransmit when there is no route

invokes congestion control enters slow start recovery

Throughput is always low

Why use TCP at all in such cases? For interactions with the Internet and seamless

portability to applications using standard TCP (file transfer, email, browsers).

04/18/23 26

Approaches to Improving TCP Hide error losses from the sender

Sender will not reduce congestion window

Determine the cause of loss If due to errors, do not reduce window size

Modifications are done at the sender only the receiver intermediate nodes only combinations of the above

04/18/23 27

ATCP in the TCP/IP Stack

TCP

IP

Link layer

TCP

IP

Link layer

ATCP

Sender Receiver

04/18/23 28

ATCP Approach Uses network layer feedback from

intermediates nodes for appropriate actions Different types of network feedback:

ICMP: “Destination Unreachable” message indicates route break Stops transmission Waits until a new route is found and resumes trx

ECN: ACK with ECN flag indicates network congestion Invokes congestion control

Retransmission time-out or 3 duplicate ACKs: Retransmits unacknowledged segments without

shrinking congestion window

04/18/23 29

Multicast Transport Reliable delivery

Feedback: ACK, NACK Congestion control

Feedback: loss rate, data rate Group-based

not scalable feedback implosion exposure

Tree-based scalable feedback implosion solved limited exposure

04/18/23 30

Multicast Transport in MANETs: a Challenging Issue

Tree-based extremely difficult to establish a tree-based structure

due to node mobility Group-based

not scalable feedback suppression: random timer, probabilistic multiple simultaneous transmissions: potential

collisions Calculation of round-trip-time: difficult due to

node mobility route break

04/18/23 31

Issues to Be Discussed Medium access control Transport layer issues Security Incentives for cooperation

04/18/23 32

Vulnerabilities of MANETs Wireless links jamming Broadcast nature eavesdropping Mobility, dynamics difficult to detect

anomalies (e.g., bogus routes) No central authorities or infrastructures

difficult key management Trade-offs between resource constraints and

security

04/18/23 33

Potential Attacks Impersonation

An attacker assumes identity and privileges of an authorized node

Denial of service (e.g., jamming, flooding) Network layer attacks

Blackhole a node falsely advertises good paths drops packets

Wormhole: 2 colluding attackers form a tunnel Byzantine: creating routing loops, non-optimal paths Resource consumption attack: generating unnecessary

packets (false route requests, beacons)

04/18/23 34

Attack Countermeasures Defense against external attacks

Authentication + encryption (needs key and trust management)

Defense against internal attacks Secure routing protocols (e.g. SEAD, ARAN, SAODV) Protect routing metrics by hashing

Intrusion detection techniques CONFIDANT

04/18/23 35

Security: Open Issues Key management

Still relies on a central authority (or a group of trusted nodes)

Consumes high power for processing Intrusion detection

Can only detect a group of potential attackers Cannot single out the true attackers or compromised

nodes High rate of false alarm due to unreliable wireless

links and randomness of channel access No solutions to flooding/jamming attacks Incentives for nodes to cooperate

04/18/23 36

Issues to Be Discussed Medium access control Transport layer issues Security Incentives for cooperation

04/18/23 37

Incentives Rewards: per-hop payment in every packet or

in counters embedded in nodes may not always be effective

Punishment punishing both selfish and malicious nodes Example: CONFIDANT [Buchegger 2002]

Based on the biological example in “The Selfish Gene” by Richard Dawkins (Oxford University Press, 1989 edition, 1976)

04/18/23 38

“The Selfish Gene” Reciprocal altruism is beneficial for every

biological system when favors are granted simultaneously

Example: survival chances of birds grooming parasites off each other’s head.

3 types of birds: Suckers: always help others Cheats: get help from but never help others Grudgers: start out by helping every bird

but bears a grudge against those that do not help and subsequently no longer groom their heads

04/18/23 39

CONFIDANT

Every node has 4 components: The monitor The trust manager The reputation system (node rating) The path manager

04/18/23 40

CONFIDANT Components The monitor: neighborhood watch

Listening to transmissions of next nodes Observing route protocol behaviors (no forwarding,

unusual route attraction, unusually frequent route updates)

Calling the reputation system when a bad behavior is detected

The trust manager: distributed and adaptive Using trust function to calculate trust levels Forwarding ALARM messages to warn others of

malicious/selfish nodes Filtering ALARM messages based on trust level of the

reporting node

04/18/23 41

CONFIDANT Components (cont.)

The reputation system (node rating): based on Own experience: greatest weight Observations: smaller weight Reported experience (from ALARM messages): weight

function according to trust level Rating lists are maintained locally, may be

exchanged with friends The path manager

Deleting paths containing malicious nodes Dealing with route requests from malicious nodes

(e.g., ignore, alert the source)

04/18/23 42

Disadvantages of CONFIDANT Authentication is a prerequisite

Requires efficient key management

Implementing all the components requires high processing power and storage incurs lots of overheads

04/18/23 43

SummaryMANETs Unpredictable mobility Low bandwidth channels Wireless channels: error-prone media, vulnerable to

attacks Devices: low power, limited resourcesIssues Medium access control Transport layer issues Security Incentives for cooperation

04/18/23 44

Thank you!